CHARACTERIZATION OF TWO CENTERS OF PORPHYRY-STYLE Cu-(Mo) ALTERATION IN THE WESTERN SUPERIOR MINING DISTRICT, ARIZONA by Sean Patrick O'Neal
A Prepublication Manuscript Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements for the Degree of
PROFESSIONAL SCIENCE MASTERS IN ECONOMIC GEOLOGY In the Graduate College THE UNIVERSITY OF ARIZONA 2015
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Abstract This study characterizes two hydrothermal systems in the Superior mining district of central Arizona that have no previous mention in the scientific literature. Detailed alteration mapping, age dating, and isotopic analysis of carbonates provide insights into the evolution of the systems and comparisons to other systems within the district, the Silver King mine, the Magma mine, and the Resolution deposit. Alteration in the hydrothermal systems of the Superior mining district display a variety of alteration types and contain different metal concentrations, despite all being hosted in rocks of the same stratigraphic sequence and likely being sourced from the same underlying batholith, the Schultze Granite. Alteration mapping of the two hydrothermal systems has identified mineral assemblages and zoning patterns typical of the distal porphyry copper environment. Alteration in the pelitic Pinal Schist and the quartzites of the Apache Group rocks is dominantly comprised of quartz + sericite + pyrite and locally includes pyrophyllite + kaolinite. Carbonate strata have altered to calcic skarn assemblages (marble + garnet ± quartz), magnesian skarn assemblages (tremolite + talc + calcite + sulfides), and quartz + serpentine + manganese oxide + calcite ± pyrite ± chalcopyrite. Porphyry dikes strongly altered to quartz + sericite + pyrite are observed within some of the zones of strong alteration and have not yet been conclusively tied to an exposed source intrusion in the immediate area, which leaves open the potential for one to lie at depth. Alteration assemblages and intensities are highly controlled by stratigraphy. Some units display intense alteration over kilometers along strike whereas others display weaker and less continuous alteration. Radiometric U-Pb zircon ages and crosscutting relationships have constrained the ages of the mineralized systems to younger than ~74 Ma and older than the midTertiary Whitetail Conglomerate. Carbon and oxygen isotopic analyses of altered carbonate 3
strata show shifts in δ13C and δ18O compositions towards values of magmatic fluids, which demonstrates that the host rocks have reacted with magmatic fluids. The western Superior mining district provides an example of exploration opportunity within a “mature” district. Further, the Superior mining district provides an example of the variation of alteration styles and metal contents that can exist within a cluster of porphyry deposits as well as within a single porphyry system.
Introduction Over 130 years of mining and exploration activity has taken place within the Superior (Pioneer) mining district of Arizona. Although it has been considered a “mature” mining district, the discovery of the world class Resolution deposit was not made until the mid-1990’s, over 100 years after the initial discovery of the district (Paul and Manske, 2005). The Resolution deposit is likely associated with the prolific Laramide Schultze Granite, which has been identified as the mineralizing intrusion at the neighboring Globe-Miami district and extends under post Laramide cover rocks to the west (Creasey, 1984; Seedorff et al., 2005b; Stavast, 2006; Maher, 2008; Hehnke et al., 2012). In addition to Resolution, other dikes that are lithologically and temporally similar to phases of the Schultze Granite have been intercepted by drilling that ultimately led to the discovery of the Superior East deposit (Sell, 1995; Seedorff et al., 2005b). Similar dikes associated with porphyry-style Cu-Mo alteration and mineralization have been located in outcrop and intersected by drilling as far as ~5 km west of the town of Superior (core preserved in the Arizona core repository, Duncan and Spencer, 1993). With the western extent of this large and highly productive batholith unknown, further exploration potential for additional porphyry centers exists to the west of Resolution.
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As geologic understanding becomes more refined with time and as mining and metallurgical technology allow for lower grades to be mined profitably, it becomes worthwhile to explore old districts with new perspectives. New understanding of the effects of post-mineral dismemberment associated with extension in the Basin and Range province creates new exploration potential in the region (Wilkins and Heidrick, 1995; Maher, 2008; Nickerson et al., 2010), along with consideration for how alteration-mineralization features in porphyry copper systems tend to vary with wall-rock composition and from proximal to distal positions, including from the roots that underlie orebodies to overlying rocks that might be barren (e.g., Einaudi, 1982a, 1982b; Seedorff et al., 2005a, 2008; Sillitoe, 2010). This study provides detailed descriptions of two, little-known centers of metasomatism, in the northwestern part of the Superior mining district, the King’s Crown and Woodcamp Canyon study areas. Detailed maps of the rocks at the King’s Crown and Woodcamp Canyon systems generated during this study have identified alteration that extends over 9.5 km2 and 5.5 km2, respectively, and that resembles the distal porphyry Cu-(Mo) environment. Carbon and oxygen isotope data indicate that magmatic fluids have interacted with the altered host rocks; however, neither system currently can be associated with a causative intrusion, which leaves open the possibility for additional undiscovered porphyry centers at depth. Radiometric dates and crosscutting relationships demonstrate that the two centers were emplaced between the midTertiary and the Laramide.
Regional Geologic Setting The Superior (Pioneer) mining district of Arizona surrounds the town of Superior, Arizona, which is located in Pinal County, approximately 100 km east of Phoenix (Fig. 1). The
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district is located within a moderately extended portion of the southeastern Basin and Range physiographic province (Dickinson, 1981, 1989, 2002). Mid- to late Tertiary extension affected the Laramide arc of southwestern North America, the latter of which produced a porphyry belt that is one of the most prolific copper provinces in the world (Lang and Titley, 1998; Leveille and Stegen, 2012). The district contains a wide variety of rock types and ages, including schist, intrusive and extrusive igneous rocks, siliciclastic rocks, and carbonate strata that were deposited or emplaced during the Proterozoic, Paleozoic, Mesozoic, and Tertiary. The basement rocks consist of Proterozoic schist and lesser amounts of diorite, granodiorite, and granite that overlie the southern flank of the Archean craton of North America (Conway and Silver, 1989; Dickinson, 1989). Later Proterozoic quartzite and carbonate strata were deposited directly on the basement, and diabase was subsequently intruded primarily as sills along bedding planes in Proterozoic strata and as subhorizontal sheets in the underlying basement rocks at 1.1 Ga (Wrucke, 1989). Paleozoic and Mesozoic quartzites and carbonate strata disconformably overlie Proterozoic rocks. Cretaceous quartzose sedimentary rocks and a thick succession of volcanic and volcaniclastic rocks are preserved in a structural graben near the Resolution deposit (Manske and Paul, 2002; Hehnke et al., 2012) but are not preserved to the northwest in either the King’s Crown or Woodcamp Canyon study areas. Laramide magmatism in the porphyry copper province persisted from ~80 Ma to ~50 Ma (e.g., Leveille and Stegen, 2012). In the Superior area, Laramide magmatism began at ~74 Ma, and various phases of the Schultze Granite were emplaced between 69 and 61 Ma (Seedorff et al., 2005b; Stavast, 2006; Hehnke et al., 2012). The Schultze Granite is associated with the porphyry copper mineralization in the neighboring Globe-Miami district and is hypothesized to be the source of mineralization at Resolution (Peterson, 1954; Stavast, 2006; Maher, 2008;
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Hehnke et al., 2012). Northwest-southeast striking reverse faults (Davis, 1979; Krantz, 1989; Dickinson, 1991), for which there is local evidence within the Superior mining district (e.g., Manske and Paul, 2002; Maher, 2008; this study), were active broadly contemporaneous with Laramide magmatism. The magmatism and tectonism of the Laramide was followed by uplift and erosion and subsequent extension and associated volcanism and sedimentation. Extension occurred in the area between ~25-15 Ma and was accompanied by normal faulting which variably dismembered and tilted the upper crustal rocks (Spencer and Reynolds, 1989; Maher, 2008). Synextensional fluvial and alluvial sedimentary rocks were deposited in the extensional basins (Maher, 2008; see also Dickinson, 1991, Fig. 31; Gawthorpe and Leeder, 2000). Tuffs and lava flows overlie the synextensional sedimentary rocks, the most notable of which is the Apache Leap Tuff, which is a welded ash-flow tuff that was emplaced at 18.6 Ma (Ferguson et al., 1998; McIntosh and Ferguson, 1998).
District Setting The King’s Crown and Woodcamp Canyon study areas encompass two distinct centers of metasomatism that are the focus of this study in the northwestern part of the Superior mining district. The study areas are located 6 km north-northeast and 9 km north of the town of Superior, respectively (Fig. 2a). Neither study area has seen significant exploration in more than 50 years or has any description in the published literature. There is limited exploration reporting and a few drill holes with depths of 500 m or less that were drilled in the Woodcamp Canyon area, and no known previous exploration drilling at King’s Crown.
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The Superior district and the eastern portion of the King’s Crown study area is divided by a large north-south trending cliff, known as Apache Leap where it bounds the eastern extent of the town of Superior, which consists primarily of Tertiary volcanic and volcaniclastic cover rocks that overlie Mesozoic, Paleozoic, and Proterozoic sedimentary rocks and sills (Fig. 2a, 2b). A high plateau of Tertiary volcanic rocks lies to the east of the cliff, and the low land to the west is mainly Proterozoic schist. The undeveloped Resolution deposit is located beneath the Tertiary volcanic cover, 7 km and 10 km to the southeast of King’s Crown and Woodcamp Canyon respectively (Fig. 2a). The Resolution deposit has an inferred resource of 1.766 Gt of material with an average grade of 1.51% Cu and 0.035% Mo (Anonymous, 2014). Exposed in the cliffs of Apache Leap along the north end of the town of Superior and extending eastward under the cover of the Tertiary volcanic rocks is the Magma mine, which historically produced 1.4 Mt of copper between 1910 and 1996 (Paul and Manske, 2005). The Silver King mine is located within the southeastern corner of the King’s Crown study area and just west of the Tertiary cover rocks (Fig. 2a) and has produced over 6.1 Moz of silver between 1875 and 1928 (Short et al., 1943). The Fortuna, Monarch of the Sea, and Black Diamond mines (Fig. 2a) are all small historic underground operations within the King’s Crown study area that produced small quantities of copper ± silver ± gold (Anonymous, 2015). A dormant marble quarry that is owned by Omya Inc., a Swiss producer of industrial minerals, is located within the east-central portion of the King’s Crown study area (Fig. 2a). In addition to these historic producers, numerous prospect pits dot the landscape within the King’s Crown study area. Several small prospect pits, tunnels, and drifts are also present throughout the Woodcamp Canyon study area; however, no modern or significant historic mining activity has taken place. A notable topographic feature, here referred to as Red Top Hill, is present roughly in the center of
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the Woodcamp Canyon study area (Fig. 2a). Red Top Hill is a local knob underlain and surrounded by basement rocks of the Pinal Schist that make up the majority of the area. The bright red color is due to an abundance of iron oxides that are so prevalent that the hill is easily visible in satellite imagery. A prominent north-south elongate mountain of pink granite occurs along the western border of the study area.
Methods This study focused on geologic mapping, in particular of the various alteration assemblages that occur within the individual hydrothermal centers, with the primary goal of better understanding the genesis of the igneous and hydrothermal activity. Detailed geologic maps were made in a style modified from the Anaconda mapping method (Brimhall et al., 2006) at a scale of 1:5,000 over a large portion of the 35-km2 mapping area. Forty-five days of mapping took place between March 2012 and February 2015, mostly from January through April 2014. A focus was put on recording (1) lithology, including formational unit, rock type, and textures, (2) the extent, mineralogy, and textures of the alteration, (3) the extent, orientations, and distribution of the various dikes, (4) crosscutting relationships between the various igneous units and alteration, and (5) structural information such as faults, folds, breccias, contacts, and bedding orientations. Mapping of lithology built upon the earlier work of Peterson (1960, 1969) and Spencer and Richard (1995). A hand lens, tungsten carbide scribe, magnet, and HCl acid bottle were used during field mapping. Observations were recorded in an illustrative manner with colored pencils on three overlapping mylar sheets. Mapping was supplemented by radiometric U-Pb age dates of igneous zircons, carbon and oxygen isotopic analyses of fresh and altered carbonates, petrographic characterization of
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thin sections, and various mineral identification techniques, including TerraSpec Mineral Analyzer, Raman spectroscopy, handheld XRF, and rock chip geochemistry. Samples for dating were collected to better constrain the ages of the various igneous dikes and intrusive bodies within the study areas. Carbon and oxygen isotopic analyses were conducted in order to provide insight regarding the origins of the hydrothermal fluids that altered the carbonate rocks within the King’s Crown study area. More detailed descriptions of the techniques used to produce the geochronological and isotopic data are provided in later sections of this paper. Transmitted and reflected light petrography was conducted on 22 polished thin sections from samples collected during field mapping in order to confirm the presence of mapped alteration and to aid in the comparison of the various observed dikes.
