Porfidos Cu Mirador SE Ecuador

©2013 Society of Economic Geologists, Inc. Economic Geology, v. Geology, v. 108, pp. 11–35

Geology, Mineralization, and Geochronological Constraints of the Geology, Mirador Cu-Au Porphyry District, Southeast Ecuador JOHN DROBE,1,† DARRYL LINDSAY,2,* HOLLY STEIN,3 AND JANET GABITES,4 1 Dorato

Resources Inc., 2300 - 1177 West Hastings Street, Vancouver, Vancouver, British Columbia, Canada V6E 2K3

 2 ExplorCobres 3 AIRIE

4 Pacific

S.A., Av. República de El Salvador #1082 y NN.UU., Ed. Mansión Blanca, Torre París, Mezanine

Program, Department of Geosciences, Colorado State University, Fort Collins, Colorado 80523-1482, and Geological Survey of Norway, 7491 Trondheim, Norway

Center for Isotopic and Geochemical Research, Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, Canada V6T 1Z4

 Abstract The Mirador porphyry Cu-Au district is located in the southernmost sector of the northern Andean Cordillera, in Zamora-Chinchipe province, southeastern Ecuador. The district contains two significant porphyry Cu-Au ± Ag ± Mo deposits, Mirador and Mirador Norte, and an interconnected series of narrow, mineralized structures known collectively as Chancho. The principal mineralization in the porphyries is disseminated to blebby chalcopyrite developed primarily in potassic alteration, with overlying chalcocite supergene enrichment zones. Prior to radiometric dating presented in this study, these deposits were considered Late Jurassic based on close similarity, and therefore assumed age equivalence, with the well-dated Panantza, San Carlos, and Sutzu porphyry copper deposits located 40 km to the north. New U-Pb zircon ages confirm a MidM iddle Jurassic age for granodiorite of the Zamora batholith at this location (163.8 ± 1.9 Ma), and Late Jurassic ages for subvolcanic intrusions (156.2 ± 1.0 and 153.1 ± 1.3 Ma). Re-Os molybdenite ages average 156 ± 1.0 Ma and indicate coeval mineralization between Mirador and Mirador Norte. Mineralization and related sub volcanic igneous activity are closely associated a t Mirador and represent the same temporal event recorded at Panantza-San Carlos and Sutzu, as well as coeval porphyry, skarn, and epithermal Au mineralization extending at least 80 km south, defining a north-south Cu-Au metallogenic belt spanning over 120 km.

Introduction The Mirador deposits were considered Late Jurassic in age THE MIRADOR porphyry Cu-Au district is located in the (Drobe et al., 2008) based on their similar geology and Cordillera del Condor, in the southernmost sector of the assumed age equivalency with Panantza and San Carlos, (Coder, 2001). Prior to northern Andean Cordillera in southeastern Ecuador, 340 km  which had been radiometrically dated (Coder, dating, sedimentary rocks of the Aptian (base at 125 Ma) south of Ecuador’s capital city of Quito (Fig. 1), in the Hollin Formation (Tschopp, 1953), which unconformably  province of Zamora-Chinchipe. Elevations range from about 800 to 1,800 m above sea level. The area has a wet equatorial overlie the south margin of the Mirador deposit, provided a Mirador.. This unconformable reclimate with an average rainfall of 2,300 mm/yr. Over the past minimum age constraint for Mirador lationship is also present at Panantza and the Sutzu porphyry  decade, this subandean region has emerged as a significant metalliferous belt bridging important, world-renowned dis- deposit, located 15 km south of Panantza. Host rocks for by Chiaradia et al. (2009) as being tricts in northern Peru and Colombia. The Mirador district these deposits are reported 40Ar/ 39Ar method), with mineralizabetween 160 to 153 Ma ( comprises two main porphyry Cu-Au ± Ag ± Mo deposits, Mirador and Mirador Norte, as well as a subparallel, inter- tion between 158 to 153 Ma (Re-Os, molybdenite). The connected series of narrow, mineralized structures known geochronological ages presented here confirm a Middle collectively as Chancho (Norte, Central, and Sur zones). At Jurassic age for plutonic rocks of the Zamora batholith (ca. the time of writing, total indicated resources for Mirador and 164 Ma), and a Late Jurassic age for both the hosting subvolMirador Norte, using a 0.4% Cu cut-off, were estimated at canic intrusions and the mineralization (156 Ma). Mineraliza609 million metric tons (Mt) of 0.58% Cu, containing 7.8 bil- tion is related to and slightly postdates the onset of subvollion pounds (Blb) of Cu, 3.2 million ounces (Moz) of Au, and canic igneous activity at Mirador. This study presents the first geochronological dates for the 22 Moz of Ag. Additional inferred resources, at the same cutoff, are estimated at 281 Mt of 0.52% Cu (Drobe et al., 2008). Mirador district and describes their significance to both the The related Panantza and San Carlos porphyry deposits, lo- local and regional geology. The local geology of these deposits cated 40 km to the north, contain additional inferred re- is based on detailed mapping and sampling of stream outsources of 463 Mt at 0.66% Cu, and 600 Mt at 0.59% Cu, re- crops and tropical saprolite profiles along drill trails and ridge spectively, using a 0.4% Cu cut-off. Thus, these four deposits crests, combined with logging of nearly 52 km of diamond drill core. Despite the heavy jungle cover at surface, drill taken together contain approximately 25 Blb of Cu. holes spaced at approximately 75-m centers at Mirador and 100-m centers at Mirador Norte permit a robust interpreta† Corresponding author: e-mail, jdrobe@doratore [email protected] sources.com *Present address: Batero Gold Corp., 3703-1011 Cordova St., Vancouver Vancouver,, tion of lithology, alteration, and mineralization relationships. By dating multiple intrusive phases and their related minBC V6C 0B2, Canada. 0361-0128/13/4080/11-25

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Submitted: April 21, 2011  Accepted: April 5, 2012

12

DROBE ET AL. 78°W

 N 

Mocoa Cu-porphyry  C O L  OM  O    B  I A  I   

0°

    n        a        e        c      O           i c                i f        c        a      P

   o     C

s

t

a

    i     S

e

r

r

a

   o   a  p

  N  o   R  i o

 ABITAGUA   ABITAGUA  BATHOLITH 162-173 Ma

Guayaquil 

 Machala

         l       a       e          R       a Z      m         r  a o   r a  r CUENCA           l       e    R    i o a          l          i    r         d   o        r    d       o    n         C   o    C

4°

Zamora

PERU

    l   e    d

   a    r    e     l     i    l    d    r    o     C

0°

CUCHILA  (LA BONITA) BATHOLITH 198-210 Ma

QUITO

   r    O

i

e

n

t

e

Tertiary to Recent sediments Tertiary to Recent Continental volcanics Cretaceous to Tertiary I-type intrusions

Mirador Cu-Au District

ZAMORA BATHOLITH 164 - 190(?) Ma

Cretaceous accreted island arc, ocean floor, and marine sedimentary rocks Jurassic I-type intrusions Triassic to Cretaceous sedimentary + volcanic rocks Paleozoic to Cretaceous metamorphic rocks

0

200km

78°W

FIG. 1. Inset shows location of the Mirador Cu-Au district, district, located in southeast Ecuador Ecuador toward the northern northern end of the Zamora batholith, the southernmost of three Jurassic I-type intrusions within the sub-Andean region. Modified from Gendall et al. (2000).

eralization at Mirador and proving the temporal association to porphyry and epithermal deposits located elsewhere in the Cordillera del Condor, we underscore the exploration significance of the ca. 156 Ma Late Jurassic, subvolcanic, calc-alkaline igneous event that affects Middle Jurassic plutonic rocks of the extensive Zamora batholith, as well as older volcanosedimentary pendants within the batholith, and volcano-sedimentary sequences unconformably overlying the batholith. The ages also help clarify and constrain Mesozoic tectonostratigraphic events in the Northern Andes.

Regional Geology  Mirador and the other documented Late Jurassic porphyry  copper deposits within 40 km are associated with subvolcanic intrusions intruding equigranular plutonic rocks comprising the regionally extensive Zamora batholith, a loosely defined Middle to Late Jurassic, calc-alkaline igneous complex that extends over 200 km along a NNE trend, between latitudes 3° and 5° S, and at least 100 km wide (Baldock, 1982; Aspden et al., 1990; Litherland et al., 1994). The batholith is the dominant geologic entity in the sub-Andean region of southeast Ecuador, especially in the Cordillera del Condor, the mountain range that forms the international border in the area, and along the Rio Zamora to the west of that range. This batholith, together with the lithologically correlative Abitagua and Cuchilla batholiths to the north and similar rocks in the Cordillera Oriental of Colombia (Fig. 1), are interpreted as remnants of a volcanic arc constructed along an Andean-type continental margin (Sillitoe, 1988, 1990) that was well-established by the Middle Jurassic. The batholith was intruded along a north-south regional structure, as evidenced by northsouth intrusive contacts with roof pendants. The current 0361-0128/98/000/000-00 $6.00

northeast orientation of the batholith is the result of dextral slip along northeast, post-Cretaceous, Andean orogeny faults. It remains unclear how many intrusive and volcanic phases the Zamora batholith comprises, their age relationships, and age range. This is due to several factors, but primarily regional-scale mapping of discontinuous, weathered exposures in a largely inaccessible area, and the inclusion within the batholith of volcanic-textured rocks ranging from Triassic to post-Cretaceous age (Litherland et al., 1994). However, the batholith can be broadly divided into two intrusive types: (1) equigranular plutonic rocks of medium-grained granodiorite, diorite, and tonalite, and very coarse, K-feldspar megacrystic, monzogranite (collectively referred to as “granodiorite” in this paper), locally with aplite and leucogranite predominating (as at Panantza); and (2) younger subvolcanic intrusions comprising feldspar (albite>>coarse microcline)-hornblende ± quartz porphyry of andesitic to dacitic composition that clearly intrude the plutonic rocks. Subvolcanic rocks occur as dikes and stocks (<2-km diam) and give the youngest of the reported ages for the batholith. Importantly, they are uniquely associated with copper and gold mineralization,  whereas plutonic intrusive margins are notably unmineralized. Textures vary with size of the intrusions. Larger intrusions, comprising plugs or stocks several hundreds of meters  wide, show seriate textures transitional between subvolcanic and plutonic; in weathered exposures these are easily confused as phases of the Zamora pluton, though the diagnostic euhedral hornblende phenocyrsts aid in distinguishing them. There is some confusion about the plutonic and subvolcanic elements of the Zamora batholith in the literature due to naming conventions applied to subvolcanic rocks, which are sometimes described using volcanic classifications (dacite,

