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Article Carboniferous Orogenic Gold Deposits at Pataz, Eastern Andean Cordillera, Peru: Geological and Structural Framework, Paragenesis, Alteration, and 40Ar/39Ar Geochronology HAEBERLIN, Yves, et al. Abstract The Pataz province forms the central part of a ≥160-km-long orogenic gold belt extending along the Eastern Andean Cordillera in northern Peru and has produced a total of 6 million ounces (Moz) gold from vein-type deposits during the last 100 yr. The deposits present several recurrent and typical field characteristics, including (1) at a regional scale, location of the mineralization in low-order structures within a 1- to 5-km-wide structural corridor east of a major north-northwest–striking lineament and in spatial association with the northnorthwest– striking margins of the 330 to 327 Ma Pataz batholith; (2) at the mine scale, strong lithological controls of the vein geometries and styles, the lodes occurring as fairly continuous ≤5-km-long quartz veins inside or along the margins of the batholith or as branching and bedding-concordant narrow ore shoots within adjacent folded Ordovician turbidite sequences; (3) consistent orientations of veins, in particular within the batholith, where more than 80 percent of the quartz veins are emplaced in north- to northwest-striking, eastdipping, brittle-ductile deformation [...] Reference HAEBERLIN, Yves, et al. Carboniferous Orogenic Gold Deposits at Pataz, Eastern Andean Cordillera, Peru: Geological and Structural Framework, Paragenesis, Alteration, and 40Ar/39Ar Geochronology. Economic Geology, 2004, vol. 99, no. 1, p. 73-112 DOI : 10.2113/gsecongeo.99.1.73 Available at: http://archive-ouverte.unige.ch/unige:21636 Disclaimer: layout of this document may differ from the published version. 1 / 1 ©2004 by Economic Geology Vol. 99, pp. 73–112 Carboniferous Orogenic Gold Deposits at Pataz, Eastern Andean Cordillera, Peru: Geological and Structural Framework, Paragenesis, Alteration, and 40Ar/39Ar Geochronology YVES HAEBERLIN,† ROBERT MORITZ, LLUÍS FONTBOTÉ, Section des Sciences de la Terre, Université de Genève, Rue des Maraîchers 13, 1205 Genève, Switzerland AND MICHAEL COSCA Institut de Minéralogie et Géochimie, Université de Lausanne, BFSH 2, 1015 Lausanne, Switzerland Abstract The Pataz province forms the central part of a ≥160-km-long orogenic gold belt extending along the Eastern Andean Cordillera in northern Peru and has produced a total of 6 million ounces (Moz) gold from vein-type deposits during the last 100 yr. The deposits present several recurrent and typical field characteristics, including (1) at a regional scale, location of the mineralization in low-order structures within a 1- to 5-km-wide structural corridor east of a major north-northwest–striking lineament and in spatial association with the north- northwest–striking margins of the 330 to 327 Ma Pataz batholith; (2) at the mine scale, strong lithological controls of the vein geometries and styles, the lodes occurring as fairly continuous ≤5-km-long quartz veins inside or along the margins of the batholith or as branching and bedding-concordant narrow ore shoots within adjacent folded Ordovician turbidite sequences; (3) consistent orientations of veins, in particular within the batholith, where more than 80 percent of the quartz veins are emplaced in north- to northwest-striking, eastdipping, brittle-ductile deformation zones; (4) a consistent Au, Ag, As, Fe, Pb, Zn, ±Cu, ±Sb, ±Bi-Te-W metal association and a sulfide-rich paragenetic sequence, with a first stage composed of milky quartz, pyrite, ar- senopyrite, and ankerite and a second stage of blue-gray microgranular quartz, galena, sphalerite, chalcopyrite, Sb sulfosalts, electrum, and native gold, followed by barren calcite-dolomite-quartz veinlets; and (5) hy- drothermal alteration of the vein wall rocks, consisting of pervasive muscovite alteration with minor chlorite, carbonate minerals, and pyrite associated with strong bleaching in plutonic rocks, and of weak muscovite and chlorite alteration in sedimentary rocks. Structural analysis of the deposits outlines four synchronous sets of mineralized fractures in the Pataz district. The predominant north- to northwest-striking, east- to northeast-dipping system, which is generally located in reactivated reverse faults, accounts for more than 80 percent of the gold resource of the district. Three subordinate systems include, in decreasing order of economic importance, (1) east-west–striking flat extensional veins, (2) bedding-concordant veinlets in east-west–striking and north-dipping to north-south–striking and east-dipping limbs of long wavelength folds in Ordovician sedimentary rocks, and (3) weakly mineralized roughly east-west–striking, sinistral vertical faults. The vein orientations of the four structural sets are compat- ible with a triaxial strain model, with the main shortening axis P oriented at 080°/15°, an intermediate axis oriented at 165°/00°, and a subvertical extensional T axis oriented at 255°/80°. Under these conditions, the rich- est ore shoots are preferentially sited in sinistral pull-aparts, which occur at the intersection of either north-south–striking lodes or extensional lodes with roughly east-west–striking vertical faults. 