Rock Units and Stratigraphic Framework Introduction A wide variety of rock types are present in the Superior mining district (Fig. 3a, 4a). Understanding the differences in compositions and textures of the various rock types is important because of the lithologic control on the styles of alteration. Rock units observed in the Superior mining district are described below from oldest to youngest. Estimated thicknesses in the district are taken from Peterson (1969). Nomenclature for grain sizes follows the classes of the Wentworth scale (Wentworth, 1922). Percentages are visual estimations expressed in terms of volume percentage. Proterozoic Pinal Schist The oldest basement rock in the district is the highly variable, Proterozoic (~1.7 Ga) Pinal Schist. Throughout the Superior and neighboring Globe-Miami district, as well as the rest of
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southeastern Arizona, the schist takes on many different varieties and no one variety can be considered typical (Peterson, 1962). It does, however, have some distinctive characteristics within the study areas, mainly a quartz-phyllosilicate-rich composition and an abundance of 1–8cm thick, highly contorted, milky white bull quartz veins. Within the Woodcamp Canyon study area, the schist dominantly consists of strongly foliated muscovite flakes that are ~1-5 mm across, quartz, fine-grained disseminated magnetite, and andalusite grains 1 to 3 cm long (Fig. 5a). This muscovite-andalusite variety also dominates the northern part of the schist at the King’s Crown study area. In contrast, the schist in the southwestern portion of the King’s Crown study area is predominantly a dark, chlorite-rich variety. Within this variation of the schist is a bed of rhyolite schist that crops out over an area of 0.1 km2. A fourth variation of the schist that is locally encountered has weakly foliated beds of psammitic schist consisting of medium-grained quartz and muscovite sand. Still in other places, blue colored psammitic varieties exist with very fine-grained quartz and muscovite. Proterozoic granodiorite Proterozoic granodiorite, ~1.6 Ma, intrudes the Pinal Schist to the west-northwest of the Woodcamp Canyon area (Wilson, 1939; Livingston, 1968). The unit is equigranular, moderately foliated, and contains coarse grained feldspar, muscovite, quartz, and biotite. Proterozoic Pioneer Formation The Proterozoic Pioneer Formation lies unconformably on the Pinal Schist and has a stratigraphic thickness of 90 m. The basal unit is the Scanlan Conglomerate member, which is a conglomerate that is 0–1 m thick with subangular cobbles of bull quartz vein material derived from the underlying schist, 2 to 5 cm in diameter, set in a coarse-grained sandy matrix. The Scanlan Conglomerate is overlain by a medium-grained, dark purplish-gray arkosic quartzite
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with well developed, 15 cm-tall cross bedding and common thin interbeds of felsic volcaniclastic sediments. Greenish gray clots 1-4 mm diameter of fine-grained intergrowths of chlorite and magnetite or ilmenite are observed throughout this unit and are diagnostic of the unit (Peterson, 1969). The Pioneer Formation is a cliff-forming unit with well developed blocky fracture patterns. Where observed to the north of the quartz monzonite porphyry in the Woodcamp Canyon study area, the formation occurs as fissile red shale. Proterozoic Dripping Springs Quartzite The Dripping Springs Quartzite, which is 220 m thick, overlies the Pioneer Formation. The contact between the two units is marked by the Barnes Conglomerate, a distinctive 2- 4-m thick highly resistant marker bed which consists of 3-10 cm cobbles hosted in a coarse-grained, sandy, arkosic matrix. The rest of the Dripping Springs Formation is predominantly a light colored, medium to fine-grained, arenitic to arkosic massive quartzite with local cross beds and subtle ripple marks. The upper half to two thirds of the unit contains frequent 0.1- to 2-meter thick beds of brownish-black arenaceous shale. Proterozoic Mescal Limestone The Mescal Limestone lies atop the Dripping Springs Quartzite. The unit is 110 m thick, highly variable lithologically, and thinly bedded. Beds commonly consist of alternating sandy dolomite, limestone, and shale. Where observed fresh, the limestone weathers to a distinctive chalky white color. The limestone and dolomite commonly contain abundant chert nodules, thin layers of chalcedony, and matted stromatolite fossils. The highly variable textures and the chalky white weathered surfaces are diagnostic of the Mescal Limestone.
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Proterozoic basalt A 20-m thick sill of Proterozoic basalt was emplaced above the strata of the Apache Group within the King’s Canyon study area. The basalt is dark brownish-black in color and is commonly glassy and aphanitic. The rock primarily consists of plagioclase, pyroxene, and olivine that was subsequently altered to iddingsite (Peterson, 1969). Locally the basalt is vesicular that are filled with amygdules. Proterozoic diabase Diabase dated at 1.1 Ga occurs in the Superior district primarily as sills intruding bedding planes and contacts within the Apache Group rocks and locally in the upper portions of the underlying Pinal Schist (Peterson, 1969; Wrucke, 1989). The thicknesses of the sills are highly variable, but generally range between 25 and 75 m. The diabase predominantly consists of medium-grained ophitic intergrowths of plagioclase and interstitial clinopyroxene It is mostly medium-grained, although locally it can be very coarse-grained and commonly has very finegrained chilled margins. Common fine-grained accessory minerals include magnetite, apatite, olivine, and quartz. Local uralitization of the pyroxene phenocrysts to fibrous actinolite needles is observed. The diabase is easily identified by white plagioclase laths against a background of dark green-gray pyroxene. Precambrian Troy Quartzite The Troy Quartzite is a cross-bedded sandstone and quartzite with a basal siltstone layer that overlies rocks of the Apache Group (Wrucke, 1989). This unit is present at Resolution (Manske and Paul, 2002) and to the south of the town of Superior, but is absent in the study areas.
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Cambrian Bolsa Quartzite Bolsa Quartzite, 130 m thick, unconformably overlies strata of the Apache Group, the Troy Quartzite, and diabase. A dark reddish-gray conglomerate 3 to 5 m thick that contains poorly sorted (very fine to coarse-grained,) subangular clasts forms the base of the unit. The majority of the Bolsa Quartzite consists of interbedded layers of moderately sorted feldspathic sandstones with subrounded grains and plane-parallel laminations (Middleton, 1989). The grain sizes vary widely in the different beds from fine-grained to very coarse-grained. Devonian Martin Limestone The Devonian Martin Limestone is a light to medium gray unit, 120 m thick, consisting of many different layers of limestone, dolomite, and thin interbedded sandstone and shale. Some of the limestone beds are fossiliferous, with numerous crinoids and brachiopods (Beus, 1989). There are dull green shale beds, 2 cm thick, are common. Mississippian Escabrosa Limestone The Mississippian Escabrosa Limestone is a light gray, 140-m thick unit of limestone and dolomite. These beds vary widely in texture from micritic to sparry, sandy, shaly, or fossiliferous. The upper half of the unit generally contains abundant chert nodules and interbedded shale partings. Locally, certain beds are very fossiliferous and contain abundant crinoid and brachiopod fragments (Beus, 1989). Permian Naco Limestone The Naco Limestone is only present in a few places in the district. Beds totaling 60 m in thickness are present to the south of the Silver King mine and bordering the northern side of the Conley Spring fault. The unit is 45 m thick near the Magma mine. The unit is a grayish-white, layered limestone with chert nodules and a variety of corals and fossils (Blakey and Knepp,
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1989). The contact between the Naco Limestone and the underlying Escabrosa Formation is marked by a basal, 4-m thick, dusky-red shale (Peterson, 1969). Laramide quartz diorite A stock of Laramide quartz diorite that crops out over an area of 3.3 km2 is present in the center of the King’s Canyon study area. The quartz diorite is a composite pluton that consists of five distinct phases that, from oldest to youngest, include the porphyritic phase, the mediumgrained phase, the fine-grained phase, the dacite porphyry (also known as the Silver King stock, which hosts the Silver King mine), and granitic aplite dikes and dikelets (Puckett, 1970; Fig. 3a). Primary biotite in the quartz diorite yields a 40Ar/39Ar radiometric date of 75 Ma, and igneous zircons from the Silver King stock yield a U-Pb date of 74 Ma (Hehnke et al., 2012). The porphyritic phase is present in the northeastern corner of the intrusion and contains 1 cm long phenocrysts of hornblende and smaller phenocrysts of biotite and plagioclase hosted in a finegrained matrix. The fine-grained phase comprises the majority of the northern half of the intrusion and contains phenocrysts of biotite, plagioclase, and lesser hornblende. The mediumgrained phase comprises the southern half of the intrusion and also contains phenocrysts of biotite, plagioclase, and lesser hornblende. Interstitial quartz ranges between 0% and 15% in these early three phases (Puckett, 1970). The Silver King stock is an east-west elongated elliptical body, 700 m by 350 m, which intrudes into an east-west oriented finger of the mediumgrained phase that emanates from the southeastern flank of the main intrusive body. The stock contains, rounded phenocrysts of quartz, 5-10 mm long, and smaller phenocrysts of plagioclase. The late granitic aplite dikes are light pink to white and have phenocrysts of quartz and biotite (Fig. 6a-b). The dikes range from 1 cm to 3 m in width and are present throughout the intrusion and the rocks immediately surrounding the body; however, the densest concentration occurs
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within the southeastern corner of the intrusion (~0.5%). These dikes are observed cutting a different set of thin, gray, aphanitic dikelets that intrude the Silver King dacite porphyry. These aphanitic dikes are gray in color and plagioclase rich. Contacts between the quartz diorite and the surrounding rocks commonly form clast-supported, non-rotated igneous breccias. Numerous diorite porphyry dikes are observed to the east and west of the intrusion, generally with eastnortheast to east-southeast strikes and steep dips. The groundmass of diorite porphyry dikes is rich in fine-grained plagioclase and phenocrysts of biotite and hornblende. An igneous breccia phase of the intrusion is present along the northeastern corner of the stock and in a circular outcrop off the northeastern corner of the main body (Fig. 3a). The breccia phase contains rounded clasts of schist, diabase, and occasional quartzite of uncertain age (Fig. 6c). Laramide (?) quartz monzonite porphyry A quartz monzonite porphyry (QMP) intrusion intrudes the Pinal Schist and a section of overturned beds of the Apache Group and bounds the Woodcamp Canyon study area to the north. The age of the QMP is uncertain. It’s outcrop pattern is highly elongate east-west, with a prominent northwest-southeast finger emanating from its western edge (Fig. 4a). The intrusion crops out as a large hill along the northern boundary of the Woodcamp Canyon study area. A small, early igneous breccia phase is present along the western finger of the QMP and contains clasts of Proterozoic granodiorite and Dripping Springs Quartzite within a matrix of fine-grained greenish-gray material and phenocrysts of plagioclase and quartz (Fig. 7a). Clasts of the breccia phase are common in the main phase of the QMP, which is tannish-white in color and contains large, 5-mm quartz and plagioclase phenocrysts in a fine-grained groundmass. The QMP along its border has an aphanitic groundmass. A number of 1-2 m wide, north-northwest oriented dikes
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with compositions similar to the QMP are present beyond the northwestern corner of the main QMP intrusion. Tertiary Woodcamp Canyon Granite The Tertiary Woodcamp Canyon Granite intrudes the Pinal Schist along the western border of the Woodcamp Canyon study area. This intrusion crops out as a prominent hill with roughly a north-south elongated oval shape and covers an area of 4 km2. The stock is pink in color but locally white along its border. Its eastern edge is rimmed by a thin, fine-grained, white border facies. The main intrusive body takes on a fine-grained, porphyritic texture along its perimeter but quickly grades to coarse-grained equigranular within its interior. Phenocrysts are primarily quartz, sanidine, and plagioclase with lesser fine-grained biotite (~1%). The stock has a K-Ar biotite date of 18.35 Ma on a sample from the southeastern corner of the intrusion (Shafiqullah et al., 1980). The boundary of the stock is commonly brecciated and cemented by a magnetite-rich matrix. Where its boundary is not brecciated or faulted, numerous fine-grained dikelets, 0.5-5 cm wide emanate from the granite. Tertiary sedimentary and volcanic rocks The youngest rocks in the study area are thick units of sedimentary and volcanic rocks that cover the eastern half of the Superior mining district (Peterson, 1969). These rocks were not mapped during this study. These rocks entirely cover the Resolution deposit and the eastern part of the Magma mine, whereas the western part of the Magma mine, the Silver King mine, and the King’s Crown study area border the western extent of these Tertiary strata. The rocks constitute the Apache Leap cliffs that bound the city of Superior to the east. The oldest of these Tertiary units is the Whitetail Conglomerate, alluvial to fluvial sediments that generally formed in upward-fanning growth sequences that were deposited in syn-
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extensional sedimentary basins (Maher, 2008; see also Dickinson, 1991, Fig. 31; Gawthorpe and Leeder, 2000). The Whitetail Conglomerate is overlain by rhyolite lava flows and then by the Apache Leap Tuff (Peterson, 1969), which is dated at 18.6 Ma (Ferguson et al., 1998; McIntosh and Ferguson, 1998). The total thickness of these Tertiary rocks is thinnest (~160 m) along their western edge where they are exposed in the Apache Leap cliffs and increases eastward to greater than 1,200 m over the Resolution deposit (Peterson, 1969; Manske and Paul, 2002). The volcanic rocks are overlain by quaternary conglomerates, gravels, and alluvium. Other dikes Numerous other dikes, besides the ones described above, are present in the King’s Crown and Woodcamp study areas. These dikes are of unknown ages and cannot be confidently tied genetically to any known source. These dikes are described in later sections.
Structure The King’s Crown and Woodcamp Canyon study areas and the rest of the Superior area contain numerous normal and reverse faults that offset units ranging in age from Proterozoic to Tertiary (Hammer and Peterson, 1968; Peterson, 1960, 1969; Spencer and Richard, 1995). Structures in the district can be broadly categorized into three groups: (1) easterly striking normal faults, (2) folds and reverse faults, and (3) northerly striking normal faults that truncate mineralization. The easterly striking normal faults and reverse faults are commonly mineralized. Fault displacements are particularly well exposed where they displace the well stratified units. Faults in the Pinal Schist are more difficult to identify due to the general absence of marker units that can be used to easily observe offsets, but faults are likely present. For this reason, fewer faults are mapped in the Woodcamp Canyon study area. Two primary orientations of faults
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predominate in the study areas: an older, easterly striking set and a younger, northerly striking set. The slip direction and amount of displacement on these faults has not been fully quantified. The easterly striking normal faults in the study areas are commonly marked by breccia that contain homogeneous, angular clasts cemented in a silicious matrix with ~0.2-1.5% disseminated, oxidized pyrite. Easterly striking normal faults Faults in the easterly striking set dip steeply to both the north and south in the district. These faults tend to host mineralization in the district, as is the case at the Magma mine, the Silver King mine, and various other small veins and fractures, commonly marked by prospect pits, throughout the district (Ransome, 1914; Short et al., 1943; Hammer and Peterson, 1968). The faults are also cut by a younger set of north-south striking normal faults and are covered by Tertiary volcanic rocks (Hammer and Peterson, 1968). In the King’s Crown study area, these faults are commonly observed in the Apache Group and Paleozoic sedimentary rocks and tend to have displacements of tens to a few hundred meters. Because they host Laramide mineralization, these faults must have formed pre or syn Laramide. Folds and reverse faults Laramide contractional features are locally observed along a northwesterly trend throughout the region and district, including reverse faults at the Resolution deposit (Manske and Paul, 2002; McCarrel, 2012), the Elm Canyon overthrust and syncline south of the Magma mine (Short et al., 1942), and folds and reverse faults in and near the Ray mine (Creasey et al., 1983; Keith, 1986; Richard and Spencer, 1998; Maher, 2008). This trend traverses through the eastern portion of the King’s Crown study area and the center of the Woodcamp Canyon study area.