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MIRADOR Cu-Au PORPHYRY DISTRICT, SE ECUADOR

andesite, etc.; as in Litherland et al., 1994), and sometimes as plutonic (granodiorite, etc.; as in Gendall et al. 2000). We argue that the volcanic nomenclature is more useful, as it helps divide the subvolcanic units that were intruded after significant uplift, erosion, and partial burial of the plutonic rocks and that are associated with mineralization. The subvolcanic units are perhaps more usefully included within the Misahuallí Member of the epicontinental Chapiza Formation (Tschopp, 1953), rather than the Zamora batholith. There is some precedence for this, as Litherland et al. (1994, p. 24) included porphyritic rocks of the “highest-level igneous facies of the Rosa Florida pluton” within the Misahuallí Member. The porphyritic, subvolcanic stocks and dikes form small complexes every 15 to 20 km along the NNE axis of the Zamora batholith, and almost all are associated with significant mineralization (Fig. 2). There appear to be at least three controlling north-south−oriented structures, with the most active aligned with the Panantza-San Carlos, Mirador, Fruta del Norte, and Chinapintza deposits; dikes continue s outh on both sides of the border for at least another 20 km. A parallel structure 20 km to the west is evident at the Nambija Au skarn and just west of Yantzatza. Another parallel structure occurs 15 km east of the main trend and hosts the Warintza porphyry Cu-Mo deposit (Fig. 3); this one is the least well studied due to its remoteness but shows up on regional sediment geochemistry maps. All the porphyry Cu deposits in the region are associated with these feldspar-hornblende-quartz porphyries, which show varying degrees of mineralization. Notably, NNW- and NW-trending dikes are mineralized,  whereas NE-trending dikes are post- or late mineralization and therefore younger. Initially the dikes were differentiated based on their degree of mineralization into three categories of early, intra-, and late mineralization dikes. Additional drilling indicates it is only  possible to differentiate the dikes as pre- (or “early”) and post- (or “late”) mineralization. Based on drill core observations, an early dike can have varying degrees of mineralization along strike, and the degree of mineralization appears to be more a function of degree of fracturing prior to the mineralization event, rather than the apparent timing of intrusion relative to mineralization; metal grades change little or gradually  at contacts. Late or postmineral dikes have sharp changes in grade across their contacts and are essentially barren of copper, though some show minor pyrite mineralization and chlorite-epidote alteration. Along its eastern margin the Zamora batholith intrudes marine sedimentary and minor andesitic volcanic rocks of the Lower Jurassic Santiago Formation (Tschopp, 1953; Baldock, 1982; Litherland et al., 1994; Gaibor et al., 2008; Fig. 2). While defined within Ecuador as having a Lower Jurassic base, the correlative strata in northwestern Peru, the Pucará Group, extend into the Upper Triassic. The calcareous units are intensely hornfelsed and calcsilicate altered to a fine-grained, dark rock that has been misidentified as andesite of the Misahuallí Member by past workers (e.g., Litherland et al. , 1994; Gendall et al., 2000). Steeply W dipping, N-S−trending pendants and large xenoliths of calc-silicate−altered, thin-bedded marine shale and sandstone occur north of Mirador and south of Chancho Norte along the Rio Tundayme; vesicular, aphyric andesite cobbles are common in the Rio Quimi drainage. 0361-0128/98/000/000-00 $6.00

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Farther south, at Nambija, intermediate volcano-sedimentary rocks of the Piuntza unit of the Santiago Formation form a roof pendant within the Zamora granodiorite (Litherland et al., 1994; Paladines and Rosero, 1996; Chiaradia et al., 2009). The upper and lower contacts of the enigmatic Piuntza unit are unknown, as it occurs as inliers within the batholith. Based on lithology and tentative fossil evidence of a Triassic age (Litherland et al, 1994), it may correlate better with the Pucará Group of Peru, specifically with the volcaniclastic Oyotún Formation intermediate volcanic rocks. These overlie carbonates of Upper Triassic age and are thought to extend into the upper Lower Jurassic (Jaillard et al., 199-0). While most Triassic volcanic rocks in Peru are considered of intraplate origin, in northwest Peru these volcanic rocks are reported to have a calc-alkaline arc component by the Early  Jurassic (Romeuf et al., 1995; Rosas et al., 1996). The volcanic rocks continue north of Nambija and are  widespread west and northeast of Yantzatza. These were originally mapped as probable Misahuallí unit (Litherland et al., 1994), but based on our limited mapping of this area they are probably better included with the Piuntza unit as they are intruded by granodiorite of the Zamora batholith. Calk-alkaline  volcanic rocks are common over the extent of the Zamora batholith, both intruded by and unconformably overlying the batholith, and are currently all (with the exception of those at Nambija) grouped into the Misahuallí unit, a convention initiated by Litherland et al. (1994). The original definition of  the Misahuallí Member by Tschopp (1953) was as the upper  volcanic Member of the dominantly continental-type, coarseclastic sedimentary, Upper Jurassic Chapiza Formation. Litherland et al., (1994) placed both sedimentary and volcanic successions into the Santiago Formation and included all continental-type, calc-alkaline volcanic rocks that are (1) spatially associated with the Zamora batholith, and (2) underlie the Cretaceous Hollin Formation and overlie the Santiago Formation, and therefore of Jurassic age, as the Misahuallí  unit. The underlying sediments of the Chapiza Formation  were redefined as the Chapiza unit. As Coder (2001) pointed out, this scheme puts a marine rift succession (Santiago Formation) together with collisional volcano-sedimentary sequence (Misahuallí Member of Tschopp) and ignores the major regional igneous event of the intrusion of the Zamora batholith. There is now sufficient mapping and dating in the region to return to the more restrictive, original definition of the Misahuallí (and Chapiza Formation) by Tschopp (1953). This  works better to separate Late Jurassic subvolcanic and volcanic rocks, which are closely associated with significant metal deposits, from both plutonic rocks of the Zamora batholith and intermediate volcanic rocks intruded by the batholith, the latter of which are better grouped with the Piuntza unit of the Santiago Formation. Therefore, the mainly  andesitic, calc-alkaline volcanic rocks on the west and north side of the batholith that were included as Misahuallí unit by  Litherland et al. (1994) and Romeuf et al. (1995; 172 Ma by  40Ar/ 39Ar) are in this study assigned to the Piuntza unit of the Santiago Formation, as they are intruded and altered by  Zamora granodiorite. Late Jurassic volcanic and volcaniclastic rocks equivalent and coeval with the subvolcanic units are preserved in a re-

13

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DROBE ET AL.

the southern extent of the Zamora batholith at approximately  4°42' S. The belt is roughly 30 km wide in the east-west direction, including the Warintza Cu-Mo deposit at the easternmost limit, and the Nambija Au skarn district at the western limit. Porphyry Cu deposits The Cu-Au and Cu-Mo porphyry deposits within the northern half of the 120-km-long Zamora Cu-Au belt share many  similarities in geology and mineralization. Most importantly, all are typical calc-alkaline, granodiorite-hosted, Late Jurassic-aged deposits in which chalcopyrite is the principal copper mineral at currently known depths, with chalcocite forming locally important, but relatively thin, enrichment blankets between a leached cap of saprolite and the primary s ulfide mineralization below. Rock exposure is poor and limited to creek drainages that have cut down through the saprolite zone and exposed both the enriched and primary mineralization. All the deposits have drainages cutting through their centers and are easily recognizable as Cu porphyry systems. Channel sampling of these drainage outcrops has provided reliable estimates of the hypogene grades of the mineralization below. None of the deposits has significant iron-oxide lithocaps, and only Panantza and parts of San Carlos show significant oxide copper within the saprolite. The main difference between the deposits is that at Mirador gold is of significant concentrations, whereas at Panantza and San Carlos molybdenum is significant; both deposit districts bear a similar tenor of Ag (Table 1). The Panantza and San Carlos porphyry Cu deposits, as well as the nearby Kutucus Cu skarn, from the north end of the Zamora Cu-Au belt were first described by Gendall et al. (2000). As detailed by Gendall et al. (2000), the large geochemical surface anomalies of these deposits were discovered through detailed pan concentrate and silt sampling of  streams, with follow-up soil and rock sampling, by Gencor

and Billiton S.A. from 1994 to 1998. Also during this time, additional areas of porphyry Cu mineralization were discovered along the overall N-S strike of the belt at San Luis, San Miguel, San Marcos, and Sutzu (Fig. 3). The Warintza deposit (Ronning and Ristorcelli, 2006) is anomalous in that it is located 13 km east of the main belt (Fig. 3). The porphyry deposits occur in pairs that are separated 4 to 6 km in a northwest direction, usually along strike of the older subvolcanic dikes, with the larger deposit of the pair to the southeast. The northwest trend to every deposit can be attributed to transtensional openings on northwest-oriented  jogs along a north-trending, regional (probably crustal-scale) sinistral strike-slip fault. However, the geologic significance of  the pairings is not well understood. From north to south, the pairs are Panantza-San Carlos, San Marcos-Sutzu, and Mirador Norte-Mirador (Fig. 3.). North of Panantza, the mineralization continues as the narrow, more structurally controlled zones of Trinidad, San Miguel, La Florida, and San Luis, before disappearing beneath apparent Misahuallí volcanic rocks. West of Mirador, the Chancho system (Chancho Norte, Chancho, and Chancho Sur) is similarly narrow and structurally controlled, with the south end opening to a “horse tailing” of diffuse mineralization before disappearing beneath Hollin Formation cover rocks. Each deposit is defined very well by soil geochemistry, with Au + Mo highs centered on Zn lows; Cu anomalies are less useful for targeting due to the high mobility of Cu in the tropical soils. The spatial coincidence of the anomalies is very important: the most intense mineralization is where all three soil anomalies coincide. Some prospects, such as Sutzu and San Marcos, have broader, less defined, and offset anomalies and the mineralization appears to be of lower grade, based on stream channel sampling. Panantza and San Carlos deposits were initially scout drilled between 1998 and 1999, and Panantza was advanced

TABLE 1. Porphyry Copper Resources within the Zamora Cu-Au belt—Mirador and Panantza Districts Project

Category

Metric tons (t)

Cu (%)

Cu (lbs)

Au (oz)

Ag (oz)

Measured and indicated resources Mirador1 Mirador Norte1

Measured and indicated Indicated

437,670,000 171,410,000

0.61 0.51

5,887,000,000 1,921,000,000

2,740,000 489,000

21,530,000 -

Total measured and indicated

609,080,000

0.58

7,808,000,000

3,229,000

21,530,000

Project

Category

Tonnes

Cu%

Cu (lbs)

Inferred resources Mirador Mirador Norte Panantza2 San Carlos2 Subtotal Panantza-San Carlos

Inferred Inferred Inferred Inferred

235,400,000 45,820,000 463,000,000 600,000,000 1,063,000,000

0.52 0.51 0.66 0.59 0.62

2,708,000,000 513,000,000 6,737,000,000 7,740,000,000 14,477,000,000

1,250,000 101,000

9,900,000 -

Total inferred

1,344,220,000

0.60

17,698,000,000

1,351,000

9,900,000

Note: - = not estimated 1 See the Technical Report “Update on the Copper, Gold and Silver Resources and Pit Optimizations: Mirador and Mirador Norte Deposits,” dated No vember 30, 2006, available on SEDAR 2 Panantza: see the Technical Report titled “Panantza Copper Project—Update on Inferred Resource Estimate,” dated July 10, 2007 available on SEDAR; San Carlos— see the Technical Report titled “Corriente Copper Belt Project—Order of Magnitude Study (Preliminary Assessment) dated June 22, 2001, available on SEDAR; does not include copper oxide mineralized material that was included in the 2001 resource estimate and is recalculated using a block model at a 0.4% Cu cutoff  0361-0128/98/000/000-00 $6.00

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MIRADOR Cu-Au PORPHYRY DISTRICT, SE ECUADOR

considerably further in follow-up drill programs in 2000 and 2006, with drilling now totaling almost 17,000 m in 53 holes. At the time of drilling the Panantza and San Carlos deposits in 1998 to 1999, the Mirador deposit remained a stream-sediment anomaly, as the border conflict in the mid-late 1990s had prevented follow-up exploration in the area. Mirador and the neighboring Chancho zones were initially followed-up by  rock sampling and drilled by Corriente Resources Incorporated (“Corriente”) in April 2000, as Corriente and Lowell Mineral Exploration assumed management of the project. Mirador Norte is the most recent discovery made, in March 2003, during additional mapping at the limits of the geochemical data. The Mirador and Mirador Norte deposits differ significantly from those to the north in that gold is present in economic quantities (over 0.2 g/t).

