Received 14 July 1999; accepted in revised form 20 January 2000

Transcription

1 Ž. Ore Geology Reviews Metallogeny of the southern Sierras Pampeanas, Argentina: geological, 40 Ar 39 Ar dating and stable isotope evidence for Devonian Au, Ag Pb Zn and W ore formation R.G. Skirrow a,), A. Camacho a,1, P. Lyons a, P.E. Pieters a,2, J.P. Sims a,3, P.G. Stuart-Smith a,4, R. Miro b a Minerals DiÕision, Australian Geological SurÕey Organisation, GPO Box 378, Canberra, ACT 2601, Australia b SerÕicio Geologico Minero Argentino, AÕ. Poeta Lugones 161, B. NueÕa Cordoba, Cordoba 5000, Argentina Received 14 July 1999; accepted in revised form 20 January 2000 Abstract Early to mid-paleozoic metamorphic and igneous rocks of the southern Sierras Pampeanas tectono-stratigraphic terrane, northwestern Argentina, host diverse metallic mineral deposits of Au, Ni Cu, Ag Pb Zn, W, Li, Be, Nb, LREE and Th. Integration of deposit geology, 40 Ar 39 Ar geochronology and stable isotope studies with regional geological mapping and U Pb zircon dating, has led to the resolution of three principal metallogenic epochs. Ordovician intrusion-hosted Ni Cu sulfide deposits and rare-metal-enriched pegmatites were succeeded by a province-wide metallogenic epoch and concurrent tectonic processes during the Devonian. This epoch was characterized by mesothermal lode Au deposits, high-level Ag Pb Zn veins, W" Cu-bearing veins and replacements, and various pegmatite- and granite-related lithophile element deposits. Finally, Miocene Pliocene epithermal Au Ag mineralization developed during Andean tectonic events. Sericitic alteration associated with mesothermal lode Au Ž "Cu" Ag. in the Sierra de Las Minas and Sierras de Cordoba yields Devonian 40 Ar 39 Ar step heating ages in the ranges and Ma, respectively. These ages span the timing of Ag Pb Zn sulfide quartz veining in the El Guaico district Ž 386" 4 Ma, sericite., and W-bearing quartz vein mineralization in the Aguas de Ramon and El Morro districts which yielded muscovite ages of 366"1 and 363"1 Ma, respectively. Oxygen isotope d 18 OSMOW compositions of the Au, Ag Pb Zn and W ore-forming fluids, calculated from quartz, white mica and chlorite isotope ratios, range from 5.2 to 10.8 Ž ns32., whereas calculated d DSMOW compositions range 34 from y61 in the La Rioja Au systems to y141 in the W veins Ž ns9.. Sulfide d S values range from 1.5 to 10.0, averaging 6.5 Ž ns15.. The oxygen and hydrogen stable isotope data suggest contributions to the ore fluids of either D-depleted meteoric waters that have reacted extensively with metasedimentary rocks, or fluids equilibrated with degassed felsic magmas, or mixtures of both. This mineralization formed during and up to Ma after widespread emplacement of high-level peraluminous to metaluminous granites, which yielded U Pb zircon ages of 404" 6 382" 6 ) Corresponding author. address: roger-skirrow@agso.gov.au Ž R.G. Skirrow.. 1 Present address: School of Geology, University of New South Wales, NSW 2052, Australia. 2 Present address: 13 Arabana Street, Aranda, ACT 2614, Australia. 3 Present address: Geoverde, PO Box 479, Jamison Centre, ACT 2614, Australia. 4 Present address: SRK Consulting, PO Box 250, Deakin West, ACT 2600, Australia r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž. PII: S