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Contractional features are observed in the Woodcamp Canyon and King’s Crown study areas. Isoclinal folds and small-scale faults with apparent reverse relationships are observed in the pit walls of the Omya marble quarry within the Escabrosa Limestone (Fig. 8a). Southwest of the marble quarry, small mesoscale folds are observed in thin, 2-3 cm-thick, interbedded shale partings in the Martin Limestone (Fig. 8b). Within the Woodcamp Canyon study area, the quartz monzonite porphyry body is bordered to the north by a gently folded, overturned section of the Apache Group (Peterson, 1960). A zone of northwesterly striking structural features cuts through the middle of the Woodcamp Canyon study area and passes between the Woodcamp Canyon Granite and the quartz monzonite porphyry. These structures are strongly to weakly altered to sericite + pyrite and are locally intruded by quartz monzonite porphyry dikes. The timing and dips of these structures have not been determined; however, their strikes suggest that they may be Laramide. Northerly striking normal faults The remaining normal faults in the Superior district generally strike north to northwest and tilt Proterozoic, Paleozoic, Mesozoic, and older Tertiary rocks easterly by ~30-50° and the overlying younger Tertiary rocks easterly by ~5-25° (Hammer and Peterson, 1968; Peterson, 1969; Spencer and Richard, 1995; Maher, 2008). These faults are generally barren of mineralization and cut mineralized rocks (Hammer and Peterson, 1968). Evidence indicates that they consist of more than one set of faults, but all are of Tertiary age (Maher, 2008). The most prominent of the Tertiary faults in the district is the Concentrator fault, which bounds the town of Superior to the east. To the north of Superior, the Concentrator faults, branches, and splays such as the Main fault and the Conley Spring fault (Fig. 3a, Fig. 4a), the latter of which runs through the southern portion of the King’s Crown study area (Peterson,
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1969). Northerly striking normal faults are present to the east and west of the Woodcamp Canyon and King’s Crown study areas. Faults of this set with major displacements are not observed within the study areas themselves; however, ones with minor offsets are likely present.
Hydrothermal Alteration within the King’s Crown Study Area and the Silver King Mine Area Overall distribution and intensity of alteration Hydrothermal alteration within the King’s Crown study area and the Silver King mine area is widespread, and finding a completely unaltered rock is a challenge (Fig. 3b-f). Alteration is highly stratigraphically controlled and manifests itself differently in each rock unit; consequently the description of hydrothermal features is organized by host rock, from oldest to youngest units. General alteration intensities are greatest within Proterozoic and Paleozoic strata in a strip of exposures to the east of the quartz diorite intrusion (Fig. 3b-f). Alteration intensity gradually becomes weaker to the north and south of this zone. A notable zone of intense alteration is evident around the Silver King mine as well; however, there is a noticeable decrease in intensity in the region between these two areas. Alteration is weak within the quartz diorite itself and weakest in the schist to the north of the body (Fig. 3c, e, f). To the south and west of the quartz diorite, the schist is largely altered to a calc-silicate assemblage. The distances over which alteration intensities change differ between all of the various rock formations. For example, alteration in the Dripping Springs Quartzite is continuous over ~4 km along strike, whereas alteration in the neighboring unit, the Pioneer Formation, is only continuous over ~2.5 km. The alteration observed within the study areas is described below by rock type. All percentages are visual estimates expressed as volume percentages, unless otherwise noted.
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Alteration of Proterozoic Pinal Schist Alteration in the schist can be subdivided into two different styles: calc-silicate alteration in the dark chlorite schist and sericite + chlorite + pyrite ± biotite alteration in the other varieties of schist. The calc-silicate assemblage dominates the dark chlorite schist variety, which is everywhere moderately to pervasively altered to this assemblage (Fig. 3f). Variations in the assemblage occur over 1 to10-m intervals but always consist of some combination of the following minerals (listed in order of abundance): chlorite, actinolite, garnet, epidote, diopside, calcite, magnetite, specular hematite, mushketovite, clinozoisite, and rhodochrosite. It is worth noting that sulfides are not a part of this assemblage. The calc-silicates are coarse grained and occur in bands that mimic the pinching, swelling, and curving of the Proterozoic bull quartz veins in the schist (Fig. 5d). Contacts between the dark chlorite schist and the other varieties of schist are sharp. The other compositional and textural varieties of the schist are variably altered. The most intense alteration is mapped along the northeastern corner of the quartz diorite and surrounding the Fortuna mine where the schist is altered to sericite + pyrite ± chalcopyrite ± quartz and contains abundant limonite veins (Fig. 3c). Weaker zones of this alteration style are observed along the eastern side of the quartz diorite. The muscovite-andalusite schist to the north of the quartz diorite is weakly altered to epidote. The rhyolite schist is uniformly moderately altered to chlorite + sericite + pyrite, with pyrite making up roughly 0.5 % of the rock. Rare pods of biotite alteration occur along east-west striking structures to the east of the quartz diorite. Occasional lineations of intense silica with accessory magnetite are observed in the schist (possibly along fault planes?) to the west of the quartz diorite (Fig. 5e).
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Alteration of Proterozoic Pioneer Formation Alteration in the Pioneer Formation is primarily vein-controlled. Where most intensely altered, quartz + sericite + pyrite veins, 1-10 mm wide, constitute up to 6 % of the rock (Fig. 9a) and are accompanied by weak to moderate sericite and up to 0.5 % pyrite disseminated throughout the groundmass. The quartz + sericite + pyrite veins in this intense alteration eventually grade into greisen veins of coarse muscovite + quartz + pyrite accompanied by weaker disseminated alteration to the north and south. Although these two vein types locally occur together, crosscutting relationships have not been observed. However, in a few places sparse chlorite veins are observed cutting and offsetting quartz + pyrite + sericite veins. Weak pyrite + sericitization of the groundmass pervades well beyond the zone of veined rock. Alteration of Proterozoic Dripping Springs Quartzite The Dripping Springs Quartzite is altered to sericite + pyrite ± quartz of variable intensity. Where most intensely altered, the abundance of oxidized pyrite sites can exceed 5%. The style of alteration varies between the different bed types, although the assemblage is always sericite + pyrite ± quartz. The majority of the Dripping Springs Quartzite is medium- to finegrained sandstone, which exhibits pervasive disseminated sericite + pyrite alteration of the matrix (Fig. 9b). Numerous sub-millimeter quartz veins are present in these sandy beds. The thin, shaly layers present throughout the unit display minimal alteration of the matrix but contain abundant, thin quartz-sericite-pyrite veins. These veins typically have many orientations, although the dominant orientation is always parallel to bedding surfaces. Where the texture is preserved, the oxidized pyrite sites in these veins are typically quite large (up to 5 mm) (Fig. 9c). The matrix of the basal Barnes Conglomerate alters to coarse grained, globular, recrystallized quartz along with lesser amounts of sericite and pyrite.
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Alteration of Proterozoic Mescal Limestone The Mescal Limestone, which can contain significant dolomite, exhibits a wide variety of alteration styles (Fig. 3b). Massive tremolite develops in the most intensely altered beds and tends to be densest and coarsest grained around chert nodules (Fig 10a). Tremolite is variably accompanied by sheets of chalcedony, talc, specular hematite, fine grained recrystallization of limestone and dolomite, pyrite, and rare chalcopyrite. Serpentine veins, manganese oxide veins, calcite ± quartz veins, and recrystallization of the carbonates occur outward of the intensely altered zones (Fig 3b). Serpentine and manganese oxide veins are also observed cutting the tremolite-bearing assemblages. A few small local zones of strong tremolite alteration in the middle of less intensely altered carbonates are centered on mineralized dikes. To the south of the Silver King mine, magnetite replacement bodies and manganese oxide veins are observed. Alteration of Proterozoic diabase and basalt Where alteration of diabase and basalt is most intense, the rocks contain disseminated biotite + pyrite alteration of the pyroxene and magnetite and lesser sericitic alteration of the plagioclase along with sub-millimeter biotite veins (Fig. 11a). Biotite is also observed forming as thin, 1-mm wide veins. Overlapping with and outboard of this biotitic alteration, diabase is altered to chlorite ± pyrite. Where these assemblages overlap, chlorite partially or completely replaces biotite (Fig. 11b). In areas of weak alteration, there are sparse quartz ± calcite veins that are 1–3 cm wide. Diabase locally is altered to actinolite, as is the case near the Fortuna mine. Albite + chlorite + epidote veins are locally present in the less intensely altered diabase (Fig. 11c). The outcrops immediately to the north of the Silver King mine contain occasional quartz + pyrite + chalcopyrite + hematite + mushketovite veins. Where observed in contact with the quartz diorite, thick veins (1–3 cm) and pods of magnetite are observed.
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Alteration of Cambrian Bolsa Quartzite The Cambrian Bolsa Quartzite is typically the least altered quartzite within the King’s Crown study area but nonetheless contains upwards of 1 % disseminated oxidized pyrite (Fig. 9e). Where the Bolsa Quartzite is most intensely altered, the matrix is altered to sericite. As alteration intensity decreases, the relative percentages of disseminated sericite + pyrite decrease until the rock is fresh. The thin basal conglomerate unit is typically less altered than the rest of the unit. Alteration of Devonian Martin and Mississippian Escabrosa Limestones The Martin and Escabrosa Limestones are fresh in the northern portion of the King’s Crown Study area and become progressively more altered southward. The first visible evidence of alteration is observed just north and west of the marble quarry, where the beds contain randomly oriented networks of undulating calcite ± quartz ± pyrite veins (Fig. 10b). To the east of the marble quarry, beds are moderately altered to tremolite, weakly recrystallized to finegrained marble, and contain trace oxidized pyrite sites, but the alteration dies out before these units disappear under the cover of Tertiary volcanic rocks. The marble quarry itself is hosted in a structurally thickened section of Escabrosa Limestone and is intensely altered, primarily to coarse-grained marble. Outward of this central core of coarse-grained marble, rocks exposed in the quarry are altered to fine-grained marble with numerous other alteration products, including tremolite, actinolite, magnetite, hematite, trace pyrite, rare chalcopyrite and sphalerite, and occasional pods of phlogopite. The siliciclastic beds have been silicified to hornfels and jasperoid, and magnetite is generally present in the numerous thin shaly beds. Manganese oxide veins, calcite ± quartz ± pyrite veins, and calcite veins with inner hematite halos and outer serpentine halos are observed cutting the fine-grained marble (Fig. 10d). To the south of the
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marble quarry, the Martin and Escabrosa Limestones remain coarsely to finely recrystallized. Pods and veins of jasperoid which occasionally contain sulfosalts (tennantite and tetrahedrite) are intermittently present (Fig. 10e). Within ~150 m of the eastern finger of the quartz diorite, coarse-grained green garnets are present. Carbonate rocks are recrystallized to marble (Fig. 10c) south of the quartz diorite finger all the way to the Conley Spring fault, which displaces the beds by several hundred meters (Fig. 3b). Pods and thin beds of massive hematite, specular hematite, magnetite, bornite, native copper, and cuprite locally occur south of the finger of quartz diorite, such as at the Black Diamond mine (Fig 10h). These assemblages are reminiscent of the mantos replacement bodies present at the Magma mine located 2 km to the southeast. Alteration of Laramide quartz diorite The quartz diorite is variably altered (Fig. 3c, e). The most intense alteration mapped is within the Silver King stock, the host of the Silver King mine. Within and immediately adjacent to the mine area, the stock has been pervasively altered to quartz + sericite + pyrite (Fig. 6f) along with a dense stockwork of quartz veins containing siderite + barite as well as an abundance of ore minerals including chalcopyrite + argentite + argentiferous sphalerite and galena + stromeyerite + native silver + tetrahedrite (Blake, 1883; Ransome, 1914). Other reported ore minerals include anglesite, bornite, cerargyrite, chalcocite, native copper, covellite, cuprite, gold, and malachite (Anonymous, 2015). The veins and alteration in the mine are controlled by a 080°striking fault zone (Haynes and Reynolds, 1980; this study). Beyond the mine area, the dacite porphyry is intruded by thin, curving, anastomosing bodies of dark, aphanitic dikelets (1-3 cm in width) that display biotitic alteration of their mafic sites (Fig. 12a). These dikes are crosscut by small aplite dikes.
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A highly altered breccia pipe, 120 x 140 m across at the surface, is located 450 m to the east of the Silver King mine. The breccia pipe was first noted by King (1971) in an ASARCO internal report and later described by Haynes and Reynolds (1980) in a Fischer-Watt Mining Company internal report. The breccia pipe has sharp contacts with the medium-grained phase of the quartz diorite and cuts off diorite porphyry dikes. In and adjacent to the pipe are widespread quartz + chlorite + k-spar + chalcopyrite + pyrite + magnetite veins that have been subsequently cut off by quartz + sericite + pyrite veins (Haynes and Reynolds, 1980). Pyrite can be as abundant as 6% in the pipe. Other veins include quartz + barite + galena. The pipe is hosted in a steeply dipping, 050° striking fault zone, and many of the veins have a 025° strike, both orientations of which are different than the fault zone that is host to the Silver King mine (Haynes and Reynolds, 1980). Disseminated alteration in the breccia pipe dies out 15–30 m from the pipe, beyond which only sparse chlorite veins are present. A study of fluid salinities and homogenization temperatures shows that the fluids at the Silver King mine were dilute (~2 equiv. wt% NaCl), whereas the fluids at the breccia were highly saline (up to 40 equiv. wt% NaCl) (Haynes and Reynolds, 1980). The study also determined that K-feldspar veins in the breccia pipe formed between 360-440°C and that veins in the Silver King mine formed between 150-300° (Haynes and Reynolds, 1980). The mafic sites adjacent to the eastern edge of the main quartz diorite body are moderately to weakly altered to biotite, but this alteration quickly grades into weak chlorite ± epidote alteration interior to the body (Fig. 3e). The interior and western half of the intrusion contain numerous chlorite + quartz + epidote veins of varying densities (0 – 2%) (Fig. 6e). These veins emanate from the aplite dikelets (Fig. 6a), although the aplites themselves are unaltered. Along the eastern boarder of the quartz diorite, the aplites are cut and offset by quartz + chlorite
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+ pyrite + chalcopyrite veins (Fig. 6b). The density of aplite is greatest in the southeastern corner of the main intrusion (Fig. 3a).The southeastern finger of the quartz diorite is weakly altered to albite + epidote alteration of plagioclase and chlorite after mafic phenocrysts. Adjacent to this finger are coarse-grained marble and garnet alteration in the Martin and Escabrosa Limestones. Both the clasts and the matrix of the breccia phase of the quartz diorite in the northeastern corner of the intrusion appear unaltered in hand sample. The southwestern corner of the quartz diorite body hosts many prospect pits centered on veins of quartz + pyrite + chalcopyrite + sericite ± chlorite ± magnetite ± calcite ± galena. These veins all share a similar east-northeast strike, and some are hosted in fault zones. A pebble dike located roughly in the center of the main intrusion also strikes east-northeast (Fig. 6d) and contains large (~10 cm) rounded clasts within a sericite matrix. The diorite porphyry dikes west of the quartz diorite are fresh or are weakly altered to chlorite and epidote and contain sparse quartz + chlorite + epidote veins (Fig. 6g). On the eastern side of the quartz diorite, the dikes are variably altered to a similar intensity as the host rocks around them. The exception is calc-silicate alteration in the schist, in which the dikes cut the alteration. Where altered, diorite porphyry dikes are altered to chlorite + sericite + pyrite and contain veins of quartz + chlorite.
