UNIVERSITY OF GENEVA - DEPARTMENT OF MINERALOGY

The Pataz gold province is situated 500 km north of Lima on the Eastern Cordillera of the North Peruvian Andes, and constitutes the northernmost part of a Carboniferous mesothermal Au and Sb belt, which extends along the entire cordillera down to northern Argentina. The Pataz mineralized belt covers an area 160 km long and 1 to 3 km wide, extending first along the eastern side of the Marañón Valley from Bolívar to Pataz (Schreiber et al., 1990; Haeberlin et al., 1999; Haeberlin, 2000), then striking southeastwards to the Parcoy (Vidal et al., 1995; Macfarlane et al., 1999) and Buldibuyo districts. It includes numerous quartz-sulfide veins, located within the external part of the granodioritic Pataz Batholith close to the contact with a Pre-Carboniferous basement. The grades in the oreshoots vary between 7 to 15 Au g/t, exceptionally up to 120 Au g/t. The annual production of the province, including the operating mines in the Parcoy district, amounted in 1999 to 360'000 ounces, which represents one tenth of the gold supply of Peru.

The present contribution based on our ongoing investigation (Haeberlin et al., 1999; Haeberlin, 2000) redefines with the help of ⁴⁰Ar/³⁹Ar dating the geological and structural frame of the Pataz gold deposits, and attributes the Pataz metallogenic belt to the mesothermal shear-zone hosted gold deposit class. Fluid inclusion data, accompanied by stable and radiogenic isotope determinations, set constraints on the composition and origin of the hydrothermal fluid(s) involved in the genesis of the gold lodes.

Between 7° and 8° S, Wilson and Reyes (1964), and Schreiber (1989) consider that the pre-Carboniferous basement of the Eastern Andean Cordillera, mapped as the Marañón Complex, encompasses from bottom to top three entities: (1) a basal tightly deformed metapelite member, (2) a middle metavolcanite unit, and (3) an upper turbiditic sequence. However, diverging from the former nomenclature, we will adopt in this paper the term Marañón Complex only for the lower polymetamorphic basement, as it presents a different structural style, characterized by polyphased strong deformations, compared to the weak foldings observed in the two overlying units.

The redefined Marañón Complex outcrops near Pataz as bad land terrains at the bottom of the Marañón Valley and mainly comprises grey phyllites and minor intercalations of micaschists, eyed-gneiss, and graphitic layers with a total tickness over 1 km. It underwent regional greenschist to lower amphibolite metamorphism with four superposed stages of deformation (Schreiber, 1989). The only radiometric age available for equivalent sequences in Central Peru is an imprecise K-Ar age of 600 Ma for the main foliation (Dalmayrac et al., 1980). Thus, the attribution of a late Proterozoic age to this metamorphic event remains doubtful and, following the example of the Argentinean Sierras Pampeanas and Eastern Cordillera, where the basement series, were metamorphized during the Pampean orogeny (Rapela et al., 1998), a mid-Cambrian for this polyphased tectonics is also conceivable. Superposed to the metapelites, there is a thick unit of volcanoclastics, locally named the Vijus Unit (Haeberlin, 2000). The series groups not only a broad spectrum of lava flows, from rhyolites and dacites in the lower part to andesites in the upper part, but also some intercalations of metaconglomerates and metagrauwackes. In absence of fossils, the attribution of an age to the Vijus Unit remains imprecise and only a time interval from mid-Cambrian to early Ordovician can be suggested. Over an angular unconformity follows the 200 to 600 meter-thick siliciclastic sediments of the Contaya Formation. In the studied area, the lithostratigraphic column begins with alternating thin layers of massive quartzites, dark sandstones, grey slates, and minor limestones. Upwards, the Contaya outcrops mimic the traditional Ordovician facies described in Peru, and consist of black slates, sandstones and few quartzites, assembled in decametric turbiditic sequences. The presence of graptolite fauna (Wilson and Reyes, 1964), in the aforementioned middle and upper levels of the Contaya Formation, indicates at once a Llandvirn age and a deep marine sedimentation. The early Paleozoic outcrops, in particular the Contaya slates, are characterized by a low-grade regional metamorphism with two moderate folding phases. Since the Silurian and the Devonian are missing, it is not possible to determine with accuracy the age of these deformations, except that they took place sometime between Ordovician and late Devonian.