2 40 ( ) R.G. Skirrow et al.rore Geology ReÕiews Ma. The Devonian Au deposits are localised in transcurrent and reverse fault and shear zones that may be related to collision of the Chilenia terrane with the western margin of Gondwana during the newly defined Achalian orogeny. These deposits are considered to be members of the mesothermal lode Au family of systems found worldwide. q 2000 Elsevier Science B.V. All rights reserved. Keywords: metallogeny; southern Sierras Pampeanas, Argentina; 40 Ar 39 Ar 1. Introduction The Sierras Pampeanas Ž Pampean Ranges., situated east of the Andes in northwestern Argentina, form a tectono-stratigraphic terrane comprising of mainly early- and mid-paleozoic metamorphic and igneous rocks Ž Fig. 1.. Crystalline basement was uplifted during Andean tectonism in the Tertiary as a series of meridionally trending tilt block ranges, separated by Mesozoic to Recent sedimentary basins Ž Jordan and Allmendinger, The southern Sierras Pampeanas, in the provinces of San Luis, Cordoba and La Rioja, forms part of an extensively mineralized metallogenic domain that has been a significant historical producer of W as well as Be, Li, Nb and Ta from pegmatites. Of increasing interest, however, is the potential of the region for precious and base metal mineralization. A large number of Au-bearing quartz vein and Ag Pb Zn vein deposits have been mined, albeit on a small scale, since the late 19th century, yet the region remains relatively underexplored. The metallogenic diversity of the region is further exemplified by the presence of epithermal Au Ag mineralization associated with Miocene Pliocene magmatism Ž Urbina et al., 1997., and Ordovician Ni Cu sulfide deposits hosted by mafic ultramafic intrusions in the Sierra de San Luis Ž Skirrow and Sims, 2000., both of which are attracting the interest of explorationists. The mineral deposits and metallogenic evolution of the southern Sierras Pampeanas have been the subject of few published studies. Metallogenic investigations have been hampered, until recently, by a paucity of accurate spatially controlled regional geological and geochronological data. Systematic evaluation of the mineral potential of the Sierras Pampeanas requires the development of a comprehensive regional geological and metallogenic framework, to which the present work contributes through documentation of mineralization styles, and appraisal of ore genesis and metallogenic evolution. Attention is focused on three key deposit types: lode Au, Ag Pb Zn and W vein deposits, and the following metallogenic problems are addressed: Ž. i the styles of mineralization present; Ž ii. the relative and absolute timing of Au, Ag Pb Zn and W ore formation; and Ž iii. the origins of the ore fluids in the hydrothermal deposits. The study was part of the Geoscientific Mapping of the Sierras Pampeanas Cooperative Project, carried out between the Australian Geological Survey Organisation Ž AGSO. and the Servicio Geologico Minero Argentino Ž SEGEMAR., in the period The multidisciplinary, integrated program of regional geological mapping, U Pb Th and 40 Ar 39 Ar geochronology, whole rock geochemistry, and metallogenic studies, was supported by airborne magnetic and radiometric surveys extending over 2 approximately 20,000 km in three areas Ž Fig. 1.. The results were presented as a series of digital geological, geophysical and metallogenic prospectivity maps, published reports on 20 1:100,000 scale sheet areas and three 1:250,000 scale sheets 5 ŽLyons et al., 1997; Pieters et al., 1997; Sims et al., 1997., unpublished reports and a metallogenic atlas 5. Aspects of the results are also presented by Sims et al. Ž and Stuart-Smith et al. Ž in press.. Mineral deposit investigations included field documentation of the geology of principal deposits in all major mining districts of the project areas, and were supported by petrography, X-ray diffractometry, electron microprobe mineral analysis, and whole rock geochemistry. Data on more than 700 mineral occur- 5 Ž. rences were compiled in a database ARGMIN. In this paper, we present the first 40 Ar 39 Ar dating of hydrothermal alteration, and oxygen, hydrogen and sulfur isotope geochemistry to be reported for Au, Ag Pb Zn and W deposits of the southern Sierras 5 Available from Servicio Geologico Minero Argentino, Avda. Julio A. Roca 651, Buenos Aires 1322, Argentina.