Alteration within the Woodcamp Canyon Study Area Overall distribution and intensity of alteration Alteration within the Woodcamp Canyon area is concentrated along a north-northwest trending strip between the Tertiary Woodcamp Canyon Granite and the QMP, measuring 3.8 x 1.5 km (Fig. 4b). Within this domain of alteration, virtually all fresh sulfides have been oxidized
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leaving behind only boxwork, iron oxides (predominantly goethite but locally abundant in jarosite) with anomalous base metals (Cu, Mo, Pb, Zn) detected by XRF and rock chip geochemistry. Large quantities of ferricrete are exposed in modern and paleodrainages (Fig. 4b, Fig. 5f). Centered within the zone of most intense alteration is Red Top Hill, which consists of schist that is so highly altered that the limonites are visible in satellite imagery (Fig. 2). Alteration of the Proterozoic Pinal Schist Alteration in the schist occurs over a broad north-northwest trending band between the southern and western edges of the quartz monzonite porphyry and the northeastern edge of the Woodcamp Canyon Granite (Fig. 4b). Alteration is inconsistent or patchy and sporadically alternates in intensity across the beds within the schist and across small-scale fractures and faults. Alteration consists of sericite + kaolinite + oxidized pyrite and veins of sericite + kaolinite + oxidized pyrite ± quartz (Fig. 5h). Many different orientations of sheeted vein sets are observed. Handheld XRF readings commonly yield elevated levels of Cu, Pb, and Zn in the 100’s to 1,000’s of ppm and elevated Mo values in the 10’s to 100’s of ppm (Fig. 4b). A more consistently intense zone of alteration is centered on Red Top Hill, in which blue-green and white colored pyrophyllite (identified by TerraSpec Mineral Analyzer) is part of the assemblage (Fig. 4c, Fig. 5c). Abundant jarosite is also present in this area of intense alteration. Alteration is magnetite-destructive; even the weakest alteration observed destroys all of the magnetite in the schist. Weak veins of chlorite + magnetite + specularite occur outwards of sericite alteration. Alteration of Proterozoic diabase and strata of the Apache Group A section of overturned Apache Group and diabase occurs on the northern side of the quartz monzonite porphyry (Fig. 4a). The Pioneer Formation consists of unaltered red shale, although the Mescal Limestone is weakly veined with serpentine and the Dripping Springs
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Quartzite is weakly altered to sericite + pyrite. Where the diabase is in contact with the western finger of the quartz monzonite porphyry, the mafic sites of the diabase are moderately altered to epidote, and the diabase is cut by quartz + albite + epidote veins (Fig.11c). Alteration of the Laramide (?) quartz monzonite porphyry The quartz monzonite porphyry (QMP) intrusion is mostly unaltered; however, it is strongly altered to disseminated sericite + pyrite and sericite + pyrite veins along its southern contact and its western finger (Fig. 4b, 7b). More internal to the body, the alteration quickly grades to fresh rock. Within the western finger of the QMP, the feldspar sites are weakly altered to albite and epidote. The diabase adjacent to this alteration is also altered and veined to albite + epidote + quartz. The north-northwest trending QMP dikes that emanate from the southern portion of the main stock are all strongly altered and cut by pyrite veins with sericitic envelopes (Fig. 7c). Alteration of the Tertiary Woodcamp Canyon Granite The Woodcamp Canyon Granite is largely fresh (Fig. 4b); however, it contains widespread 1-4 mm wide magnetite-specular hematite veins throughout the majority of the intrusion, averaging 0.2% of the intrusion. The northeastern contact with the schist is silicified; however, this contact is likely fault controlled. Sparse quartz + sericite + pyrite veins are locally present in small zones along the eastern and southwestern contact of the Woodcamp Canyon Granite.
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Characterization and Alteration of Other Dikes In the Study Areas Introduction Numerous other dikes are present in the study areas besides those associated with the quartz diorite in the King’s Crown study area and the QMP in the Woodcamp Canyon study area described in the sections above (Fig. 3a, Fig. 4a). These other dikes are not correlated to an exposed intrusion. Mineralized dike in the Mescal Limestone A subcrop of a mineralized porphyritic dike, ~4-m wide, is present in the Mescal Limestone 1.4 km northwest of the marble quarry (Fig. 3a). The dike is pervasively altered to sericite + quartz + pyrite + calcite, and the original texture of the rock has been destroyed (Fig. 12b). Alteration in the Mescal Limestone adjacent to the dike is more intense than in the surrounding rocks (Fig. 3b). Mineralized dike in the marble quarry An elliptical outcrop of a mineralized dike, 6-m wide, is present on a bench in the marble quarry (Fig. 3a). The dike contains abundant phenocrysts of plagioclase and much less biotite that have been weakly to strongly altered to sericite. The fine-grained groundmass is quartz and plagioclase rich and is weakly altered to sericite (Fig. 12d). The dike contains up to 5% oxidized pyrite sites. The dike crosscuts the contractional deformation observed in the highwalls of the marble quarry. Mineralized dike in diabase A mineralized porphyritic dike, ~0.5-m wide, is present in a small outcrop bordering the quartz diorite to the east. The dike contains plagioclase and mafic phenocrysts which have been
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altered to sericite and biotite respectively. Disseminated hydrothermal pyrite constitutes ~3-5% of the dike. Mineralized dike in the Pioneer Formation A mineralized porphyritic dike, ~7-m wide, is present in the Pioneer Formation northeast of the center of the quartz diorite (Fig. 3a). The dike contains ~30% plagioclase phenocrysts, and ~5% mafic phenocrysts. The dike has been strongly altered to chlorite + sericite + quartz + pyrite (Fig. 12g). Mineralized dike in the quartz diorite A mineralized porphyritic dike, 3-m wide, with plagioclase and lesser biotite phenocrysts intrudes into a small hydrothermal breccia within the southeastern corner of the quartz diorite intrusion (Fig. 3a). The phenocrysts in the dike are strongly altered to sericite and chlorite and the groundmass is strongly altered to sericite + quartz + pyrite (Fig. 12c). The breccia has a quartz + pyrite matrix. The quartz diorite bordering the hydrothermal breccia and dike is moderately altered to epidote and K-feldspar. Mineralized dikes in the Escabrosa Limestone Subcrops of two mineralized porphyritic dikes have been identified in the Escabrosa Limestone near its contact with the eastern finger of the quartz diorite (Fig. 3a). The dikes contain plagioclase phenocrysts and are strongly altered to sericite + quartz + pyrite (Fig. 12e). The dike contains ~5% pyrite that has oxidized to goethite. Garnet-bearing porphyry dikes and sills A few mineralized, garnet-bearing, porphyritic dikes and sills with plagioclase phenocrysts are observed intruding diabase and Bolsa Quartzite east of the quartz diorite body, commonly along bedding planes (Fig. 3a). The dikes are strongly altered to sericite and chlorite
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and contain up to 3% pyrite. Phenocrystic garnet occurs as red euhedral grains up to 8 mm in diameter where unaltered. Garnet maintains its euhedral crystal boundaries but occurs as recrystallized, fine-grained, polycrystalline intergrowths of garnet and manganese oxide, as determined by Raman spectroscopy, where altered (Fig. 12f). The Grandfather Lead The Grandfather Lead is a felsic dike, 1-2 m wide, with a near-vertical dip exposed across ~3.5 km along a northeasterly strike (Peterson, 1969; (Fig. 3a). The dike is pervasively altered to quartz + sericite + pyrite oxidized to goethite, which has obliterated the original rock texture (Puckett, 1970). Alteration does not extend more than a few centimeters beyond the dike. Unmineralized sill in the marble quarry An unmineralized sill is observed in a high wall in the marble quarry intruding along a bedding plane in the Escabrosa Limestone (Fig. 3a). The dike strikes 050° and dips 50° southeast and has a medium-grained, white and gray groundmass and white feldspar phenocrysts. Porphyritic dike south of Red Top Hill A porphyritic dike with hornblende, biotite, and lesser plagioclase phenocrysts crops out south of Red Top Hill (Fig. 4a). The dike intrudes weakly altered Pinal Schist and has a strike of 315° and a dip of 65° to the northeast. The mafic phenocrysts are weakly altered to biotite, and the fine-grained groundmass is weakly altered to sericite (Fig. 12h). No other dikes with a similar composition have been observed in the Woodcamp Canyon study area.
Geochronology U-Pb radiometric age dating was performed on three samples to test the hypothesis of the presence of Schultze Granite-aged magmatism (~69-61 Ma) in the King’s Crown study area
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(Seedorff et al., 2005b; Stavast, 2006; Hehnke et al., 2012). Three samples of igneous zircons were collected from (1) the mineralized porphyritic dike present on the bench in the marble quarry, (2) a mineralized diorite porphyry dike intruding diabase adjacent to the eastern border of the quartz diorite intrusion, and (3) an unmineralized and unaltered granite porphyry dike intruding the medium-grained phase of the quartz diorite. U-Pb zircon geochronology by laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) was conducted on samples 1 and 2 at the University of British Columbia’s Pacific Centre for Isotopic and Geochemical Research, and sample 3 at the University of Arizona LaserChron Center. Methods are discussed in Appendices A and B. Results Results are shown in Table 1. Uncertainties shown in the table are at the 2-sigma level and include only measurement errors. Inheritance was tested in the samples by examining both the core and tip of each zircon where possible. Ages older than Cretaceous were interpreted to represent inheritance in the samples. Many of these ages are Proterozoic in age, which would be expected due to the Proterozoic age of the country rock in the study area. Sample 1 yielded an age of 72 ± 2 Ma, and sample 2 yielded an age of 69 ± 4 Ma (Table 1). Both of these ages are younger than the quartz diorite (~74.8 ± 0.3 Ma, as determined by Hehnke et al., 2012). However, the age of the quartz diorite and sample 1 fall within the error of one another. Sample 2 has a large margin of error and may fall within the age of the Schultze Granite, the quartz diorite, or somewhere in between. Sample 3 yielded an age of ~73.5 ± 4.2 Ma, which is slightly younger than the age of primary biotite in the quartz diorite but ~5 Ma older than the oldest phases of the Schultze Granite (Table 1).
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Stable Isotopic Analyses Purpose The isotopes of carbon and oxygen are useful in the study of hydrothermal ore deposits as a way to identify the sources of alteration fluids, as marine carbonate, magmatic fluids, and meteoric water all have significantly different initial isotopic ratios (Ohmoto and Rye, 1979; Taylor, 1979). Metasomatism of carbonate should result in a shift in isotopic composition due to mass transfer and equilibrium isotopic fractionation between the fluids and host rocks. A total of 12 samples of calcite and dolomite, representative of four broad alteration types, were taken from carbonate strata within the King’s Crown study area and were analyzed for their C and O isotope ratios. The alteration types are (A) fresh ± weak serpentine veins, (B) marble + serpentine + quartz + manganese oxide and calcite ± pyrite veins, (C) tremolite + silica + marble ± talc ± pyrite ± chalcopyrite, and (D) marble + garnet. The results of these analyses are shown in Figure 13, which plots δ 13C vs. δ 18O. The ranges of magmatic C and O and meteoric O are also shown for comparison. Results The different alteration types roughly cluster together (Fig.13). Alteration type A yielded the heaviest δ13C value of 1.81 ‰. Alteration type B generally yields slightly lighter δ13C values than alteration type C. Vein material generally yields lighter δ13C values than non-vein material. The arrow shown in Figure 13 indicates the isotopic difference between a sample of recrystallized limestone (-0.42‰ δ13C and -8.64‰ δ18O) and a sample of calcite vein material (2.83‰ δ13C and -16.38‰ δ18O) that are located a few millimeters apart. The two samples of alteration type D yielded δ13C values of -5.73‰ and -14.11‰, both of which are significantly lighter than the other samples. The sample of the unknown lithology yielded the lightest δ18O 35
value of -22.38‰. This sample was taken from carbonate strata along the western border of the quartz diorite.
Geochemical Environment of Alteration in Carbonate Strata Purpose The stability of alteration assemblages can provide information about geologic environments and processes, which in turn can provide insights for mineral exploration. Alteration assemblages observed in the carbonate units of the Mescal, Martin, and Escabrosa Limestones within the King’s Crown study area were plotted on T vs. XCO2 and T vs. logfCO2 phase diagrams (see Einaudi et al., 1981 Figs. 5-7). These diagrams are used to approximate relative temperatures of formation of the alteration fluids that interacted with the host rocks. These relative temperatures are then plotted on the alteration map and used to identify zoning patterns along the north-south elongated strip of carbonate strata in the eastern portion of the study area (Fig. 14). Assumptions for the pressure and XCO2 conditions under which the alteration assemblages formed needed to be made before the diagrams can be used. Assumptions Pressure is estimated by approximating the depth of emplacement of the systems by adding the thickness of the overlying stratigraphic units. Calculations assume one kilobar of lithostatic pressure for every three kilometers of crust, and hydrostatic pressure is estimated to be one third of the lithostatic pressure (Turcotte and Schubert, 1982). The carbonate strata in the King’s Crown study area are estimated to have been ~2 km below the paleosurface during alteration using the stratigraphic thicknesses reported by Peterson (1969), which roughly yields a hydrostatic pressure of alteration formation of ~250 bars. Pressure has a significantly smaller impact on the stability fields of the assemblages in the phase diagrams than other variables, such 36
as temperature, sulfur and oxygen fugacities, and host rock compositions (Einaudi et al., 1981); therefore, this estimate is sufficiently precise. The stability of certain mineral assemblages can constrain XCO2 (e.g., Einaudi et al., 1981, Fig. 6), For example, serpentine-bearing assemblages are stable only at low values of XCO2 (generally <0.1). Moreover, XCO2 estimates are required in turn to estimate temperature. In the absence of assemblages whose stabilities are useful for constraining XCO2, XCO2 is regarded as either “high” or “low” based on a combination of alteration textures observed in the host rocks and the results of the C and O isotopic analyses. Assemblages that are primarily vein-controlled are assumed to have a relatively low XCO2, as there would have been water-rich fluids coursing through the veins during the reaction. This is consistent with the fact that serpentine alteration, which is indicative of low XCO2, occurs almost entirely as veins within the King’s Crown study area (Fig. 10f). The opposite was assumed for massive alteration assemblages that lack significant veins. In contrast, tremolite-bearing assemblages occur almost entirely as massive replacement of the carbonate host rock and are densest and coarsest-grained around chert nodules, where the assemblage presumably scavenged the silica necessary for the reaction to occur (Fig. 10a), which suggests that the assemblage had minimal direct interaction with hydrothermal fluids. These assumptions are further supported by isotopic data that show that tremolite-bearing assemblages (alteration type C) generally exhibit small isotopic shifts and that massive alteration assemblages generally exhibit smaller isotopic shifts than vein-controlled alteration assemblages (Fig. 13). The garnet-bearing alteration assemblages (alteration type D) exhibit the largest shift in isotopic composition, and are therefore assumed to have formed under low XCO2 conditions (Fig. 13). Alteration assemblages that presumably formed in the presence of water-rich fluids (i.e., the vein assemblages) are all assumed to have a XCO2 value of 0.1 (Huang,
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1976; Taylor and O'Neil, 1977). Mineral assemblages determined to have “high XCO2” values were assigned an XCO2 value of 0.3 because the effect of XCO2 on temperature is minimal for XCO2 values much greater than 0.3. Results Using these assumptions, the alteration assemblages observed in the carbonate strata are plotted on the diagrams, and ranges of temperatures of formation are estimated. The temperature ranges are then plotted to create a map of interpreted hydrothermal fluid temperatures (Fig 14). Some of the mineral assemblages were too poorly constrained to be useful for this exercise, and the locations of these assemblages are marked on the map with question marks. The resulting map highlights three centers of relatively high alteration fluid temperatures (Fig 14).