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and indicated resources of 5.7 Moz Au, 7.3 Moz Ag, and inferred resources of 6.1 Moz Au, 7.9 Moz Ag (Henderson, 2009). Other important systems include the subepithermal, Au-Ag sulfide vein deposits (Sillitoe, 2009) of the Chinapintza district, having inferred resources of 0.8 Moz Au (Eason and Oviedo, 2004), and the Jerusalem camp having measured and indicated resource of 0.58 Moz Au, 3.38 Moz Ag, and inferred resource of 0.71 Moz Au, 6.27 Moz Ag (Holly, 2006). Fruta del Norte has a Late Jurassic minimum age of mineralization of ca. 155.4 Ma, based on an interpreted overlying volcanic unit, coeval with Mirador and the porphyry deposits to the north (Stewart, 2008). The age of sulfide-hosted gold mineralization at Chinapintza is enigmatic, as there are both Late Jurassic dates (K-Ar dates of 153−156 Ma, Litherland et al., 1994), with a recent U-Pb date from mineralized dacite porphyry of 157.7 ± 1.4 Ma (McClelland, 2010), and an Skarn and epithermal Au Oligocene U-Pb date of 30.6 ± 1.4 Ma (Gaschnig, 2009) from Geologic and radiometric dating evidence indicates skarn rhyodacite subvolcanic rock that is in part mineralized. Chimineralization in the Zamora Cu-Au belt formed both distally  napintza bears much resemblance to the Late Jurassic porand later than the main porphyry Cu-Au mineralization. No phyry Cu systems, as the auriferous sulfide veins trend mostly  economically significant skarn deposits adjacent to porphyry  NW and are hosted by the Zamora batholith, with mineraldeposits are known to occur, mainly because the porphyry de- ization genetically associated with younger subvolcanic units. posits are hosted entirely within intrusive rocks. Skarns are Local Geology and Mineralization hosted by Triassic-Lower Jurassic Santiago Formation marine sedimentary and volcanic rocks where they are intruded by  The Mirador porphyry Cu-Au district comprises the Mithe Late Jurassic igneous rocks. Conversely, the significant rador and Mirador Norte deposits, both with block modelepithermal Au deposits in the belt are temporally related to based resource estimates, and the Chancho prospect compristhe porphyry Cu deposits, though none are spatially associ- ing Chancho, Chancho Sur, and Chancho Norte zones (Figs. ated with them due to the level of erosion affected in the 3, 4). Mirador and Mirador Norte are connected along a NWEarly Cretaceous. trending structure that was tested near the mid-point with an The discontinuous skarns within the Nambija Au skarn dis- 800-m drill hole, which intersected several narrow, weakly  trict (Prodeminca, 2000; Chiaradia et al., 2009), located 60 mineralized structures. The Chancho system trends NNW  km southwest of Mirador (Fig. 2), have produced an esti- and appears to be more structurally controlled than Mirador. mated 62 t Au (Gemuts et al., 1992), all by informal mining. They are hosted by a north-south, elongate roof pendant of  Mirador  the Piuntza unit of the Santiago Formation within the Zamora Resources: At the time of writing, the Mirador resource esbatholith (Litherland et al., 1994). As rich and widespread as timation (Sivertz et al., 2006a) was 438 Mt of measured and the skarn mineralization is, there is only minor, insignificant indicated mineral resources grading 0.61% Cu, 0.19 g/t Au, associated porphyry Cu mineralization (David prospect at the and 1.5 g/t Ag, at a 0.40% Cu cutoff grade. Additional inGuaysimi skarn; Chiaradia et al., 2009). While subvolcanic in- ferred mineral resources, also at a 0.40% Cu cutoff, are estitrusions lithologically similar to those at Mirador are present mated as 235 Mt grading 0.52% Cu, 0.17 g/t Au, and 1.3 g/t and closely related to the skarn mineralization, the gold min- Ag. The Mo grades are low and were not included in the eseralization at Nambija has been precisely dated at 145 Ma timate, but, at the same Cu cutoff, average about 0.006 ppm. (Chiaradia et al., 2009), or about 10 m.y. younger than the This estimate, and the geologic interpretation presented in porphyry Cu deposits in the belt. The fact that Nambija is 20 this study, is based on 36,284 m of core drilling in 143 diakm west of the main NNE trend of deposits with the mond drill holes. Cordillera del Condor may be evidence of younger activity   Lithologies: Plutonic rock of the Zamora batholith is the confined to a parallel structure; no other dating of mineral- main host rock of the Mirador system (Fig. 5a). The pluton ization to the north or south exists. comprises mainly medium-grained, equigranular Zamora graThe Kutucus Cu skarn prospect, 5 km north of San Carlos nodiorite (unit “Jzgd”), with leucogranite phases common (Fig. 3), is on the contact of Santiago Formation calc-silicate along the west and southwest margins. There are also scataltered units and Zamora granite. The dacite porphyry dikes tered xenoliths of calc-silicate altered shale. A typical cross related to the mineralization have been K-Ar dated as coeval section is presented in Figure 6a. Hornblende and biotite are  with those at the San Carlos deposit (Gendall et al., 2000), but mostly replaced by brown to black secondary biotite, which is no associated porphyry Cu mineralization has been found to the most obvious indicator of potassic alteration in the deposit date, although the preliminary exploration was focused on the (Fig. 7a). skarn potential. The oldest rocks that intrude equigranular granodiorite are The continuation of the metallogenic zone to the south of  feldspar-hornblende porphyry dacite dikes, with crowded, euMirador is dominated by the epithermal, intermediate sulfi- hedral albite typical of subvolcanic units (Fig. 7b; unit “Jefp”). dation Au deposit of Fruta del Norte, having total measured They strike north and northwest and are subvertical. These 0361-0128/98/000/000-00 $6.00

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DROBE ET AL.

NW

SE

leached PROP

POT

    8     8 M127      M

enriched

M36    5    8   2    M  1

   8    4 M77    M

   5   9    6   4    M    M

M     3     4    

   MM    7    5    M45

M80

M15

PROP POT    0    4    M

     4      7      M

M     3    9   

M103

   8    2    1    M

M93

1300mRL

1200mRL

1100mRL

1000mRL

0

100

200

metres

 A. MIRADOR

SW

NE

POT

SER-PY 

MN59

MN40

PROP

POT

MN58

MN64

   1 M        0 N        N 0    2       M

MN38

   6    3    N MN56 M

900mRL

leached

800mRL

enriched

700mRL

600mRL

0

100 metres

200 500mRL

B. MIRADOR NORTE

FIG. 6. Cross sections for (A) Mirador and (B) Mirador Norte (see Fig. 4 for legend). Heavy bars along the drill stems mark intercepts >0.4% Cu. Green contours from block model show >0.4% Cu and red contours show >0.6% Cu. Inner limit of propylitic alteration zone (PROP), sericite-pyrite (SER-PY), and outer limit of potassic alteration (POT) are also shown. Sections are 100 m thick. 0361-0128/98/000/000-00 $6.00

20

21

MIRADOR Cu-Au PORPHYRY DISTRICT, SE ECUADOR

biotite replacing HBL

 A 

B

1 cm

1 cm

 AN

PY CP

Jzgd

C

CP-PY

1 cm

D

1 cm

CP PY 

 Au

 Au MG

PY 

PY

CP

CP

1 cm

CP

CP  Au

E

 Au

PY 

F  AB HBL

QZ lith

QZ OR

G

1 cm

20 pm

Jzgd chlor. HBL

H

S

Jhbp

FIG. 7. Lithologic units at Mirador. A. Potassic-altered equigranular granodiorite of the Zamora batholith. B. Potassic-altered early feldspar-hornblende porphyry, showing complete replacement of hornblende (HBL) by secondary brown biotite. C. Early mineralized breccia, with fragments of Zamora granodiorite (Jzgd) in a matrix of rock flower cemented by anhydrite (AN) and chalcopyrite + pyrite (CP-PY). D. Evenly disseminated chalcopyrite (yellow) and pyrite (silver) in Mirador granodiorite with diffuse, early vein stockwork. E. Disseminated and fine-fracture chalcopyrite (yellow) and pyrite (silver) in Mirador Norte early porphyry dacite. F. Photomicrographs of polished sections of sulfide concentrate from Mirador metallurgic testwork, showing Au grains in various associations with pyrite (PY), chalcopyrite (CP), and minor magnetite (MG). G. Postmineral rhyodacite dikes: left is rhyodacite porphyry with phenocrysts of albite (AB), hornblende (HBL), quartz (QZ), and orthoclase (OR), right is tuffaceous equivalent, with argillic matrix, increase in quartz, lesser and chloritized hornblende, and sparse lithic fragments; this is from the dike dated in this report. H. Outcrop of postmineral breccia saprolite, showing fragments of angular shale (S), subangular granodiorite (Jzgd), and late dike (Jhbp). 0361-0128/98/000/000-00 $6.00

21

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DROBE ET AL.

fragments. The matrix also contains coarse fillings of chalcopyrite-pyrite and anhydrite, which, together with the quartz vein fragments, help distinguish the unit in weathered surface exposures. Fragments are angular to subangular and show an even potassic alteration with no alteration rims; they  range in average size from centimeters to more than a meter that are clearly observable in outcrop. Northeast-striking, NW- and SE-dipping hornblendefeldspar-quartz porphyry dacite dikes (Fig. 7g; unit “Jhbp”) cut the breccia and other mineralized units of the deposit; they are clearly postmineralization and therefore selected for dating the close of volcanic activity. The largest dike swarm is along the southeast margin of the mineralization, and from southwest to northeast has textures transitional from porphyritic to tuffaceous. Where the dike resembles a crystal tuff, it is quartz rich with minor lithic fragments, and is distinguished from the dacite flow portion of the dike by the intense argillic alteration of the matrix giving it lighter, buff colors. These rocks are sparsely fractured relative to the mineralized rocks, lack any quartz veining, and are fresh to chlorite altered. Outcrops are blocky and resistant and  weather to a characteristic bright red clay due to the oxidation of abundant magnetite. Large rhombs of orthoclase are common in the main dikes. Smaller dikes in the northwest portion of the deposit are dark gray with albite phenocrysts dominating the texture. A large central dike has abundant coarse hornblende phenocrysts, in addition to subhedral albite, orthoclase, and quartz phenocrysts. Late phreatic breccia (Fig. 7h; unit “brpm”) occurs at the margins of most late dacite dikes and as irregular diatremes around the north and northwest margins of the mineralized zone. The breccias are characterized by a polymictic, angular to subrounded fragment assemblage of mineralized and unmineralized rock, the relative quantity of each fragment type being dependent on whether the breccia intruded mainly  mineralized rocks or postmineral intrusions. Common fragments of black shale and fresh Zamora granite, which are not known to occur within several kilometers of the deposit, indicate the fragments have traveled significant distances; the large diatreme north of the deposit and outside of mineralization is composed almost exclusively of shale fragments. Black shale, similar to the Yuquianza Member of the Santiago Formation (Gaibor et al., 2008), is not known from surface outcrops anywhere on the property, suggesting a sharp change in geology possibly across a regional high-angle re verse fault beneath the deposit at depths below the current drilling. The matrix is mostly finely ground rock where the breccia occupies a postmineral dike contact but contains significant milled sulfide minerals in bodies that intrude mineralized Zamora granite. Copper grades within the late breccia range from very low to slightly less than the deposit average, depending on the amount of mineralized rock incorporated. Outcrops of this breccia are massive and very sparsely fractured. In drill core, the breccia is the least fractured lithology  in the deposit. All the intrusive rocks are unconformably overlain by  quartzite sandstone and interbedded shale of the Hollin Formation, an Aptian-Albian-aged transgressive, continental shelf sequence with an eastern provenance. This indicates the mineralization and associated subvolcanic units were exposed 0361-0128/98/000/000-00 $6.00

at surface by the middle Early Cretaceous (about 127 Ma). Mineralization and alteration: Most of the Mirador mineralization is exposed as tan to brown saprolite, with residual silica and abundant iron oxides, in the numerous drill trail and road exposures. The deep weathering has left well-defined geochemical footprint of the deposit, with Au and Mo acting as largely immobile elements with their anomalies coincident  with mineralization at depth. Cu is highly mobile in saprolite and forms a patchy, displaced anomaly, tending to deposit on propylitic-altered and postmineralization units due to their carbonate content. There is a well-defined Zn depletion anomaly coincident with the Au and Mo anomalies. Zones of  supergene Cu enrichment have formed beneath the saprolite and have relatively flat upper boundaries and more uneven lower boundaries with the hypogene mineralization. Primary  and supergene mineralization are only exposed where the drainages have cut down through the overlying saprolite, and so the leached zone is thickest under ridge crests and nonexistent in the valleys with perennial streams (see Fig. 6a). Channel sampling of potassic-altered rock exposures along the two main drainages returned slightly above-average hypogene grades due to weak supergene enrichment. The transition from leached zone to supergene or directly  to primary can be sharp, on a centimeter scale, or “mixed” (mottled) over several meters near fracture zones where uneven clay alteration persists to greater depths. Secondary  chalcocite coating the primary sulfides forms the supergene, enriched mineralization. This zone is intensely argillic altered, with the alteration (and chalcocite mineralization) diminishing gradually with depth. Argillic alteration extends to greater depths within the breccia, likely as a result of deeper penetration of meteoric waters along the easily dissolved breccia matrix. While the supergene zone forms less than 10% of the total resource, its high Cu grade, low hardness, and shallow depth make it important to the economics of the deposit. Primary Cu-Au mineralization at Mirador is mostly as disseminations and fine fracture fillings of chalcopyrite and pyrite in potassic-altered Zamora granodiorite and early porphyry dacite, and as coarse blebs of these same sulfides together with purple anhydrite filling interstices in the matrix of  the early breccia diatreme (Fig. 7c-e). There is no statistical difference in Cu or Au grades inside and outside the breccia, despite the difference in mineralization style. Total sulfide concentrations are almost constant across the deposit at about 4%, with chalcopyrite greater than pyrite within the central potassic zone. Bornite is only present in weak, sporadic amounts deep in the southeast quadrant. Potassic alteration, in the form of secondary biotitization of mafic minerals and anhydrite fillings in the breccia matrix, is dominant, with only  local quartz-sericite overprinting, usually along late pyritic structures. Abundant magnetite occurs along the northwest edge of the deposit but is disassociated with Cu-Au mineralization. A deep (>300 m), narrow (ca. 100 m) zone of massive, milky quartz flooding occurs near the southwest edge of the early breccia diatreme and has lower copper grades, presumably due to its impermeability. The deepest drill holes at Mirador intersected homogeneous hypogene Cu grades (0.6% Cu) to 1,000 m below surface, indicating a vertical geometry  of the mineralization.