3 ( ) R.G. Skirrow et al.rore Geology ReÕiews Pampeanas. These data, integrated with geological constraints, provide a new framework in which to reassess the metallogeny of this historic mining region. 2. Geological setting and tectono-magmatic evolution of the southern Sierras Pampeanas The metamorphic and igneous basement of the southern Sierras Pampeanas comprises three lithostratigraphic domains, characterised by metasedimentary rocks deposited in the late Neoproterozoic to Cambrian, Cambro-Ordovician, and the Ordovician, respectively Ž Sims et al., All three domains share a common tectonic history since the Early Devonian. The late Neoproterozoic to Cambrian domain is exposed principally in the eastern part of the southern Sierras Pampeanas, and consists of pelitic and psammitic gneiss, marble and schist with subordinate orthogneiss Ž metamorphic basement, Fig. 1.. The protoliths of these metasedimentary rocks are interpreted to have accumulated on a passive margin during the separation of Laurentia from Gondwana Ž Sims et al., Deformation, upper amphiboliteto-granulite-facies metamorphism and granitoid magmatism associated with the Pampean orogeny occurred at ca Ma during convergence on the western margin of Gondwana Ž Rapela et al., The Cambro-Ordovician domain consists of pelitic gneiss and schist derived from sediments that were deposited in a possible back-arc basin associated with east-dipping subduction along the margin of Gondwana in the latest Cambrian to earliest Ordovician Ž Sims et al., 1998; metasediments, Fig. 1.. The Ordovician domain includes a magmatic arc to the west of the back-arc basin and is preserved as an extensive belt of igneous complexes that is well exposed in the southern ranges of La Rioja Province ŽPankhurst et al., 1996, 1998; Pieters et al., 1997; Ordovician granite, Fig. 1.. Magmatism was closely followed by the Famatinian orogeny during the Early Ordovician. Emplacement of mafic and ultramafic intrusions, including the host to Ni Cu sulfide mineralization at Las Aguilas in the Sierras de San Luis Ž Fig. 1., occurred at ca. 480 Ma in a collisional setting ŽSims et al., 1997, 1998; Skirrow and Sims, Towards the close of the Famatinian orogeny Ž ca. 450 Ma; Sims et al., 1998., extensional shear zones developed under greenschist facies conditions accompanied by S-type granite, Li"Be"Nb"Tabearing pegmatites, and widespread metamorphic retrogression. The resumption of convergence on the western margin of Gondwana in the mid-paleozoic resulted in compressive deformation and the development of an Early Devonian magmatic arc extending over much of the southern Sierras Pampeanas. This tectonism has been termed the Achalian Orogeny ŽSims et al., 1998; Stuart-Smith et al., in press., and probably corresponds to the Fase Precordilleranica Ž Astini, in the Precordillera west of the Sierras Pampeanas where it is related to the amalgamation of the Chilenia terrane Ž Ramos et al., In the Sierras Pampeanas, the deformation was dominated by orthogonal westerly directed thrusting, with a component of sinistral shearing, both at greenschist facies, and the development of regionally extensive ductile and brittle ductile, conjugate faults and shear zones. Some of these faults and shears were significant in localising precious and base metal vein deposits, as described in detail below. Peraluminous to slightly metaluminous granites were widely emplaced in the metamorphic basement during and after Achalian shear zone development Ž Devonian granite, Fig. 1.. These granites are of particular significance because of similarities in age with the mineralization discussed herein. Uranium lead zircon dating of the granites suggests that initial plutonism was around 404 Ma ŽStuart-Smith et al., in press.. Ar Ar ages from greenschist facies mylonite zones and brittle ductile strike slip faults and fractures suggest that deformation continued through most of the Devonian Ž Sims et al., Felsic magmatism, however, may have continued into the Carboniferous Ž e.g. Rapela et al., Some of the Devonian shear zones Že.g. in the Sierra de San Luis and Sierras de Comechingones. were the loci of granite that was multiply injected into active shears. In other areas, large, circular to ovoid, zoned, and fractionated plutons crosscut early, greenschist facies shear zones. The granites Ž sensu lato. range from monzogranite and syenogranite to granite Žsensu stricto. and leucogranite, and are variably magnetic with outer phases of zoned plutons typically more magnetic than core phases. Most are two-mica granites Ž sensu lato., although hornblende has been re-