Interpretations - King’s Crown Study Area Porphyry Cu-(Mo) style of alteration Porphyry-style sericitic alteration marked by the addition of quartz + pyrite, the replacement of feldspars to sericite, and the replacement of biotite, pyroxene, and hornblende to chlorite (Seedorff et al., 2005a) is observed in all of the rock units older than the Tertiary. Higher temperature, more proximal porphyry-style potassic alteration marked by the replacement of biotite, pyroxene, and hornblende to secondary biotite (Seedorff et al., 2005a) is observed in the diabase and the eastern margin of the Laramide quartz diorite and is commonly overprinted by chlorite of the sericitic alteration assemblage. A total of ~7 km2 of porphyry-style alteration is exposed at the surface at King’s Crown and is observed in all but the Tertiary rocks within the study area (Fig. 3b-f).
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Einaudi (1982b) notes that in porphyry systems the formation of metasomatic skarn in carbonate-rich rocks are broadly contemporaneous with potassium-silicate alteration in the associated stock, and alteration of skarn and carbonate-rich rocks to quartz + clay + carbonate + pyrite accompanies sericitic alteration in the stock. Hydrous skarn alteration marked by the replacement of carbonate strata to marble + silica + magnetite + hematite + pyrite + chalcopyrite and additionally tremolite + talc + serpentine + phlogopite in the magnesian strata and garnet in the calcic strata is observed in all three carbonate units at King’s Crown. Styles of skarn alteration similar to those at King’s Crown are observed at the nearby Resolution porphyry Cu(Mo) deposit located ~4 km southwest of the study area, where tremolite and talc are the two most common minerals of the magnesian skarn; chlorite, serpentine, magnetite, hematite, and manganese oxide also typically are present (McCarrel, 2012; Hehnke et al., 2012). The anhydrous calcic skarns at Resolution typically have andraditic garnet ± diopside ± wollastonite and are partially to completely overprinted by hydrous minerals, including chlorite, actinolite, and epidote, with calcite (McCarrel, 2012; Hehnke et al., 2012). Isotopic data show a shift in the δ13C and δ18O compositions of the alteration products in the carbonate strata towards magmatic water values (Fig. 13). Further, analysis of phase diagrams indicate that alteration products formed under high temperature conditions, some up to at least 525 degrees. These lines of evidence in addition to the highly saline fluids observed by Haynes and Reynolds (1980) are consistent with a magmatic source of fluids. In summary, the observed styles of hydrothermal alteration and geochemical environment in the King’s Crown study area are consistent with the distal porphyry Cu-(Mo) environment in a setting where carbonate rocks are widespread (e.g., Einaudi, 1982a).
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Alteration centers in the carbonate strata As shown above, one or two sizeable centers of higher temperature alteration are present at King’s Crown (Fig. 14). These three centers may all be sourced from one porphyry center at depth, and the centers may be attributed to the fluids exploiting multiple permeable zones or structures. Alternatively, there may be a cluster of two or more smaller porphyry centers at depth that each created their own alteration center exposed at the surface. Center 3 of Figure 14 encompasses a porphyritic dike intensely altered to sericite and pyrite, which is interpreted to have locally caused the higher temperature tremolite-bearing alteration assemblage. It is this author’s interpretation that the majority of tremolite alteration observed in the King’s Crown study area primarily formed from contact metamorphism during the emplacement of porphyry dikes and less so from the interaction with hydrothermal fluids. This interpretation is corroborated by the isotopic analyses which show that carbonate material taken from strata that has been highly altered to tremolite did not undergo a large shift in isotopic composition, which shows that the alteration products were not primarily metasomatic in nature. If true, this assumption would mean that zones of massive tremolite alteration would be closely associated with intrusions which, if not exposed at the surface, may lie directly below the subsurface. Center 1 contains ~1.3 km of massive tremolite along the strike of the carbonate host rock but is only associated with a few small mineralized intrusions on the surface (Figs. 3a, 14). It is hypothesized that a larger porphyritic stock or dike swarm lies at depth below this center. Center 2 is spatially associated with the eastern finger of the quartz diorite and the Silver King stock. Center 2 is separated from Center 1 by a lull in alteration intensity (Fig. 14). Coarsegrained garnet vectors towards the contact with the quartz diorite, which suggests that the center
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is associated with the emplacement of the quartz diorite. Alternatively, the contact of the finger of quartz diorite may have acted as a structure that concentrated upward-flowing magmatic fluids of a younger intrusion that spread out laterally after reaching the more permeable carbonate strata. The lateral extent of skarn formation from the causative intrusion is largely dependent on the thickness of limestone beds and the presence of permeable zones, such as faults, interbedded sandstone, or fractured hornfels beds (Einaudi, 1982b). According to Einaudi (1982b) large, well-mineralized skarns rarely extend more than 200 to 400 m from their associated stocks; however, distal, unmineralized to weakly mineralized skarns are found as far as 500 to 900 m from the central stocks of many porphyries and tend to be associated with dikes and sills. These distances of 200 to 900 m rarely extend as far as biotitic (potassium-silicate) alteration extends into felsic to intermediate igneous rocks surrounding the porphyry stocks. At Resolution, distal skarn characterized by weakly marbleized limestones and weak serpentine and proximal skarn characterized by garnet and pyroxene are estimated to extend laterally to the west between 2001,000 m and 100-500 m respectively outward of the 1% ore shell (McCarrel, 2012). Based on these estimates from Einaudi (1982b) and comparison to Resolution (McCarrel, 2012), skarn at King’s Crown is expected to extend 400-900 m from a central stock and 100-700 m away from mineralization. Ages of alteration Centers 1 and 2 postdate the emplacement of the quartz diorite (~74 Ma) and the diorite porphyry dikes, both of which are crosscut by mineralized veins (e.g., Fig. 6b) and predate emplacement of the oldest Tertiary Whitetail Conglomerate, which covers the eastern extent of alteration. The various phases of the Schultze Granite, the prolific mineralizing intrusion of the
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Globe-Miami mining district, formed between ~69-61 Ma (Stavast, 2006), which falls within the realm of possible ages for the alteration observed in the King’s Crown study area. Felsic porphyries that resemble certain phases of the Schultze Granite have been encountered at the nearby Resolution deposit and yield radiometric ages within the range of the Schultze Granite (Seedorff et al., 2005b; Stavast, 2006; Hehnke et al., 2012). Geochronology sample 3, the granite porphyry dike, also resembled certain phases of the Schultze granite. Puckett (1970) concluded that the granite porphyry dikes observed in the King’s Crown study area represented the final phase of crystallization of the quartz diorite. The granitic dike yielded an age of ~73.5 ± 4.2 Ma, which is slightly younger than the age of primary biotite in the quartz diorite of ~74.8 ± 0.3 Ma, as determined by Hehnke et al. (2012) but ~5 Ma older than the oldest phases of the Schultze Granite (Table 1). These results are consistent with Puckett’s interpretation that the granite porphyry dikes are late stage differentiates of the quartz diorite rather than offshoots of the Schultze Granite. Sample 1, the mineralized dike in the OMYA marble quarry, yielded an age of 72 ± 2 Ma, and sample 2, the mineralized dike in diabase adjacent to the eastern border of the quartz diorite, yielded an age of 69 ± 4 Ma (Table 1). Both of these ages are younger than the quartz diorite. For this reason, it is considered likely that a buried intrusion causative of the alteration within the study area is possibly related to the Schultze Granite. However, the age of the quartz diorite and sample 1 fall within the error of one another. Sample 2 has a large margin of error and may fall within the age of the Schultze Granite, the quartz diorite, or somewhere in between. The calc-silicate alteration in the Pinal Schist to the west, south, and east of the quartz diorite intrusion is interpreted to predate the porphyry style alteration and the alteration associated with the Silver King system. Xenoliths of calc-silicate-altered Pinal Schist are
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observed within the quartz diorite intrusion (Fig. 5g). Further, unaltered diorite porphyry dikes crosscut the calc-silicate assemblages. The Silver King system and the porphyry style alteration both demonstrably postdate emplacement of the quartz diorite and diorite porphyry dikes, meaning that the calc-silicate alteration must have occurred as a separate event. The lack of sulfides in the calc-silicate assemblage further suggests that the calc-silicate alteration is unrelated to the other sulfide-rich porphyry related alteration within the study area. The calcsilicate alteration may either be metasomatic or metamorphic in origin. The alteration displays abrupt discontinuities in bulk composition within the compositionally homogenous host rock (the quartz + chlorite rich variety of the Pinal Schist), which are indicative of metasomatism as opposed to metamorphism, which does not show such discontinuities (Einaudi et al., 1981). The alteration intensity of the calc-silicates roughly correlates with the abundance of the highly folded bull quartz veins in the schist, which suggests that the bull quartz veins may have served as a greater silica source for mineral growth and/or acted as impermeable surfaces that concentrated fluids along their boundaries. Alternatively, the calc-silicates may have formed concurrently with the quartz veins, in which case the alteration would predate metamorphism. The calc-silicate alteration occurs most commonly in the dark chlorite-rich variety of the Pinal Schist, which is sharply juxtaposed against the stratigraphically lower, unaltered, muscoviteandalusite variety of the Pinal Schist and the stratigraphically higher Apache Group and diabase. The calc-silicate alteration within this horizon is continuous over ~8 km along strike and continues southwest of the study area across the Concentrator Fault and onto the Picketpost quadrangle (Spencer and Richard, 1995). The contact between the Middle Proterozoic Pinal Schist and the Late Proterozoic Apache Group may have once concentrated fluid flow. Alternatively, the dark chlorite-rich variety of the Pinal Schist may have contained thin layers of
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carbonate strata that, during metamorphism, interacted with the surrounding siliceous material and altered to calc-silicates. Structural interpretations King’s Crown contains few easterly striking faults with offsets greater than 100 m and no northerly striking faults of significant offsets, which suggests that the area, though tilted, may constitute a mostly intact structural block. Northerly striking Tertiary faults with significant offsets bound the structural block to the west of the study area (Peterson, 1969). The folds and reverse faults observed within the study area lie along strike with other known Laramide contractional features in the region and are also presumed to be associated with Laramide contraction. Reverse faults in the marble quarry may have acted as permeable conduits that were exploited by the alteration fluids that marbleized and altered the Escabrosa Limestone. Strata in the King’s Crown study area dip easterly, and dips vary between 30° and 50° in the Apache Group, between 25° and 35° in the Whitetail Conglomerate and overlying rhyolite, and between 5° and 15° in the Apache Leap Tuff (Peterson, 1969). The maximum dip in the synextensional Whitetail Conglomerate provides a limit for the minimum amount of tilting of the hanging wall related to Tertiary extension of 35°. The average dip observed in the Apache Group and Paleozoic strata of 45° would represent the full amount of Tertiary tilting if the strata are presumed to be flat-lying pre-Tertiary. However, Laramide contractional deformation may have tilted the beds easterly or westerly, which would cause the net Tertiary tilting to be either greater than or less than 45°. The most common relationship observed elsewhere in the region is that the pre-Tertiary strata were close to flat-lying (Maher, 2008); thus 45° is a reasonable estimate of the total amount of Tertiary tilting. With this assumption, a vertically emplaced Laramide porphyry
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system would have been tilted 45° east and presently would have a westerly plunge of 45°, as shown in Figure 15. Exploration targeting Highly altered porphyritic dikes are spatially associated with some of the strong skarn development and other alteration assemblages in the King’s Crown study area. These porphyritic dikes are not associated with a known causative intrusion and could potentially be sourced from a buried intrusion at depth below the surficial expression of alteration. If the hypothesized intrusion is Laramide, then it would have been tilted by Tertiary normal faults and would currently plunge ~45° to the west-southwest. Rocks more proximal to this hypothesized buried intrusion would be a target for porphyry Cu-(Mo) mineralization. The relationship observed in alteration Center 3 suggests that tremolite in the carbonate strata is closely associated with intrusive bodies. Center 1 contains ~1.3 km of massive tremolite alteration along the strike of the carbonate host rock but is not associated with an exposed intrusion. Because of tilting associated with Tertiary extension, a target for porphyry Cu-(Mo) mineralization would lie to the west-southwest of this center. Further evidence in support of this hypothesis is provided by the zoning patterns of the distribution and intensities of the various porphyry-style alteration intensities observed in the other rocks in the study area, all of which vector towards this same center (Fig. 3b-f).