22

MIRADOR Cu-Au PORPHYRY DISTRICT, SE ECUADOR

Gold occurs as fine inclusions divided equally between chalcopyrite and pyrite, with minor native gold (Fig. 7f). Molybdenite is concentrated in an outer halo of quartzmolybdenite veins outboard of, but partly overlapping with, the main copper mineralization (Fig. 5a). Paragenesis: The paragenesis of Mirador is illustrated in Figure 8. The mineralization and alteration initiated as an east-west−oriented stockwork of barren, milky, A-type quartz  veins following emplacement of the early porphyry dacite dikes in a fault and/or fracture zone (Fig. 8a, b). Initial Cu + Au + Ag + Mo mineralization and corresponding potassic alteration closely followed the early quartz stockwork, as predominantly disseminated and fine fracture controlled chalcopyrite-pyrite within granite and early porphyry dacite host rocks. Mineralization intensity was variable within the early  porphyry dikes, which seem to have been differentially fractured and open to hydrothermal fluids. Some dikes of this unit did not fracture well and were not as permeable to copper fluids as the older granodiorite. The early phreatic breccia diatreme appears to have formed after the initial disseminated mineralization, based on the disseminated chalcopyrite within the fragments (Fig. 8c). Copper-gold mineralization continued postemplacement of  the breccia and deposited coarse chalcopyrite, pyrite, anhydrite, and rare bornite in open spaces between breccia fragments, mixed with fine comminuted rock matrix, and added disseminated and fine fracture-fill chalcopyrite in areas peripheral to the breccia (Fig. 8d). Mo was concentrated in a halo outboard of the Cu-Au mineralization. Toward the waning of mineralization, NE-striking, NWdipping hornblende-feldspar-quartz porphyry dikes intruded all mineralized units within the deposit, followed closely by  phreatic “pebble” dikes along reactivated dike margins as well as isolated diatremes (Fig. 8e). The consistent association of  the dacite dikes with postmineral breccia dikes suggests the two units are at least in part coeval. The larger, late breccia diatremes on the northwest margin differ in that they are dominated by shale fragments, and while they may be the same age as dacitic breccias, they seem to be rooted in rocks not exposed at surface. Sparse, thin (<10 cm), subvertical veins of massive pyrite, chalcopyrite ± galena ± sphalerite that are relatively gold rich (i.e. grades >10 g/t Au) cut the late hornblende-feldspar dikes (Fig 8e). They are insignificant in volume relative to the porphyry mineralization but are evidence of a minor, very late mineralization event. Notably, they are identical in sulfide assemblage as the Au-bearing sulfide veins mined in the Chinapintza district. Mineralized units within the upper approximately 300 m of  the deposit at Mirador are highly fractured, with most drill core broken in pieces less than 10 cm long. The fracturing is the result of the volume expansion associated with hydration of hypogene anhydrite to gypsum by meteoric water (Fig. 8f). The gypsum veinlets subsequently dissolve leaving loose fractures. The drill core is relatively competent below the level  where anhydrite and gypsum are affected by weathering and leaching. Argillic alteration penetrates to depth within the newly created fracture system, and decreasing from very  strong within the supergene zone, to weak at the gypsum-anhydrite front about 300 m below. 0361-0128/98/000/000-00 $6.00

23

Mirador Norte Mirador Norte is located 4 km of Mirador, along strike of  the soil geochemical anomaly (Fig. 4a). It is relatively lowlying and very poorly exposed, albeit gossanous saprolite of  the phyllic alteration halo is exposed in road cuts just east of  the Mirador camp. It was discovered during follow-up of a single anomalous molybdenum silt sample. Mineralization over 1% Cu is exposed in a single small drainage that was pre viously overlooked. The current resource estimate for Mirador Norte at a 0.4% Cu cut-off is 171 Mt at 0.51% Cu and 0.09 g/t Au indicated, with additional inferred resources of 46 Mt of 0.51% Cu and 0.07 g/t Au (Sivertz et al., 2006b). The geology of the Mirador Norte deposit is simpler than that of Mirador, lacking a breccia diatreme and any postmineral units. The host rocks are the same equigranular granodiorite intruded by NW-striking, hornblende-feldspar porphyry dacite dikes. The mineralization, dominant alteration, and metal ratios are similar in composition to Mirador but are more structurally controlled, without the coincident circular zoning of  metals and alteration. Copper grades are similar in both granodiorite and porphyry dikes, although at the south margin of  the deposit the copper grades in porphyry show some variation relative to the granite: copper grades both increase and decrease across dike contacts, along strike, or up- and downdip. It appears that the dike contacts controlled fluid flow more at the margins than at the center of the deposit, where fracturing was perhaps more pervasive and less prone to control by  lithology. Similar changes in mineralization intensity along strike within dikes are observed at the Panantza deposit. Mirador Norte mineralization consists mainly of disseminated and stockwork hypogene chalcopyrite. As at Mirador, there is a superficial leached zone up to 40 m thick overlying the secondary enrichment blanket that averages 14 m thick. The enrichment zone is immature, with chalcocite coatings on chalcopyrite and pyrite. The enriched zone grades into primary, disseminated chalcopyrite mineralization. Higher grade areas are associated with structurally controlled, f ine-grained, dark-gray silica flooding that can contain more than 5% chalcopyrite. Alteration is mostly potassic in the form of black to brown secondary biotite and is almost completely overprinted by propylitic (chlorite + epidote) alteration, which, unlike the fringing chloritic alteration at Mirador, is spatially coincident  with it. Local coarse anhydrite is preserved at deeper levels below the gypsum front. The potassic alteration assemblage transitions to intense quartz-sericite-pyrite alteration along the west side of the deposit, while along the northeast side propylitic alteration extends to the north past the potassic alteration. Early and barren quartz veining is only significant in the northwest third of the deposit. Chancho The Chancho prospect consists of three narrow zones along strike of each other over a distance of 6 km (Fig 3). The northern two of these zones, Chancho and Chancho Norte,  were drilled by Corriente in 2000, with 20 holes totaling 2,006 m. In both zones the mineralization is narrow and structurally  complex, forming a series of small lenses of mineralization  with grades similar to Mirador, and no formal resources have been calculated.

23

24

DROBE ET AL.

155.8 ± 0.5 Ma

163.8 ± 1.9 Ma

156.5 ± 0.5 Ma

156.2 ± 1.0 Ma

 A. Intrusion of early feldspar-hornblende porphyry into  Zamora granodiorite along structural zone at ~158 Ma.

B. QZ- stockwork following east-west fractures, followed closely by start of potassic alteration with dissemina ted CP+PY+MO+Au+AN at ~156Ma.

C. Phrea tic brecciation, with vapour inflating and brecciating mineralized rock, then collapsing back leaving vugs between fragments and more fractures.

D. Finish of main mineralization with coarse CP+PY+AN in breccia interstices, changing laterally to disseminated and fracture controlled CP outside of the breccia.

 GP dissolves  AN -> GP

153.1 ± 1.3 Ma latest Au-rich CP+PY GL±SL veins

E. Intrusion of late porphyry dacite, breccia dikes and diatremes. Late CP+PY±SL±GL+Au veinlets may be significantly younger than porphyry mineraliza tion.

F. Last event is meteoric waters entering surface fractures, converting AN to GP, causing fracture front to propagate down. GP is dissolved leaving open fractures.

Late breccias

potassic alteration with disseminated CP

Late porphyry

blebby CP

Early porphyry

fracture/vein CP

Zamora granite

fracture/vein MO+QZ

CP = chalcopyrite MO = molybdenite PY = pyrite GP = gypsum SL = sphalerite  AN = anhydrite GL = galena dates from this study:

0361-0128/98/000/000-00 $6.00

24

U-Pb Re-Os

FIG. 8. Mirador paragenesis in a series of schematic cross sections.

25

MIRADOR Cu-Au PORPHYRY DISTRICT, SE ECUADOR

The southern zone, Chancho Sur, is well exposed over several hundred meters in road cuts along the El Condor military  access road and in several small drainages that cross this route. The host rock is mostly Zamora granite, though the alteration extends into a large pendant of Santiago Formation calc-silicate altered shale. Potassic alteration in the granite is only weakly developed as patches within an intense, pyritic, quartz-sericite alteration envelope. Weak disseminated chalcopyrite is dominated by abundant disseminated pyrite in outcrop in the creeks cutting the zone. Primary mineralization at Chancho crops out in a 150-m wide exposure in the Rio Tundayme canyon, as well as small tributaries to the south. It comprises disseminated chalcopyrite and pyrite in sheared, brecciated, and potassic-altered Zamora granite and the “Chancho porphyry,” which is an early hornblende-feldspar subvolcanic dike similar to units at Mirador. Local argillic fault zones appear to postdate the mineralization and indicate that the structure was reactivated in part. The mineralization on both sides of the structure grades sharply over a few meters into an envelope of intense pyritic quartz-sericite alteration and then weak propylitic alteration in pink Zamora granite. Although surface channel sampling returned 145 m of 0.92% Cu, the best hole (CH01), drilled directly under the surface sampling, returned only 51 m of  0.96% Cu, which thinned to 1.05% Cu over 34 m in another hole drilled 100 m to the south under the same zone; both intercepts had <100 ppb Au. The soil geochemical anomaly of the Chancho zone was traced northward along its N-S strike for 2.5 km into Chancho Norte, where the mineralized structure crops out in small drainages. Here it is even narrower, with grades above 1% Cu intercepted over only 18 m in drill hole CHN01. The chalcopyrite mineralization occurs as narrow lenses within sheared, potassic-altered Zamora granite and is cut s harply in places by late, northeast-trending, quartz-rich, hornblendefeldspar dikes, similar to dikes at Mirador. The weakly propylitic Zamora granite from drill hole CHN01 was chosen for UPb dating for this study.

Uranium-Lead (U-Pb) Geochronology  Sample selection Zircons from intrusive units that bracket the mineralization in the Mirador and Mirador Norte deposits were dated by UPb, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) at the University of British Columbia Pacific Center for Isotopic and Geochemical Research laboratory. The units chosen are the main host for mineralization in the Zamora granodiorite (unit “Jzgd”), mineralized hornblende-feldspar porphyry (unit “Jefp”), and postmineralization, hornblende-feldspar-quartz porphyry (unit “Jhbp”). The latter is associated with coeval to slightly younger, polymictic, phreatic breccias. Samples details are shown in Table 2. Methodology LA-ICP-MS dating of zircons is a routine procedure at Pacific Center for Isotopic and Geochemical Research laboratory. Zircons are separated from their host rocks using con ventional mineral separation methods. For igneous rocks, approximately 25 of the coarsest, clearest, most inclusion-free 0361-0128/98/000/000-00 $6.00

TABLE 2. Sample Descriptions for U-Pb Dating Sample no.