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5 ( ) R.G. Skirrow et al.rore Geology ReÕiews Table 1 Major mineralization types of the southern Sierras Pampeanas Ž a Mineralization type Ore and associated elements Age Principal districts and provinces. Mafic ultramafic intrusion- Ni, Cu, Co, PGE, Au, Cr b Ordovician Las Aguilas Virorco Ž SL. hosted Ni Cu Au" Ag" Cu veins Au, Ag, Cu, W, U c Devonian Sierras de Las Minas Ž LR. Rıo Candelaria Ž CD. Santo Domingo Ž SL. Ag Pb Zn veins Ag, Pb, Zn, V c Devonian El Guaico Ž CD. Wolframite quartz veins W c Devonian Aguas de Ramon Ž CD. San Roman Ž SL. Disseminated scheelite in W, Cu, Zn, Mo, Bi, F, Au Devonian c Ž and older?. Sierra del Morro Ž SL. El Zinqui Ž CD. calcsilicates and other hosts La Florida Santo Domingo Ž SL. Pegmatite-related rare metals Li, Be, Nb, Ta, Sn, W; Ordovician to Devonianr Sierras de San Luis Ž SL. and industrial minerals feldspar muscovite, quartz Carboniferous Sierras de Cordoba Ž CD. Cu skarn Cu, W, Fe Devonian c Ž.? Sierra Grande Ž CD. Sediment-hosted U U d post-carboniferous Sierras de Chepes Ž LR. Epithermal Au Au, Ag, Pb, Zn e Miocene Pliocene La Carolina Ž SL. Porphyry Cu Cu, Au e Miocene Pliocene Diente Verde Ž SL. a Principal districts only those situated within the three study areas are listed. Abbreviations SL: San Luis Province; LR: La Rioja Province; CD: Cordoba Province. b Sims et al. Ž 1997, 1998., Skirrow and Sims Ž c This study. d Pieters et al. Ž e Sruoga et al. Ž 1996., Urbina et al. Ž 1995, ported in a few intrusions. Accessory ilmenite is present in some Devonian granites; others contain accessory magnetite. Alumina saturation indices are close to the peraluminousrmetaluminous boundary of 1.1, or are slightly lower. Ratios of Fe2O3rFeO suggest that almost all of the granites formed from relatively oxidized melts, although the widespread weak hematitic Ž "sericite " chlorite. alteration overprint ŽLyons et al., 1997; Sims et al., 1997; Stuart-Smith et al., in press. may have led to modification of primary magmatic Fe2O3rFeO ratios. The radiometric responses Ž K, Th, U. are generally highly elevated in both the low-magnetic cores and in the magnetic outer regions of zoned plutons. The eruption of high-k intermediate to felsic volcanics in several centres across the southern Sierras Pampeanas occurred during the period 9.5" " 0.2 Ma ŽRamos et al., 1991; Urbina et al., 1995; Sruoga et al., Mineralization of the southern Sierras Pampeanas A wide range of metallic and industrial mineral occurrences are present in the southern Sierras Pampeanas Ž Angelelli, 1984.; The principal metallic types occurring within the three study areas in the provinces of San Luis, Cordoba and La Rioja are summarized in Table 1. The earliest mineralization recognized in these areas are deposits of Ni Cu sulfides hosted by Ordovician mafic ultramafic intrusions in the Sierra de San Luis Ž Skirrow and Sims, No unequivocally pre-ordovician mineralization was observed in the study areas, although late Precambrian syngenetic W mineralization was proposed by de Brodtkorb and Brodtkorb Ž We are also aware of Mn" Au possibly associated with Cambrian dacites in the northern Sierras de Cordoba. The second major period of mineralization occurred dur- Fig. 1. Generalised geology of the southern Sierras Pampeanas, and location of the three study areas Ž dashed boxes. of the AGSO SEGE- MAR geoscientific mapping project. Locations of Figs. 2 and 3 are indicated by boxes with solid outlines. Mineral deposits outside the areas of Figs. 2 and 3 that are mentioned in the text and in tables are numbered: Ž. 1 Santa Maria Au Ž Cu Ag.; Ž. 2 La Argentina Au Ag; Ž. 3 Loma Blanca W Ž Sierra de Los Morillos.Ž. ; 4 Santo Domingo Au and El Duraznito W Au; Ž. 5 Las Aguilas Ni Cu; Ž. 6 La Carolina district Au Ag; Ž. 7 San Francisco del Monte de Oro.

