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Haeberlin, Y., Moritz, R. and Fontboté, L. (2000): Late Paleozoic orogenic gold deposits in the Central Andes (extended abstract). Submitted to: A GEODE-GEOFRANCE 3D workshop on orogenic gold deposits in Europe with emphasis on the Variscides, November 7 - 8, 2000, Orléans-La Source (BRGM), France. To be published in a special issue of the Chronique de la recherche minière.

Late Paleozoic orogenic gold deposits
in the Central Andes, South America

Yves Haeberlin, Robert Moritz and Lluís Fontboté

Section des Sciences de la Terre, Université de Genève
Rue des Maraîchers 13 - 1211 Genève 4 - Switzerland

Abstract

In the Central Andes, abundant brittle-ductile quartz veins in Ordovician to Carboniferous granites and numerous saddle-reefs in Early Paleozoic turbidites represent a coherent group of Paleozoic structurally-hosted gold deposits, that are part of an Au-(Sb-W) orogenic belt. This belt, extending from North Peru to central Argentina along the Eastern Cordillera and further south in the Sierras Pampeanas, includes historical districts such as Pataz-Parcoy, Ananea, Yani-Aucapata, Amayapampa and Sierra de la Rinconada. Three epochs of mineralization were identified, from south to north: Early Devonian in the Sierras Pampeanas, presumably Late Devonian for the northwestern Argentinean, Bolivian and southern Peruvian turbidite-hosted lodes, and 312-314 Ma for the Pataz province in North Peru. The latter 160-km long province consists of sulfide-rich quartz veins hosted by brittle-ductile shear zones that have affected a 329 Ma old granodiorite. This mineralization, associated in space and time with a Basin and Range setting, appears to be related to a tectono-thermal reequilibration of the Cordilleran margin in response to its overthickening subsequently to the subduction of a young oceanic crust and spreading ridges.

Introduction

Despite the common metallogenic heritage of the proto-Pacific margin of Gondwana from Eastern Australia, host of well-known orogenic provinces, via New Zealand and Antarctica to South America (Sillitoe, 1992), only little attention has been devoted to the study of orogenic gold deposits in the Central Andes. The recent reevaluation (Zappetini and Segal, 1998; Haeberlin et al., 1999; Skirrow et al., 2000) of several historical gold occurrences, such as the turbidite-hosted Sierra de La Rinconada deposits in NW Argentina, the granite-hosted Pataz province in North Peru, and the Au-Ag-Cu veins in the southern Sierras Pampeanas, Argentina, highlights that the Early and Middle Paleozoic units of the Central Andes, from northern Peru to central Argentina, are host of a belt of orogenic gold, antimony and tungsten deposits.

In this contribution, after an overview of the Pataz province, situated in the northernmost part of this belt, a comparison between the Pataz lodes and the numerous structurally-hosted Au-(Sb-W) deposits of the Eastern Andean Cordillera and the Sierras Pampeanas is outlined. A parallel is also drawn with some of the major peri-Gondwanian Paleozoic orogenic gold provinces.

 

Fig. 1 - (a) Schematic geological map of the Pataz gold province with the location of the main mines. (b) Situation of the Pataz province in the frame of the orogenic Au-(Sb-W) belt of the Eastern Andean Cordillera and the Sierras Pampeanas.

The Pataz gold deposits

The Pataz gold province is situated 500 km north of Lima in the Eastern Cordillera of the North Peruvian Andes (Fig. 1). The mineralized belt is 160 km long and 1 to 3 km wide, extending first along the right side of the Marañón Valley from Bolívar to Pataz (Schreiber, 1989; Schreiber et al., 1990; Haeberlin et al., 1999; Haeberlin et al., 2000; Haeberlin, 2001, in prep), then striking to the southeast to the Parcoy (Vidal et al., 1995; Macfarlane et al., 1999) and Buldibuyo districts. It includes numerous quartz-sulfide veins, located within the margins of the 329 Ma old (Vidal et al., 1995) calk-alkaline Pataz Batholith close to the contact with a Late Proterozoic-Early Cambrian low-grade metamorphic basement overlain by Cambro-Ordovician volcano-sedimentary rocks (Fig. 1a). The ore grades in the oreshoots vary between 7 to 15 g/t Au, exceptionally up to 120 g/t Au. The annual production of the province, including the operating mines of the Parcoy district, was 360'000 ounces in 1999, which represents 10% of the gold supply of Peru.

Deposit styles

The auriferous veins of the Pataz province are classified as orogenic gold deposits, also named mesothermal or lode-gold deposits. The lodes share over the entire mineralized belt several common and typical field characteristics, including:

(1) a strong lithological and in particular rheological control by the host-rock; the lodes occurring as continuous km-long quartz veins if enclosed by the Carboniferous batholith, and as split and bedding-concordant oreshoots if hosted by the folded Ordovician slates.