Location

Description

Unit

CHN01

Chancho Norte drill hole CHN01 at 77-m depth Mirador drill hole M16 at 110-m depth

Zamora granodiorite; pink, weak propylitic alteration Early hornblendefeldspar porphyry; strong Cu mineralization and potassic alteration Postmineral, mauve quartz-feldspar-hornblende porphyry; youngest of the postmineralization dikes at Mirador

Jzgd

M16

M108

Mirador drill hole M108 at 50-m depth

Jefp

Jhbp

crystals are selected for analysis. The selected zircons are mounted in an epoxy puck along with several crystals of internationally accepted standard zircon (Plesovice, PL) and Pacific Center for Isotopic and Geochemical Research laboratory internal standard (KL), and brought to a very high polish. High-quality portions of each crystal are selected for each analysis. The surface of the mount is washed for 10 min with dilute nitric acid and rinsed in ultraclean water. Cathodoluminescent imaging was not available; however a visual inspection under microscope allowed recognition of inclusions, fluid inclusions, and cracks. Analyses are performed with a New Wave 213-nm Nd-YAG laser coupled to a Thermo Finnigan Elements2 high-resolution ICP-MS. Ablation takes place within a New Wave “Supercell” ablation chamber which is designed to achieve very  high efficiency entrainment of aerosols into the helium carrier gas. Typically a 30- µm spot is used with 35% laser power, and line scans rather than spot analyses are run to avoid  within-run elemental fractionation. Each analysis consists of a 10-s background measurement (laser off) followed by 35 s of  data acquisition. Analyses of the standard zircons are interspersed between the samples throughout the run sequence. For igneous rocks lines are run on 16 to 20 of the crystals. Data are reduced using the GLITTER software marketed by the GEMOC group at Macquarie University (Van Achterbergh et al., 2001). The software automatically subtracts background measurements, propagates all errors, and calculates isotopic ratios and ages. Close scrutiny of the plots of the analyses in GLITTER pointed to cores in some of the crystals and possible areas of lead loss in others. The analyses of PL  with a conservative assigned error of 1% are used to calculate the in-run drift and fractionation correction that is applied to the samples. KL is used as an independent monitor. Reported ages are based on the weighted mean of the calculated 206Pb/ 238U ages for relatively young zircons (Phanerozoic). Errors on the ages are reported at 95% confidence level. ISOPLOT software written by K.R. Ludwig at Berkeley Geochronology Center is used for plotting and final interpretation of the analytical results. Results The reported ages were derived from means calculated using ISOPLOT of the 206Pb/ 238U ages for the 16 to 20 zircon analyses for each sample, using averages weighted by analytical errors. Results are presented in Table 3; analyses in italics

25

26

DROBE ET AL. TABLE 3. U/Pb Analytical Results

Analysis no.1 CHNO1L1 CHNO1L2 CHNO1L3 CHNO1L4 CHNO1L9 CHNO1L10 CHNO1L11 CHNO1L12 CHNO1L13 CHNO1L14 CHNO1L15 CHNO1L16 CHNO1L17 CHNO1L18 CHNO1L19 CHNO1L20 M16L1 M16L2 M16L3 M16L4 M16L5 M16L6 M16L7 8M16L8 M16L9 M16L10 M16L11 M16L12 M16L13 M16L14 M16L15 M16L16 M16L17 M16L18 M16L19 M16L20 M108L1 M108L2 M108L3 M108L4 M108L5 M108L6 M108L7 M108L8 M108L9 M108L10 M108L11 M108L12 M108L13 M108L14 M108L15 M108L16 M108L17 M108L18 M108L19 M108L20

207Pb/ 235U

(Ma) 166.5 168.9 163.8 169.8 153.9 172.3 164.8 150.2 162.7 177 155.9 163.7 162.9 159.4 166.2 155.8 156.6 165 171.3 155.5 155.4 150.8 158.1 155.1 155.5 145.8 137.1 150.8 157.5 154.3 162.8 156.8 168.3 160.3 167.6 157.2 160.8 172.9 158.6 163.2 158.8 154.3 155.1 156.6 159.4 159.7 154.3 155.3 166.6 159.9 150.4 147.8 154.3 149.7 155 159.4

207Pb/ 235U (±1 σ  error)

3.69 4.16 9.06 4.3 19.99 9.2 10.63 7.04 7.27 6.47 7.57 5.46 7.74 10.87 24.23 12.41 4.51 4.89 5.28 6.63 3.4 3.38 5.74 13.28 4.55 3.34 7.6 4.36 3.68 5.03 5.03 4.58 11.45 7.9 7.96 9.85 5 8.1 4.3 8.71 4.33 3.23 3.41 3.8 11.37 5.39 3.72 5.1 8.98 7.61 6.1 5.54 3.79 5.34 5.52 4.77

206Pb/ 238U

206Pb/ 238U (±1 σ  error)

207Pb/ 206Pb

(Ma)

(Ma)

207Pb/ 206Pb (±1 σ  error)

Preferred age (Ma)

(±1 σ  error)

161.8 158.7 165.8 164.8 168.3 173.7 169.9 163.9 156.6 167.6 160.3 167.3 162.8 166.1 165.5 166.1 156.1 155.9 156.9 157.8 156.3 153.8 157.4 156.9 156.6 155.3 158.4 160 156.1 155.5 156.1 153.2 154.8 158.5 154.8 155.7 148.8 156.8 149.5 154.3 152.7 149.3 154.2 150.1 153 151.7 155.3 156.1 156.5 155.8 156.7 155.7 154.5 153.6 155 151.7

1.54 1.59 2.78 1.72 5.42 2.78 2.81 2.3 2.34 2.28 2.62 1.99 2.46 3.03 6.74 4.12 2.12 2.15 2.25 2.46 1.92 1.88 2.36 4.82 2.22 1.91 2.71 2.09 1.94 2.21 2.16 2.07 3.35 3.11 2.61 3.32 2.07 3.03 1.99 2.7 2.03 1.79 1.87 1.93 3.93 2.38 1.94 2.36 3.38 2.58 2.5 2.29 1.94 2.35 2.28 2.1

225.1 262.8 219 204.6 19.1 169.2 150.2 0.1 309.5 358 171.3 92.6 146.7 25.9 24.1 11.9 113.9 291.1 369.2 123 136.8 67.8 186 108.4 145.6 38 0.1 39.7 141.8 183.5 265.3 196.9 348.4 185.5 318.3 203.6 279.7 327.1 265 199.7 219.8 235.1 147.4 257.3 247.6 231.7 124.3 147.5 314.6 179.7 60.7 22.5 103.3 86.2 154 271.6

53.99 59.68 133 62.04 305.34 128.51 155.05 111.84 105.33 85.18 116.36 82.03 114.82 166.59 338.27 192.85 70.88 70.9 72.89 104.42 53.28 55.79 88.25 204.02 71.48 55.76 36.12 71.99 57.08 79.15 74.07 70.74 158.26 118.77 112.37 149.29 73.54 109.99 64.1 127.99 65 48.98 52.49 57.07 167.42 80.6 58.08 79.46 126.19 114.68 100.19 92.75 59.68 88.24 86.26 71.11

161.8 158.7 165.8 164.8 168.3 173.7 169.9 163.9 156.6 167.6 160.3 167.3 162.8 166.1 165.5 166.1 156.1 155.9 156.9 157.8 156.3 153.8 157.4 156.9 156.6 155.3 158.4 160.0 156.1 155.5 156.1 153.2 154.8 158.5 154.8 155.7 148.8 156.8 149.5 154.3 152.7 149.3 154.2 150.1 153.0 151.7 155.3 156.1 156.5 155.8 156.7 155.7 154.5 153.6 155.0 151.7

1.5 1.6 2.8 1.7 5.4 2.8 2.8 2.3 2.3 2.3 2.6 2.0 2.5 3.0 6.7 4.1 2.1 2.2 2.3 2.5 1.9 1.9 2.4 4.8 2.2 1.9 2.7 2.1 1.9 2.2 2.2 2.1 3.4 3.1 2.6 3.3 2.1 3.0 2.0 2.7 2.0 1.8 1.9 1.9 3.9 2.4 1.9 2.4 3.4 2.6 2.5 2.3 1.9 2.4 2.3 2.1

1 Samples

were analyzed by laser ablation and ICP-MS (Thermo-Finnigan ELEMENT) at the University of British Columbia; italicized analyses were excluded from the age calculations

 were rejected as outliers as described below. Concordia diagrams and weighted mean plots are provided in Figure 9. The errors in both diagrams are plotted as 2 σ . Preferred ages chosen from each of the three samples are in Table 4. CHN01 (Zamora granodiorite): Zircons in this rock are extremely small, between 100 and 200  µm in length. They are 0361-0128/98/000/000-00 $6.00

clear and colorless, but some contain tiny fluid inclusions. The range of ages obtained from the 16 reported analyses is 156.6 ± 2.3 to 173.7 ± 2.8 Ma; however two outlying analyses can be excluded from the calculations. Analysis L10 is on a zircon that has a dark zone that is visible under the center portion of the laser track. The isotopic ratios and count rates 26

27

MIRADOR Cu-Au PORPHYRY DISTRICT, SE ECUADOR a a-poin error ellipses are

166

M108 Jhbp

0.026

-

162

160

      U 

158

0.025

  a    M   e 154   g    A

        8          3          2

      /        b        P

0.024

        6          0          2

150

150

0.023 146

error ellipses 142

0.022 0.13

0.15

0.17

0.19

Mean = 153.1 ± 1.3 [0.82%] 95%conf. Wtd by data-pterrs only, 0 of 20 rej. MSWD = 1.4, probability = 0.10 (erro r bars are 2 )

0.21

207Pb/ 235U data-pointerror ellipses are 2

0.027

data-pointerror symbolsare 2

170

M16 Jefp

168

0.026

164

      U 

        8          3          2

      /        b        P

        6          0          2

160

160

  a    M   e 156   g    A

0.025

0.024

150

152

0.023 148

error ellipses

Mean = 156.2 ± 1.0 [0.66%] 95%conf. Wtd by data-pterrs only, 0 of 19 rej. MSWD = 0.51, probability = 0.95 (error bars are 2 )

144

0.022 0.12

0.14

0.16

0.18

0.20

0.22

207Pb/ 235U data-pointerror symbolsare 2

data-pointerror ellipses are 2

0.030

CHN01 Jzgd

180

176

180

      U          8 

0.028 172

        3          2

      /        b        P

  a 168    M   e 164   g    A

0.026

        6          0          2

160

160

0.024

156

152

error ellipses

Mean = 163.8 ± 1.9 [1.4%] 95%conf. Wtd by data-pterrs only, 2 of 16 rej. MSWD = 2.1, probability = 0.012 (error bars are 2 )

148

0.022 0.08

0.12

0.16

0.20

0.24

207Pb/ 235U FIG. 9. Concordia diagrams of zircon analyses from Zamora granodiorite (Jzgd), early porphyry dike (Jefp), and postmineralization porphyry dike (Jhbp), with mean weighted 206Pb/ 238U ages and 2 σ  errors. The latter are sorted by youngest to oldest before plotting. 0361-0128/98/000/000-00 $6.00

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DROBE ET AL.

granodiorite with silicification and ill-defined stockwork quartz-chalcopyrite-pyrite veining. Re-Os is now a well-established and reliable method for dating molybdenite (Stein et al., 1997, 2001). The method has significant application to ore geology, as milligram quantities permit the direct dating of ore deposits, provided the occurrence of molybdenite is paragenetically constrained. The technique has been previously applied to other deposits in the Zamora Cu-Au belt, including the Panantza, San Carlos, Sutzu, and Nambija districts (Coder, 2001; Chiaradia et al., 2009; Vallance et al., 2009). The analytical work for the named deposits (published in Chiaradia et al., 2009), for the Mirador and Mirador Norte (this work), and for Fruta del Norte (see “Discussion”) was carried out under the AIRIE program at Colorado State University.

TABLE 4. Summary of U-Pb Geochronology at Mirador Sample No.