6 44 ( ) R.G. Skirrow et al.rore Geology ReÕiews ing the Devonian, and included the mesothermal lode Au, Ag Pb Zn and W vein deposits that are the focus of the present contribution, as well as several other styles of W and Cu mineralization. These other W deposits, pegmatite-related rare metal deposits and Tertiary epithermal Au and porphyry Cu mineralization are described elsewhere Že.g. Lyons et al., 1997; Pieters et al., 1997; Sims et al., 1997; Urbina et al., The districts described below are representative of Au, Ag Pb Zn and W vein mineralization in the southern Sierras Pampeanas. 4. Lode gold mineralization Quartz vein systems hosting Au"Ag"Cu mineralization, henceforth termed lode Au, were investigated in the Rıo Candelaria and San Ignacio districts in the Province of Cordoba Ž Fig. 2., the southern ranges of the Province of La Rioja Ž Fig. 3., and in the Santo Domingo district in the Province of San Luis Ž Fig Rıo Candelaria and San Ignacio gold districts, ProÕince of Cordoba The Rıo Candelaria district Ž Fig. 2. comprises 25 identified small Au-quartz vein deposits and numerous unnamed workings dispersed in a belt ;20 km long that were mined up to the 1930s ŽCaminos and Cucchi, The region is dominated by pelitic to psammitic gneiss, migmatite, amphibolite and subordinate metacarbonate rocks, which were intruded by syn-metamorphic Cambrian S-type granites ŽLyons et al., 1997; Rapela et al., 1998., and by Devonian granite in the east Ž Fig. 2.. The gold occurrences are situated within variably sheared Ž in places mylonitic. quartz feldspar biotite muscovite" garnet gneiss known as the Guamanes shear zone ŽMartino, 1993; Lyons et al., This regionally extensive north- and northwest-trending zone attains widths of several kilometres. Biotite defining the shear fabric yields a 40 Ar 39 Ar plateau age of 358"2 Ma Ž Sims et al., Narrow mylonite zones within the sheared gneiss are characteristically chlorite hematite-altered and overprint the higher grade biotite shear fabric. The gold deposit characteristics are summarized in Table 2. The geometry and morphology of quartz veining indicate formation in zones of subvertical extension within the Guamanes shear zone. Movement on the steeply east-dipping retrogressive mylonite zones was generally reverse, and represents reactivation of the Guamanes shear zone. Gold occurs with sulfides in the primary ore zones and as coarse grains Ž up to 200 mm diameter. in near-surface zones of oxidation Ž - 40 m depth., where gold grades are highly variable. Intense sericitisation of host rocks occurs within ;1 m of major veins, and in wall rock fragments within veins. Distal chlorite hematite sericite" carbonate alteration Ž Fig. 4a. is similar to that in retrogressive mylonite zones in the Guamanes shear zone, and exhibits low magnetic susceptibility relative to less altered gneiss. In conjunction with hematitic mylonite zones, this alteration may partly account for the relatively low regional magnetic response of the Guamanes shear zone. Gonzalez and Mas Ž 1997, identified H 2O CO2 primary and pseudosecondary fluid inclusions and H 2O-rich secondary inclusions in mosaictextured deformed quartz in the Las Higueritas deposit Ž Table 2.. Based on the wide range of CO 2 r H 2O ratios, phase separation of H 2O CO2 fluid was proposed by Gonzalez and Mas Ž The characteristics of gold deposits in the Rıo Candelaria district are commensurate with syn-deformational vein formation and gold deposition, in a mesothermal environment at relatively high crustal levels. A contrasting style of Au mineralization, at the Paso de La Quinta prospect, comprises a siliceous zone reportedly