Alteration Patterns and Structural Controls of the El Espino IOCG Mining District, Chile

Miner Deposita (2014) 49:235 – 259 259 DOI 10.1007/s00126-013-0485-0

ARTICLE

Alteration patterns and structural controls of the El Espino IOCG mining district, Chile G. P. Lopez  M. W. Hitzman  E. P. Nelson &

&

Received: 8 February 2012 /Accepted: 19 August 2013 /Publish Received: / Published ed online: 14 Septem September ber 2013 # Springer-Verlag Berlin Heidelberg 2013 Abstract  The El Espino IOCG mining district is character-

ized by several mineralized bodies the largest of which is the El Espino deposit, which has an estimated geologic resource of 123 Mt at 0.66 % Cu and 0.24 g/t Au. Mineralized bodies are distributed in a 7×10 km 2 area throughout a 1,000-m vertical section. They range from single veins to stockworks and bre brecci ccias as to man mantoto-typ typee dep depos osits its.. The ore bod bodies ies are ho hoste stedd  primarily by volcanic volcanic,, volcanic volcaniclastic, lastic, and sedimentar sedimentaryy rocks of the Early Cretaceous Arqueros and Quebrada Marquesa formations, with a few mineralized zones within Late Cretaceouss dior ceou dioritic itic intr intrusio usions. ns. The faul faultt and vein arch architec itecture ture show showss that El Espino IOCG system was localized within within a dilatational jog along a major transtensional dextral fault system. Sodic altera alt eratio tionn (al (albit bite) e) is the mos mostt ex exten tensiv sivee sty style le of alt altera eratio tionn in the district, and it is bounded by major NS –  NNE trending faults. Sodic – calcic calcic (epidote – albite) albite) alteration occurs at deep to medium elevations (1,000 – 500 500 m) and grades inward into calcic alteration. Calcic alteration alteration surrounds dioritic intrusions of the Llahuin plutonic suite. Significant iron oxides are associated with later calcic alteration associations (actinolite – epidote epidote –  hematite). The upper portions of the alteration system (0 – 500 500 m) display hydrolytic alteration associations with abundant hematite. Hydrolytic veins are feeders to zones of  manto-type alteration and mineralization within favorable volcano-sedimentary lithologies that formed El Espino deposit. Sulfides are largely confined to calcic and hydrolyt hydrolytic ic alteration associations. Hydrothermal fluids responsible for  hematite and sulfide mineralization had salinities between 32

Editorial handling: R. P. Xavier  G. P. Lopez (*) : M. W. Hitzman : E. P. Nelson Department of Geology and Geological Engineering, Coloradoo School of Mines, Golden, Colorad Golden, CO 80401, USA e-mail: [email protected]

and 34 wt% NaCl eq and temperature of approximately 425 °C at an estimated depth of 3 – 4 km. Geochronological U – Pb Pb and 40 39 Ar/  Ar data indicate that hydrothermal alteration was coeval with magmatic intrusive activity. One particular dioritic intrusion (88.5 Ma) preceded the calcic stage (88.4 Ma), which was accompanied by iron oxide copper and gold mineralization. Hydrolytic alteration, related to economic iron oxide copper and gold mineralizati mineralization, on, came immediately after  at 87.9 Ma.

Introduction

Iron oxide copper gold (IOCG) deposits are magnetite- and/or  and/or  hematite-rich hydrothermal systems with economic copper  and gold. Their classification and genesis are still contentiou contentiouss (Haynes  2000  2000;; Hitzman 2000 Hitzman  2000;; Barton and Johnson  2000  2000;; Sillitoe 2003 Sillitoe  2003;; Williams et al. 2005 al.  2005;; Pollard 2006 Pollard  2006;; Chiaradia et al. 2006 al. 2006;; Groves et al. 2010 al. 2010). ). Andean-type IOCG deposits, which are the youngest major province of the IOCG class, formed in an extensional to transtensional tectonic regime within the Jurassic – Cretaceous Cretaceous magmatic arc along the convergent margin of the Central Andes — from from southern Peru to north central Chile (Sillitoe  (Sillitoe   2003). 2003). Andean IOCG deposits display disp lay variable variable morp morpholo hologies gies and alteration alteration styl styles es and have a variety of relationships to possible causative intrusive rocks. Thee El Esp Th Espino ino dep deposi ositt loc locate atedd at 31 31°23 °23′ S in th thee Co Coas asta tall Ra Rang ngee of Northern Chile is one of the southernmost known IOCG systems in Chile (Fig. 1 (Fig. 1). ). Currently, the El Espino deposit has a measured and indicated total resource of 145 Mt at 0.55 % Cu and 0.22 g/t Au (Explorator Resources  2011  2011). ). The El Espino deposit is located at the west margin of a much larger  mining district (El Espino mining district) that contains a number of small vein, stockwork, manto, and irregularly shaped iron oxide-bearing copper  –  gold deposits within a  – gold mixed sequence of Mesozoic volcanic and sedimentar sedimentaryy rocks

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Miner Deposita (2014) 49:235 – 259 259 Methods and material materialss

Fig. 1 Locatio Locationn of El Espino deposit deposit in relation of other Chilean Chilean IOCG

deposits. deposit s. Two NS tren trendin dingg belts belts of ore depo deposit sitss over overlap lap alon alongg the the Chi Chilea leann Iron Belt: magnetite – apatite apatite deposits and IOCG deposits

Projectt go Projec goals als wer weree ach achiev ieved ed thr throu ough gh a com combi binat natio ionn of de detai tailed led field mapping and laboratory studies. Geological, structural, and alteration mapping were done throughout the 70-km 2 study stu dy are areaa at 1: 1:20 20,00 ,0000 sc scale ale du durin ringg 20 2004 04 an andd 20 2005 05.. Add Additi ition onal al informat info rmation ion was gath gathered ered from mapp mapping ing pits and unde undergro rground und adits and from logging of drill core from the El Espino deposit (Fig. 2). Field mapping and loggi logging ng conce concentrate ntratedd on establishing the time – space space relationships and structural and stratigraphic controls of alteration and mineralization. Samples collected during the fieldwork were examined  petrographical  petrogr aphically ly at Colora Colorado do School of Mines using transmitted and reflected light microscopy and with scanning electron microscopy (SEM) and a PGT Spirit analyzing system attached to the SEM for X-ray energy dispersive spectrometry spectro metry.. Detailed petrography petrography was condu conducted cted on samples for fluid inclusion analyses using the methodology of Goldstein and Reynolds (1994 ( 1994)) at Colorado School of Mines Mines.. Major and trace element geochemical analysis were  performed on relatively unaltered samples of volcanic rocks (7) and intrusive (13) rocks, together with hydrothermally altered and mineralized samples from throughout the district  to investigate chemical changes associated with hydrothermal alteration. The samples were crushed and pulverized at ALS Chemex, La Serena, Chile, and then were sent to ALS Chemex, Vancouver, Canada, for geochemical analyses. Whole rock analysis involved lithium metaborate/lithium tetraboratee (LiBO2/Li2B4O7) fus tetraborat fusionand ionand an analy alysis sis by in induc ductiv tiveely coupled plasma (ICP) atomic emission spectroscopy. Trace element analysis was conducted by a combination of ICP mass spectrometry and ICP atomic absorption spectroscopy after four acid near total digestions. Fluid inclusions were analyzed on three double-sided  polished thick sections sections (50  μ m) m) from the Espino and Romero areas at Colorado School of Mines and Fluidinc, Denver. Denver. The analyses were performed using a US Geological Survey-style gas-flow heating/freezing stage mounted on an Olympus microscope equipped equipped with a 40× objective objective (N.A.= 0.55) and 10× 10× oculars. The heating and freezing stages were calibrated at  374.1 and 0.0 °C using synthetic pure H 2O fluid inclusions andd at  56 an 56.6 .6 °C us using ing syn synthe thetic tic CO2 fluid inclu inclusion sions. s. The Thermal rmal cycling was used to bracket the liquid – vapor vapor homogenization temperatures temperatur es (T h) and melting temperatures (T m) of the fluid inclusion assemblag assemblages. es. Sulfur isotope analysis was performed by G. Lopez at  Colorado School of Mines on pyrite (13), chalcopyrite (22), and gypsum (1) separates derived from micro drilling of core and rock samples across the district. Approximately 25 –  100 μ g (weight dependent dependent on the mineral analyzed) analyzed) of sample were combusted in a Eurovector 3000 elemental analyzer at  Colorado School of Mines, yielding sulfur dioxide that was −

cut by intermediate composition intrusions. The El Espino mining district has a vertical relief of approximately 1,000 m allowing investigation of alteration and mineralization styles through a significant vertical section. This paper presents information on lateral and vertical zonation of alteration and mineralization in the district, the structural setting of the mineralization, mineralizati on, the relative ages of igneous i gneous and hydrothermal hydrothermal events, and the chemistry of the hydrothermal fluids inferred from petrographic and isotopic investigations. investigations.

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Fig. 2 Geological framework and location of Espino district. Regional map is simplified from the 1:1,000,000 scale Chilean Geolog ical Survey map (Servicio Nacional de Geología y Minería, 2002).  Rectangle  shows location of Fig. 3 Fig.  3

delivered to a Isoprime mass spectrometer using continuousflow techniques, with helium as the carrier gas. Repeat analyses of a lab working standard (Colorado School of Mines Barium Sulfate) yield a precision of 0.2  ‰ . The isotopic data are reported using the  δ  notation in units of per mile, relative to the Cañón Diablo Troilite standard.

Tectonic and geological setting

The segment between 27° and 33° S in Chile is characterized  by a Mesozoic – Cenozoic Cenozoic magmatic arc built mostly over a Late Paleozoic to Triassic accrecionary prism and arc along a subducting margin. The Mesozoic Andean evolution was characterized by a Jurassic – Early Early Cretaceous magmatic arc accompanied accompan ied by a sedimentary back-arc basin and an aborted marginal basin followed by a Late Cretaceous magmatic arc with a fold and thrust belt to the east (Fig.  2  2). ). The Jurassic –  Early Cretaceous magmatic arc comprised coeval inner and

outer arcs developed in the present Coastal Cordillera and in the Chile – Argentina Argentina border, respectively (Mpodozis and Ramos 1990 Ramos  1990). ). The El Espino deposit is located at the eastern margin of this inner magmatic arc. The Early Cretaceous times were marked by an extensional regime, regim e, marin marinee trans transgres gression sion,, decre decreased ased plut plutonism onismin in the Coast Coast-al Cordillera, and extensive subaerial volcanism — with with marine intercalations — associated associated with intra-arc rifting (Åberg et al. 1984;; Vergara et al.  1995 1984  1995). ). The volcanic rocks are high-K, calc-alkaline to shoshonitic basaltic andesite to andesite. Strontium and neodymium isotopic studies suggest that the volcanic rocks and coeval intrusive rocks were derived from depleted mafic magmas and metasomatized subducted sediments with inclusion of partially melted Jurassic plutonic rocks (Morata and Aguir Aguirre re 2003 2003). ). Th These ese roc rocks ks un under derwen wentt hig highh sub subsid sidenc encee and low-grade burial metamorphism concurrent with extensionrelated plutonism (Levi and Aguirre 1981 Aguirre  1981;; Parada et al. 2005 al.  2005). ). At approximately 100 Ma, the tectonic regime shifted from extensional to compressional between 32 and 33° lat. S,