(2) a homogeneity of the vein strikes, in particular within the batholith, where the quartz veins are emplaced in N-S to NW-SE oriented brittle-ductile deformation zones, dipping 30 to 60° to the E-NE and with a sinistral-reverse sense of motion. Subordinate, they occur as flat relay structures and as conjugate E-W south or north-dipping extensional veins in listric faults.

(3) a spatial association either with brecciated shear-zones and their related brittle-ductile features such as tension gashes and Riedel structures or, for the subordinate systems, with tensional structures showing crack and seal textures.

(4) a simple two-stage ore sequence, with a first paragenesis composed of milky quartz, pyrite and arsenopyrite, and a second one postdating a fracturation event with blue-grey quartz, galena, sphalerite, Sb-sulfosalts, electrum and native gold. A final post-sulfide paragenesis with calcite and dolomite in veinlets crosscuts the earlier parageneses.

(5) a pervasive low-temperature hydrothermal alteration of the wallrock, consisting in plutonic rocks of sericitization with minor chloritization, carbonitization, and pyritization.

The overall uniformity in H, O, C, S and Pb isotope composition on the ore/gangue/alteration minerals within the entire province is indicative of a large-scale fluid migration over several tens to hundreds of kilometers (Haeberlin et al., 2000). Lead isotopes suggest that the hydrothermal fluids migrated through upper crustal rocks, and acquired most of their elements through strong interaction with the host-rocks, in particular the Pataz Batholith. The ore fluids were moderately saline, aqueous and CO₂-poor. Primary fluid inclusions, involved in the gold stage, are H₂O-NaCl, with salinities lower than for the first-stage inclusions, ranging from 4 to 18 wt.% NaCl_eq, reflecting probably the income of a dilute water, which mixed with a saline brine (Haeberlin et al., 2000). Gold precipitated, according to delta³⁴S and delta¹⁸O geothermometry, at 330 ±50°C for the granitoid-hosted lodes, and at ~250°C for the turbidite-hosted oreshoots, suggesting for the latter an emplacement either at a shallower structural level or at the margin of the mineralized system. Based on isochore calculations, the granitoid-hosted lodes were formed under a pressure of 3.5 ±1 kbar, which corresponds under a pure lithostatic regime to a depth of 13 ±4 km. ⁴⁰Ar/³⁹Ar dating on sericite from the alteration zone of the gold lodes yields ages between 312 and 314 Ma (Haeberlin et al., 1999).

The timing of the regional events demonstrates that the 312-314 Ma auriferous mineralization is related neither to the ca. 329 Ma Pataz Batholith, nor to the Early Paleozoic regional metamorphism. The regional setting prevailing immediately before gold ore formation is a Cordilleran-type margin with about a 20 Ma-long Basin and Range evolution. This context is characterized by arc-related calk-alkaline plutonism, molasse-type sedimentation in pull-apart basins, dextral strike-slip faults, and sea-floor spreading (Haeberlin, 2001, in prep.). The gold event, postdating the injection of lamprophyres, occurs in a compressive tectonic frame during a temporary and sudden uplift of the convergent margin possibly in response to its overthickening.

Implications for the genesis of gold deposits

The orogenic gold deposits of Pataz appear to be not genetically related to high-level felsic intrusions or metamorphic devolatilization, but are the result of deep fluid circulation proximal to first-order NNW-SSE oriented structures, possibly consecutive to tectono-thermal reequilibration of an unstable convergent margin. Crustal processes, intimately linked to the subduction of a young and hot buoyant oceanic plate with spreading ridges, such as slab break-off and asthenospheric upwelling, are potentially the driving forces at the origin of the lode gold deposits (e.g. Qiu and Groves, 1999). According to our hypotheses, the first mechanism modified the stress conditions required for opening or re-opening older fractures, and the second produced the high heat flow susceptible to provoke the devolatilization of the upper crust, and thus the release of the large fluid amounts involved in the generation of gold veins.

Orogenic Au-(Sb-W) deposits in the Central Andes

A large number of small structurally-hosted Paleozoic Au-(Sb-W) deposits is documented in the Central Andes from 6° to 32°S, along the Eastern Cordilllera in Peru, Bolivia and northern Argentina and in the Sierras Pampeanas (Fig. 1b), occurring either as saddle-reefs in Early Paleozoic turbiditic series or as brittle-ductile veins in Ordovician to Carboniferous granites. Apart from the Pataz province, the major auriferous districts are, from north to south, Ananea and Santo Domingo in southeastern Peru, Yani-Aucapata in the Cordillera Real of Bolivia, Amayapampa in Central Bolivia, Sierra de la Rinconada in northwestern Argentina and Sierra de las Minas in west-central Argentina (Fig. 1b). These primary deposits are likely the dominant gold sources for the giant placer deposits of the Madre de Dios and Beni basins.