containing up to 160 grt Au and 460 grt Ag Ž Deantonio, This zone is unusual in the Rıo Candelaria district as it dips steeply and consists of fine mesh-like vein stockworks of cockade and crustiform quartz" hematite within silicified, sericitised breccia Ž Table 2.. The host rock may have been pre-existing mylonite. Chalcedony is abundant as late infill, and there is no evidence of extensive recrystallization of silica minerals. The Fig. 2. Geology of part of the Sierras de Cordoba, and location of Au, Ag Pb Zn, W and other mineral occurrences Ždata sources: Lyons et al.,

7 ( ) R.G. Skirrow et al.rore Geology ReÕiews

8 46 ( ) R.G. Skirrow et al.rore Geology ReÕiews Fig. 3. Geology of the Sierra de Las Minas Ž La Rioja and San Luis provinces., and location of Au"Cu"Ag deposits Žbased on Pieters et al.,

9 ( ) R.G. Skirrow et al.rore Geology ReÕiews Table 2 Characteristics of lode Au deposits, Cordoba and La Rioja provinces Deposit characteristic Rıo Candelaria district Paso de La Quinta San Ignacio district Sierra de Las Minas Host rocks c Quartz biotite feldspar Replaced mylonite? Granitic gneiss, Granodiorite, granite, migmatite muscovite"garnet sheared gneiss biotite paragneiss Vein thickness, -1.0 m, m 3 7 m, )1.6 km m, m m, m strike m, m Vein geometry Pinch-and-swell, gash-shaped, Siliceous zone is planar Pinch-and-swell, gently sigmoidal Gash-like, linear to sigmoidal gently sigmoidal Vein arrays Single and multiple, subparallel, Fine veinlet stockwork in Four principal veins Subparallel sets, en echelon en echelon altered host rock Vein dip, strike east, Siliceous zone is steep Steep, NE to E Steep, NW and NE; rare E W Vein quartz Milky white, recrystallized, Fine-grained cockade and Massive to weakly banded, Massive to banded anhedral and crustiform crustiform quartz; milky white, anhedral to subhedral, subhedral, rare cavities late chalcedony infill partly recrystallized, milky white; crack-seal anhedral and subhedral, fibre quartz; rare cavities; later clear grey quartz later clear quartz networks Ore minerals, Gold, pyrite, sphalerite, Gold, hematite Gold, pyrite, galena, chalcopyrite, Electrum, gold, pyrite, sulfides, oxides galena, chalcopyrite, bornite, cerussite, vanadinite, chalcopyrite, galena arsenopyrite, rare pyrrhotite rare wolframite a Alteration Sericitization within Chlorite sericite Intense sericite"pyrite Intense sericite"pyrite; ;1 m of veins; distal pyrite up to in aureoles 1 2 m wide less common chlorite hematite several metres from chlorite"epidote"carbonate sericite carbonate siliceous zone Extension direction Sub-vertical? Sub-horizontal Sub-horizontal Sub-horizontal Shear sense East-side-up?? Sinistral Ž on NW set.; dextral Ž on NE set. Late-stage Brecciation, fracturing, Chalcedony infill Supergene alteration Fracturing, brittle faulting, processes supergene alteration supergene alteration Fluid inclusion H 2O CO 2 primary and?? H 2O CO 2: 3308C ; average, T pseudosecondary: C d h C H2 O secondary: 2208C b b b Fluid inclusion H 2O CO 2: %?? H 2O CO 2: salinity average 12% c Eq. wt.% NaCl H 2 O: % b e f a d Gold grades, grt 0.2 2; uncommonly Up to Up to 177; average 5.7 f a Resources 60,000 t at 20 grt 1 Mt? 170,000 t at 10.2 grt 15,000 t at 5 grt Au, Ž. Ž e. Ž g representative Puigari Monserrat 15 grt Ag Grupo Sur. a Miro and Torres Ž b Gonzalez and Mas Ž c Cravero et al. Ž d JICA-MMAJ Ž e Zolezzietal. Ž f Deantonio Ž g Marcos Ž

