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Pdf/108/1/11/3467353/11-35.Pdf by Guest on 01 October 2021 12 DROBE ET AL ©2013 Society of Economic Geologists, Inc. Economic Geology, v. 108, pp. 11–35 Geology, Mineralization, and Geochronological Constraints of the 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, British Columbia, Canada V6E 2K3 2 ExplorCobres S.A., Av. República de El Salvador #1082 y NN.UU., Ed. Mansión Blanca, Torre París, Mezanine 3 AIRIE Program, Department of Geosciences, Colorado State University, Fort Collins, Colorado 80523-1482, and Geological Survey of Norway, 7491 Trondheim, Norway 4 Pacific 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 por- phyry Cu-Au ± Ag ± Mo deposits, Mirador and Mirador Norte, and an interconnected series of narrow, min- eralized structures known collectively as Chancho. The principal mineralization in the porphyries is dissemi- nated 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 Mid- dle 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 at 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, northern Andean Cordillera in southeastern Ecuador, 340 km which had been radiometrically dated (Coder, 2001). Prior to south of Ecuador’s capital city of Quito (Fig. 1), in the dating, sedimentary rocks of the Aptian (base at 125 Ma) province of Zamora-Chinchipe. Elevations range from about Hollin Formation (Tschopp, 1953), which unconformably 800 to 1,800 m above sea level. The area has a wet equatorial overlie the south margin of the Mirador deposit, provided a climate with an average rainfall of 2,300 mm/yr. Over the past minimum age constraint for Mirador. This unconformable re- decade, this subandean region has emerged as a significant lationship is also present at Panantza and the Sutzu porphyry metalliferous belt bridging important, world-renowned dis- deposit, located 15 km south of Panantza. Host rocks for tricts in northern Peru and Colombia. The Mirador district these deposits are reported by Chiaradia et al. (2009) as being 40 39 comprises two main porphyry Cu-Au ± Ag ± Mo deposits, between 160 to 153 Ma ( Ar/ Ar method), with mineraliza- 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 subvol- Mirador Norte, using a 0.4% Cu cut-off, were estimated at canic intrusions and the mineralization (156 Ma). Mineraliza- 609 million metric tons (Mt) of 0.58% Cu, containing 7.8 bil- tion is related to and slightly postdates the onset of subvol- lion pounds (Blb) of Cu, 3.2 million ounces (Moz) of Au, and canic igneous activity at Mirador. 22 Moz of Ag. Additional inferred resources, at the same cut- This study presents the first geochronological dates for the off, 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 out- sources 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 taken together contain approximately 25 Blb of Cu. drill core. Despite the heavy jungle cover at surface, drill holes spaced at approximately 75-m centers at Mirador and 100-m centers at Mirador Norte permit a robust interpreta- † Corresponding author: e-mail, [email protected] *Present address: Batero Gold Corp., 3703-1011 Cordova St., Vancouver, tion of lithology, alteration, and mineralization relationships. BC V6C 0B2, Canada. By dating multiple intrusive phases and their related min- Submitted: April 21, 2011 0361-0128/13/4080/11-25 11 Accepted: April 5, 2012 Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/108/1/11/3467353/11-35.pdf by guest on 01 October 2021 12 DROBE ET AL. 78°W N COLOMBIAMocoa Cu-porphyry 0° CUCHILA 0° (LA BONITA) QUITO BATHOLITH 198-210 Ma o ap N Rio Costa ABITAGUA Sierra BATHOLITH 162-173 Ma Guayaquil Oriente Tertiary to Recent sediments Tertiary to Recent Continental volcanics Z mora CUENCA io a Cretaceous to Tertiary I-type intrusions R Pacific Ocean Cretaceous accreted island arc, ocean floor, Machala dillera Real Mirador and marine sedimentary rocks Cor Jurassic I-type intrusions Condor Cu-Au Triassic to Cretaceous sedimentary + del District volcanic rocks 4° Zamora a ZAMORA Paleozoic to Cretaceous metamorphic rocks er BATHOLITH 0 200km PERU 164 - 190(?) Ma ordill C 78°W FIG. 1. Inset shows location of the Mirador Cu-Au district, located in southeast Ecuador toward the northern end of the Zamora batholith, the southernmost of three Jurassic I-type intrusions within the sub-Andean region. Modified from Gen- dall et al. (2000). eralization at Mirador and proving the temporal association to northeast orientation of the batholith is the result of dextral porphyry and epithermal deposits located elsewhere in the slip along northeast, post-Cretaceous, Andean orogeny faults. Cordillera del Condor, we underscore the exploration signifi- It remains unclear how many intrusive and volcanic phases cance of the ca. 156 Ma Late Jurassic, subvolcanic, calc-alka- the Zamora batholith comprises, their age relationships, and line igneous event that affects Middle Jurassic plutonic rocks age range. This is due to several factors, but primarily re- of the extensive Zamora batholith, as well as older volcano- gional-scale mapping of discontinuous, weathered exposures sedimentary pendants within the batholith, and volcano-sedi- in a largely inaccessible area, and the inclusion within the mentary sequences unconformably overlying the batholith. batholith of volcanic-textured rocks ranging from Triassic to The ages also help clarify and constrain Mesozoic tectonos- post-Cretaceous age (Litherland et al., 1994). However, the tratigraphic events in the Northern Andes. batholith can be broadly divided into two intrusive types: (1) equigranular plutonic rocks of medium-grained granodiorite, Regional Geology diorite, and tonalite, and very coarse, K-feldspar megacrystic, Mirador and the other documented Late Jurassic porphyry monzogranite (collectively referred to as “granodiorite” in copper deposits within 40 km are associated with subvolcanic this paper), locally with aplite and leucogranite predominat- intrusions intruding equigranular plutonic rocks comprising ing (as at Panantza); and (2) younger subvolcanic intrusions the regionally extensive Zamora batholith, a loosely defined comprising feldspar (albite>>coarse microcline)-hornblende Middle to Late Jurassic, calc-alkaline igneous complex that ± quartz porphyry of andesitic to dacitic composition that extends over 200 km along a NNE trend, between latitudes 3° clearly intrude the plutonic rocks. Subvolcanic rocks occur as and 5° S, and at least 100 km wide (Baldock, 1982; Aspden et dikes and stocks (<2-km diam) and give the youngest of the al., 1990; Litherland et al., 1994). The batholith is the domi- reported ages for the batholith. Importantly, they are nant geologic entity in the sub-Andean region of southeast uniquely associated with copper and gold mineralization, Ecuador, especially in the Cordillera del Condor, the moun- whereas plutonic intrusive margins are notably unmineral- tain range that forms the international border in the area, and ized. Textures vary with size of the intrusions. Larger intru- along the Rio Zamora to the west of that range. This sions, comprising plugs or stocks several hundreds of meters batholith, together with the lithologically correlative Abitagua wide, show seriate textures transitional between subvolcanic and Cuchilla batholiths to the north and similar rocks in the and plutonic; in weathered exposures these are easily con- Cordillera Oriental of Colombia (Fig. 1), are interpreted as fused as phases of the Zamora pluton, though the diagnostic remnants of a volcanic arc constructed along an Andean-type euhedral hornblende phenocyrsts aid in distinguishing them. continental margin (Sillitoe, 1988, 1990) that was well-estab- There is some confusion about the plutonic and subvol- lished by the Middle Jurassic.
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