40Ar/39Ar dating of the granodiorite-monzogranite bodies of the Pataz batholith provides good plateau ages at 329.2 ±1.4 and 328.1 ± 1.2 Ma for biotite separates, which are similar to a published 329 Ma U/Pb age for the granodiorite. A muscovite and a biotite sample from an aplite dike yielded plateau ages at 322.1 ± 2.8 and 325.4 ± 1.4 Ma, respectively. Muscovite samples from alteration intimately associated with the gold mineralization yielded three 40Ar/39Ar spectra with low-temperature staircase-shaped patterns followed by plateau seg- ments at 314 to 312 Ma. These ages, analytically indistinguishable at the 2σ level, are considered to be the most probable ages for the mineralization event. Three other plateaulike ages between 305 and 288 Ma have been obtained and are interpreted to reflect partial argon loss during late fluid circulation associated with the intrusion of Late Cretaceous monzonite porphyries. The age determinations are inconsistent with a genetic link between the 314 to 312 Ma Pataz gold deposits and the 330 to 327 Ma calc-alkaline Pataz batholith, or with the 327 to 319 Ma aplite dikes, or the Late Creta- ceous porphyry magmatism. Instead, the overall homogeneity of the structural, mineralogical, and geochemi- cal characteristics of the deposits over the ≥160-km-long mineralized belt and the geotectonic evolution sug- gest that gold mineralization is linked to a large-scale thermal event that occurred in a thickened collisional belt undergoing uplift tectonics. Introduction terranes that were deformed along the circum-Pacific margin OROGENIC gold deposits, also referred to as mesothermal de- (Goldfarb et al., 1998, 2001; Bierlein and Crowe, 2000) but posits (Groves et al., 1998), are widely distributed in accreted remain relatively unrecognized on the western margin of South America. The reevaluation of several historical Peru- † Corresponding author: e-mail, [email protected] vian, Bolivian, and Argentinean gold deposits (Zappettini and 0361-0128/01/3402/73-40 $6.00 73 74 HAEBERLIN ET AL. Segal, 1998; Haeberlin et al., 1999; Skirrow et al., 2000; Hae- (1998) suggested that the gold deposits were coeval with a berlin, 2002) has revealed that this class of deposits is actually late pulse of the composite batholith and tentatively classified widely distributed in the eastern Central Andes and its fore- the Pataz-Parcoy gold deposits as intrusion related. land, along the exposures of early to middle Paleozoic mobile In this paper, we examine the deposits of the Pataz district belts, such as the Eastern Andean Cordillera and the Sierras and the Culebrillas deposit in the southern Parcoy district Pampeanas (Fig. 1; Haeberlin et al., 2003). Only a limited (Fig. 1). We report the structural and geometric framework of number of metallogenic and regional geologic studies exist on the deposits, describe the ore and gangue mineralogy and the these deposits, in part because few among them are presently alteration styles, discuss the regional, lithological, and struc- being mined and because most of them are located in areas tural features that controlled the distribution of gold veining, that are largely difficult to access. An exception is the Pataz and propose reconstructions of the syn- and postmineraliza- orogenic gold province, situated on the eastern side of the tion strain orientations that are responsible for the present- Marañón Valley in the Eastern Cordillera of northern Peru, day deposit geometry. We also present new 40Ar/39Ar dating where three medium-sized mining corporations are active on the plutonic host rocks and the alteration related to the and where ~6 Moz gold have been extracted during the 20th gold lodes that permit a reevaluation of the possible genetic century. This province encompasses, from north to south, the links between the batholith and the gold mineralization and Pataz, Parcoy, and Buldibuyo districts and contains sulfide the presently conflicting orogenic (Bohlke, 1982; Groves et mineral-rich quartz lodes hosted by the giant calc-alkaline al., 1998, and references therein) versus intrusion-related Pataz batholith and, to a lesser extent, its immediate metsed- (Sillitoe, 1991; McCoy et al., 1997; Sillitoe and Thompson, imentary rocks (Fig. 1; Schreiber, 1989; Schreiber et al., 1998; Thompson et al., 1999; Lang et al., 2000; Thompson 1990a, b; Vidal et al., 1995; Macfarlane et al., 1999; Haeber- and Newberry, 2000) models for the deposits. lin, 2002, Haeberlin et al., 2003). The first written record of auriferous veins in the Pataz area Local Geology dates back to the second part of the 18th century, but historical studies quoted in De Lucio (1905) place their discovery Pre-Silurian units during the Inca (15–16th centuries) or even earlier. De Lucio In the Pataz-Parcoy region, the Eastern Cordillera (Fig. 1) (1905), in a descriptive work, first pointed out the economic is characterized by a pre-Silurian metamorphic basement and potential of the region.

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