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resulting in crustal shortening, basin closures, eastward migration of magmatism, and uplift (Arancibia   2004; 2004; Parada et al. 2005 al.  2005). ). The change from extensional to compressional tectonics has been attributed by Arancibia (2004 ( 2004)) to higher  spreading rates in both the southeast Pacific and the south Atlantic (Wilso (Wilsonn 1992  1992). ). Mid-Cretaceous uplift of the Coastal Range Ran ge in thi thiss po porti rtion on of Chi Chile le ha hass been been do docu cumen mented ted by ap apati atite te fission track data from Jurassic and Early Cretaceous plutonic rocks that cooled to approxim approximately ately 80 – 125 125 °C by 110 – 90 90 Ma (Gana and Zentilli 2000 Zentilli  2000;; Parada et al. 1999 al.  1999). ). The Early Cretaceous Arqueros and Quebrada Marquesa formatio form ations ns were deposite depositedd in the Coastal Coastal Range regi region on betw between een 30° and 32° S. The Neocomian Arqueros Formation consists of  lava, volcanic breccia, tuff, and agglomerate with lenticular  intercalations of conglomerate, sandstone, and locally thin fossiliferous limestone. It has an estimated thickness of 3,500 –  4,000 m and is interpreted to have formed near a continental margin with associated volcanic activity and recurrent marine transgressions (Aguirre and Egert  1965 Egert  1965). ). The Barremian to Albian Quebrada Marquesa Formation consists of mixed volcanic and sedimentary rocks (Aguirre and Egert  1965).  1965). The formation was subdivided in two members: the lower Espino Membe Me mberr an andd th thee up uppe perr Que Quelen len Mem Member ber (Ri (Riva vano no and Se Sepu pulv lveda eda 1991). 1991 ). The Espino Member consists of limestone, siltstone, sandstone, and conglomerate with local gypsum lenses. Vertical and lateral facies variations of the Espino Member made it  difficult to define a representative stratigraphic column. The Espino member is interpreted to have been deposited in relatively small marine to transitional basins with sea level variationss (Rivano and Sepulve variation Sepulveda da 1991  1991). ). The Quelen Member, with estimated thickness of 1,200 m, consists of red colored volcaniclastic volcanicl astic and sedimenta sedimentary ry rocks including lava, pyroclastic  breccia,  brec cia, and volcanica volcanically lly derived derived sandstone sandstone and conglome conglomerate rate.. The Quelen member represents deposition in a continental environment environ ment accompanied by intense volcanism. The Early Cretaceous Espino and Quelen members in the El Espino district are covered by the Late Cretaceous Salamanca Formation (initially defined as the Viñita Formation by Aguirre and Egert   Egert   1965, 1965, but later included into Salamanca Formation by Rivano and Sepulveda  1991  1991). ). The Salamanca Format For mation ion is pre prese serve rvedd fro from m the Coa Coasta stall Ran Range ge up to th thee Ma Main in Cordillera and ranges in thickness from approximately approximately 3,500 –  4,000 m. The formation is subdivided into two units: lower  Santa Virginia Member and upper Rio Manque Member  (Rivano and Sepulveda  1991  1991). ). The Santa Virginia Member  consists of conglomerate and red, hematitic sandstone with minor siltstone and lacustrine limestone intercalations. These sedimentary rocks are interpreted to have been deposited as alluvial fans and debris flows with finer-grained sediments deposited in the basin center. The overlying Rio Manque Member consists of tuff and volcanic breccia; it represents deposition during a period of intense and explosive latest  Cretaceous volcanism (Rivano and Sepulved Sepulvedaa  1991  1991). ).

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Cretaceous intrusions are exposed over an area of approximately 3,000 km2 along the Coastal Range between 31° and 32° S (Fig. 2 (Fig.  2). ). This area contains three large plutonic bodies (Fig. 3) an andd a nu numb mber er of sma smalle llerr bod bodies ies of sim simila ilarr co compo mposit sition ion (Rivano and Sepulveda  1991  1991). ). The plutons include diorite, tonalite, tona lite, amph amphibol ibolee and pyro pyroxene xene gran granodio odiorite, rite, and amph amphibol ibolee monzodiorite. monzodio rite. Potassium – argon argon geochronology available indicates that the Quilatapia – El El Durazno pluton (Fig. 3 (Fig.  3)) ranges in age from 134 to 108 Ma, whereas the Illapel – Caimanes Caimanes  pluton (Fig. 3) ag ages es ra rang ngee fr from om 13 1300 to 86 Ma an andd yo youn ungg to th thee east (Rivano and Sepulve Sepulveda da 1991  1991). ).

El Espino district geology

The El Esp Espino ino min mining ing dis distri trict ct con contai tains ns the Cre Cretac taceou eouss vol volcan canoosedimentary Arqueros, Quebrada Marquesa, and Salamanca formations. Only the upper portion of the Arqueros Formation and the basal portion of the Salamanca Formation are exposed in the area (Fig. 3 (Fig.  3). ). Thes Th esee fo form rmat atio ions ns ar aree in intr trud uded ed by a se seri ries es of La Late te Cr Cret etac aceo eous us intermediate stocks of granodioritic to dioritic composition (Llahuin pluton in the north-central area and Illapel – Caimanes Caimanes  plutonn to the south; Fig. 3  pluto Fig. 3). ). Stratigraphy The Arqueros Formation in the El Espino district has a minimum thickness of 1,500 m and consists of gray-greenish andesitic lava, andesitic breccia, and andesitic lapilli tuff with lenses len ses up to 100 m th thick ick of and andes esiti iticc co congl nglome omerat rate, e, san sands dston tone, e, siltstone, and limestone (Fig. 4). The unit displays a high degree of lateral facies variability variability.. The Quebrada Marquesa Formation (Figs. 3 and 4) dis plays a conformabl conformablee contact with the underlying Arqueros Formation in the southern portion of the El Espino district   but is in fault contact with the Arqueros Formation in the north. nort h. The Queb Quebrada rada Marq Marquesa uesa Form Formatio ationn cons consists ists of a mixe mixedd sequence of volcanic and sedimentar sedimentaryy rocks. The Espino Member of the Quebrada Marquesa Formation is composed of continental to marginal marine volcaniclastic and sedimentary rocks. The marine sedimentary rocks of  Espino member were deposited in a relatively small, structurstructurally controlled basin that is approximately 13 km long in a north – south south dire directio ctionn and and appr approxim oximatel atelyy 5 km wide wide.. The base of the Espino Member consists of a 5 – 10 10 m thick, wellstratified red andesitic conglomerate that grades upward into red gr grayw aywack ackee bed beds, s, th then en sil siltst tston one, e, and fin finall allyy lim limest eston one. e. Th This is  basal unit is overlain by two additional, normally graded sequences each with a base of sandstone that grades upward to siltstone, limestone, and local gypsum beds. Graywacke is dominantly dominant ly gray in color and is composed of feldspar, quartz, and lithic clasts of andesite. Sandstone is yellow to pink and

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Fig. 3 Geological map of El Espino mining district, showing the location of the three mineralized mineralized areas that were examined in detail: Espino, Romero,

and Llahuin

fine to coarse grained. There is complete gradation from volcanic-derived volcanic-d erived arkosic graywacke to feldspathi feldspathicc sandston sandstone. e. Siltstone beds range from dark gray to yellow with the former   being locally organic rich with relatively abundant abundant diagenetic  pyrite. Limestone is thick bedded and commonly contains chert nodules. The largest gypsum bed in the district is 100 m thick and has been mined over a strike length of  800 m (San Enrique mine). In the central portion of the district, the sedimentary rocks within these sequences are intercalated with volcanic lavas, pyroclastic beds, and volcaniclastic sedimentary rocks. Volcanic and pyroclastic rocks include gray to green colored porphyritic lava flows

with pyroxene and hornblende phenocrysts, lapilli tuff, and  breccia.  brecci a. Major elemen elementt chemis chemistry try of volcani volcanicc rocks indicates these rocks are mediu mediumm- to highhigh-K K basalt basaltic ic andesite to andesite (Fig. 5 (Fig.  5). ). The uppermost sequence in the Espino Member consists of inversely graded sandstone beds, with subsidiary limestone and siltstone. Sandstone is yellow to gray and fine to medium grained. Limestone beds are a few meters thick and commonly contain chert nodules. The thickness of the Espino Member is estimated to be approximately 900 – 1,000 1,000 m in the southern southern portion of the district and approximately approxim ately 600 m in the northwest northwest where it is truncated by the Llahuin pluton.

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Fig. 4 Cretaceous stratigraphy in

the El Espino mining district  showing the stratigraphic intervals of the Espino, Romero, and Llahuin areas

The Quelen Quelen Member Member of the Quebra Quebrada da Marq Marquesa uesa Form Formatio ationn is characterized by several centimeter thick medium- to coarse-grained sandstone base that grades upward to a regionally extensive coarse conglomerate with clasts of wellrounded porphyritic porphyritic andesite, andesite, andesitic breccia, breccia, and phaneritic phaneritic dioritic intrusive rocks. The conglomerate is overlain by a thick greenish lapilli tuff of the Late Cretaceous Salamanca Formation. The Late Cretaceous Salamanca Formation (Figs. 3 (Figs.  3 and  and 4  4)) is in fault contact with the Quebrada Marquesa Formation in the El Espino mining district, although a few outcrops at  higher elevations show an erosional unconformity between red conglomeratic sandstone of the Quelen Member and lapilli tuff of the Santa Virginia member of Salamanca Formation. In the study area, the Salamanca Formation contains a basal unit that comprises volcanic breccia and

Fig. 5   Plot of SiO 2  vs. K 2O (Tatsumi and Eggins 1995) showing

chemical compositions of volcanic rocks in the Arqueros and Quebrada Quebra da formations. formations. Rocks are classified classified as medium- to high-pot high-potassium assium andesite and basaltic andesite

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locally gray thin-bedded siltstone. Higher in the sequence, this formation contains red to gray polymictic conglomerate and red sandston sandstone. e.

Intrusive rocks Approximately 30 % of the El Espino mining district is underlain by intrusive rocks (Fig. 2 (Fig.  2). ). The northern portion of  the study area exposes the large, multiphase Llahuin pluton. The central portion of the area exposes a number of isolated diorite plutons, and the southern portion of the area exposes the large multiphase Illapel – Caimanes Caimanes pluton (Fig. 3 (Fig.  3). ). The Llahui Llahuinn plut pluton on (or Llahui Llahuinn plut plutonic onic suit suite) e) comp comprise risess at  least nine individual intrusions, which are distinguished by texture and mineralogy, and geographical location. Few intrusive contacts between intrusions were observed due to soil, debris, and/or vegetation cover, so some textural and mineralogical variations observed in the district might   be due to zonation within a singl singlee stock stock.. Some of the diorit dioritic ic intrusion intru sionss have cause causedd local contac contactt metam metamorphi orphism sm of adjacent  adjacent  volcano-sedimentary rocks. Biotite-pyroxene hornfels occurs adjacent to sills in drill cores in the El Espino deposit. Locally andesitic breccias in the Llahuin area display a meter-wide metamorphic aureole composed of fine-grained recrystallized quartz with lesser magnetite and epidote adjacent to a quartz monzonite intrusion. Three intrusions present in the El Espino deposit area include a hornblende-bearing porphyritic dioritic stock and sills; an hornblende-bearing, fine-grained monzodiorite; and an equigranular, fine-grained hornblende and pyroxene bearing diorite. A sharp contact was observed between the amphibole-bearing monzodiorite and the hornblende and  pyroxene-be  pyroxe ne-bearing aring diori diorite. te. A mediu mediumm- to coarse coarse-grai -grained, ned, equigranular quartz-dioritic to tonalitic intrusion is exposed at higher elevations north of the El Espino deposit. East of  the El Espino deposit at low to medium elevations, a hornblende and pyroxene-bearing microdiorite, NS-strike gray dioritic dikes, and a NS-trending tabular body of leucomonzodiorite are present. Dikes display a sharp contact with the leuco-monzodiorite. leuco-monzodiorite. Northeast of the El Espino deposit an equigranular, equigranu lar, medium-grained, olivine and pyroxene pyroxene-bearing -bearing gabbro diorite, an equigranular medium-grained quartz monzonite, and a monzodiorite occur in topographically high areas. The monzodiorite unit displays a transitional contact   between a dark gray idiomorphic medium-grained unit and a  pink xenomorphic xenomorphic finer-grained unit. U – Pb Pb geochronology on zircons from a weakly altered diorite from the Llahuin pluton at the El Espino deposit  yielded an age of 88.5±1.7 Ma. A quartz diorite from the Llahuin area in the northern portion of the district yielded an age of 88.1±1.1 Ma. More deta detailed iled descriptio descriptions ns of samples analyzed, and age results are reported in Lopez (2012 ( 2012). ).