Interpretations - Woodcamp Canyon Study Area Hypogene and supergene porphyry Cu-(Mo) styles of alteration A total of ~5 km2 of porphyry Cu-(Mo) style alteration is observed in the Pinal Schist and part of the QMP within the Woodcamp Canyon study area. The Pinal Schist is weakly to
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strongly altered to sericitic alteration, which is marked by the addition of quartz + pyrite and the replacement of feldspars by sericite at moderate temperatures (~300° to 450°C) in the hypogene porphyry environment, and the local presence of pyrophyllite and kaolinite is indicative of even more intense hydrogen metasomatism and base-cation leaching at moderate to low (~200° to 360°C) temperatures (Seedorff et al., 2005a). Advanced argillic alteration is also observed at the Resolution deposit where it is centered on and occurs in the high portions of the system (Hehnke et al., 2012). The advanced argillic alteration at both the Woodcamp Canyon study area and at Resolution overprints quartz + sericite + pyrite alteration. Following subsequent uplift and erosion, oxygenated groundwater reacted with altered schist in the supergene or weathering environment. Where meteoric groundwater in the vadose zone interacts with rocks with high pyrite content, fluid acidities are high, which results in the oxidation of sulfides and the leaching and transport of base metals to the top of the water table (Anderson, 1982; Titley and Marozas, 1995; Sillitoe, 2005). Fresh sulfides are exceptionally rare within the Woodcamp Canyon study area, and all veins are oxidized to goethite and locally to jarosite. Ferricretes and manganocretes are observed in modern and paleodrainages (Fig. 5f), which is indicative of multiple events of lateral flow of these strongly acidic fluids (Hayes, 2004). Oxidized vein surfaces in the schist commonly yield elevated Cu, Mo, Pb, and Zn anomalies measured by hand-held XRF at levels of 100’s to 1,000’s of ppm (Fig. 4b), which are the remnants of the mineralization that was present in the veins before weathering. If base metals were indeed leached from the altered schist and remobilized, they would have been transported downward, perhaps with an important lateral component, and precipitated at the top of the water table. Supergene redistribution of metals may have occurred more than once-prior to, during, and/or after tilting by Tertiary normal faulting.
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In summary, the Woodcamp Canyon study area resembles the distal, hydrolytic porphyry Cu-(Mo) environment in a setting where carbonate rocks are absent at the present level of exposure. Moreover, hypogene features having been subsequently modified by supergene processes, with the present surface representing the leached cap of a porphyry system (e.g., Anderson, 1982; Sillitoe, 2005), albeit in a geometry perhaps modified by Tertiary faulting and tilting. Structural interpretations Faults are observed in the schist within the Woodcamp Canyon study area; however, no constraints on dips or amount of offset can easily be determined due to the lack of marker units. A northwest-striking structural zone passes through Red Top Hill and between the Woodcamp Canyon Granite and QMP (Fig. 4a). Alteration in the study area is most intense within this zone, suggesting that it predated mineralization and alteration. The orientation of the zone and its age relationship to the alteration suggest that it might be related to the Laramide thrust features that are observed in the Superior and neighboring districts and throughout southern Arizona. The lack of marker units and constraints on faulting makes estimating the amount of tilting in the study area uncertain. Cleavage orientations in the schist are erratic and do not yield any easily identifiable patterns. Dips in the Apache Group and Paleozoic strata to the east of the study area range from 30°-50° to the east-northeast. Dips in the Apache Group to the north of the QMP range between slightly overturned (~70° to the west) to 40° to the east-northeast. In contrast, dips in the Apache Group 2 km to the south of the Woodcamp Canyon Granite range from 30°-50° to the east-southeast. Finally, dips in the Apache Group 5.5 km to the west of the Woodcamp Canyon Granite range from 20°-55° (Peterson, 1960, 1969; Spencer and Richard,
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1995). These variations in dip suggest that it is likely that both normal and reverse faults have variably dismembered and tilted the rocks within the study area. Age of alteration events Alteration likely occurred during the Laramide and predates intrusion of the unaltered Woodcamp Canyon Granite, which cuts off quartz + sericite + pyrite alteration of Pinal Schist. The southern border of the QMP of presumed Laramide age is altered, although the rest of the body is fresh. The northwest-striking structural zone of probable Laramide age is altered. The presence of modern and ancient ferricretes suggests that multiple events of supergene fluids have occurred in the modern setting; however, this does not rule out the potential for older supergene events to exist. Exploration targeting The advanced argillic alteration around Red Top Hill is the most intense alteration observed in the study area and likely marks the top-center of the hydrothermal system. If the system is Laramide and if 45° of Tertiary tilting to the east-northeast is assumed, then a center of porphyry-style hypogene mineralization would lie at depth to the west-southwest of Red Top Hill (Fig. 16). The Woodcamp Canyon Granite also lies to the west-southwest of Red Top Hill and would have cut through the projection of the Laramide system at depth. Therefore, an exploration target lies at depth between the Woodcamp Canyon Granite and Red Top Hill. The Woodcamp Canyon Granite is the same age as the Apache Leap Tuff (~18 Ma; Shafiqullah et al., 1980) which has been tilted eastward 5-15° by subsequent Tertiary extension, and therefore likely has been tilted by approximately the same amount, in which case the zone of potential hypogene mineralization would extend at depth under the eastern portion of the surface expression of the granite.
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The location of a chalcocite enrichment blanket target is more difficult to predict. The supergene blanket could have formed any time between the deposition of the porphyry system and present day. If the blanket formed prior to or during Tertiary extension, it would have undergone subsequent tilting (e.g., Maher, 2008). Documentation of tilted chalcocite blankets is noted at Resolution (Manske and Paul, 2002) and in the neighboring Globe-Miami District (e.g., Maher, 2008). If an enrichment blanket formed after Tertiary tilting, then it would underlie the current surficial expression of the leach cap. It is also possible that multiple events of supergene enrichment occurred at different times, as is suggested by the presence of abundant ferricretes in modern and ancient drainages, in which case some blankets may be variably tilted while others may be flat-lying.
Discussion Lithologic control on alteration and mineralization Wall-rock composition is one of several factors that produces variability in the expression of alteration and mineralization in hydrothermal systems (e.g., Meyer and Hemley, 1967; Barton et al., 1991a; Seedorff et al., 2005a); therefore, a hydrothermal system emplaced into lithologically heterogeneous crust results in variability of the expression of alteration. For example, potassium-silicate alteration and hydrolytic alteration in igneous rocks are expressed as skarn formation and skarn destruction or silica-pyrite alteration in carbonate strata, respectively (Einaudi, 1982a). This relationship has been documented at Santa Rita (Nielsen, 1970), Ely (James, 1976), and Bingham (Atkinson and Einaudi, 1978). Ore grades are also documented as varying between host lithologies, as at Resolution, where skarns average between 1.7 and 3.2% Cu, diabase and Cretaceous volcanic rocks average ~1.6% Cu, and quartzites average ~0.8% Cu
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(Hehnke et al., 2012). Controls on the extent of alteration are partially attributed to permeability and reactivity/buffering capacity of the wall-rock, where increasing permeability and decreasing reactivity/buffering capacity of the wall-rock results in a greater lateral extent of alteration (e.g., Einaudi, 1982b). Wall-rock composition has a particularly pronounced impact on the alteration expression and extent within the King’s Crown study area. Where carbonate strata are altered to hydrous skarn assemblages, the quartzite units above and below are strongly altered to hydrolytic assemblages. Sericitic alteration in quartzites is expressed as chlorite in adjacent diabase, which is richer in Mg and Fe. The lateral extent of alteration in the more-reactive carbonate wall rocks is less than the less-reactive quartzite wall rocks (Fig. 3b-e). Variation in alteration styles even exists between adjacent rocks of similar compositions within the same zone of alteration type, such as the hydrolytic alteration observed in the adjacent Pioneer Formation and the Dripping Springs Quartzite. The sandy beds of the Dripping Springs Quartzite widely display disseminated sericite + pyrite alteration of the matrix and sub-millimeter quartz veinlets (Fig. 9c). The thin shaly layers present throughout the unit, however, show minimal alteration of the matrix and instead exhibit veins of coarse-grained pyrite cubes with sericite envelopes (Fig. 9d). The quartzite of the neighboring Pioneer Formation displays yet another style of alteration characterized by a stockwork of 1 to 2-cm thick veins of quartz + sericite + pyrite (Fig. 9a). In contrast, the hydrothermal system within the Woodcamp Canyon study area was emplaced into more lithologically homogeneous rocks, which exhibits more uniform styles of alteration (Fig. 4b). Permeability played an important role in controlling the extent of alteration within the Woodcamp Canyon study area, as alteration is widespread in the permeable, mica-rich Pinal Schist but only pervades a short distance into the less-permeable QMP (Fig. 4b).
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Recognition of the effect of wall-rock composition on alteration style, alteration distribution, and ore grade is important for mineral exploration, as the effects of lithology must be accounted for to interpret alteration zoning patterns when targeting an ore body. Copper grades at Resolution are highest in the skarns, followed by the diabase and Cretaceous volcanic rocks, and lowest in quartzites. A similar relationship could likely be observed in the King’s Crown study area. Contrasting styles of alteration-mineralization in the Superior district The Superior district contains a variety of mineral deposits, including porphyry copper deposits and related skarns, veins and mantos related to felsic porphyry bodies, and stockwork veins related to quartz diorites (Table 2). The Resolution deposit is a large porphyry system rich in Cu and Mo, relatively rich in F, relatively poor in As, and displays a wide variety of alteration styles including potassic, phyllic, advanced and intermediate argillic, propylitic, with anhydrous skarn, hydrous skarn, and skarn-destructive alteration (Manske and Paul, 2002; McCarrel, 2012; Hehnke et al., 2012). The Magma mine consists of base metal lodes and mantos that are rich in Cu and As, with lesser Ag and Au (Hammer and Peterson, 1968; Paul and Knight, 1995). The Silver King mine is a small, high-grade, fault-controlled quartz stockwork body that is rich in Ag, Cu, Pb, Zn, and locally Au (Blake, 1883; Hammer and Peterson, 1968; this study). The Woodcamp Canyon and King’s Crown systems are examples of additional diversity in the district (Table 2). Woodcamp Canyon is primarily hosted in schist and exhibits intense phyllic and advanced argillic alteration. It has a well-developed leached cap and also has potential for a chalcocite blanket at depth. King’s Crown was emplaced into a wide variety of host rocks and exhibits a wide variety of alteration styles, including, potassic, phyllic, propylitic and hydrous skarn.
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Much of the variation in alteration style and metal concentrations of the deposits of the Superior district can be attributed to the different host rocks that the systems were emplaced into and the differences in the parts of a porphyry system represented by the deposits. Resolution represents the heart of a giant porphyry system emplaced into stratified sedimentary and volcanic rocks. The present exposures of Woodcamp Canyon and King’s Crown likely represent the distal alteration expressions of the sides of two separate porphyry systems emplaced into (1) schist and (2) schist, diabase, sedimentary rocks, and intrusive rocks, respectively. The base metal lodes of the Magma vein likely represent the top of a porphyry system emplaced into carbonate strata, diabase, and quartzites (Bartos, 1989), the source of which may be at depth, west of the mine (Maher, 2008). The Silver King mine is rich in Ag, Pb, and Zn that were introduced by lowsalinity fluids and might represent a system above and distal to a porphyry system emplaced into igneous host rocks. The compositions of the causative intrusions are unknown across the district, although recent workers speculate that a phase of the Schultze Granite, an intrusion that is unusually felsic compared to most porphyry copper intrusions, is the likely source for the Resolution deposit (e.g., Stavast, 2006; Maher, 2008; Hehnke et al., 2012). Variations in compositions of source intrusions may be responsible for some of the variation observed. These variations in the expressions of the different systems have strong implications for exploration. An exploration program should not limit itself by only looking for specific metal ratios and alteration assemblages or by examining only certain host rocks. Diversity of porphyry systems within single porphyry copper clusters Porphyry copper deposits are known commonly to occur in clusters (e.g., Seedorff et al., 2005a; Sillitoe, 2010). Sometimes the deposits in the cluster bear a strong similarity to one another, whereas in other cases the deposits may differ considerably in intrusive rock
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compositions, metal ratios, alteration style, etc. One example of a porphyry cluster that displays variation among the deposits is the Oyu Tolgoi trend, which is a north-south alignment of six major discrete porphyry systems all within 12 km of each other along strike (Kavalieris and Wainwright, 2005; Crane and Kavalieris, 2012). The Oyu Tolgoi deposits are diverse in terms of alteration assemblages, sulfide mineralogy, and Au/ Cu ratios despite all being sourced from compositionally and temporally similar intrusions quartz monzodiorite dated at 361 to 374 Ma) and being emplaced into largely similar host rocks (mafic volcanic rocks with lesser siltstone and intermediate plutonic rocks) (Crane and Kavalieris, 2012). Potassic, sericitic, propylitic, and advanced argillic alteration assemblages are all variably present in the different systems. Au/Cu ratio in the different systems ranges from 0.1 to 1.2. Hypogene ore minerals in the different deposits range from chalcopyrite dominant, to bornite dominant, to mixed, and may or may not contain significant molybdenum, enargite, tennantite-tetrahedrite, and covellite. Another porphyry cluster that displays similar variation between the different deposits is the Escondida mining district in northern Chile (Hervé et al., 2012), in which the deposits vary in alteration style and metal ratios despite all being sourced from Eocene biotite granodiorite or hornblende dioritic intrusions. In contrast, some porphyry clusters exhibit more similar characteristics between the various deposits, such as the deposits of the Globe-Miami mining district, which were partially emplaced into similar rock strata as the neighboring Superior mining district. The Globe-Miami district exhibits broad similarities in alteration style and deposit types between its three major porphyry centers (Pinto Valley, Miami-Inspiration, and Copper Springs), although the Cu grade and total contained Cu differ notably between the systems (Maher, 2008). All three systems exhibit deeper-level potassic alteration underlying phyllic alteration, higher-level base metal lode
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veins, and supergene enrichment blankets. Chalcopyrite is the primary hypogene ore mineral in all three systems. The Ertsberg-Grasberg mining district is another example of a district that displays broad similarities between the various deposits within the district where the deposits are all sourced from compositionally and temporally similar dioritic intrusions emplaced into similar host rocks and exhibit similar metal concentrations (~0.65% Cu, ~0.5 ppm Au, and ~4 ppm Ag), alteration styles (potassic core that grades into phyllic, distal propylitic halo, prograde skarn, and weak development of advanced argillic alteration), and ore mineralogies (dominantly covellite in phyllic alteration, chalcopyrite in potassic alteration, and bornite at depth) (Leys et al., 2012). The Ertsberg-Grasberg district is sourced from two separate intrusions of similar K-rich dioritic compositions (Leys et al., 2012) and the Globe-Miami district is entirely sourced from the felsic Schultze Granite (Creasey, 1984; Seedorff et al., 2005b; Stavast, 2006; Maher, 2008). Variation appears to be the norm rather than the exception within the Superior mining district, which contains a wide variety of metal ratios and styles of alteration (Table 2). The various systems of the Superior district contain some combination of Cu, Ag, Mo, Au, As, Pb, Zn, and F as either economic resources or tracer elements. Advanced argillic alteration, potassic alteration, and supergene enrichment may or may not be present in each system. Major ore sulfides that may or may not be present in the different systems are chalcopyrite, argentite, bornite, and chalcocite. This is in marked contrast to the neighboring Globe-Miami district where the three porphyry centers (Pinto Valley, Miami-Inspiration, Copper Springs) display broadly the same alteration styles, albeit in different proportions, and have chalcopyrite as the dominant hypogene ore mineral (e.g., Maher, 2008).