The Pataz Batholith, main host-rock of the lodes, is part of a giant undeformed intrusion belt, which extends from 6°S to 10°S along the Eastern Cordillera of the Andes, intruding the pre-Carboniferous metamorphic basement along a fracture-controlled NNW-SSE strike. It consists of a suite of calk-alkaline rocks, from diorite-tonalites to monzogranites, with granodiorites as intermediary term. Both the granodiorites and the monzogranites hold abundant mafic enclaves, and basement metamorphized xenoliths. Two crosscutting dyke populations, representing late magmatic activities of the batholith, were identified: (1) abundant tilted aplite and vertical rosa granite dykes, well exposed in the apical part of the batholith, and (2) rare melanocratic lamprophyres, outcropping in close spatial relationship with the N-S striking oreshoots. For the granodiorite, Vidal et al. (1995) lay out a zircon U/Pb age of 329 ± 1 Ma near Parcoy. Haeberlin et al. (1999) report for the diorites and granodiorites several ⁴⁰Ar/³⁹Ar ages on hornblendes, clustered between 319.6 and 323.0 ± 2.0 Ma, and for the aplite dyke an ⁴⁰Ar/³⁹Ar plateau age of 322 ± 3 Ma on a muscovite. Since the ⁴⁰Ar/³⁹Ar cooling ages are consistent with the former U/Pb zircon date, they conclude that both the magmatic differentiation of the batholith and the emplacement of the aplite dykes occurred during a short time interval close to 329 Ma.

The Pataz Batholith has no direct contact with the Upper Paleozoic and Mesozoic units, and is only covered unconformably by the Pliocene calk-alkaline lava flows of the Lavasén Volcanites. Permo-Carboniferous molasses and an incomplete Mesozoic series outcrop in half grabens along the Marañón River, and form an independent structural domain limited by regional NNW-SSE faults. The Mississippian Ambo Group is a molassic deposit related to the erosion of the early Paleozoic belt. It marks the opening of intracontinental basins. No sedimentation is recorded in the Pennsylvanian, probably as consequence of a general uplift. From the mid-Permian to the early Triassic, a horst and graben setting predominates with the deposition of the Mitu conglomerates and volcanites. After this intense block tectonics, the Mesozoic appears as a quiet marine sedimentation period, conditioned by the Marañón Geoanticline, which, during its history, acts as a structural high (Benavides, 1999). Thus, the reduced Mesozoic-Cenozoic lithostratigraphic column (Wilson and Reyes, 1964; Schreiber, 1989; Vidal et al, 1995) holds major erosion gaps and consists only of Upper Triassic limestones and dolomites (Pucará Group), Neocomian greyish sandstones (Goyllarisquizga Group), Middle Albian marls and limestones (Crisnejas Formation), and Upper Cretaceous-Eocene continental clastic red beds (Chota Formation).

The auriferous veins of the Pataz province are classified as structurally-hosted lode gold deposits, most commonly termed "mesothermal" or "orogenic" gold deposits. The lodes, hosted by second and third-order structures, appear predominantly as kilometer-long continuous quartz veins enclosed at the margin of the batholith, and less abundantly as split and sheared bedding-concordant ore shoots in the adjacent phyllites and slates. They occur as N-S to NW-SE oriented structures from 0.1 to 8 meter thick with a dip of 30 to 60° to the E-NE, and only exceptionally as flat structures or as E-W lodes dipping to the south (Haeberlin, 2000). Lithological contacts, aplite and lamprophyre dykes, and bedding planes were favorable sites for vein opening. The lodes display the typical brittle-ductile characteristics of the mesothermal gold deposits including (Fig. 1a,b,c,d): (1) shear zones along the hangingwall, (2) hydraulic breccias at the footwall, (3) wallrock slivers and sulfide-laminated textures, indicating several episodes of vein opening and filling, and (4) decimetric tension gashes and Riedel structures (Haeberlin et al., 1999).

Superposed Andean brittle tectonics is responsible for discontinuities, decametric displacements and duplications of veins (Fig. 1.e). The first reverse throw on the fault planes indicates NE-SW compression, and is followed by a second one suggesting SE-NW compression. These movements are coeval with the Miocene folding phase. The late ubiquitous normal movements, are related to the E-W Pliocene extension tectonics.

Fig. 1 - Mine photographs illustrating the typical brittle-ductile characteristics of the Pataz lodes. All views are taken towards the gallery roof. (a) Hydraulic breccias and strong sericitization at the footwall of a decametric N-S striking 45° east dipping lode hosted by lamprophyre dyke (Consuelo vein). (b) Low-mineralized 40 cm thick sinistral shear-zone crosscutting a hornfels enclave (La Lima vein). (c) Sigmoidal closing of a 45° east dipping vein along a vertical aplite dyke (Mercedes vein). (d) Multiple open-space filling textures with quartz I and pyrite I ribbons enclosing elongated wallrock slivers (Mercedes vein). (e) Two superposed late Andean brittle reverse movements (Mercedes vein).