241 Structure

A fa fault ult arc archit hitec ectur turee mod model el of the El Esp Espino ino min mining ing di distr strict ict was developed by mapping topographic lineaments on Landsat  and Aster imagery that were then field checked during geological mapping (Fig. 6 (Fig.  6). ). Fault orientation data from outcrop show two principal strike sets: NNW –  NS, and NE (Fig. (Fig. 7  7). ). Fault dips are mostly >45° (Fig.  7d  7d). ). Fault cores are mostly about 0.5 – 1 m thick, but ranging r anging up to 20 m thick and consist  of matrix-supported breccia, gouge, and locally a penetrative fracture cleavage. Damage zones of fractured and locally folded rocks can be up to 150 m thick. Analysis of slickenline rake (Fig. 7d (Fig.  7d)) shows that, although the majority of faults are strike-slip faults, oblique-slip and dip-slip fault also are present. Multiple slickenlines on some major faults indicate a  protracted faulting history. history. This is supporte supportedd by the presence of hydrothermal alteration (specular hematite) and copper  mines along NNW-strike faults that were probably active  previously  previous ly during basin formation. The Quebrada Marquesa formation formation has a general NS trend and is bounded to the west by the Espino – Illapel Illapel fault that  extends for  20 km from near the city of Illapel to the El Espino area. Although the fault trends generally N to NNE, about 8 km south of the area this structure changes trend to  NNW and becomes a set of right-stepp right-stepping ing   en echelon   fault  segments along the west boundary of the basin (Fig. 3 (Fig.  3). ). Within the basin, strata generally have low homoclinal dip to the  NE, but are broadly folded folded in the southern southern portion. Boudinage in limestone and fine-grain f ine-grained ed sandstone units of the El Espino member suggests beddingbedding-parallel parallel extension during diagenet diagenet-ic compaction or during subsequent folding. Folds have EW trend in the east and NE trend in the west (Fig.  6  6). ). Near the southern edge of the basin, a broad open NE plunging anticline cli ne is pr prese esent nt an andd str strata ata dip to the E an andd SE. In add additi ition on,, dip dipss are locally steep where strata are drag folded within about  10 m of some major faults. A  π -analysis, which modeled the average axis of rotation of bedding in the Quebrada Marquesa Formation (in this case caused by fault-block rotation), suggest ge stss th that at th thee ea east ster ernn ma marg rgin in of th thee ba basi sinn ma mayy be bo boun unde dedd by a  NNW-strike  NNW -strike fault ( 350°) (Fig. 7a (Fig.  7a). ). A fault with this approximate strike, although not the basin margin fault, is present  east of the Romero area (Fig. 6 (Fig.  6). ). The Quebrada Marquesa strata are also cut by a set of NEstrike, left-steppin left-steppingg  en echelon  faults (Fig. 3 (Fig.  3). ). Most mineralized veins in the district, including those in the El Espino, Romero, and Llahuin areas, strike NS to ENE, and many are related to these NE-strike faults (Fig. 7). Veins are defined here as fractures with infill of more of 5 cm width, and veins that display a fault plane or fault zone with gouge and/or   breccia are termed fault-vein fault-veins. s. Several types of veins were identified based on their ore and/or gangue mineralogy in the district and include iron oxide sulfide (with or without-q without-quartz) uartz) veins, vei ns,qua quartz rtz sul sulfid fidee vei veins, ns, and andcal calcit citee- and and/or /or bar barite ite sul sulfid fidee vei veins. ns. ∼

∼

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Fig. 6 Struct Structural ural map of El

Espino mining district showing faults in  blue , lineaments in black , folds in  black , and veins in red .  Blue triangles indicate inverse fault, dots indi  indicate cate normal faults, and  blue arrows  indicate sense of movemen movementt along faults faults.. The district district is bounded on the west we st andeast by ma majorfaul jorfaultt zon zones es

Iron ox Iron oxid idee su sulfi lfide de ve vein inss ar aree th thee mo most st co comm mmon on fo follo llowe wedd by qu quar artz tz sulfide veins. Iron oxide sulfide veins range from 5 to 2 m in width, whereas quartz sulfide veins range from 5 cm to 1 m. In the El Espino deposit area, iron oxide and quartz sulfide veins occur in the Espino Member and intrusive units of Llahuin pluton. Orientations are variable but average 240°/75° (right-hand (right-hand rule); quartz sulfide veins have a wider  ∼

dispersion in orientation than iron oxide sulfide veins (Fig. 8 (Fig.  8). ). Veins frequent frequently ly termin terminate ate at conta contacts cts with over overlying lying limest limestone one units where the calca calcareous reous rock above is mostly silicified silicified and the rock beneath shows stratabound metal disseminations, indicating that locally iron oxide and sulfide mineralization was focused immediately beneath these contacts. The wall rocks surrounding iron oxide sulfide veins are commonly altered to

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243

calcic and hydrolytic associations depending on their structural level. The true width of the alteration zones range from 20 cm to 5 m. Wall rocks surrounding quartz sulfide veins are altered to hydrolytic to argillic associations with widths ranging from 20 cm to 1 m. In the Llahuin area iron oxide and quartz sulfide veins occur in the Quelen Member of Quebrada Marquesa Formation. Veins have an average strike of  020°; quartz sulfide veins have an average dip of  55° (towards the east), while iron oxide sulfide veins are steeper with average dip of  90° (Fig. 8). Barite sulfide veins occur in the Arqueros Formation and in the Espino Member of Quebrada Marquesa Formation. They are distal to the El Espino deposit area and have scattered orientations orientations.. In the Romero area, iron oxide and quartz sulfide veins occur mostly in the Quelen Member and have strikes ranging from NNW to NNE and averaging 185°. Dips of the veins average 65° (Fig. 8). Vei einn de dens nsit ityy is hi high gher er in th thee Ro Rome mero ro ar area ea (up to 6/km) than in the Espino area ( 3/km), where stockworks of specular hematite are a common feature related to major veins. ∼

∼

∼

∼

∼

∼

Fig. 7   Lower hemisphere, equal

area stereonets showing bedding and fault orientation data.  a Bedding poles in Quebrada Marquesa Formation excluding areas of drag folds near exposed major maj or faul faults. ts. The aver average age bedd bedding ing orientation is 000°/20°. Fault  rotation rotati on axis (π -axis  perpendicular  perpendi cular to best fit great  circle) is nearly horizontal and trends 350 – 170°. 170°.  b  Pole density contourss for 66 faults with gouge contour width >5 cm. There are three main fault set orientations: 135°/90°, 205°/80°, 350°/70°. c  Major faults orientations shown as great circles . Slickenlines in faults are depicted as  dots with arrows  showing the relative motion of the hanging wall.  Black   squares  squar es labe  labeled led 1, 2, and 3 are are the the instantaneous strain axes calculated by Faultkin program (Allmendinger et al. 2001 al.  2001)) that  correspond to σ 3,  σ 2, and  σ 1  principal  princi pal stress axes, axes, respectively respectively,, if minimal rotational strain is assumed.  d  Histograms showing dip of major faults ( top ) and the rake of major fault slickenlines (bottom)

a

Kinematic data indicate that the NNW- and NE-striking faults in the district had both dextral and sinistral strike-slip and normal and reverse dip-slip motion (Fig. 7c (Fig.  7c). ). Fault-vein kinematic data display dextral and sinistral strike-slip components, but all dip-slip components show normal fault motion (Fig. 8c, (Fig.  8c, d). d). The orientations of the instantaneous principal strain axes were modeled from fault kinematic data using the  2001). ). The results  Faultkin   program (Allmendinger et al.  2001 (Fig. 9 (Fig.  9)) show near vertical shortening and NNW – SSE SSE extension for the major faults and near vertical shortening and WNW – ESE ESE exte extensio nsionn for the fault fault veins. veins. Alth Although ough the results results are only an approximation due to the likelihood of multiple faulting events, the models suggest that the fault architecture was established prior to mineralization during NNW – SSE SSE crustal transtension with a dextral component and that the fault veins formed during extensional faulting and WNW –  ESE oriented crustal extension. Thee ov Th overa erall ll ge geome ometry try and kin kinem emati atics cs of the fau faults lts an andd vei veins ns suggests the district contains a negative flower structure developed in a right-stepping jog locally along a major N-strike dextral fault system. This architecture also controlled the

b

2%

∼

4%

6%

∼

c

d

244 Fig. 8   Lower hemisphere, equal

area stereonets showing vein orientation data.  a  Poles to veins; Espino IOCG veins (black  circle ), Espino quartz-sulfide and calcite-quartz-sulfide veins (open circle ), Romero IOCG (black   squaree ), Romero quartz-sulfide  squar veins (open square ), Llahuin IOCG veins (black triangle), Llahuin quartz-sulfide veins (open triangle ), and barite-sulfide veins ( star ). ). b  Pole density contour cont ourss for for 44 IOC IOCG G vein veinss in in the El Espino mining distri district; ct; average average IOCG vein orientation is 210°/85°. c  Major veins orientations plotted as  great  circles with dots indicating slickenlines and  arrows on the dots  showing relative motion of  the hanging wall. Numbers 1 – 3 are the instantaneous strain axes calculated by Faultkin program (Allmendinger et al. 2001 al.  2001)) that  correspond to σ 3,  σ 2, and  σ 1  principal  princi pal stress axes, axes, respectively respectively,, if minimal rotational strain is assumed.  d  Histograms showing dip of major fault veins ( top ) and the rake of major fault-vein fault-vein slickenlines (bottom)

Miner Deposita (2014) 49:235 – 259 259

a

b 2%

4%

6%

8%

∼

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6 4 2 0 10

20

50

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Fault-vein dip

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80 90

Slickenline rake

localization of later IOCG alteration and mineralization around the Llahuin pluton (Fig. 10 (Fig.  10). ). The Espino vein system formed mostly as NE-strike extension fractures and normal faults within the jog, whereas the Romero and Llahuin veins formed along the eastern N- to NNE-strike basin margin fault  system. The Romero area contains NS-striking dioritic dikes that locally controlled the location of IOCG fault veins.