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Future work is required to further define the sources of variation. Given the apparent age range in some districts (e.g., Seedorff et al., 2005b), not all intrusions in a cluster necessarily are related to the same large, batholithic-sized batch of magma.
Conclusions Detailed 1:5,000 scale mapping has revealed the presence of porphyry Cu-(Mo)-style alteration at two additional locations in the Superior mining district at King’s Crown and Woodcamp Canyon, with the former containing abundant carbonate rocks, and the latter hosted largely by siliciclastic rocks. The King’s Crown study area contains abundant calcareous rocks, and the metasomatic features include skarn alteration of carbonate strata, potassic (biotitic) alteration of the quartz diorite and diabase, and widespread acid alteration (primarily in the form of quartz + sericite/chlorite + pyrite). Two separate centers of higher temperature alteration in the area have been distinguished: a center to the west-southwest of the marble quarry, and another centered on the Silver King stock. Isotopic analyses of alteration products in carbonate strata show that the host rocks have reacted with magmatic fluids. Radiometric age dates and crosscutting relationships constrain the age of alteration to younger than 74 Ma and older than the Tertiary Whitetail Conglomerate. Laramide igneous activity between ~69-61 Ma is known to occur nearby in the Superior and Globe-Miami mining districts. The rocks have been variably tilted and dismembered by contractional features of probable Laramide age and Tertiary extensional faults. Precambrian and Paleozoic rock strata currently dip between 30°-50° to the east-northeast due to post middle Tertiary tilting related to extension. A Laramide porphyry system would, therefore, presently plunge ~45° to the west-southwest. Several mineralized and highly altered
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porphyritic dikes are observed within some of the zones of intense alteration. Two of these dikes were dated at ~72 Ma and ~69 Ma. These dikes are not tied to a causative intrusion and could potentially be sourced from a buried intrusion at depth below the outcropping alteration. The Woodcamp Canyon study area is hosted primarily by siliciclastic rocks. Detailed mapping has revealed hypogene porphyry Cu-(Mo)-style alteration with a strong supergene overprint. Metasomatic features include widespread quartz + sericite + pyrite alteration in the Pinal Schist and the southern portion of the QMP, advanced argillic alteration (marked by the presence of pyrophyllite and kaolinite) in the schist at Red Top Hill, the pervasive oxidation of sulfides to goethite ± jarosite, and the presence of ferricretes and manganocretes. The observed alteration is younger than the QMP (of presumed Laramide age) and older than the Woodcamp Canyon Granite (~18 Ma). The ferricretes occur in multiple horizons, suggesting that multiple events of supergene fluids have occurred in the modern setting; however, the potential for additional older supergene events also exists. The rocks in the study area have likely been dismembered and tilted by contractional features of probable Laramide age and by Tertiary extensional faults, although there are not marker units within the immediate study area to constrain the amount of post-mineral tilting. Strata near the study area dip between slightly overturned (70° to the west) to 30° to the east-northeast. If a Laramide porphyry system had been tilted 45° to the east-northeast by Tertiary extension, it would presently plunge 45° to the westsouthwest. A chalcocite blanket formed by modern day supergene fluids would lie directly below the surface footprint of the leach cap; however, older Tertiary or Laramide aged chalcocite blankets would be variably tilted, depending on the time of formation. The variability in the expression of alteration and mineralization at King’s Crown is attributed in part to the presence of a wide range of wall-rock compositions. Further variation in
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alteration style as well as metal contents and deposit types is also observed between the different deposits within the Superior mining district. Future work is required to further define the sources of variation.
Acknowledgments I thank Desert Star Resources (DSR) and Bronco Creek Exploration (BCE) for their financial support of this research. Additional funding was provided by a generous Society of Economic Geologists (SEG) Student Research Grant. The study greatly benefitted from previous detailed mapping of the local stratigraphy by Donald W. Peterson of the U. S. Geological Survey. I thank my advisors Eric Seedorff and Mark Barton, for their guidance, encouragement, and geologic insights. I thank David Maher for helping formulate and bringing this project to fruition. Stimulating geologic conversations with David Maher, Doug Kreiner, and Michael McCarrel of BCE and Alan Wainwright, Daniel MacNeil, and Leanne Smar of DSR greatly improved the results of this study. I also thank Leanne Smar, David Maher, Michael McCarrel, Leif Hammes, Doug Kreiner, Honza Catchpole, J.D. Mizer, and Christian Rathkopf for their help and contributions to field mapping. Access to properties for geologic mapping and sampling for this project by Omya Inc. and the Silver King of Arizona Mining Company is appreciated. Conversations about the Resolution deposit with Hamish Martin of Resolution Copper Company provided useful context for this study.
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APPENDIX A. U-Pb GEOCHRONOLOGIC METHODS—UNIVERSITY OF ARIZONA LASERCHRON CENTER Zircon crystals were extracted from samples by traditional methods of crushing and grinding, followed by separation with a Wilfley table, heavy liquids, and a Frantz magnetic separator. Samples were processed such that all zircons were retained in the final heavy mineral fraction. A split of these grains (generally 50-100 grains) were selected from the grains available and incorporated into a 1” epoxy mount together with fragments of our Sri Lanka standard zircon. The mounts were sanded down to a depth of ~20 microns, polished, imaged, and cleaned prior to isotopic analysis. Cathodoluminescence (CL) images of the grain mounts were obtained using a Cameca Camscan Series II scanning electron microscope (SEM). The CL images revealed heterogeneities in zircon grains, such as zoning, inheritance, or inclusions, and the images were later used to map the locations of points analyzed on each grain. After CL images were obtained, the carbon coating was removed, and the sample was repolished with diamond powder and scratched to aid in focusing the laser beam during analysis, when the grains are observed under combined reflected and transmitted light microscopy. U-Pb geochronology of zircons was conducted by laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) at the Arizona LaserChron Center (Gehrels et al., 2008). The analyses involve ablation of zircon with a New Wave UP193HE Excimer laser (operating at a wavelength of 193 nm) using a spot diameter of 30 microns. The ablated material is carried in helium into the plasma source of a Nu HR ICPMS, which is equipped with a flight tube of sufficient width that U, Th, and Pb isotopes are measured simultaneously. All measurements are made in static mode, using Faraday detectors with 3x1011 ohm resistors for 238U, 232Th, 208Pb, 206Pb, and discrete dynode ion counters for 204Pb and 202Hg.
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Ion yields are ~0.8 mv per ppm. Each analysis consists of one 15-second integration on peaks with the laser off (for backgrounds), 15 one-second integrations with the laser firing, and a 30 second delay to purge the previous sample and prepare for the next analysis. The ablation pit is ~15 microns in depth. For each analysis, the errors in determining 206Pb/238U and 206Pb/204Pb result in a measurement error of ~1-2% (at 2-sigma level) in the 206Pb/238U age. The errors in measurement of 206Pb/207Pb and 206Pb/204Pb also result in ~1-2% (at 2-sigma level) uncertainty in age for grains that are >1.0 Ga, but are substantially larger for younger grains due to low intensity of the 207Pb signal. For most analyses, the cross-over in precision of 206Pb/238U and 206Pb/207Pb ages occurs at ~1.0 Ga. 204
Hg interference with 204Pb is accounted for measurement of 202Hg during laser ablation
and subtraction of 204Hg according to the natural 202Hg/204Hg of 4.35. This Hg is correction is not significant for most analyses because our Hg backgrounds are low (generally ~150 cps at mass 204). Common Pb correction is accomplished by using the Hg-corrected 204Pb and assuming an initial Pb composition from Stacey and Kramers (1975). Uncertainties of 1.5 for 206Pb/204Pb and 0.3 for 207Pb/204Pb are applied to these compositional values based on the variation in Pb isotopic composition in modern crystal rocks. Inter-element fractionation of Pb/U is generally ~5%, whereas apparent fractionation of Pb isotopes is generally <0.2%. In-run analysis of fragments of a large zircon crystal (generally every fifth measurement) with known age of 563.5 ± 3.2 Ma (2-sigma error) is used to correct for this fractionation. The uncertainty resulting from the calibration correction is generally 1-2%
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(2-sigma) for both 206Pb/207Pb and 206Pb/238U ages. Concentrations of U and Th are calibrated relative to our Sri Lanka zircon, which contains ~518 ppm of U and 68 ppm Th. Results are shown in Table 1. Uncertainties shown in the table are at the 2-sigma level and include only measurement errors. Inheritance was tested in the samples by examining both the core and tip of each zircon where possible. Ages older than Cretaceous were interpreted to represent inheritance in the samples. Many of these ages are Proterozoic in age, which would be expected due to the Proterozoic age of the country rock in the study area. The resulting interpreted ages are shown on weighted mean diagrams using the routines in Isoplot (Ludwig, 2008) (Fig. A1). The weighted mean diagrams show the weighted mean (weighting according to the square of the internal uncertainties), the uncertainty of the weighted mean, the external (systematic) uncertainty that corresponds to the ages used, the final uncertainty of the age (determined by quadratic addition of the weighted mean and external uncertainties), and the MSWD of the data set.
References Gehrels, G.E., Valencia, V., Ruiz, J., 2008, Enhanced precision, accuracy, efficiency, and spatial resolution of U-Pb ages by laser ablation-multicollector-inductively coupled plasma-mass spectrometry: Geochemistry, Geophysics, Geosystems, v. 9, Q03017 doi:10.1029/2007GC001805. Ludwig, K., 2008, Isoplot 3.6: Berkeley Geochronology Center Special Publication 4, 77 p. Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial lead isotope evolution by a two-stage model: Earth and Planetary Science Letters, v. 26, p. 207-221.
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Figure A1. Geochronology results for the granitic dike in the quartz diorite.
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APPENDIX B. U-Pb GEOCHRONOLOGIC METHODS—UNIVERSITY OF BRITISH COLUMBIA PACIFIC CENTER FOR ISOTOPIC AND GEOCHEMICAL RESEARCH Zicrons were analyzed using laser ablation (LA) ICP-MS methods, employing methods as described by Tafti et al. (2009). Instrumentation employed for LA-ICP-MS dating of zircons at the PCIGR comprises a New Wave UP-213 laser ablation system and a ThermoFinnigan Element2 single collector, double-focusing, magnetic sector ICP-MS. All zircons greater than about 50 microns in diameter were picked from the mineral separates and were mounted in an epoxy puck along with several grains of the 337.13 ± 0.13 Ma Plešovice zircon standard (Sláma et al., 2007), together with a Temora 2 reference zircon, and brought to a very high polish. The surface of the mount was washed for 10 minutes with dilute nitric acid and rinsed in ultraclean water prior to analysis. The highest quality portions of each grain, free of alteration, inclusions, or possible inherited cores, were selected for analysis. Line scans rather than spot analyses were employed in order to minimize elemental fractionation during the analyses. A laser power level of 40% was used. A 25 micrometer spot size was used. Backgrounds were measured with the laser shutter closed for ten seconds, followed by data collection with the laser firing for approximately 35 seconds. The time-integrated signals were analyzed using Iolite software (Patton et al., 2011), which automatically subtracts background measurements, propagates all analytical errors, and calculates isotopic ratios and ages. Corrections for mass and elemental fractionation were made by bracketing analyses of unknown grains with replicate analyses of the Plešovice zircon standard. A typical analytical session at the PCIGR consists of four analyses of the Plešovice standard zircon, followed by two analyses of the Temora2 zircon standard (416.78 ± 0.33 Ma), five analyses of unknown zircons, two standard analyses, five unknown analyses,
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etc., and finally twoTemora2 zircon standards and four Plešovice standard analyses. The Temora2 zircon standard was analyzed as an unknown in order to monitor the reproducibility of the age determinations on a run-to-run basis. Final interpretation and plotting of the analytical results employed the ISOPLOT software of Ludwig (2003). Thanks to Hai Lin for mineral separation, Vivian Lai for ICP-MS/laser set-up, and Dave Newton for help with all other steps of the dating process.
References Ludwig, K., 2003, Isoplot/Ex, version 3: A geochronological toolkit for Microsoft Excel: Berkeley, California, Geochronology Center, Berkeley. Patton, C., Hellstrom, J., Paul, B., Woodhead, J., and Hergt, J., 2011. Iolite: Freeware for the visualization and processing of mass spectrometry data: Journal of Analytical Atomic Spectroscopy, v. 26, p. 2508-2518. Sláma, J., Košler, J., Condon, D.J., Crowley, J.L., Gerdes, A., Hanchar, J.M., Horstwood, M.S.A., Morris, G.A., Nasdala, L., Norberg, N., Schaltegger, U., Xchoene, B., Tubrett, M.N., and Whitehouse, M.J., 2007, Plešovice zircon — A new natural reference material for U–Pb and Hf isotopic microanalysis: Chemical Geology, v. 249, p. 1-35. Tafti, R., Mortensen, J.K., Lang, J.R., Rebagliati, M., and Oliver, J.L., 2009, Jurassic U-Pb and Re-Os ages for newly discovered Xietongmen Cu-Au porphyry district, Tibet: Implications for metallogenic epochs in the southern Gangdese belt: Economic Geology, v. 104, p. 127–136.
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Figure B1. Geochronology results for the porphyritic dike in the Pioneer Formation.
Figure B2. Geochronology results for the porphyritic dike in the OMYA marble quarry.
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Tables Table 1. Summary of U-Pb geochronology samples from King’s Crown. Table 2. Summary of ore deposits and hydrothermal systems of the Superior mining district.