The veins present a two-stage relatively sulfide-rich sequence with a first generation of dodecahedral-shaped pyrite and arsenopyrite building ribbons through the euhedral milky quartz (stage I), and a second one postdating a fracturation phase with blue greyish fine-grained quartz, sphalerite containing chalcopyrite exsolutions and pyrrhotite, and late galena with tiny Sb-sulfosalt inclusions (stage II). Electrum is mainly hosted in sphalerite (Fig. 2a); pure gold precipitates later, generally with galena, either in fractured pyrite (Fig. 2b), or at boundaries with arsenopyrite. Carbonates are minor in the ore paragenesis, except in the lodes hosted by diorites and lamprophyre dykes. Euhedral ankerite forms early multiple open-space filling textures between coarse milky quartz. Dolomite and calcite grow as idiomorphic crystals.

Fig. 2 - Photomicrographs of gold and base metal sulfide textural relationships in the Pataz ores. As = arsenopyrite, Au = native gold, el = electrum, gn = galena, py = pyrite, qz = quartz, sl = sphalerite. (a) Sphalerite veinlet with chalcopyrite exolutions, galena, and electrum crosscutting arsenopyrite and pyrite of stage I (Mercedes vein). (b) Galena and gold filling cracks in strongly fractured pyrite I (La Lima vein).

A 10-cm to 10-m wide greenish pervasive hydrothermal alteration halo with visible bleaching occurs in the walls of mineralized quartz veins hosted by plutonic rocks or by hornfels. This alteration consists of sericitization, with minor chloritization, carbonitization and pyritization, which, towards the outer margin of the vein, sharply changes in a few centimeters to minor propylitization before reaching the fresh rock. Almost no alteration is visible in slates and phyllites, where just a bleaching of the strongly sheared wallrock occurs.

Vidal et al. (1995) were the first to undertake radiometric dating of the alteration related to the mineralization, and obtained on sericite an age of 286 ± 6 Ma for the Cabana vein near Parcoy by the conventional K/Ar method. Haeberlin et al. (1999) dated by ⁴⁰Ar/³⁹Ar four sericite separates from different veins in the Pataz district, and one from the Culebrillas mine in the Parcoy district. The five spectra, including the major La Lima vein (Fig. 1), show disturbed initial heating steps and an almost plateau shape for heating steps above 1000°C. The total fusion ages, which include all the steps, as the former K/Ar age of 286 ± 6 Ma [Vidal, 1995 # 223], are therefore too young and geologically meaningless. The spectrum configuration is interpreted as a superposition of two events: the ore-forming event, recorded by the plateau-like steps, in the Carboniferous, and a reheating, affecting at least the low-temperature steps, around 140 Ma.

The weighted mean averages of the plateau-like steps representing from 34 to 52 percent of the total ³⁹Ar released gas yield a bimodal distribution of the ages, with three ages between 312 and 314 Ma and two significantly younger ages at 305 Ma (Table 1). The maximal ages, which overlap considering 2

errors, are considered as the age of the formation of the sericite alteration and subsequently as the age of the gold mineralization. The two younger sericite dates at 305 Ma reflect cooling through a low-blocking temperature during hydrothermal activity and late fluid circulation. Such behaviors at low temperatures are typical of sericite because of the fine-grained size of the mineral and of the deformation it suffers during crystallization.

Fig. 3 - Typical step-heating ⁴⁰Ar/³⁹Ar spectrum of a sericite from a Pataz gold vein.

Table 1 - Summary results of the ⁴⁰Ar/³⁹Ar incremental heating analyses of the hydrothermal sericite separates. The ages considered as the age of the mineralization are emphasized in bold characters.

The initial staircase shape of the age spectra is interpreted as a partial Jurassic rejuvenation of the sericites, which is consistent with a ⁴⁰Ar/³⁹Ar dating of a K-feldspar separated from a dacite near Pataz yielding an age of 137 ± 3 Ma. The absence of disturbance of the spectra of the fresh host-rocks suggests that the thermal overprint is the result of fluid circulation along the auriferous veins during the intrusion of Jurassic porphyric stocks in the Eastern Cordillera.