Alteration and mineralization

The El Espino mining district displays a variety of different  alteration types and styles (Figs. 11 (Figs.  11 and  and 12  12). ). The paragenetic sequence of alteration (Fig. 11 (Fig.  11)) was built based on crosscutting relationships between alteration minerals observed either  in the field or petrographically with the microscope. An early sodic alteration event affected much of the district. It was followed foll owed by sodi sodicc – calcic calcic and potassic alteration alteration events. These alteratio alte rationn even events ts are asso associat ciated ed with iron oxid oxidee mine minerali ralizati zation. on. A later calcic alteration event was synchronous with copper 

sulfide mineralization. The district displays a complex series of late hydrolytic alteration styles that also have associated copper and gold mineralization. Structural controls on alteration and mineraliza mineralization tion The distribution of hydrothermally altered and mineralized zones in the El Espino district indicates a strong structural control. The NNE-striking Lagarrigue fault forms a fundamental eastern boundary to the district with altered rocks occurring to the west but not to the east of this fault. Sodic hydrothermal alteration appears to have occurred between Lagarrigue and Illapel – Espino Espino faults. Major alteration zones are distributed along, or subparallel to, the NNW- to NE-striking faults in the district. Although the NNW-strike faults may contain iron oxides and sulfides, these faults rarely contain ore bodies. Many of the iron oxide and sulfide-bearing veins in the district have a generally N to NE strike. At the El Espino deposit steeply dipping sulfide-bearing veins strike NE to  NNE. Similar veins in the Romero area also have an N- to

Miner Deposita (2014) 49:235 – 259

245

Fig. 9   Lower hemisphere, equal

area stereonets showing fault   a  major faults  plane solutions solutions for  a and b  IOCG fault veins in El Espino mining district. Labels σ 3, σ 2, and  σ 1  correspond to  principal  princi pal stress axes

3

2

3 2

1

 NNE strike that extend upward into the Llahuin area. Vein density is higher at Romero and veins stockworks, commonly containing abundant specular hematite, are more prominent  here than in other prospects. The width and intensity of  alteration around NE-striking faults and veins are commonly comparable to, or better developed than, alteration around the  NW striking faults. The N- to NE-striking fault veins in the district are related to a transtensional stress regime during the prolonged hydrothermal event. The strike of these structures suggests WNW –  ESE extension with an associated dextral component. Calcic and hydrolytic alteration together with the majority of iron oxide and sulfide mineralization appear to be synchronous with this extensio extensional nal structural event. Fig. 10   Proposed structural

model for El Espino mining district.  a  Satellite image (Landsat 741) with lineament  interpretation.  b  Fault jog model showing general orientation and location locati on of veins (red ) in El Espino (e ), Romero (r ), and Llahuin (l ) areas. Plutons shown as pink ellipses : d  Llahuin diorite and equivalents, g  dioritic pluton along southern margin of basin

1

The NW-strike Tamara fault (Correa   2003) 2003) apparently  postdatess both alteration and mineraliza  postdate m ineralization. tion. While it locally contains sericite, it is never mineralized. Instead, the fault  appears to have acted as a conduit for meteoric waters that  resulted in supergene alteration of sulfide-rich zones that it  transects. Sodic alteration Sodic alteration affected a large portion of the El Espino district. It is best developed along a north-striking corridor  from the Caimanes – Illapel Illapel pluton to the El Espino deposit. The style and mineralogy of sodic alteration associations vary by rock type but generally resulted in the formation of 

246

Miner Deposita (2014) 49:235 – 259 259

Fig. 11 Paragenetic sequence of alteration minerals. Continuous lines represent a major phase,  segmented lines represent a minor phase, and a  dotted  line  represents a trace mineral

albite (Fig. 13a 13a). ). Igneous rocks generally display pervasive alteration with complete replacement of plagioclase by albite and commonly wholesale destruction of igneous mafic mineral er alss by ch chlo lori rite te.. Vei einl nlet etss of ma mass ssiv ivee al albi bite te up to 1 cm in wi widt dthh are common in intrusive rocks. Sedimentary rocks generally display selective selective replacement of fine-graine fine-grainedd beds by albite or  albite and chlorite, where albite replaces plagioclase plagioclase and chlorite replaces amphibole and pyroxene. In conglomerate and volcanic breccias, albite replaces the matrix and/or forms rims around clasts. Sodic – calcic calcic alteration Sodic alte Sodic alteratio rationn asso associati ciations ons were overprin overprinted ted by sodi sodicc – calcic calcic alteration associations of albite, epidote, actinolite, titanite, scapolite, and minor apatite. apatite. The area area affected by sodic – calcic calcic alteration is more restricted than that affected by sodic alteration. As with sodic alteration associations, associations, sodic – calcic calcic alteration formed textural and mineralogi mineralogical cal differences related to rock type. The most common alteration minerals are epidote, actinolite, and albite. The most common mineral associations are epidote – albite, albite, actinolite – albite, albite, epidote – actinolite actinolite – chlorite chlorite –  albite – ( titanite), and actinolite – scapolite scapolite – albite albite (Fig (Fig.. 11 11). ). Al Al- bite parti partially ally or total totally ly repla replaces ces plag plagiocla ioclase. se. Epid Epidote ote parti partially ally −

replaces the cores of plagioclase, pyroxene, and hornblende crystals. crys tals. Actin Actinolite olite repla replaces ces hor hornblen nblende de and pyro pyroxene xene.. Volcan olcanic ic rocks display pods of epidote grading outward to albite. Brecci Bre ccias as and con conglo glomer merate atess sho show w cla clasts sts rep replac laced ed by int inter ergro grown wn epidote and albite, or surrounded by an albite rim. Thin-bedded siltstone and sandstone are altered to albite and epidote zones along bedding (Fig. 14b 14b). ). Some siltstone samples show irregular, <1-mm-wide epidote veinlets that cut across albite-altered beds to connect parallel epidote-altered beds. Intrusions contain epidote veinlets with albite selvages or   pseudo-breccia  pseudo-b reccia textures t extures formed due to irregular replacement  of the rock by albite, actinolite, and chlorite. Subhedral disseminated magnetite (1 – 5 %) is commonly present in sodic –  calcic alteration associations, associations, and it is often partially to totally martitized. Intense sodic – calcic calcic alteration with scapolite is restricted to zones within the leucodiorite intrusion located in the Romero area. Potassic alteration Potassic alteration assemblages are present in the El Espino, Romero, and Llahuin areas. These assemblages are locally and weakly developed and/or highly affected by subsequent  calcic and hydrolytic alteration. Subsequent alteration of potassic assemblages has commonly resulted in biotite altering

Miner Deposita (2014) 49:235 – 259

Fig. 12 Alteration map of El Espino mining district 

Fig. 13   Outcrop photographs

showing examples of alteration. a  Sodic alterated rock as albite stockwork veinlets in quartz diorite from Caimanes – Illapel Illapel  pluton. b  Sodic – calcic calcic altered red sandstone from Romero area with some beds partially altered to albite and epidote

247

248 Fig. 14   Photographs showing

examples of potassic alteration.  K-spar  potassium   potassium feldspar, bt   biotite,  biotit e, chl  chlorite,  chlorite, mt  magnetite.  magnetite. a  Hand sample of sandstone from El Espino prospect with disseminated secondary biotite and magnetite alteration assemblage.  b  Hand sample of  andesite from El Espino prospect  with K-feldspar and chlorite alteration association. c  Photomicrograph (crossed   polars) showing leucodiorite from Romero area replaced mostly by K-feldspar and lesser  epidote and biotite. Primary  plagioclase  plagioc lase is replaced by K-feldspar and biotite. d  Photomicrograph (crossed   polars) showing secondary  biotitee replaced by chlorite  biotit

Miner Deposita (2014) 49:235 – 259 259

a

b

bt + mt mt

K-spar

2 cm

cal

c

to chlorite and potassium potassium feldspar generally being being replaced by sericite, clays, and chlorite; potassium feldspar is more commonly preserved than biotite. Potassium feldspar either re places original plagiocla plagioclase se or replaces albite formed during earlier sodic or sodic – calcic calcic alteration. Secondaryy biotite accompanied by disseminat Secondar disseminated ed subhedral magnetite was found in several deep drill holes at the El Espino deposit (Fig. 14a (Fig.  14a). ). The style of alteration is typical of  that observed in other Chilean IOCG deposits such as Candelaria (Marschik and Fontbote  2001  2001). ). However, its extent is poorly constrained. Potassic alteration assemblages containing potassium feldspar, commonly with hematite (Fig. 14b (Fig.  14b), ), are most common in the El Espino district. The age relationships between the biotite-bearing and potassium feldspar-bearing potassic assemblages are currently unclear. While it is possible that the different potassic mineral assem blages simply represen representt originally different protolitths (Barton and Johnson  1996  1996), ), they may also represent fundamentally different assemblages formed at different times and in different structural levels. Volcanic and intrusive rocks affected by potassic alteration contain interstitial metasomatic potassium feldspar. In pyroclastic and sedimentary rocks, potassium feldspar partially or  totally replaces albite either in the matrix or in previously altered fragments. Potassium feldspar alteration is selective, forming bands in thin bedded siltstone or rims around clasts. Locally, potassium feldspar has pervasively replaced all the  primary minerals in clastic sedimentary rocks. The eastern side of Romero area displays extensive evidence of potassic and potassic – calcic calcic alteration. A conglomerate of the Quelen Member in the Romero area contains rounded pods (3 – 5 cm diameter) of epidote grading outward to interstitial pink potassium feldspar as haloes around the

2 cm

d K-spar

bt

bt

chl

200 m

200 m

 pods. Some pods have specular hematite in their centers. The least altered leuco-monzonite sample located at medium elevations in this area displays interstitial potassium feldspar  (10 % of the rock), disseminated magnetite, and ferromagnesian minerals that are replaced by actinolite and chlorite. A more altered sample displays secondary biotite and potassium feldspar.. Potassium feldspar replaces plagioclase phenocrysts feldspar and is commonly intergrown with minor biotite, actinolite, epidote, and/or chlorite (Fig.   14c, d). d ). Secondary biotite accompani acco mpanied ed by disse dissemina minated ted subhe subhedral dral magn magnetite etite was found deep at El Espino deposit (few meters of core) and its distribution is unclear. Calcic alteration Calcic alteration affected a northwest-trending area of approximately 25 km2 extending from the El Espino deposit to the Romero prospect (Fig. 12 (Fig.  12). ). Calcic alteration mineral associations tio ns con contai tainn epi epidot dote, e, act actino inolite lite,, and chl chlori orite te with les lesser ser cal calcit cite, e, titanite, apatite, and quartz and minor garnet (andradite) in calcareous sedimentary rocks. The most common calcic alteration associations are epidote – actinolite actinolite and epidote – chlorite chlorite (Fig. 15a (Fig.  15a). ). A first stage of calcic alteration alteration is charac characterized terized by  partiall replace  partia replacement ment in volcan volcanic ic rocks and perva pervasive sive replac replaceement in sedimentary rocks that locally obliterates their primary texture. Calcic alteration epidote replaces igneous feldspar  and mafic minerals and cuts hydrothermal albite and potassium feldspar. Epidote occurs as both fine-grained anhedral aggreg agg regate atess an andd wel well-d l-deve evelop loped ed cry crysta stals ls up to 10 mm in len length gth.. It occurs as veinlets, disseminations, or massive replacemen replacements ts of the rock. Actinolite occurs as acicular aggregates with crystal lengths averaging 0.5 – 1 cm, but ranging up to 5 cm; it is commonly disseminated but locally can pervasively