Unit

CHN01 M16 M108

Zamora batholith (Jzgd) Mineralized porphyry (Jefp) Postmineral porphyry (Jhbp)

Preferred age (Ma) 163.8 ± 1.9 156.2 ± 1.0 153.1 ± 1.3

change over this zone, and the 206Pb/ 238U age integrated over the entire analysis is older than the other analyses, with  younger (~160 Ma) zones at either end. The second outlier, L13, is from an extremely small crystal that appears to have a damaged zone or core. The 206Pb/ 238U age of 156.6 ± 2.3 Ma is younger than the mean age; however, the 207Pb/ 206Pb age suggests older zircon within the crystal. Excluding these two analyses does not change the calculated weighted mean age and error of 163.8 ± 1.9 Ma, but does change the MSWD from 3.5 to 2.1. The MSWD value allows statistical evaluation of the broad range of dates obtained in LA-ICP-MS zircon dating. Although the scatter in the results for this sample is almost outside the limit of reliability, we consider the reported age to be the best estimate of the age of the intrusion. M16 (early dacite): Zircons in this rock are between 150 and 300  µm in length. They are colorless to pale pink, mostly  clear, but some contain tiny fluid inclusions, and more than half are not complete crystals or not euhedral. The range of  ages obtained from the 20 analyses is 153.2 ± 2.1 to 160.0 ± 2.1 Ma. Despite the 7-m.y. spread of ages, the calculated  weighted mean of the 206Pb/ 238U ages is 156.2 ± 1.0 Ma (MSWD = 0.52), which is consistent with the Re-Os ages for the mineralization. Several of the analyses could have been excluded based on visual inspection of the zircons, but there  was no analytical justification to do so. M108 (late dacite): Zircons in this rock are between 150 and 500  µm in length. They are light brown to pink, lighter crystals are clear, darker crystals are not transparent. Fluid inclusions are visible, and most are not euhedral nor complete crystals. The range of ages obtained from the 20 analyses is 148.8 ± 2.1 to 156.8 ± 3 Ma. The calculated weighted mean of the 206Pb/ 238U ages is 153.1 ± 1.3 Ma (MSWD = 1.4). Several of the analyses could have been excluded based on visual inspection of the zircons, but there was no analytical justification to do so. The results are consistent with observations of crosscutting relationships of the three units, with the Zamora granodiorite  yielding an age of 163.8 ± 1.9 Ma (MSWD = 2.1), the early  porphyry dacite 156.2 ± 1.0 Ma (MSWD = 0.52), and the late porphyry dacite 153.1 ± 1.3 Ma (MSWD = 1.4).

Methodology The AIRIE program provides geochronology using an occurrence-driven methodology (Stein et al., 2003; Stein, 2006). This means paragenetically constrained occurrences of molybdenite are targeted for mineral separation. Details of methodology are similar to those reported in Zimmerman et al. (2008). Briefly, Re-Os data for Mirador and Mirador Norte were acquired by isotope dilution. Molybdenite separates were made using a small hand-held drill. Powdered molybdenite was  weighed and transferred to a Carius tube for dissolution and simultaneous sample-spike equilibration in aqua regia. A mixed Re-double Os spike is applied to correct for common Os (almost always negligible in molybdenite) and to correct for mass fractionation (Markey et al., 2003). Re and Os are chemically  isolated and Os is purified through a series of distillations, using HBr and Re purified using anion-exchange column chemistry. Re data were acquired using the total evaporation method. Isotopic ratios were measured on a Triton instrument at AIRIE using negative thermal ion mass spectrometry (NTIMS).

Results The Re-Os data for three molybdenite samples from Mirador and Mirador Norte are presented in Table 5. Sample size was 2 to 9 mg with excellent agreement among the three Re-Os ages. Although some labs advocate threshold values for sample size (e.g., Selby and Creaser, 2004), the Re-Os data in this study are clear proof that sample size is not relevant to obtaining robust Re-Os ages; it is the occurrence that matters (Stein, 2006). Reported Re concentrations are minimum values as the fine-grained, molybdenite-rich powders drilled from the core samples were diluted up to 90% by silicate. Therefore, actual Re concentrations in these molybdenites are likely in the 1,000 ppm range and the quantity of molybdenite for the Re-Os analyses was at the 1-mg level. Rhenium-Osmium (Re-Os) Geochronology  Reported Re-Os data are fractionation and blankcorrected. Blanks at the time of these analyses were Re = 2.55 ± 0.04, Os Sample selection = 0.443 ± 0.005, and 187Os/ 188Os = 0.931 ± 0.016 pg. Blank Two molybdenite samples from Mirador drill cores and an- corrections are insignificant to the calculated age for these other from Mirador Norte drill core, all representative of  high Re molybdenites. The measured common Os in these main stage Cu-Au-Mo mineralization, were selected for Re- molybdenites ranges from 0.3 to 1.2 ppb. The reported radiOs dating. The Mirador samples are from argillic, mineral- ogenic 187Os is corrected for common Os with 187Os/ 188Os = ized, early breccia (sample M105) and granodiorite (sample 0.2. The common Os in these samples is insignificant relative M131) with vuggy quartz-molybdenite-pyrite-chalcopyrite to radiogenic Os, and thus, the correction is extremely minor  veining. The Mirador Norte sample is from strongly argillic and insignificant to the age calculation. 0361-0128/98/000/000-00 $6.00

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MIRADOR Cu-Au PORPHYRY DISTRICT, SE ECUADOR TABLE 5. Re-Os Data for Molybdenite from Cu-Au Deposits, Mirador and Mirador Norte Sample no.

Drill hole, depth (m)

M105 M131 MN65

M105, 121.16 M131, 140.1 MN65, 105.2

Deposit

AIRIE Run no.

Re (ppm)

187Os,

Mirador Mirador Mirador Norte

MDT-953 MDT-962 MDT-954

165.2 (1) 144.43 (9) 414.8 (2)

(ppb)

Age (Ma)

269.75 (9) 236.94 (8) 677.3 (2)

155.8 ± 0.5 156.5 ± 0.5 155.7 ± 0.5

Notes: Samples analyzed by Carius tube dissolution using double Os spike and ID-NTIMS (Triton); all uncertainties reported at 2  σ  and absolute for last decimal place indicated in table; Re blank = 2.55 ± 0.04 pg, Os blank = 0.443 ± 0.005 pg, and 187Os/ 188Os blank composition = 0.931 ± 0.01; numbers in parentheses are the ± errors for the last digit of the element concentrations

Discussion U-Pb zircon dating A summary of isotope dates from the Zamora Cu-Au belt porphyry Cu deposits in the following discussion are presented in Figure 10. Equigranular granodiorite from the Chancho Norte prospect, representative of typical equigranular plutonic rock of the Zamora batholith, gives a U-Pb zircon age of 163.8 ± 1.9 Ma. This is the first reported U-Pb age for plutonic rock of the Zamora batholiths, and statistically  coeval with the 160.6 ± 1.6 Ma 40Ar/ 39Ar hornblende age obtained by Coder (2001) from equigranular granodiorite, and

published by Chiaradia et al. (2009) on Zamora granodiorite from San Carlos. It is in closer agreement with the unpublished SHRIMP U-Pb date of 164.7 ± 2.2 Ma from coarse Zamora granodiorite on the Peruvian side of the Chinapintza  vein district, 55 km south of Mirador (McClelland, 2010). As discussed by Chiaradia et al. (2009), the San Carlos hornblende showed some Ar loss in the low-temperature steps,  which was attributed to degassing of minor chlorite. The slightly lower age relative to the U-Pb dates at Mirador and Chinapintza suggests that the hornblende at San Carlos was slightly thermally reset during intrusion by the later porphyritic dikes. Note that Zamora granodiorite K-Ar ages from

ZCGB Porphyry Geochron Northern Deposits   s   u    k   u    t   u    K

  s   o    l   r   a    C   n   a    S

  s   u    k   u    t   u    K

  s   o    l   r   a    C   n   a    S

  s   o    l   r   a    C   n   a    S

146

   )   a 156    M    ( 158   e   g    A160

162

  s   o    l   r   a    C   n   a    S

  a   z    t   n   a   n   a    P

  u   z    t   u    S

  r   o    d   a   r    i    M

  r   o    d   a   r    i    M

  r   o    d   a   r    i    M

  r   o    d   a   r    i    M

  e    t   r   o    N   r   o    d   a   r    i    M

151.9 152.0

152 154

  s   o    l   r   a    C   n   a    S

Mirador District

Qtz-Ser Alt (muscovite) San Carlos

148 150

  s   o    l   r   a    C   n   a    S

  e    t   r   o    N   o    h   c   n   a    h    C

late dike (hbl)

early dike (whole rock)

154.0

Zamora

153.5

late dike Panantza

153.1

153.3

155.7

155.8

154.9

Sutzu

156.2 157.0

157.0

San Carlos

early dike

156.5

157.8 160.6 Zamora Zamora (hbl)

164

163.8 Zamora granodiorite

166 168 170

K-Ar: Gendall et al. (2000 )

 Ar-Ar: Coder (2001)

Re-Os Mo: Coder (2001)

U-Pb Zircon this study 

Re-Os Molybdenite this study 

Fig 10. Age relationships for porphyry Cu deposits in the Zamora Cu-Au belt. Re-Os ages of mineralization from all the deposits falls between 153 and 158 Ma, with Mirador overlapping that of San Carlos and Sutzu. Porphyry dikes are coeval  with mineralization, with early dikes i ntruded at the onset of mineralization. The 160.6 Ma 40Ar-39Ar hornblende date and two younger K-Ar dates from Zamora granodiorite at San Carlos and nearby Kutucus skarn are likely disturbed by late magmatism. Evidence for this can be seen in the relatively young K-Ar age for an early dike at San Carlos. 0361-0128/98/000/000-00 $6.00

29

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DROBE ET AL.

the Kutucus skarn prospect, just east of San Carlos, yield even  younger ages, between 152 to 157 Ma (Gendall, 2000), and likely also reflect disturbed Ar systematics. Early, mineralized hornblende porphyry at Mirador yielded an age of 156.2 ± 1.0 Ma, in temporal and geologic agreement  with the 156 Ma Re-Os ages for Mirador and Mirador Norte mineralization presented in this study (Fig. 10; Table 5). That is, veins cutting an intrusion must be younger than the intrusion. This date is contemporaneous with a 157.7 ± 1.4 Ma SHRIMP U-Pb age determination of similar mineralized dacite porphyry from the Peruvian side of the Chinapintza Au-Ag vein district (McClelland, 2010). The early dikes at Mirador are younger than early dacitic intrusions dated at 160.1 ± 0.2 Ma at the Fruta del Norte epithermal Au deposit, 20 km to the south (U-Pb zircon, Stewart, 2008; Fig. 2). They  are contemporaneous with mineralization at San Carlos, 40 km to the north (Coder, 2001; Chiaradia et al., 2009), The central dacite dike at San Carlos (Fig. 4), which is similar to early porphyry at Mirador in that it is thoroughly mineralized, albeit with reduced copper grades relative to the granodiorite it intrudes, yielded an Ar-Ar hornblende age (153.5 Ma) between the Re-Os age of mineralization (ca. 157 Ma) and quartz-sericite alteration (151.9 Ma). Mineralized, early porphyry dikes at Panantza and Sutzu have not been dated, but Re-Os dating of mineralization hosted by the dikes indicates they must be older than 153.3 ± 0.5 and 154.9 ± 0.5 Ma, respectively (Fig 3). Late, postmineral dacite dikes and related phreatic breccias at Mirador mark the end of volcanic activity there and are dated at 153.1 ± 1.3 Ma. Thus, the ages of the earliest and latest subvolcanic intrusions span 5.4 to 0.8 Ma. More dikes would have to be dated before we can say whether there was continuous or episodic volcanism over this period. The late dikes can be considered coeval with late dacite porphyry at San Carlos (ca. 153.5 Ma), and porphyry Cu-Mo mineralization at Panantza (ca. 153.3 Ma). They are younger than ca. 155.4 Ma andesite overlying mineralization at Fruta del Norte (based on two Ar-Ar hornblende dates; Stewart, 2008). These dates indicate that the equigranular Zamora granite is 4.2 to 11 Ma older than the earliest dacite dikes and porphyry mineralization. This is more than the 0.6 to 5.8 Ma range in Ar-Ar ages between Zamora granodiorite and mineralization at San Carlos but is close to the 3.4 to 10.6 Ma U-Pb age gap for the Chinapintza granodiorite and dacite porphyry. The ca. 8 Ma gap is consistent with the observation that the shallowly emplaced subvolcanic units are superimposed on deeply emplaced plutonic rock, implying significant uplift between igneous events. While the larger, genetic relationship between batholith and younger, shallow intrusive activity is not well understood, we note that younger subvolcanic intrusions exploit long-lived structural zones occupied by older, larger batholiths that have barren margins. Subvolcanic complexes that mark the final magmatic stages of these batholiths are associated with porphyry Cu ± Au ± Mo, skarn Au ± Cu, and epithermal Au-Ag mineralization worldwide (Sillitoe, 1997; Tosdal and Richards, 2001; Richards, 2003). Whether there is a real association of waning igneous activity and mineralization, or just a preservation of the final systems as uplift of the batholith wanes, remains to be proven. 0361-0128/98/000/000-00 $6.00