Quartz and transparent yellow-brown sphalerite samples were prepared as doubly polished 100

m thick plates and analyzed for microthermometric data under a Leica DMLB, with a Nikon 100x objective. All the trapped fluid inclusions are two-phase with high liquid to vapor ratios and range in size from 2 to 6

, exceptionally up to 20

. Four fluid inclusion populations of saline composition were recognized based on their ubication, their morphology and their chemical composition (Haeberlin, 2000):

These populations present two substantial discrepancies compared to the assemblages documented in : (1) the presence of a CO₂ fluid, a phase which was not previously observed, and (2) the lack of strong evidences for the entrapment of KCl salts in the analyzed samples. The fluid inclusion petrography indicates that the first three populations are related to the hydrothermal activity. Type 1 aqueous fluid inclusions, with moderate salinities (12 to 14 wt.% NaCl_eq.) and homogenization temperatures ranging between 170 and 260°C, represent the earliest mineralizing fluid, in equilibrium with the first sulfide stage of the ore sequence. Type 2 inclusions describe the fluid(s) involved during the lead-zinc mineralizing stage and subsequently in the gold precipitation. They share many common compositional characteristics with the first population, but their homogenization to a liquid phase occurs at lower temperatures (130 to 180°C) and their salinity estimates display a mixing trend with a moderate saline upper-end member (17.5 wt.% NaCl_eq.), close to the nature of the earliest fluid, and a low-salinity pole (4.6 wt.% NaCl_eq.), representing a dilute water. The aqueo-carbonic inclusions, forming the third population, are interpreted to reflect a late episode of fluid entrapment, coeval either with the end of stage II or/and with stage III. This carbonic fluid has a NaCl equivalent salinity, determined from clathrate melting temperatures, ranging between 4 and 8 wt.%, and homogenizes into liquid phase at temperatures between 185 and 260°C.

With both sulfur and oxygen geothermometry, we establish that the gold precipitation occurred at a temperature of 330 ± 50°C and, based on the isochore calculations, at an average pressure of 3.5 ± 1 kbar, which corresponds under pure lithostatic regime to a depth of 13 ± 4 km. According to the fluid inclusions, the proposed scenario responsible for the gold precipitation is a sudden change in the nature of the fluid, possibly due to the income of a dilute low-carbonic water, which mixed with the early crustal-derived saline brine.

To better evaluate the possible source(s) of the ore fluid(s), a multi-isotopic study was carried out on the ore and alteration minerals. The isotope data are characterized by an overall homogeneity throughout the mineralized belt, but their interpretation is not univocal (Haeberlin, 2000). Sulfur isotopes of galena, sphalerite and pyrite, with values around 0 per mil, are generally interpreted to indicate a magmatic source but they do not allow to discriminate between a local plutonic source, such as the Pataz Batholith, or another igneous-derived sulfur source. The average hydrogen and oxygen isotope compositions of the hydrothermal fluid (

D_f = -25 ± 10 per mil and

¹⁸O_f = 7 ± 2 per mil) overlap the metamorphic and magmatic ranges. The strong similarity between these values and the Pataz Batholith rock composition suggests either that the hydrothermal water was derived from the same deep crustal reservoir or/and that it has strongly interacted with the plutonic wallrocks in a rock-buffered system.

Lead isotope determinations on galena are slightly less radiogenic than the common lead of the intrusion and indicate that most of the lead is derived from the batholith but also that a substantial part has an upper crustal origin. Finally, the scattered Sr compositions of the carbonates suggest that the hydrothermal fluid strongly interacted with the pluton. The primary strongly radiogenic signature of the hydrothermal fluid (⁸⁷Sr/⁸⁶Sr around 0.715) is not consistent with any of the local reservoirs, and possibly reflects the involvement of an external Sr source.

Summarizing the available data, it could be concluded that the mineralization occurs at 312 to 314 Ma, i.e. about 40 Ma after the regional very low-grade metamorphism, and 15 Ma after the calk-alkaline magmatism. It is emplaced in an accretionnary context, characterized by a general uplift in the Pennsylvanian suceeding to the Mississippian molassic sedimentation in transtensional basins. The Pataz Batholith only served as a favorable rheological and chemical locus for the genesis of economic and regular veins. The veins formed at about 13 km of depth along oblique-sinistral brittle-ductile shear zones resulting from a local NW-SE compression. Neither magmatism nor metamorphism are directly related to the genesis of the Pataz mesothermal lodes. The outstanding paragenetic, structural and isotopic homogeneity of the auriferous veins along the 160-km long belt argues for a large-scale fluid circulation. On the basis of stable and radiogenic isotopes, the ore-forming fluid(s) derived from the upper crust but acquired also most of its/their elements through a strong interaction with the conduits and host-rocks, in particular the granodioritic host (Haeberlin et al., 1999 and Haeberlin et al., 2000). The mixing of two brines, one crustal and one more superficial, as suggested by the fluid inclusions, could be the cause of gold deposition.