Miner Deposita (2014) 49:235 – 259

249

replace host rocks. Chlorite replaces igneous ferromagnesian minerals as well as hydrothermal actinolite and biotite. Epidote and actinolite associations, sometimes with calcite, are accompanied accompan ied by abundant magnetite magnetite or less frequently hematitee (F tit (Fig. ig. 15b 15b). ). Thes Thesee minera minerall associat associations ions may contain magnetite, net ite, ilme ilmenite nite,, pyr pyrite, ite, and cha chalco lcopyri pyrite te with mino minor  r   bornite and covell covellite ite (Fig.  15c, d). d ). Sulfides replace iron oxide ox ide min minera erals ls or fo form rm min minute ute inc inclus lusion ionss in qu quart artzz (Fig.   15c, 15c, d). Min Minor or cha chalco lcopyr pyrite ite,, bor bornit nite, e, pyr pyrite ite,, and native gold, together with acicular actinolite, were found in quartz grains around or within replacive epidote – actinolite actinolite  pods. Chalcopyrite either formed later as inclusions in idio blasticc pyrite (Fig.   15e, f ) or as partial replacement of   blasti  pyrite..  pyrite A second stage of calcic alteration is characterized by iron oxide- and sulfide-bearing veinlets with actinolite selvages that cut earlier calcic, sodic – calcic, calcic, and sodic alteration associations. These veinlets range in width from 1 to 10 mm and contain actinolite, epidote, and quartz with either magnetite or  specular hematite and pyrite and chalcopyrite. Specular hematite in some veinlets has been replaced by mushketovite (magnetite pseudomorphs of hematite) related to carbonate Fig. 15   Photomicrographs

showing examples of calcic alteration associations.  ab  albite, act  actinolite,   actinolite,  cal  calcite,   calcite,  epi epidote, qz  quartz,  hm  specular  hematite,  mt  magnetite,  bn  bornite,,  cpy  chalcopyrite, py  bornite  pyrite..  a  Conglomerate from  pyrite Romero area replaced by actinolite and epidote overprinting earlier albite. b  Same conglom conglomerate erate as A showing actinolite, epidote, calcite, and hematite (crossed   polars). c  Conglomerate from Romero area showing quartz with inclusions of acicular actinolite crystalss and opaques (crossed  crystal  polars). d  Same as  f , but with reflected light showing that  opaques are bornite and chalcopyrite replacing hematite. e  Mineralized sandstone from El Espino area showing pyrite and chalcopyrite (reflected light). f  Pyrite   Pyrite and chalcopyrite replacing magnetite in siltstone from El Espino deposit   pervasively  pervasi vely altered altered to actinolite actinolite and min minor or epid epidote ote (ref (reflect lected ed lig light) ht)

deposition. Many of the veins display actinolite selvages that  are common commonly ly 5 – 10 10 mm wid wide. e. Som Somee act actin inoli olite te sel selvag vages es gr grade ade outwards to epidote with minor intergrown chlorite. Calcic alteration associations also occur in specular  hematite-bearing breccias found at low elevations within the leuco-monzonite unit in the Romero prospect area. These breccias contain fragments replaced by albite, scapolite, potassium feldspar, or quartz in a matrix of actinolite with specu specular lar hemati hematite te and disseminated disseminated chalc chalcopyri opyrite te and pyrite. Hydrolytic alteration Hydrolytic alteration associations occur within hydrothermal  brecciaa zones, around a minera  brecci mineralized lized manto body at the El Espino deposit, and in veins (Fig.   16a, b). b ). Hydrolytic alteration associations contain variable amounts of quartz, calcite, sericite (illite and phengite), chlorite, iron oxide minerals, dominantly hematite, and sulfides (Fig.  1  16a 6a – d); some hydrolytic associations contain minor rutile and Na –  Fe tourmaline. Hydrolytic alteration associations contain the vast majority of sulfides in the El Espino mining district.

a

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Hydrolytic stage hydrothermal hydrothermal breccias are found at the El Espino, Espi no, Rome Romero, ro, and Lla Llahuin huin area areas. s. The These se hydr hydrothe othermal rmal brec brec-cias contain fragments with muscovite replacing earlier  formed alteration minerals such as albite and potassium feldspar in a matrix of chlorite and specular hematite. Alteration zones adjacent to El Espino manto commonly contain a white mica – chlorite chlorite – specular specular hematite assemblag assemblage. e. Both hydrothermal breccias and mantos are commonly overprinted by 1 – 2 cm wide veinlets of specular hematite, minor quartz, and chalcopy chalcopyrite. rite. Three stages of hydrolytic alteration/mineralization were recognized from cross-cutting vein relationships. The earliest veins contain quartz and sulfides. These are cut by veins containing hema he matit titee an andd su sulfi lfide dess wit withh va varia riabl blee am amou ount ntss of qu quar artz tz (F (Fig. ig.16d 16d). ). The hematite-sulfide veins have well to poorly developed selvages of white mica and chlorite. The latest veins are composed of quartz, calcite, sulfide and minor iron oxide (Fig.  16c  16c). ). Hydrolytic alteration assemblages commonly contain chalcopyrite, pyrite, and specular hematite. Sulfides and hematite at the El Espino deposit occur in veins and within a man manto to with within in a fav favora orable ble fin fine-g e-grain rained ed vol volcan canicla iclastic stic unit of the Espino Member. Pyrite and chalcopyrite are coexisti coex isting ng with chal chalcopy copyrite rite form formed ed late laterr repl replacin acingg pyri pyrite te Fig. 16   Photographs showing

hydrolytic alteration associations. cal  calcite,   calcite,  chl  chlorite,   chlorite,  cpy chalcopyrite,  hm  specular  hematite, qz  quartz,   quartz, py  pyrite, ser  sericite.  a  Hematite-sulfide veinlet with chlorite selvages in sandstone from Romero area. b  Brec  Breccia cia wit within hin 1 m wide wide vein in Romero area, showing massive chalcopyrite accompanied by chlorite chlori te and serici sericite te and cut by hematite veinlet.  c  Calcite –  chalcopyrite veinlet in diorite from El Espino deposit  overprinting specular hematite –  sericite – chlorite chlorite – quartz quartz veinlet. d  Veinlet in sandstone from El Espino deposit showing two stages of hematite, the first shows actinolite altered to chlorite and the second shows sericite, quartz and minor chlorite (reflected light).  e  Mineralized sandstone from Romero area showing chalcopyrite inclusions in pyrite (reflected light).  f  Mineralized sandstone from El Espino deposit  showing chalcopyrite after pyrite (reflected light)

(Fig.   16e, 16e, f ). ) . Mi Mino norr am amou ount ntss of bo born rnite ite,, ga gale lena na,, an andd hypogen hypo genee chalc chalcocit ocitee are pres present ent loca locally lly as repl replacem acements ents of chalcopyrite. Trace native gold is present as inclusions in chalcopyrite. Calcite veins and argillic alteration The final hypogene alteration and mineralization event in the Espino district resulted in the formation of calcite –   barite – (quartz) (quartz) veins that may contain chalcopyrite, pyrite, and/or and /or ga galen lena. a. Th These ese ve veins ins hav havee se selva lvages ges co compo mposed sed of va vario rious us combinations of chlorite, quartz, calcite, illite, clinochlore, montmorillonite, and laumontite. Thick veins of this have  been exploited by small miners for copper and locally silver,  presumablyy hosted in the galena. These veins are distal to the  presumabl main zones of hydrolytic alteration and mineralization and occur occ ur foun foundd at hi highe gherr ele elevat vation ionss in the Llah Llahui uinn and no north rth of of El Espino deposit areas. Supergene Superge ne alteration and mineralization Weathering of sulfide-rich associations in the El Espino district has resulted in the formation of supergene copper 

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Miner Deposita (2014) 49:235 – 259

oxides and limi oxides limited ted amo amount untss of sup superg ergene ene cha chalco lcocit cite. e. Sup Super ergen genee alteration also resulted in the formation of clays. Supergene alteration has been observed along fault zones and is particularly well developed along the Tamara fault zone (Fig. 6 (Fig.  6). ).

Vertical and lateral distribution of alteration and mineralization

Alteration assemblages in the El Espino mining district  display a distinct vertical and lateral zonation (Fig. 17 (Fig.  17). ). Both the El Espino and Romero prospects contain areas at low to intermediate elevations (1,000 – 500 500 m deep from Llahuin area) that contain sodic alteration assemblages that have been overprinted by sodic – calcic, calcic, potassic, and calcic alteration assemblages. The generally high elevation Llahuin area contains well-developed hydrolytic alteration assemblages. Hydrothermal alteration was centered on high angle faults withh alt wit altera eratio tionn gen genera erally lly dec decrea reasin singg in int inten ensit sityy and com comple plexit xityy outward from these structures. Sodic alteration affected over 100 km 2 in the district. Sodic alteration was most intense along north – southsouth- and NEtrending regional lineaments (Fig. 12 (Fig.  12). ). Sodic alteration is best  developed at low elevations and only minor zones of sodicaltered rocks are found at higher elevations, primarily in the Llahuin area. Sodic – calcic calcic altered zones are spatially more restricted than sodic-altered rocks and cover an area of  approximately 50 km2. Sodic – calcic calcic altered rocks surround dioritic intrusions located in the southern end of the Llahuin  plutonic suite and are present in i n structural zones within and  between the Espino and Romero areas. Sodic – calcic calcic altered zoness are best deve zone develope lopedd at low to medi medium um elev elevatio ations ns (1,0 (1,000 00 –  5000 m dee 50 deepp fro from m Lla Llahui huinn are area) a) but may extend extend to hig higher  her  elevations in the Llahuin area and in the very far north of  the district. Calcic altered rocks occur primarily primarily in an area 5 km long× long × 2 km wide from El Espino deposit on the west to the Romero area on the east (Fig. 12 (Fig.  12). ). Calcic altered rocks occur at low Fig. 17  Schematic EW cross-

section showing vertical distribution of alteration types in the El Espino mining district.  Note that intense intense calcic alteration alteration was confined largely to mixed volcanic and sedimentary rocks adjacent to the mostly sodically and lesser potasically altered Llahuinn pluton. The location of  Llahui the section is illustrated in figure 11 figure  11.. Mineral abbreviations: act  actinolite,   actinolite,  chl  chlorite,   chlorite, hm specular hemati hematite, te, ser  sericite,  sericite, tm  tourmaline

251

elevations (1,000 – 800 800 m depth from Llahuin area) in the El Espino deposit area and at medium to low elevations (1,000 –  500 m depth) in the Romero area. While surface exposures exposures of  calcic-altered rocks contain minor specular hematite or magnetite, pyrite, and traces of chalcopyrite, deep exposures of  this alteration type at the El Espino deposit contain zones of  massive magnetite cut by specular hematite veins with locally significantt amounts of pyrite and chalcopyrite. significan Potassic Pota ssic-alt -altered ered rock rockss occur as smal smalll zones both with within in and outside zones of sodic alteration (Fig. 11 (Fig.  11). ). Potassically altered rocks are found at both low (El Espino) and high (Llahuin) elevations. Extensive zones with hydrothermal potassium feldspar are common present within intrusions in the Llahuin area. Hydrothermal biotite accompanied by K-feldspar occurs deep at the eastern margin in the Romero area. Hydrothermal  biotite at the Romero prospect prospect area appears appears to have been more widesp wid esprea readd bu butt was alm almost ost com comple pletel telyy rep replac laced ed by lat later er cal calcic cic and hydrolytic alteration assemblages. Hydrolitic alteration assemblages are most common at  medium to high elevations throughout throughout the district. Hydrolytic alteration is concentrated in veins that served as feeders for El Espino manto deposit. Hydrolytic assemblages appear to change mineralogy with depth. Iron oxide-rich hydrolytic veins vei ns at El Esp Espino ino an andd Rom Romero ero gr grade ade up upwar wardd int intoo qua quartz rtz vei veins ns with little to no hematite at Llahuin. At the highest elevations in the dis distri trict, ct, ve veins ins are do domin minate atedd by ar argil gillic lic ass assem embla blage gess an andd contain cont ain domi dominant nantly ly calc calcite ite and bari barite te with mino minorr arge argentife ntiferou rouss sulfides..