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Table 1: Summary of U-Pb Geochronology of Samples from King's Crown Sample #
UTM Easting1
UTM Northing1
1
493538
3689156
2
491954
3689178
3
490727.45
3688220.7
Sample Description
Alteration and Mineralization
Plagioclase-rich porphyritic dike in marble quarry Plagioclase-rich porphyritic dike in diabase Granite porphyry dike in quartz diorite
Highly altered to quartz + sericite + pyrite Highly altered to quartz + chlorite + sericite +pyrite
1
Unaltered
Age Error (Ma) (2σ)
Laboratory2
72
2
PCIGR
69
4
PCIGR
73.5
4.2
ALC
WGS84 datum/ UTM zone 12 Abbreviations: ALC = Arizona LaserChron Center at University of Arizona, PCIGR = Pacific Center for Isotopic and Geochemical Research at University of British Columbia. 2
77
78
not reported
disseminated quartz + sericite + quartz + sericite + pyrite pyrite alteration veins and disseminated alteration; local specular hematite
hydrous skarn
not observed
biotitic alteration of the biotitic alteration of the diabase diabase and quartz and quartz diorite diorite
quartz + sericite + quartz + sericite + pyrite veins pyrite veins and and disseminated alteration disseminated alteration
quartz + chlorite + quartz + chlorite + epidote +/chlorite alteration outward of the epidote +/- albite veins albite veins and chlorite alteration Magma vein and chlorite alteration of the diabase and quartz diorite of the diabase and quartz diorite
schist
n/a: no carbonate strata present
carbonate replacement n/a: no carbonate bodies/mantos strata present
not observed
quartz + sericite + pyrite veins and disseminated alteration minor chlorite + epidote alteration of the diabase
skarn alteration
potassic alteration
phyllic alteration
propylitic alteration
quartz + epidote + albite + chlorite +/- pyrite +/calcite +/- adularia +/specularite +/rhodochrosite alteration of Cretaceous volcanic rocks
deep, early K-feldspar in quartz veins and biotite alteration of the diabase and intrusive rocks
specular hematite + pyrite + distal manganese oxide chalcopyrite + bornite + enargite replacement bodies + tennantite replacement bodies
specular hematite + magnetite + bornite replacement bodies
anhydrous skarn, hydrous skarn, skarn destructive alteration
diabase, quartzite, intermediate volcanic rocks, intermediate to felsic intrusive rocks, limestone, schist
not reported
diabase, limestone, quartzite, intermediate intrusive rocks
anhydrous skarn
limestones, quartzites, intermediate intrusive rocks diabase, schist, intermediate intrusive rocks
Winant 2010 McCarrel 2012 Hehnke et al., 2012
host rocks
Paul and Knight, 1995 Troutman, 2001
Manske and Paul, 2002 Harrison, 2007 Schwarz, 2007
Ransome, 1914 Gustafson, 1961 Hammer and Peterson, 1968 Bartos, 1989
Blake, 1883 Hammer and Peterson, 1968 King, 1971 Haynes and Reynolds, 1980 This study
This study
References
This study
Resolution
Magma Vein
Silver King Mine
Woodcamp Canyon King’s Crown
Table 2. Summary of Ore Deposits and Hydrothermal Systems in the Superior Mining District
79 not reported
distal fluorite
frequent elevated Zn frequent elevated Zn sphalerite in quartz veins XRF readings and XRF readings and rock outwards of the quartz vein rock chip geochemical chip geochemical stockwork core assays assays
F
Zn
Pb
arsenic rich; frequent tennantite and enargite
arsenic poor
common elevated As rare tennantite XRF readings
As
frequent elevated Pb frequent elevated Pb XRF readings and XRF readings and rock rock chip geochemical chip geochemical assays assays
not observed
frequent elevated Cu trace chalcopyrite, rare chalcopyrite, bornite, malachite, chalcopyrite, enargite, bornite, XRF readings, rare tennantite, (chalcopyriteazurite, native Cu, cuprite chalcocite chalcopyrite at depth?) (chalcopyrite + chalcocite at depth?)
Cu
occurs in biotite, topaz, fluorite, sericite, and zunyite
galena in quartz veins outwards of minor galena with Cu and As the quartz vein stockwork core; mineralization supergene oxide and carbonate lead
minor distal galena in propylitic alteration
early sphalerite mineralization in distal sphalerite in the Magma vein propylitic alteration
not reported
arsenic poor; rare enargite and As in solid solution in sulfides
chalcopyrite, bornite, chalcocite, digenite
hematite-dominated leach cap; minor supergene chalcocite along fractures; abundant copper likely remobilized into Whitetail Conglomerate, and deposited as native copper
minor supergene chalcocite and leach cap
Ag chlorides, Pb oxides and carbonates
well developed leach not observed cap, goethite >> jarosite > hematite; hypothesized chalcocite blanket at depth
supergene alteration
sericite + kaolinite ± dickite ± quartz ± topaz ± zunyite (scarce) ± pyrophyllite (scarce) ± APS minerals, (scarce) ± alunite + pyrite + chalcocite + digenite + bornite
kaolinite + dickite ± alunite ± topaz ± pyrophyllite ± zunyite
not reported
sericite + kaolinite + not observed quartz + pyrite + pyrophyllite
advanced argillic alteration
Resolution
Magma Vein
Silver King Mine
Woodcamp Canyon King’s Crown
Table 2. (Continued)
80
rare elevated XRF readings
Ag
rare elevated XRF readings
not observed
Au
Silver King Mine
not observed
not observed
high grade Ag hosted in quartz vein stockwork in the form of argentite, argentiferous galena, argentiferous sphalerite, argentiferous tetrahedrite, stromeyerite, and supergene Ag (cerargyrite chlorides up to 5 ppm in very rare; locally chlorobromide) structures small
barite present in rare, some barite in metal-bearing large, east-northeast quartz veins striking quartz + sulfide veins
common elevated Mo common elevated Mo minor molybdenite in ore body XRF readings and XRF readings and rock rock chip geochemical chip geochemical assays assays
Ba
Mo
Woodcamp Canyon King’s Crown
Table 2. (Continued)
rare barite in advanced argillic alteration
barite occurs with in late stage alteration
0.86 ppm Au as intermolecular <0.1 ppm in Cu-Mo zone dispersion in Cu-Zn-Ag sulfides
1.93 opt Ag, likely as 0.07 opt in ore shell stromeyerite intergrowths in Cu and Zn sulfides
0.03% Mo in ore shell, primarily in potassic alteration
Resolution
not reported
Magma Vein
Figure Captions Figure 1: Simplified regional geologic map showing the locations of the study areas and copper mines and occurrences in relation to the Schultze Granite. Inset map shows the location of the Superior mining district in relation to the Basin and Range extensional province and the Laramide arc of the southwestern United States (e.g., Barton, 1996). Modified after Maher (2008, Hehnke et.al, 2012).
Figure 2: Maps of the Superior mining district showing locations of key geographic features and regional geology. A. Locations of the Woodcamp Canyon and King’s Crown study areas, major and minor historic producing mines with total metal production (after Short, 1943; Paul and Manske, 2005), the Resolution deposit and its inferred resource (after Anonymous, 2014), the town of Superior, Arizona, and Apache Leap escarpment. B. Locations of the Woodcamp Canyon and King’s Crown study areas overlain on lithology, which is modified after Peterson (1960, 1969) and Spencer and Richard (1995).
Figure 3: Dikes, alteration, and veins in the King’s Crown study area overlain on lithology, which is modified after Peterson (1969). A. Locations of the various types of dikes, geochronology samples, cross section A-A’, prospect pits, and the Conley Springs Fault. Dike abbreviations: porph = porphyry, fsp = feldspar, py = pyrite, gt = garnet, bt = biotite. B. Alteration of carbonate strata.
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C. Disseminated sericite alteration. D. Veins in the Pioneer Formation. E. Shreddy biotite and chlorite alteration. F. Calc-silicate alteration in the Pinal Schist.
Figure 4: Dikes, alteration, and veins in the Woodcamp Canyon study area overlain on lithology, which is modified after Peterson (1960, 1969) and Spencer and Richard (1995). A. The distribution of QMP dikes and ferricretes, the locations of Red Top Hill, a porphyritic dike, the Concentrator Fault, a northwest-trending structural zone, the names of major rock units, and the location of cross section B-B’. B. Distribution and intensities of alteration products including sericite, chlorite, and serpentine, and the location of ferricrete occurrences. Also shown are results of handheld XRF analyses (Cu and Mo, n = 189) from oxidized vein surfaces in the leached Pinal Schist. C. TerraSpec mineral analyzer survey of the Pinal Schist over the Red Top Hill area showing occurrences of pyrophyllite, kaolinite, illite, and muscovite.
Figure 5: Photographs of the Pinal Schist. A. Unaltered muscovite-andalusite variety of the Pinal Schist. B. Sericite + pyrite alteration with small zones of pyrophyllite in the Pinal Schist at Red Top Hill. C. Blue-green pyrophyllite in the Pinal Schist at Red Top Hill.
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D. Calc-silicate alteration of the dark chlorite variety of the Pinal Schist. Alteration minerals include garnet, epidote, chlorite, and actinolite. E. Silica replacement lineations in the Pinal Schist, possibly along a fault. F. Ferricrete at Woodcamp Canyon. G. Clast of calc-silicate altered Pinal Schist within the porphyritic phase of the quartz diorite. H. Stockwork quartz + sericite + pyrite veins in the Pinal Schist at Woodcamp Canyon.
Figure 6: Photographs of the quartz diorite. A. Quartz + chlorite + epidote vein emanating from an aplite dike in the medium-grained phase of the quartz diorite. B. Aplite dike in the fine-grained phase of the quartz diorite cut by a quartz + chlorite + pyrite + chalcopyrite vein. C. Breccia phase of the quartz diorite with clasts of diabase, Pinal Schist, and quartzite. D. Pebble dike in the medium-grained phase of the quartz diorite with rounded clasts within a sericite + pyrite + quartz matrix. E. Quartz + chlorite + epidote veins in the eastern edge of the medium-grained phase of the quartz diorite. F. Pervasive quartz + sericite + pyrite alteration in the Silver King stock (XPL 10X). G. Unaltered diorite porphyry dike west of the quartz diorite.
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Figure 7: Photographs of the QMP. A. Early igneous breccia phase of the QMP along the western finger with clasts of Proterozoic granodiorite and Dripping Springs Quartzite within a matrix of fine-grained greenish-gray material and phenocrysts of plagioclase and quartz. B. Sericite + pyrite alteration along the southern boundary of the QMP. C. Sericite + pyrite alteration of a QMP dike.
Figure 8: Photographs of contractional deformation features in the Martin and Escabrosa Limestones. A. Isoclinal folds in the Escabrosa Limestone within the pit walls of the Omya marble quarry. B. Small mesoscale folds in interbedded shale partings in the Martin Limestone.
Figure 9: Photographs of the quartzites. A. Quartz + sericite + pyrite vein stockwork in the Pioneer Formation. B. Close-up of quartz + sericite + pyrite veins in the Pioneer Formation. C. Coarse-grained muscovite + quartz + pyrite vein surface in the Pioneer Formation. D. Sericite + pyrite alteration in a sandy layer of the Dripping Springs Quartzite. E. Disseminated sericite + pyrite vein in a shaly layer of the Dripping Springs Quartzite. F. Quartz + sericite + pyrite alteration in the Bolsa Quartzite.
Figure 10: Photographs of the carbonate strata. A. Massive tremolite alteration concentrating around chert nodules in the Mescal Limestone.
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B. Calcite ± quartz ± pyrite veins in the Martin Limestone. C. Coarse-grained calcite vein cutting through coarse-grained marble in the Escabrosa Limestone near the northern contact with the eastern finger of the quartz diorite. D. Calcite vein with an inner hematite envelope and outer serpentine envelope in the Escabrosa Limestone within the Omya marble quarry. E. Pod of jasperoid + minor sulfosalts in the Martin Limestone. F. Serpentine (chrysotile) veins in the Mescal Limestone. G. Garnet and marble in the Escabrosa Limestone adjacent to the southern contact of the eastern finger of the quartz diorite. H. Magnetite + specularite + Cu oxides from a carbonate replacement body in the Black Diamond mine area located east-southeast of the Silver King mine.
Figure 11: Photographs of the diabase. A. Biotite alteration replacing primary magnetite and pyroxene in the diabase (PPL 10X). B. Chlorite alteration overprinting biotite alteration of primary magnetite and pyroxene in the diabase (PPL 10X). C. Albite + epidote + chlorite veins in the diabase.
Figure 12: Photographs of various porphyritic dikes in the study area. A. Dark aphanitic dikes in the Silver King stock. B. Mineralized dike in the Mescal Limestone pervasively altered to sericite + quartz + pyrite + calcite.
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C. Mineralized dike altered to sericite + pyrite intruding a hydrothermal breccia in the southeast corner of the quartz diorite. D. Mineralized dike altered to sericite + pyrite in the marble quarry. E. Mineralized dike altered to quartz + sericite + pyrite in the Escabrosa Limestone near the northern contact with the eastern finger of the quartz diorite. F. Garnet-bearing sill altered to sericite + chlorite in the Bolsa Quartzite. G. Mineralized dike altered to chlorite + sericite + quartz + pyrite in the Pioneer Formation. H. Dike weakly altered to biotite + sericite south of Red Top Hill.
Figure 13: Plot of δ13C (PDB) vs. δ18O (PDB) showing the isotopic compositions of 12 carbonate samples from the King’s Crown study area. Figure modified after Barton et al. (1982). The range for meteoric water is from Taylor (1979), and the magmatic carbon range is from Ohmoto and Rye (1979). Alteration Units are (A) fresh ± weak serpentine veins, (B) marble + serpentine + quartz + manganese oxide and calcite ± pyrite veins, (C) tremolite + silica + marble ± talc ± pyrite ± chalcopyrite, and (D) marble + garnet.
Figure 14: Plan-view map of the King’s Crown study area showing the estimated ranges of temperatures of formation of the alteration assemblages observed in the carbonate strata and the three centers of higher fluid temperatures discussed in the text (base modified from Peterson, 1969).
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Figure 15: Cross section A-A’ from west-northwest to east-southeast through the center of the King’s Crown study area, showing hypothesized tilted Laramide porphyry center and causative intrusion at depth (modified after Cross section B-B’ from Peterson, 1969).
Figure 16: Cross section B-B’ from west to east through the center of the Woodcamp Canyon study area, showing hypothesized tilted Laramide porphyry center, hypothesized supergene enrichment blankets, and the surficial expression of intense quartz + sericite + pyrite + pyrophyllite alteration over the Red Top Hill area.
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