We are grateful to the Peruvian mining company Cía Minera PODEROSA S.A. and in particular Ing. J. C. Alcalde, Ing. M. Santillana for providing financial support and full access to their mines. Thanks are also due to Ing. F. Cueva and Ing. L. Ruiz for their help during two field campaigns and for the interesting talks we had at the mine office. This project, part of a Ph.D. thesis, also benefited from the financial support of the Swiss National Science Foundation (Grant No. 20-47260.96).

Benavides, V. (1999) Orogenic evolution of the Peruvian Andes: the Andean cycle. In B.J. Skinner (ed.), Geology and ore deposits of the Central Andes. Econ. Geol. Sp. Publ. Series, v. 7, p. 61-107.

Dalmayrac, B., Laubacher, G., Marocco R. (1980) Géologie des Andes péruviennes. Travaux et Documents de l' ORSTOM, v. 122, 501 p.

Haeberlin, Y. (2000, in prep.) Structural geology, geochemistry, and geochronology of gold veins in the northern part of the Pataz Batholith (La Libertad, Peru). Ph.D. thesis, University of Geneva.

Haeberlin, Y., Moritz, R., Fontboté, L., and Cosca, M. (1999) The Pataz gold province (Peru) within the frame of a mesothermal gold and antimony belt of the Eastern Andean Cordillera. In C.J. Stanley (ed.), Mineral deposits: processes to processing, v. 2, p. 1323-1326.

Haeberlin, Y., Moritz, R., Fontboté, L., and Cosca, M. (2000, submitted) Relationships between the structurally-controlled mesothermal gold deposits of Pataz (Peru) and its granodioritic host. Proceeding of the IGCP-373 fieldconference to Urals, Ekaterinburg, 18-30 July 2000.

Macfarlane, A. W., Tosdal, R. H., Vidal, C. E., and Paredes, J. (1999) Geologic and isotopic constraints on the age and the origin of auriferous quartz veins in the Parcoy mining district, Pataz, Peru. In B.J. Skinner (ed.), Geology and ore deposits of the Central Andes. Econ. Geol. Sp. Publ. Series, v. 7, p. 267-279.

Rapela, C. W., Pankhurst, R. J., Casquet, C., Baldo, E., Saavedra, J., Galindo, C. (1998) The Pampean orogeny of the southern proto-Andes; Cambrian continental collision in the Sierras de Córdoba. In: R. J. Pankhurst & C. W. Rapela (eds), The proto-Andean margin of Gondwana. Geol. Soc. of London Special Publ., v. 142 ,p. 181-217

Schreiber, D.W. (1989): Zur Genese der Goldquarzgängen der Pataz-Region im Rahmen der geologischen Entwicklung der Ostkordillere Nordperus (unter besonderer Berücksichtigung der Distrikte Parcoy, La Lima und Buldibuyo). Heidelberger Geowiss. Abh., v. 29, 235 p.

Schreiber, D. W., Fontbote, L., Lochmann, D. (1990) Geologic setting, paragenesis, and physicochemistry of gold quartz veins hosted by plutonic rocks in the Pataz region. Econ. Geol. v. 85, p. 1328-1347.

Vidal, C. E., Paredes, J., Macfarlane, A. W., and Tosdal, R. H. (1995) Geología y metalogenia del distrito minero Parcoy, provincia aurífera de Pataz, La Libertad, Volumen jubilar Alberto Benavides: Lima, Sociedad Geológica del Perú, p. 351-377.

Wilson, J.J., Reyes, L. (1964): Geologia del Cuandrángulo de Pataz. Lima, Perú, Com. Carta Geol. Nac., v. 9, 91 p.

[Publications]

Veins	Steps	% ³⁹Ar	Plateau ages	Total ages
	T (°C)	released	(Ma ± 2 )	(Ma ± 2 )

Picaflor	1000-1200	49.9	304.8 ± 1.4	280.5 ± 0.4
Mercedes	1000-1100	51.7	304.9 ± 3.0	279.6 ± 0.4
La Lima	1000-1100	46.1	312.1 ± 0.8	294.3 ± 0.6
Consuelo	1000-1200	50.2	313.5 ± 1.4	294.9 ± 0.4
Pencas	1000-1100	34.4	314.1 ± 1.2	289.8 ± 0.4