Table 1 - Geological setting, structural characteristics, ore, gangue and alteration parageneses, and fluid inclusion data of the main Au-(Sb-W) Late Paleozoic orogenic provinces in the Central Andes.

Table 1 summarizes the geological frame, the ages and the styles of the aforementioned Late Paleozoic orogenic deposits. The majority of them is located in the southern Central Andes and presents similarities with the Pataz province deposits in their tectonic settings, their quite saline and relatively CO₂-poor ore fluids, if data available, and their sulfide-rich ore sequences, consisting successively of early pyrite-arsenopyrite and minor W-minerals with milky quartz, gold occurring in the second stage with lead-zinc-copper sulfides and fine-grained quartz and, in some cases, late Sb-sulfosalts. In some localities, antimony constitutes the main ore of the Bolivian lodes, suggesting an emplacement at shallower structural levels. The variety in deposit geometry, including straight brittle-ductile veins, bedding-concordant veinlets and disseminated ores; and in alteration minerals, with pervasive sericitization in granite and weak chloritization in slate, reflects respectively the rheological and chemical controls of the host-rocks. At a regional scale, the striking fact is either the presence of the lodes in low-order structures close to a major tectonic boundary or the location of oreshoots along anticline hinges.

According to relative ages of folding phases, and scarce radiometric data on intrusions and ore-related alterations, the three following periods of orogenic gold veining, correlable with the distinct tectonic evolution of the main cordilleran domains, are recognized:

(1) In the Argentinean Sierras Pampeanas (south of 26°S), the Au-event takes place between ~390 Ma and ~360 Ma, and is apparently concurrent with the compressional tectonics associated with the accretion of the Chilenia terrane (Skirrow et al., 2000).

(2) The maximal age of the northwestern Argentinean, Bolivian and the southern Peruvian deposits (12 to 26°S) is provided by the Late Devonian collisional phase, and the younger limit by the Permo-Triassic intrusions, which are devoid of gold lodes. The oreshoots are emplaced in turbiditic rocks affected by thrusting, and are probably related to the uplift period subsequent to the collision of the Arequipa-Antofalla craton.

(3) At Pataz, and by extention presumably in the entire Eastern Cordillera north of 12°S, the gold lodes are associated in space and time with a Basin and Range setting prevailing during Early Pennsylvanian along the proto-Andean margin of North Peru.

Discussion and conclusions

(1) Considering the ubiquity of the Au-Sb-W lodes, some of them known since the Inca epoch, and the size of the auriferous placers in the Subandean basins, it should be emphasized that the Central Andes offer stimulating perspectives for the discovery of new deposits and for the reevaluation of old mining areas, either in Peru, Bolivia or western Argentina. For mineral exploration, the brittle-ductile quartz veins in Ordovician to Carboniferous batholiths and the saddle-reefs in the Early Paleozoic anticlines represent most promising targets for high-grade orogenic gold deposits with subsidiary antimony and tungsten.

(2) On a worldwide scale, the Pataz deposits and the other Andean orogenic deposits show more affinities with the shear-zone hosted gold deposits of the French Massif Central (Bouchot et al., 1997), the Australian Lachlan (Ramsay et al., 1998) and the Thomson fold belts (Solomon and Groves, 1994), in particular in their tectonic contexts, ages and parageneses, than with the classical orogenic Archean deposits of the Abitibi and Yilgarn provinces. The Andean deposits are characterized by a relatively sulfide-rich paragenesis and more saline ore-fluids.

(3) The Eastern Cordillera and its southern prolongation, the Sierras Pampeanas, are the South American slice of the giant orogenic gold belt that is emplaced in the terranes that circumscribed the Gondwana craton. Interestingly, this circum-Gondwana margin and the continental masses around the closing Paleo-Tethys Ocean, that at the Cambrian were attached to the Gondwana supercontinent (Fig. 2) account for the location of all the major Paleozoic gold orogenic provinces in the world, such as the Tasman orogenic belt of Australia, Westland in New Zealand, the Meguma province in Nova Scotia, the European Hercynides, and the Tian Shan in Central Asia (Goldfarb et al., 2000).

Fig. 2 - Paleogeographic sketch of the Gondwanian craton at Cambrian times after Courjault-Radé et al. (1992) with the circum-Gondwana and peri-Tethysian location of the terranes that will host major Late Paleozoic orogenic gold provinces. The ages of the gold belts are after Goldfarb et al. (1998) and Groves et al. (1998).

Acknowledgements

Our research on the Pataz deposits was launched in 1996 under the proposal of Ing. W. Sologuren, and benefited from the assistance of the Peruvian mining company Cía Minera PODEROSA S.A., Lima. Ing. J. C. Alcalde and Ing. M. Santillana should receive our best thanks for providing financial support and full access to the mines. Thanks are also due to the staff of the geological office, in particular Ing. F. Cueva and Ing. L. Ruiz for their help during the field campaigns. 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). This is a contribution to the European Science Foundation GEODE programme.

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