Re-Os dating Re-Os ages for three molybdenite samples from Mirador and Mirador Norte overlap within their 2 σ  uncertainties (Table 5). The reported errors on the ages include the error in the 187Re decay constant. A weighted average of the three molybdenite ages shows that the mineralization occurred at 156.0 ± 1.0 Ma (MSWD = 2.5). This mineralization age is consistent with, and appropriately bracketed by, the U-Pb ages for the mineralized early porphyry dike (156.2 ± 1.0 Ma), and the postmineralization dikes (153.1 ± 1.3 Ma). The agreement of all three Re-Os ages indicates that both Cu-Au deposits developed contemporaneously, despite Mirador having a more complicated geologic history of postmineral volcanic activity. These new ages agree well with Re-Os molybdenite ages for porphyry-style Cu mineralization at the north end of the Zamora Cu-Au belt, at San Carlos, Panantza, and Sutzu (Fig. 10), as reported by Coder (2001) and Chiaradia et al. (2009). Mineralization at Mirador occurred between the Panantza (153.3 ± 0.5 Ma) and San Carlos (157.8 ± 0.6 Ma) events. The Re-Os ages of mineralization closely bracket the age range of subvolcanic units in these three deposits. Stewart (2008) reported Re-Os data provided by the AIRIE program. Three Late Jurassic Re-Os isochron ages from marcasite from the main zone at Fruta del Norte were obtained: 161 ± 3 Ma with duplicate of 156 ± 4 Ma for marcasite in the conglomerate matrix, and 159 ± 2 Ma for vein marcasite. Vein marcasite has LLHR (low-level Re, highly radiogenic Os) qualities and therefore, the selection of the initial 187Os/ 188Os has little effect on the calculated age (Stein et al., 2000).  While the marcasite ages are less precise, they do suggest that  vein mineralization at Fruta del Norte is coincident with earliest intrusions there, at ca. 160 Ma. Stewart (2008; AIRIE program) also reported a Middle Jurassic age (169 ± 1 Ma) from a “possibly singular occurrence” of molybdenite mineralization of uncertain affinity, located 600 m south of the Fruta del Norte epithermal mineralization, and hosted by “Misahuallí” andesite. The sample is associated with low-grade copper mineralization and propylitic alteration, but the Re content (0.33 ppm) is several orders of  magnitude lower than Re concentrations typically associated  with porphyry Cu mineralization (Stein et al., 2001; Zimmerman et al., 2008). As this age of mineralization predates the ages of the Zamora pluton presented in this study, we consider the Middle Jurassic age to reflect minor mineralization associated with a pendant of probable Piuntza unit volcanic rocks within the pluton beneath Fruta del Norte. Although the age has no association with the main metallogenic event, it does reasonably extend the age of the batholith to 169 Ma. Re-Os ages at Mirador and Mirador Norte indicate they are contemporaneous and not sequential pulses of magmatism,  within the precision of the dating method. We conclude their relationship is primarily structural. This porphyry pair is similar to the deposit pair San Carlos-Panantza to the north in the Zamora Cu-Au belt, where 3- to 4-km separation occurs along a northwest-southeast trend. The San Carlos-Panantza pair, however, has nearly 4 m.y. difference in its mineralization ages.

30

Regional implications The Zamora batholith has been extensively dated by previous workers using mostly K-Ar and Rb-Sr methods, which

MIRADOR Cu-Au PORPHYRY DISTRICT, SE ECUADOR

gave a wide range of ages from 246 to 145 Ma (Hall and Calle, 1982; Litherland et al., 1994) and encompassed the ages reported in this paper. We note that the bulk of the historic KAr and all the Rb-Sr ages reported from the Zamora batholith by Litherland et al. (1994) are older than U-Pb ages of plutonic rock at Mirador and Chinapintza presented in this study, as well as the K-Ar and Ar-Ar ages of plutonic rock at San Carlos, Kutucus, and Nambija (Chiaradia et al., 2009). The maximum Early Triassic age of 246 ± 17 Ma is an Rb-Sr date from a suite of pink K-feldspar hornblende granite and microdiorite from the Rio Pitúca area, at the south end of the Nambija Au skarn (Fig. 2; Litherland et al., 1994). Other RbSr dates from the area include 187 ± 2 Ma from hornblendebiotite granodiorite near La Paz (15 km NW of Yantzatza, Fig. 2), and 198 ± 34 Ma from hornblende-biotite granodiorite near Paquisha, east of Nambija. Litherland et al. (1994) pro vided a histogram plot of 29 K-Ar dates from the batholith showing a bimodal distribution of ages, with peaks at 150 to 160 and 170 to 180 Ma; they concluded “the bulk” of the batholith was intruded between 190 to 170 Ma. All rocks described as “porphyritic” fall under the 150 to 160 Ma peak and suggest the bimodal distribution of ages supports the di vision of the batholith into Middle Jurassic plutonic and Late Jurassic, late-stage subvolcanic intrusions. The Late Triassic K-Ar hornblende date of 230 ± 14 Ma reported by Litherland et al. (1994) from andesite south of La Paz, near Yantzatza, and the K-feldspar porphyry hornblendebiotite granite at Rio Pitúca, with an Rb-Sr date of 246 ± 17 Ma date, are likely remnants of poorly preserved, mid-Triassic volcanism unrelated to, and greatly predating, the Jurassic  volcanic arc which generated the bulk of the Zamora batholith. These pre-Norian dates correlate with the Hercynian orogeny and these rocks are perhaps better considered as not part of early igneous history of the batholith, but rather as  wall rocks or pendants within the batholith. With this in mind, the older (i.e., >170 Ma) series of K-Ar and Rb-Sr ages most likely reflect some inheritance from these older elements in the batholith. Additional evidence of Triassic material incorporated into the batholith comes from a single SHRIMP-RG microprobe analysis of a core of an oscillatory zoned zircon from mineralized dacite at Chinapintza (only 25 km east of  the Rio Pitúca sample), which yielded a 241.3 ± 3.7 Ma age (McClelland, 2009). More mapping, dating, and geochemical classification of the older batholith components is necessary, therefore, before the batholith can be considered to have initiated in the Early Jurassic. There is more evidence in the sedimentary record that the belt was a marine basin at that time. Plutonic rocks of latest Triassic-Early Jurassic ages are known from the La Bonita batholith in southwestern Colombia, which did host a volcanic arc at the time (Sillitoe et al., 1984). This magmatism appears to have propagated south ward during the Early Jurassic (Jaillard et al., 1990) from central Colombia through Ecuador into northern Peru, and by  the Late Jurassic there are widespread subvolcanic rocks and associated porphyry Cu mineralization. This latter magmatism appears to have been slightly younger in the south but dates show much overlap. Porphyry Cu mineralization at the Dolores and Mocoa deposits in the La Bonita batholith in Colombia is dated at 166 to 172 Ma (sericite and whole-rock 0361-0128/98/000/000-00 $6.00

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K-Ar dates, Sillitoe et al., 1982). To the south, the Abitagua batholith contains no known porphyry mineralization, but Litherland et al. (1994) reported a 162 ± 1 Ma age derived from a 16-point Rb-Sr isochron from three composite samples of hornblende-biotite granodiorite and felsic vein material (mineralization?). Although these samples are not described in detail, they could be at least in part from subvolcanic rocks, as Baldock (1982) described subvolcanic and “altered” volcanic units within the batholith. The 162 Ma of the Abitagua fits between that and the younger Re-Os dates of mineralization at Panantza-San Carlos (ca. 158−153 Ma) and Mirador (ca. 156 Ma, this study). Figure 11 places these new dates within a Mesozoic stratigraphic section for southeast Ecuador. Latest Triassic to Lower Jurassic Santiago Formation volcanic and marine sedimentary rocks are not well preserved in the belt, their upper and lower contacts being destroyed by the Middle Jurassic magmatic arc, but they had reached sufficient thickness to allow for plutonism by ca. 164 Ma. The Zamora Cu-Au belt then went through a cycle of uplift, erosion, and burial between 164 Ma and Late Jurassic magmatism at ca. 156 to 160 Ma, which helps bracket the age of the Chapiza Formation. Thereafter the area underwent another cycle of uplift and erosion, which lasted until about the Aptian, or ca. 125 Ma,  when deposition of the Hollin Formation was initiated. The Cretaceous transgression continued until the Andean orogeny. The greatest limitations in refining the history are the lack of dates and geochemistry from the Triassic-Early  Jurassic sedimentary and volcanic rocks, respectively, and the lack of fossils to date the base of the Hollin Formation. Figure 12 further illustrates four stages of development of  the Zamora Cu-Au belt, beginning with intrusion of the Zamora batholith into Santiago Formation and Piuntza unit marine volcano-sedimentary rocks, possibly along a deepseated high-angle fault (Gendall et al., 2000), ending by about 164 Ma. Thereafter followed ca. 8 m.y. of uplift and erosion, bringing equigranular plutonic rock to the surface, ending in the onlap of shallow Late Jurassic mixed continental-derived arenite and arc-derived arkose and conglomerate of the Chapiza Formation (Sarayaquillo Formation in Peru) that marked the onset of a regional Late Jurassic-Early Cretaceous transgression flooding the entire western South American margin (Jaillard et al., 1990). Then followed a protracted episode of igneous activity from 156 to 153 Ma that resulted in subvolcanic porphyry stocks and dikes, porphyry Cu-AuMo mineralization, epithermal Au deposits, and local Au-Cu skarns where the dikes intruded Santiago Formation wall rocks (as at Kutucus). The dacite dikes appear to record a change in tectonic stress regime, coincident with the regional Cu-Au mineralization event. Mineralized early dacite dikes uniformly have northwest strikes, parallel to the geochemical trends connecting the closely spaced porphyry Cu deposits (Panantza-San Carlos and Mirador-Mirador Norte). The late or postmineralization dikes uniformly strike northeast. If the NW strike represents dilation on structures related to N-S sinistral stresses along N-S strike-slip faults, the NE strikes could represent a reversal to dextral stress along N-NE-striking faults, consistent with changes in the Late Jurassic (Kimmeridgian-Tithonian) regional tectonics as interpreted by Jaillard et al. (1990).

31

32

DROBE ET AL. 99.6 Ma

Napo Fm.