Age of alterati alteration on and mineral mineralizatio ization n

The age of hydrothermal alteration and mineralization in the El Espino district was investigated by Lopez (2012 ( 2012)) through 40 Ar--39 Ar isotopic analysis of hydrothermal minerals Ar (Table 1 (Table  1). ). Analysis of potassium feldspar, separated from an irregular calcite vein developed within a potassically altered  biotite – magnetite magnetite assemblage from deep within the Espino

252

Miner Deposita (2014) 49:235 – 259 259

deposit, yielded yielded an age of 86.09± 0.45 Ma (Table 1 (Table 1). ). Analysis of actinolite, from a calcic alteration veinlet containing chalcopyrite, actinolite, epidote, and minor magnetite at the El Espino deposit, deposit, yielded an age of 88.4± 88.4 ± 1.2 Ma (Table (Table 1  1). ). The age of the hydrolytic alteration was determined from 40 Ar  – 39Ar analyses of white micas (Table 1). Analysis of  white mica from a core sample with a hydrolytic quartzsulfide veinlet at El Espino yielded an inverse isochron age of 87.9±0.6 Ma (Table 1 (Table  1). ). White mica from a specular hematite veinlet at Espino yielded a similar plateau age of 87.9± 0.6 Ma (Table 1 (Table  1). ). Geochronological U – Pb Pb and 40Ar/ 39Ar data indicate that  hydrothermal alteration was coeval with magmatic intrusive activity. One particular dioritic intrusion (88.5 Ma) preceded the calcic stage (88.4 Ma), which was accompanied by iron oxide copper and gold mineralization. Hydrolytic alteration, related to economic iron oxide copper and gold mineralization, came immediately after at 87.9 Ma. It also appearss that hydro appear hydrothermal thermal alteration and minerali mineralization zation may have been accomplished in a short period of two to three million years.

Sulfur isotopes of sulfides from calcic and hydrolytic alteration associations

Sulfur isotopic analyses were performed on sulfide minerals collected from calcic, hydrolytic, and late calcite vein associations across the district (Table 2). Samples were acquired from a range of locations to investigate potential changes in sulfur isotopic composition vertically and laterally within the hydrothermal system. Single analysis of gypsum from the Espino Esp ino Mem Membe berr of the Que Quebr brada ada Mar Marqu ques esaa For Format mation ion yie yield lded ed 34 a δ S value of 12.4  ‰. Single analysis of diagenetic pyrite in sedimentaryy rocks of Espino Member yielded a  δ34S value of  sedimentar 7.3  ‰ . Sulfides extracted from veins and rock replacements related to calcic alteration yielded δ34S values between 4.0 and +2.4  ‰ , with a mean value around 1  ‰ (Figs.  (Figs. 18  18 and 19 19). ). Sulfides within hydrolytic altered rock yielded 34 δ S values between 4.4 and +6.2 ‰, with a mean value −

−

−

−

Ar/ 39Ar geochronology data for alterat alteration ion ages Table 1

around 0 ‰   (Figs. 18 and 19 19). ). Copper sulfide-bearing veins lacking iron oxides yielded δ 34S values between 4.3 and +1.0 ‰, whereas sulfides from veins containing iron oxide minerals yielded slightly higher  δ34S values of   betweenn 4.  betwee 4.33 an andd +6 +6.1 .1 ‰. Sul Sulfid fides es with within in lat latee sta stage ge 34 calcite veins are distinctively lower with δ S values between 4.8 and 9.7  ‰  (Fig.  (Fig. 18  18). ). Although the sulfur isotopic values of pyrite and chalco pyrite in the calcic and hydrolytic associations overlap, there are slight differences in isotopic compositions. Pyrite values are generally higher and range from 2.2 to +6.1, whereas chalcop chal copyrite yrite valu values es rang rangee from 4. 4.44 to +4 +4.5 .5 ‰. Som Somee sam sample pless with coexisting chalcopyrite and pyrite have significantly different isotopic values, suggesting that chalcopyrite formed  by utilizing reduced sulfur in the hydrothe hydrothermal rmal fluid rather  than incorporating sulfur from pre-existing pyrite (Ohmoto and Goldhaber  1997).   1997). The wide range of sulfur isotope compositions compositions are compatible with two sources variably mixed, a sulfur source of  δ 34S close to 0 ‰  and a sulfate source with variable degrees of  reduction. The sulfur isotope composition range related to calcic alteration is relatively more restricted (range of 8 ‰) with a mean near 0 ‰. That alteration surrounds a dioritic intrusion. The relatively smaller range around 0 ‰ and  patternn of the altera  patter alteration tion suggest a magmat magmatic ic source for the former of the fluids. Hydrolytic stage sulfur isotope ratios show a wider range (11 ‰), generally higher than in the calcic stage, suggesting incorporation of sulfur derived from gypsum. High temperature reduction of sulfate may have occurred during the convective circulation of  reduced magmatic hydrothermal fluids through the gypsum beds of the Quebrada Marquesa Formation. Sulfur  isotope ratios in the late calcite veins suggest incorporation of   biogenic  bioge nic sulfur. sulfur. Hydrothermal sulfides in the district show a trend toward higher sulfur isotope values with stratigraphic height in the Quebrada Marquesa Formation (Fig. 20 20). ). The greatest  variability of sulfur isotopic values occurs within the lower  marine, locally gypsiferous sedimentary rocks of the Espino Member with δ34S values ranging from 9.7 to +2.4 ‰, whereas the sulfides in the overlying continental conglomerate −

−

−

−

−

−

−

40

Sample

Locality

Material

Age ± 2σ  Ma

Type

Alteration type

ERD-6 247.4

Espino

Actinolite

88.4 ±1 ±1.2

Inverse isochron

Calcic

ERD-6 128

Espino

Sericite

87.9 ±0 ±0.6

Inverse isochron

Hydrolytic

ERD-6 142.3

Espino

Sericite

87.9 ± 0.6

Plateau

Hydrolytic

EDH-3 246

Espino

K-Feldspar

86.21 ± 0.41

Plateau

Calcite vein

Rab-01

Espino

Muscovite

98.9 ±2 ±2.2

Inverse isochron

Hydrolytic

179682

Romero

Muscovite

94.17 ± 0.49

Total fusion

Hydrolytic

172261

Llahuin

Illite

91.71 ± 0.47

Total fusion

Hydrolytic

Miner Deposita (2014) 49:235 – 259

253

Table 2 Sulfur isotope data of mineralized rocks from El Espino mining district 

Rock description

Alteration and mineralization

S isotopes 34

Sample

Locality

Host rock

Morphology

Mineralogy

Alteration type

Sulfide

δ

EDH4 ED H4-8 -83 3

Espi Es pino no

Sil ilts tsto tone ne

Silt Si ltst ston onei eiro ron n ox oxid idee – sulfide sulfide vein

alb-actalbact-chl chl-hm -hm-mu -mushkshk-cal cal-py -py-cpy -cpy

Sodicc – calcic Sodi

py

ERD ER D6-24 242 2

Espi pino no

And ndes esiite

Iro ron n ox oxiide – sulfide vein

act-epi-calc-cpy-hm

Calcic

cpy

ERD9-137

Espino

Sandstone

Rock replacement

epi-act-chl-py-cpy

Calcic

py

ERD4-174

Espino

Sandstone

Rock replacement

epi-chl-mgt-cpy-py

Calcic

cpy

ERD4-174

Espino

Sandstone

Rock replacement

epi-chl-mgt-cpy-py

Calcic

py

179694

Espino

Siltstone

Rock replacement

epi-act-cpy-py

Calcic

cpy

ERD6-236

Espino

Siltstone

Rock replacement

chl-cal-mgt-py-cpy

Calcic

py

ERD ER D6-23 236 6

Espi pino no

Siltstone

Iro ron n ox oxiide – sulfide vein

act-qtz-cal-mushk-cpy-py

Calcic

py

ERD ER D6-23 236 6

Espi pino no

Siltstone

Iro ron n ox oxiide – sulfide vein

act-qtz-cal-mushk-cpy-py

Calcic

cpy

−1.51 −3.66 −3.51 −2.94 −1.36 −1.26 −0.89 −0.83 −0.06

ERD4-136

Espino

Siltstone

Rock replacement

alb, epi-act-mgt-py-cpy

Calcic

py

0.27

179695

Espino

Diorite

Iron oxide – sulfide vein

act-qtz-mgt-calc-py

Calcic

py

2.58

ERD5-165

Espino

Calc. siltstone Ca

Calcite vein

chl-ser-cal-cpy

Calcite vein

cpy

ERD7-179

Espino

Siltstone

Calcite vein

calc-hm-cpy

Calcite vein

cpy

ERD6-83

Espino

Diorite

Calcite vein

alb-act-chl-cpy (rock), calc (vein)

Calcite vein Ca

cpy

17227 1

Llahuin

Diorite

Quartz sulfide – iron oxide vein

chl-ser-qtz-calc-hm-cpy

Hydrolytic

cpy

ERD6 ER D6-1 -12 28

Espi Es pino no

San ands dsto ton ne

Qua uart rtzz su sulf lfid idee – iron i ron ox oxid idee ve vein in

chlch l-se serr-hm hm-m -mus ushk hk-q -qtz tz-c -cal alcc-cp cpy y

Hydr Hy drol olyt ytic ic

cpy cp y

179553

Romero

Leucodiorite

Quartz vein

chl-ser-hm-cpy-py

Hydrolytic

cpy

1722 17 2252 52A A

Rome Ro mero ro

Cong Co nglo lome mera rate te

Quar Qu artz tz – iro ron n ox oxiide ve veiin

callcca c-qt qtzz-ch chll-s -seer-hm-mus ushk hk--cp cpy y

Hydr drol olyt ytiic

cpy cp y

1796 17 9682 82

Rome Ro mero ro

Cong Co nglo lom mer erat atee

Iro ron n ox oxid idee – sulfide breccia

chl-calc-(ser)-cpy-py

Hydrolytic

py

ERD5-176

Espino

Sandstone

Rock replacement

chl, calc-ser-mushk-cpy

Hydrolytic

cpy

179553

Romero

Leucodiorite

Quartz vein

chl-ser-hm-cpy-py

Hydrolytic

py

179534A

Romero Ro

Conglomerate

Rock replacement

chl-ser-mgt-hm-cpy

Hydrolytic

cpy

179534B

Romero Ro

Conglomerate

Iron oxide vein

hm-chl-cpy

Hydrolytic

cpy

ERD5 ER D5-1 -17 76

Espi Es pino no

San ands dsto ton ne

Iro ron n ox oxid idee – quartz vein

chl-ser-cal-mushk-cpy

Hydrolytic

cpy

179550

Romero

Diorite

Rock replacement

chl-ser-(hm)-py-cpy

Hydrolytic

py

−9.69 −9.14 −4.73 −4.30 −4.20 −2.65 −2.26 −2.00 −1.79 −1.59 −1.66 −1.46 −0.78 −0.29