 Albian

      s       u       o   y    r       e    l    a       c    E       a        t       e       r         C

112

 Aptian 125

Barremian

 .    m     F    n     i     l     l    o     H

fining upward conglomerate, quartzite, shale conglomerate

Hauterivian Valanginian

136 140

Berriasian Tithonian 151 Kimmeridgian 156 Oxfordian 161 Callovian

   z     i    p    a     h     C

      c        i       s       s       a       r       u        J

   y     l    r    a     E

Bathonian

Misahuallí

Toarcian

volcanics

   l    a    t    f    n   l    e   e    n   h    i    t    S    n    o    C

redbeds rest in part on Zamora

168

169 Ma 176 183

Hiatus? unclear contact relationship

~190 Ma earliest emplacement age

190

Sinemurian Rhaetian

      c        i    e       s    t    a       s    L       a        i       r        T

197 201.6 Ma 204

Norian

 .    m     F    o    g    a     i     t    n    a Piuntza     S

marine shales, limestone

marine setting

calc-alkaline (?), intermediate volcanic rocks

unit

228

Carnian

Mo mineralization within batholith, FDN

172

Pliensbachian

Hettangian

erosion

   o youngest age    i    s for Misahuallí    o    r    e Nambija Gold Skarn    d    n 145 Ma    a late dikes    t    f 153 Ma Mirador Porphyry Cu    i    l 156 Ma    p 156 Ma    u early dikes FDN Epithermal Gold 164 Ma    B 160 Ma    G continental setting    C Zamora    Z Batholith

Bajocian  Aalenian

transgression, return to marine setting

132 Ma (K-Ar)    n

   d    e    c   t    a    r  .    A   n    i    m    m    o     F    D    a

165

   e     l     d     d     i     M

unconformity

Hiatus

130

145.5 Ma

   e     t    a     L

limestone

base not defined

235

FIG. 11. Stratigraphic section for Mesozoic rocks of southeast Ecuador, placing the dates presented in this study in context; time scale ages from Walker and Geissman (2009). U-Pb ages of intrusions are orange diamonds; Re-Os ages for mineralization are green (porphyry Cu) and yellow (gold skarn and epithermal) circles. Questionable older Rb-Sr and K-Ar dates for the Zamora batholith give it a wide range of ages, from 246 to 164 Ma. The earliest plausible age is 190 Ma, based on Sinemurian fossil evidence from the Santiago Formation, which the batholith intrudes. The regional U-Pb and Re-Os dates indicate the main plutonic phase was probably between 164 to 169 Ma. The late Early Jurassic to late Middle Jurassic appears to mark a sedimentary hiatus between marine rocks of the Santiago Formationand the mixed continental and arc-derived strata of the Chapiza Formation, but Litherland et al. (1994) interpreted the two successions as conformable, and this relationship needs more study. Volcanism dominated the area until about 132 Ma, when the area was uplifted and eroded prior to the Cretaceous transgression. This start of this event is also poorly constrained in the area.

The porphyry mineralization appears to be related to this re versal of movement; similar timing of porphyry mineralization to stress reversal has been noted elsewhere in the Andes and tied to changes between transpressional and transtensional regimes (Lindsay et al., 1995; Richards et al., 2001). Following the 156 to 153 Ma activity, coeval volcanic rocks and related epithermal gold deposits above the subvolcanic porphyries are eroded away as continued uplift occurs prior to, and possibly as a result of, a late pulse of magmatism at 145 Ma, recorded at Nambija in a parallel structure 20 km  west of the Mirador-Fruta del Norte-San Carlos-Panantza trend. This uplift and erosion helps to explain the lack of  0361-0128/98/000/000-00 $6.00

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epithermal deposits associated with the numerous porphyry  Cu deposits. The only known Late Jurassic epithermal gold deposit that survived the uplift is Fruta del Norte, which was deposited in a low-standing, north-south graben south of  Fruta del Norte (Stewart and Leary, 2007) along with calcalkaline volcanic rocks of the Misahuallí Member of the Chapiza Formation. The Nambija Au skarns formed deeper than epithermal gold deposits and were preserved (Vallance et al., 2009). The porphyry Cu, skarn Au, and Fruta del Norte epithermal Au deposit was buried and preserved by Early Cretaceous back-arc sedimentation during deposition of the Hollin and

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MIRADOR Cu-Au PORPHYRY DISTRICT, SE ECUADOR   Ma ca. 164

West Piuntza

East

~160 Ma FDN epithermal  Au

Zamora Batholith

Piuntza

164 Ma

  Ma ca. 100

West

conformab le

local grabens

Napo Fm. Hollin Fm.

FDN

Zamora Batholith

calc-silicate altered volcanic rocks

ear ly skarns ?

16 9 Ma

 A

C

Chapiza redbeds

calc-silicate altered marine rocks

redbeds

145 Ma

East Mis ahualli Volc anic arc

Santiago Fm.

calc-silicate altered volcanic rocks

Nambija  Au skarn

  Ma ca. 153

West

B

West

East continenta l shelf  distal provenance

porphyry Cu- Au

Nambi ja  Au skar n

Kutucus Cu- Au skarn

regional porphyry Cu- Au-Mo 153-158 Ma subvolcanic dikes & stocks

Zamora Batholith

~155 Ma Kutucus Cu- Au skarn

153 Ma 156 Ma

  F m. Chinapintza N apo Present Hollin Fm. 30 Ma F m.  Mirador  H ol l in

East

155 Ma local grabens

FDN

por phyry Cu- Au

Kutucus Cu- Au skarn

subvolcanic dikes & stocks

subvolcanic dikes & stocks

D

FIG. 12. Schematic historic depiction of events from Early Jurassic to Present; symbols for age dating methods as in Figure 1. A. Intrusion of the Zamora batholith into Santiago Formation marine sedimentary rocks and Piuntza unit volcanic rocks finishes ca. 164 Ma; this initializes isostatic uplift. B. Pulse of continental arc magmatism beginning ca. 156 Ma, emplacing the early subvolcanic intrusions and generating the porphyry copper deposits of the Zamora Cu-Au belt from 153 to 158 Ma, as well as epithermal gold at Fruta del Norte (FDN), and skarn at the margins at Kutucus. Following continued uplift, a last pulse at 145 Ma results in Au skarn at Nambija. C. Continued uplift and erosion until about the Aptian, at which point the region is eroded down, or tectonically subsided, to sea level; associated extension creates local grabens that fill with conglomerate and preserve minor amounts of the Jurassic volcanic pile; the porphyry deposits at their present level of erosion are all exposed at this time and then covered by Early Cretaceous sediments derived from the Guyanan shield to the east. D. Main Andean orogeny uplifts large blocks of the sub-Andean region with little tilting, except within minor subblocks. There is felsic volcanism at 30 Ma al ong structures. The porphyry deposits are once again exposed to surface, though the Fruta del Norte gold deposit remains mostly buried beneath basal Hollin units.

then Napo Formations during the continuing transgression affecting the western South American margin (Jaillard et al., 1990). The exact timing of the onset of flooding of the Zamora Cu-Au belt by continental sedimentation of the Hollin Formation is unclear due to a local lack of fossils and the fact that the onset of Early Cretaceous sedimentation is highly diachronous across the region covering eastern Ecuador (Villagomez et al., 1996), and south into Peru. North of the belt, volcanic rocks of  the Misahuallí Member of the Chapiza Formation have a KAr age as young as 132 Ma (Hall and Calle, 1982; Litherland et al., 1994) and this may be the last magmatism before transgression there. In western Peru, transgressive sandstones are as old as Early Valanginian (137 Ma; Villagomez et al., 1996). In southeastern Ecuador, closest to the Zamora Cu-Au belt, fossils indicate that the base of the Hollin is Late Aptian age (112 Ma; Villagomez et al., 1996). The flat st rata of the Hollin Formation that overlie the porphyry deposits from Panantza to Mirador indicate that the level of Early Cretaceous erosion  was relatively consistent across at least 60 km of the batholith, and there has been only very slight (<5°) tilting of the systems. Hollin basal conglomerate, with volcanic and Zamora granite 0361-0128/98/000/000-00 $6.00

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clasts, is preserved only locally, possibly within fault valleys coincident with pre-Cretaceous grabens, as at Fruta del Norte. Late Cretaceous to Tertiary (Andean orogeny) magmatism occurred mostly along the buried western margin of the Zamora batholith, but also as small bodies within the batholith. Both mafic diorite plugs and felsic sills and dikes intruded the Hollin and overlying Napo Formations. At Chinapintza, the rhyodacite dikes and plugs are U-Pb dated at ca. 30 Ma (Gaschnig, 2009); at Fruta del Norte a mafic dike was Ar-Ar dated at ca. 63 to 71 Ma (Stewart, 2008). Recent uplift and sub-Andean block faulting, with only very local tilting (but up to near-vertical rotation of beds), inverted the topography so that the Cretaceous basins are now plateaus, and the deposits are exposed on valley sides below the capping quartzite formations (Fig. 12d).

Conclusions Re-Os dating of porphyry Cu-Au mineralization and U-Pb dating of calc-alkaline, subvolcanic porphyry units at Mirador and Mirador Norte confirms that these deposits are contemporaneous with similar Late Jurassic porphyry Cu-Mo

34

DROBE ET AL.

deposits and epithermal Au deposits in the Zamora Cu-Au belt, as well as other porphyry Cu districts extending the length of the Northern Andes from southern Ecuador into central Colombia. Intrusion of the earliest (mineralized) porphyries at ca. 156 Ma into Zamora granodiorite host rock of  ca. 164 Ma age indicates a ca. 8 m.y. period of uplift and erosion prior to the shallow magmatism. Sillitoe et al. (1982) reported a similar age gap between the Late Jurassic-Early Cretaceous porphyry Cu deposits and hosting plutonic rocks in Colombia. This activity was contemporaneous with onset of  porphyry copper mineralization within the resolution of the dating methods and was focused along NW-trending structures. Igneous activity continued for another ca. 4 m.y. postmineralization along dominantly northeast structures. Re-Os ages at Mirador and Mirador Norte indicate contemporaneous mineralization, within the precision of the dating method, at ca. 156 Ma. At San Carlos-Panantza, located 40 km to the north, the mineralization was in sequential pulses ca. 4 m.y. apart between 157 and 153 Ma, respectively. On a regional scale, over much of the length of the Zamora batholith, overlapping ages for various subvolcanic intrusions from Panantza southward to Chinapintza indicate a period of probable protracted shallow magmatism from 156 to 153 Ma. After a brief hiatus, a last localized magmaticmineralization event took place at Nambija at ca. 145 Ma, prior to uplift, erosion, and burial beneath Early Cretaceous platform sediments. The range of U-Pb ages closely agrees  with the Re-Os ages for mineralization, indicating that hornblende-feldspar subvolcanic dikes are clearly associated with an important regional metalliferous volcanic event that spanned >100 km along the Cordillera del Condor. Magmatic centers are spaced roughly 15 to 20 km apart. Based on evidence from San Carlos, Mirador, and Chinapintza, equigranular plutonic rock of the Zamora batholith is significantly older (ca. 8 m.y.) than porphyritic, subvolcanic rocks associated with copper mineralization at both Mirador and San Carlos. This age gap is consistent with the juxtaposition of equigranular and subvolcanic igneous textures, which indicate that considerable uplift and erosion of the batholith took place prior to the intrusion of the subvolcanic units. Unlike other porphyry Cu belts worldwide, where mineralization appears tied to final magmatic activity ending a longlived, subduction-related igneous complex, in the Zamora Cu-Au belt the peak of metalliferous magmatic activity at ca. 156 Ma occurred some 10 m.y. prior to the final episode of  magmatism at ca. 145 Ma. Some possible causes for the end of subduction and volcanic activity at the close of the Jurassic along this segment of  the Northern Andes include allochthonous terrane accretion (Litherland et al., 1994) and changing subduction configurations resulting from changes in plate motion (Jaillard et al., 1990; Chiaradia et al., 2009). In the Zamora Cu-Au belt, intrusive and metalliferous activity along NW-trending structures was followed by unmineralized intrusions localized along NE-trending structures at ca. 153 Ma. This implies a change in plate motion and within-arc stress fields from sinistral to dextral. Conversely, there is no evidence for Late Jurassic collisional terrane accretion, since the Mesozoic units are mostly unstrained. Pratt et al. (2005) gave additional evidence supporting an autochonous geologic model for the Jurassic 0361-0128/98/000/000-00 $6.00

and/or Cretaceous portion of the Cordillera Real and the subAndean zone that encompasses the Zamora Cu-Au belt. Much work remains to refine the early history of the Zamora batholith by separating out the pre-Jurassic volcanic units from the plutonic rocks. This would help clarify local geotectonic conditions at the Triassic-Jurassic boundary. Through more detailed mapping and U-Pb dating, these units also need to be distinguished from Misahuallí Member volcanic and subvolcanic units, which are the most economically  promising rocks in the belt.

 Acknowledgments  We thank the many Mirador field personnel of EcuaCorriente S.A., especially project geologists Juan Leon, Eduardo  Vaca, and Luis Quevedo. We thank Ken Shannon of Corriente Resources Inc. for approving funding for this study and for many helpful discussions. We also thank Massimo Chiaradia for a thorough review and many helpful comments.

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