179689

Espino

Andesite

Quartz vein

chl-ser-hm-calc-py-cpy

Hydrolytic

py

0.59

1796 17 9682 82

Rome Ro mero ro

Cong Co nglo lom mer erat atee

Iro ron n ox oxid idee – sulfide breccia

chl-calc-(ser)-cpy-py

Hydrolytic

cpy

1.13

179515

Llahuin

Diorite

Quartz vein

chl-ser-(hm)-cpy

Hydrolytic

cpy

1.04

EDH4 ED H4-1 -157 57

Espi Es pino no

San ands dsto ton ne

Iro ron n ox oxid idee – sulfide vein

chl-ser-mgt-py, hm-mgt-cpy

Hydrolytic

cpy

1.87

1796 17 9696 96

Rome Ro mero ro

Cong Co nglo lom mer erat atee

Iro ron n ox oxid idee – sulfide vein

chl-ser-hm-mgt-calc-cpy-py

Hydrolytic

cpy

4.60

1796 17 9696 96

Rome Ro mero ro

Cong Co nglo lom mer erat atee

Iro ron n ox oxid idee – sulfide vein

chl-ser-hm-mgt-calc-cpy-py

Hydrolytic

py

6.38

179534B

Romero Ro

Conglomerate

Iron oxide breccia

hm-chl-cpy

Hydrolytic

cpy

0.87

179683

Romero

Conglomerate

Disseminated

ser-clays-py, cpy veinlets

Hydrolitic/argillic

cpy

0.36

172255

Romero Ro

Diorite

Quartz vein

qtz-chl-py

Hydrolytic

py

4.22

Quelen Member have heavier values of  2.7 to +6.2 ‰. These compositions suggest that isotopically heavy sulfur  was contributed to the hydrothermal fluids during upward fluid migration. Calculation of the precipitation temperature of coexisting (i.e., textural equilibrium) chalcopyrite and pyrite in calcic alteration associations using the method of Ohmoto and Rye (1979 1979)) and Campbell and Larson (1998 (1998)) yields temperatures  between 529 and 280 °C. Hydrolytic Hydrolytic alteration sulfide sulfide yielded yielded temperatures temperatur es of 216 °C (T (Table able 3  3). ). −

S

Fluid inclusion studies

Eight sam Eight sample pless of qu quart artzz fro from m ve veins ins wit withh iro ironn ox oxid ides es and sul sulfid fides es were surveyed fluid inclusions suitable for microthermometric study. Homogenization temperatures were collected only from assemblages of inclusions that showed consistent  liquid to vapor volumetric proportions and that yielded consistent results (Goldstein and Reynolds   1994). 1994). Such assemblages of inclusions were very rare, perhaps because of overprinting of hydrothermal events with concomitant 

254

Miner Deposita (2014) 49:235 – 259 259

Fig. 18   Sulfur isotope

7

compositions of chalcopyrite and  pyrite organized organized by alteration alteration stages for calcic, hydrolytic, and calcite veins. Calcite veins sulfide correspond to chalcopyrite. Both sedimentary gypsum and diagenetic pyrite sulfur isotope compositions are also included for comparison

Calcite veining

6

Hydrolytic alteration 5

Calcic alteration

     y      c 4      n      e      u      q      e 3      r        F

Diagenetic pyrite   Gypsum

2

1

0 -10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

6

7

8

9

10

11

12

34S (o /  ) oo

recrystallization of quartz and post-entrapment changes of  the fluid inclusions. inclusions. A core sample from Espino deposit (ERD-6, 79.2 m depth), is a late calcic alteration stage quartz veinlet with intergrown hematite and chalcopyrite in an amphibole diorite host rock. The veinlet displays euhedral quartz with no evidence of quartz annealing (Fig. 21 21), ), though the sample did contain many fluid inclusion assemblages (FIAs) with inconsistent liquid to vapor ratios. The FIAs measured measu red were intimat intimately ely assoc associated iated with tiny specular  hematite and acicular actinolite solid inclusions within quartz and thus are deemed primary in origin. These  primary inclusions contain translusce transluscent nt halite crystals. Three FIAs yielded consistent homogenization temperatures ( T h)  between 280 and 295 °C. Salinities were between 32 and 34 wt% NaCl equivalent. No other FIAs at higher  T h  were found in this vein sample. A second core sample from a calcically altered siltstone deeper in the El Espino deposit (ERD-6, 236.8 m depth) contains an actinolite and magnetite assemblage that is overprinted by a second alteration association composed of  calcite, quartz, mushketovite, chalcopyrite, sericite, and chlorite. Euhedral quartz associated with the second calcic alteration association is intergrown with acicular fibers of  actinolite and contains growth zones along the margins of  the vein. Liquid/vapor ratios were not consistent in these inclusions due to necking; however, salinities from these  primary inclusions yield 15 wt% NaCl equivalent. Sample number 179689 is from an intermediate elevation north of the El El Espino deposit. deposit. This sample contains a mineral mineral ∼

association of quartz, pyrite, chalcopyrite, specular hematite, epidote, actinolite, chlorite after actinolite, and sericite. Most  FIAs hosted by quartz show inconsistent liquid/vapor ratios,  but one FIA FIAwas was found that yielded consistent homogeni homogenization zation temperatures temperatur es at 340 – 350° 350° C. As there is no evidence of brine-vapor immiscibility in all samples surveyed surveyed,,  T h  only provides a minimum temperature of formation conditions. A pressure correction must be ap plied to get the true formation conditions. conditions. Although the depth of formation is unknown, sample 179689 displays textures such as wispy inclusion texture and recrystallized quartz indicative of formation under lithostatic conditions (Reynolds, 2012, personal communication). If a 350 °C temperature is  pressure corrected to 425 °C (assuming that ductile deformation styles would occur at such temperatures under lithostatic load), then the pressure required would be roughly 700 – 1,000 1,000  bars for a saline brine, corresponding corresponding to a minimum depth of  formation of roughly 3 – 4 km, consistent with the estimated thickness of the Late Cretaceous lithostatic load (Salamanca Formation) in the area.

Discussion

Early sodic alteration (albite) was the most aerially extensive and external alteration event in the district and is spatially relate rel atedd to th thee Lla Llahu huin in an andd Ill Illape apell – Caimanes Caimanes plut plutons ons espe especial cially ly along major fault zones and lineaments within and around these plutons. Although best developed at stratigraphically deep levels, i.e., 1,000 m deep from the top of the alteration ∼

Miner Deposita (2014) 49:235 – 259

255

exposed higher in the Llahuin area, the sodic alteration assemblage extends upward in the El Espino mining district to the near surface in Espino and Romero areas where it generally seems to transition into a sodic – calcic calcic assemblage com posed of albite and epidote. Although some units of Llahuin suite are affected by pervasive sodic alteration, other units such as a diorite pluton and sills in the Espino deposit are not  affected by this alteration, whereas other units higher in the Llahu Lla huin in are areaa sh show ow alb albite ite ve veini ining ng whi which ch mig might ht ha have ve dev devel elop oped ed later.. The distributio later distributionn of intense sodic alteration is compatible with it being developed within inflow zones to the broad

Fig. 19   Sulfur isotope

compositions for pyrite and chalcopyrite in a  calcic and  b hydrolytic alteration associations

a

hydrothermal system, as proposed by Barton and Johnson (2000 2000)) for a number of IOCG systems. Sodic – calcic c alcic altera alteration tion (albit (albitee – epidote epidote – actinolite) a ctinolite) is relatively spatially more restricted than sodic alteration and grades into a center of calcic alteration assemblages (actinolite – epidote epidote – iron iron ox oxide ide)) dis displ play aying ing a zo zoned ned pa patte ttern rn aro aroun undd the southern end of the Llahuin Pluton. Potassic alteration in the El Espino district is relatively restricted in extent. Deep, biotite – magnetite magnetite assemblages, similar to those at Candelaria, are present but are poorly known due to a lack of deep drilling. Higher level potassium

4

Calcic

 pyrite

3

 chalcopyrite      y      c      n      e      u      c      e      r        F

2

1

0 -8

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

6

7

8

7

8

34S (o /  ) oo

b

7

Hydrolytic 6  pyrite 5  chalcopyrite      y      c      n      e      u      c      e      r        F

4

3

2

1

0 -8

-7

-6

-5

-4

-3

-2

-1

0

1

34S (o /  ) oo

2

3

4

5

6

256

Miner Deposita (2014) 49:235 – 259 259

Fig. 20   Sulfur isotope

9

compositions organized by location and therefore, stratigraphic level. El Espino sulfidess occur in the Espino sulfide Member, whereas Romero sulfides occur stratigraphically higher in the Quelen Member 

8 7

 Llahuin (n=2)  Romero (n=13)

6

 Espino (n=20)

5

     y      c      n      e      u 4      c      e      r        F

3 2 1 0

-10 -9 -9

-8

-7 -6

-5

-4 -3 -2

-1

0

1

2

3

4

5

6

7

8

9

10

34S (o /  ) oo

feldspar-bearing potassic assemblages are better developed at  feldspar-bearing the current level of exposure of the system. Cross-cutting relationships suggest that potassium feldspar-bearing assemblages cut early sodic alteration assemblages but were locally formed synchronous with potassic – calcic calcic altered rocks. In the Romero area, spatially restricted potassic –  calcic zones developed within the calcic area that are related to iron oxide mineralization. However, potassic alteration zones do not always display a close spatial relationship to sodic, sodic – calcic, calcic, and calcic alteration zones. The spatial and temporal distribution of potassic alteration zones could  be evidence that they develope developedd as outflo outflow w zones as is hypothesized for several other IOCG districts (Barton and Johnson 2000 Johnson  2000). ). The calcic alteration zone forms the core of the large-scale alteration system in the district. It postdates sodic alteration,  but is probably gradational in time and space with sodic –  calcic and locally potassic – calcic calcic alterations. The calcic alteration zone is best developed at stratigraphically deep to

Tabl ablee 3 Est Estimat imates es of the sul sulfur fur iso isotope tope fra fracti ctionat onation ion tem temper peratur aturee deri derived ved

from sul from sulfur fur isot isotope ope com composi positio tions ns of coex coexist isting ing chal chalcop copyri yrite te and pyr pyrite ite for  each alteration alteration type {T (°K)=[(0.45×10 (°K)=[(0.45×106)/(δ34S py δ34Scpy)]1/2 −

34

Sample Sam ple

Espi Es pino no

ERD6 ER D6-2 -236 36 −0.31

−1.01

Romer Rom ero o

17955 17 9553 3

−1.58

−2.36

Espi Es pino no

ERD4 ER D4-1 -174 74 −1.43

−3.01

Romero

179696

δ

S py (‰)

34

Locatio Loca tion n

6.37

δ

Scpy (‰) Alte Altera rati tion on stage

4.50

T  (°C)   (°C)

Calcic 1st   523 stage Calcic 1st   486 stage Calcic 2nd 261 stage Hydrolytic 218

medium levels (i.e., 1,000 – 500 500 m deep from the top of the exposed alteration in the Llahuin area). The calcic alteration assemblage extends upward and outward transitioning into a sodic – calcic calcic assemblage dominated by albite – actinolite actinolite –  epidote( scapolite). Significant amounts of iron oxides are associated with calcic alteration assemblage assemblages. s. Geochron Geochronologological data indicate that calcic alteration was synchronous with intrusive magmatic activity in the district. Hydrolytic alteration assemblages (chlorite – sericite sericite –  quartz – hematite) hematite) in the El Espino district occur along faults and veins. These asemblages are best developed at medium to high stratigraphic levels, i.e., 500 to 0 m deep from the top of  the exposed alteration in the Llahuin area. Although many hydrolytic zones cut rocks that were previously sodic, sodiccalcic, or calcic altered, some hydrolytic alteration zones occur in relatively unaltered rocks along mineralized veins. Hydrol Hyd rolyti yticc ve veins ins are fee feede ders rs to a zon zonee of ma manto nto-ty -type pe alt altera eratio tionn and mineralization within favorable volcano-sedimentary lithologies in the El Espino deposit. Sulfides (pyrite and chalcopyrite) are largely confined to calcic and hydrolytic alteration assemblage assemblages. s. The paragen paragenetic etic sequen sequences ces in the El Espino Espino and Cand Candelari elariaa –  Punta del Cobre IOCG districts are distinct. The economic iron oxide and sulfide mineralization are related to calcic –  pota  potassic ssic alteration zones in Candelaria (Marschik and Fontbote  2001  2001), ), while in El Espino, the bulk of the iron oxide and copper  mineralization occurs with hydrolytic alteration. Alteration distribution and paragenesis for the early and late alteration stages in the El Espino district is similar to that seen in the Manto Ma nto Verd erdee dis distri trict ct in nor northe thern rn Ch Chile ile (Be (Bena navid vides es et al. al.2007 2007). ). Both districts show regional albitization and then local hydrolytic alteration related to economic iron oxide and sulfide mineralization. −

Miner Deposita (2014) 49:235 – 259

257

Fig. 21   Photomicrographs

showing quartz growth zones with primary inclusions (a ) and  primary inclusion inclusionss coexisting coexisting with acicular actinolite crystals withinn quartz growt withi growthh zones (b )

Extensive potassium and iron metasomatism was not seen in El Espino mining district as it has been described for other  deposits such as Candelaria and Raul Condestab Condestable le (Marschik  and Fontbote 2001 Fontbote  2001;; de Haller  2006).  2006). In the El Espino mining district, potassic metasomatism occurred only locally in the Espino, Romero, and Llahuin areas, within or outside sodic, sodic – calcic, calcic, and calcic altered areas. Isolated small zones of   potassium feldspar and magnetite were even identified in the Arqueros formation to the west of the district sometimes spatially related to dioritic intrusions. The host rocks, paragenetic sequence, and alteration patterns at El Espino and the Raul-Condestable district in Peru (de Haller  2006)   2006) are very similar. At Raul-Condestable, the calcic alteration zone is developed in a volcanic and sedimentary sequence and grades upward into a hydrolytic zone and outward into a sodic – calcic calcic zone. In both districts iron oxide and sulfide mineralization are related to calcic and hydrolytic stages of hydrothermal alteration (de Haller et al. 2006 al.  2006). ). The  potassic core (biotite) described at Raul-Condestable Raul-Condestable is either  weakly developed or not exposed in the El Espino district. Extensive biotite alteration might have taken place in the Romero area, but it has been masked by the later pervasive and extensive chloritization. There is surface evidence of  secondary biotite in that area which is overprinted by either  calcic or hydrolytic assemblages. The core of the alteration  pattern at El Espino district is given by the calcic alteration zone,, domi zone dominate natedd by acti actinoli nolite, te, epid epidote, ote, iron oxid oxide, e, and sulf sulfide. ide. This type of alteration is mineralized and pervasive in RaulCondestable deposit where it surrounds a potassic core (secondary biotite) related to tonalite intrusions (de Haller  2006), 2006 ), suggesting that a potassic core in El Espino district  might be still unexposed beneath the Llahuin area and that  significant undiscovered deposits may exist in calcic-altered zones deeper within the system. Sulfur isotopic compositions of sulfides in calcic assem blages show a wide range of values with a mean near 0 per  mile (Fig. 18 18)) that may be interpreted as consistent with a mixture of two sources, one magmatic and one external with variab var iable le mix mixing ing.. Th Thee alt altera eratio tionn pat patter ternn an andd tim timing ing of alt altera eratio tionn and magmatism suggests that dioritic intrusions played a role in the formation of the system providing heat, fluid, and

 probably sulfur sulfur.. This magmatic source might have progressively mixed with a sulfate-derived source. In contrast, in hydrolytic assemblages, sulfur isotopic compositions of sulfides show a much wider spread, more clearly suggesting the involvement of a sulfate-derived source. Under the moderate temperature, relatively oxidizing conditions indicated by the mineralogy and fluid inclusions, marine sulfate is likely to be an important sulfur source in the hydrothermal system. The individu indi vidual al data sets of sulf sulfur ur isot isotopic opic comp composit ositions ions show wide spreads, and the coexisting minerals (as indicated in the paragenesis) show that most formed with iron oxide, especially hematite. All this evidence is consisten consistentt with partial reduction of he heav avyy ma mari rine ne su sulf lfat ate. e. Th This is is wh what at is ob obse serv rved ed in ot othe herr ar area eass such as Raul Raul-Con -Condest destable able (de Hall Haller  er 2006 2006)) and mo moder dernn Sal Salton ton Sea geothermal system in Southern California. The close spatial and temporal association of well-altered zones with generally intermediate composition intrusions suggests that   part of Llahuin plutonic suite (includin ( includingg possible unexposed  plutons) probably provided heat and contributed fluid and sulfur early to the hydrothermal system, but the bulk of iron oxide, copper, and gold mineralization occurred relatively late in the system by mixing with a fluid containing marinederived sulfate. Estimates of the sulfur isotope fractionation temperature show sh ow a wid widee ran range. ge. Th These ese est estima imates tes ass assum umee tha thatt su sulfu lfurr iso isoto tope pe fractionation fractionat ion factors are correct and that samples analyzed did not have microscopic inclusions of another sulfide. The estimation of fluid temperature indicated by the high quality fluid inclusion study performed seems to better represent the conditions of the fluids responsible for iron oxide, copper, and gold precipitation. The fluid responsib responsible le for specular hematite and sulfide mineralization in El Espino deposit was saline and moderate temperature, which is consistent with results obtained by Rieger et al. (2012 (2012)) for the Manto Verde district. Homogenization temperatures (T h) measured in El Espino quartz hosted inclusions within veins containing hematite were between 280 and 350 °C, consistent with quartz hosted inclusions related to hematite formation in Manto Verde, which were in the range between 208 and 470 °C (Rieger  et al. 2012 al.  2012). ). Early magnetite formation in quartz fluid inclusions at Manto Verde deposit occured around 435 °C and

258

reached up to 530 °C (Rieger et al.  2012  2012), ), suggesting that  calcic alteration related to massive magnetite formation may have occurred at that similar range of temperature at  El Espino district.

Conclusions

The paragenetic sequence and structural/stratigraphic controls of alteration and mineralization are similar to those observed at a number of other Andean IOCG systems. As is typical of  many Andean systems (Sillitoe   2003), 2003), an early large-scale sodic alteration event was followed by a complex series of  sodic – calcic, calcic, potassic, and calcic alteration events, which in turn were followed by a late, and generally high-level hydrolytic lyt ic alt altera eratio tionn sta stage ge.. As at a nu numbe mberr of oth other er And Andean ean sys system tems, s, sulfi su lfide de pre precip cipita itatio tionn in the El Esp Espin inoo min miningdistr ingdistrict ict too tookk pla place ce with calcic and hydrolytic associations. associations. Unlike the Candelaria (Marschi (Mar schikk and Fon Fontbot tbotee 2001 2001)) and RaulRaul-Con Condest destable able depo deposits sits (de Haller et al. 2006 al. 2006), ), where the bulk of sulfides formed with calcic –  potassic and calcic alteration associatio associations, ns, respective respectively ly,, the majority of the sulfides currently observed at El Espino district deposits occur in hydrolytic alteration associations overprinting overprint ing calcic alteration. Iron oxides in the El Espino district display a general tem porall tren  pora trendd from early magnetit magnetitee ass associa ociated ted with sodic, sodic –  calcic, potassic alteration, and early calcic to later hematite associated with calcic and hydrolytic alteration. This overall change in mineralogy may reflect declining temperature of  hydrothermal fluids through time or a progressive increase of  the ox oxid idati ation on sta state te of th thee flu fluid ids, s, per perha haps ps due to flu fluid id mix mixing ing wit withh a more oxidized, meteorically derived fluid. The presence of  mushketovite with some calcic alteration mineral associations mayy re ma refle flect ct a lat latee tem temper peratu ature re inc incre rease ase dur during ing an ov overa erall ll co cooli oling ng trend tre nd or a cha chang ngee to sli sligh ghtly tly mo more re red reduc ucin ingg con condit dition ionss du durin ringg an overall higher oxidation trend. The El Espino mining district appears to lack bodies of  massive magnetite – apatite apatite that are present at a number of  Andean deposits including Manto Verde (Rieger et al.  2010  2010)) and the Marcona-Mina Justa deposits in Peru (Chen et al. 2011). 2011 ). Such deposits may be absent or may be present below the current level of erosion. Hydrothermal fluid flow in the El Espino mining district  was channeled along high angle, brittle faults and related extension fractures that developed in a right-stepping jog locally along a major north-striking dextral fault system. Mantos were develo developed ped within the mixed volcano-sedimenta volcano-sedimentary ry sequence of the district, particularly beneath relatively impermeable limestone beds. Mineralized breccias along structures and particular beds are locally present in the El Espino district   but are not well develope developedd at the current level of erosion. Distal, hydrolytic, and argillic stage calcite –  barite veins with

Miner Deposita (2014) 49:235 – 259 259

minor copper and silver are interpreted to represent the highest-level highest-le vel expression of the hydrothermal system. Fluid inclusion provides evidence of a minimum fluid temperature of 425 °C (pressure corrected) and minimum depth of formation of 3 – 4 km for the formation of quartz –  hematite – actinolite actinolite veins related to the second stage of calcic alteration. Sulfur isotopes, alteration mineralogy, alteration  patterns, and timing of magmatism magmatism and alteration suggest suggest that  early high temperature magmatically derived fluids were mixed with cooler, saline, oxidized fluids that included marine-derived marine-der ived sulfate. The close spatial and temporal association of well-altered calcic zones with generally intermediate composition 88 Ma intrusions suggests that these plutons could have provided both heat and fluid in the early stage of  the IOCG system. The Andean IOCG systems in Chile generally become younger to the south. Manto Verde ’s age range between 123 and 117 Ma, Candelaria – Punta Punta del Cobre has been constrained to 116 – 110 110 Ma (Si (Silli llitoe toe 2003 2003), ), whe wherea reass El Esp Espino ino dep deposi osit, t, wit withh an apparent age of approximately 88 Ma, represents one of the youngest and southernmost systems yet defined. The relatively shallow level of erosion of the El Espino system compared to older systems farther north in Chile suggests that significant  undiscovered deposits may exist in calcic-altered zones deeper  within the system. ∼

This work was supported by the National National Science Foundation project EAR-0207217. Assistance from Teckcominco for  field work logistics and chemical rock analysis and assistance from John Humphrey and Jim Reynolds (Fluidinc) on the sulfur isotope and fluid inclusion studies, respectively, are gratefully acknowledged. GPL also acknowledges invaluable help from the Society of Economic Geologists through Terrones and McKinstry student research grants, from a  Newcrest Resources Economic Geology Fellowsh Fellowship, ip, and from Sarah Gleeson (University of Alberta) for discussions and access to facilities. Acknowledgements

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