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11 ( ) R.G. Skirrow et al.rore Geology ReÕiews textures and mineralogy resemble those of epithermal gold deposits, although the nearest volcanic rocks occur at Volcan Pocho, ;40 km to the SW. The San Ignacio Au district is situated in the Valle de Punilla, Sierras de Cordoba ŽFig. 2; Rigal, 1934; Pastore and Methol, Gold-bearing quartz veins are hosted by massive to foliated muscovite-rich granitic gneiss and biotite gneiss within a sequence also including marble and amphibolite that was metamorphosed under high grade conditions during the Pampean orogeny. The San Ignacio district lies within a NW-trending zone of lineaments and faults that is evident in aeromagnetic imagery and which postdates the Pampean structures in the region Ž Lyons et al., Quartz vein textures and mineralogy at San Ignacio are very similar to those in deposits of the Rıo Candelaria district, although the veins dip more steeply and may have formed within a transcurrent rather than reverse shearrfault system Au"Cu deposits of the southern sierras of La Rioja ProÕince The quartz vein systems hosting Au and minor Cu and Ag mineralization occur widely throughout the Sierras de Las Minas, Ulapes and Chepes ŽFigs. 1, 3; Cravero and Rıos Gomez, 1988; Rıos Gomez et al., 1992; JICA-MMAJ, The deposits were initially worked towards the end of the 19th century, and small-scale mining has continued intermittently to the present day. Most Au deposits are hosted by calc-alkaline I-type granodiorite and granite of the Chepes Igneous Complex, which was emplaced, metamorphosed in the amphibolite facies, and deformed in the early Ordovician ŽPieters et al., 1997; Pankhurst et al., 1998; Stuart-Smith et al., in press.. Several Au"Cu"Ag occurrences in the Sierra de Chepes are hosted by pelitic and psammitic metasedimentary rocks and metamorphosed feldspathic Ž volcanic?. rocks of the early Cambrian Olta Metamorphic Complex. Major zones of high strain up to several kilometres width developed during the late Ordovician deformation, and in part controlled the emplacement of leucogranites of the Chepes Igneous Complex. Many of the high-strain zones trend north south, show very low aeromagnetic responses, and develop biotite muscovite shear fabrics associated with steep reverse movement ŽPieters et al., In the Sierras de Las Minas and Ulapes region, a conspicuous pattern of northwest- and northeasttrending lineaments is revealed in aeromagnetic imagery as zones of very low magnetic response. These structures transect all basement metamorphic rock types, including the Chepes Igneous Complex, and overprint at least some of the biotite-bearing north Fig. 4. Representative illustrations of Au, Ag Pb Zn and W deposits, southern Sierras Pampeanas. Ž. a Photomicrograph of hydrothermally altered and sheared granodioritic gneiss adjacent to Au quartz veins at the Puigari Monserrat deposit, Rıo Candelaria district. Chorite and hematite Ž chl hm. replace biotite Ž bt., which with muscovite Ž mv. and ribbon quartz define the shear fabric. Feldspar augen are sericitised. Sample A95RS014A, plane-polarised light. Ž. b Photomicrograph of quartz Ž qtz. veined, sheared, and sericite Ž ser. altered granitic gneiss at the Callana VI Au deposit, Sierra de Las Minas. Stage 1 fibrous quartz in crack-seal veins contains fluid inclusion trails parallel to vein walls, and is overprinted by stage 2 microcrystalline hematite Ž hm. chalcedony veinlets and infillings. Sample A95RS033B, plane-polarised light. Ž. c Gold mineralized vein, Callana VI deposit. Milky stage 1 quartz Ž qtz. with minor sulfides and gold is cut by bands and veinlets of stage 2q3 hematite goethite Ž hm goet. chalcedony, which contain secondary gold. Sample A95RS033, scale divisions in centimetres. Ž. d Gold-bearing vein, Vallecito deposit, Sierra de Las Minas. Stage 1 galena pyrite-bearing milky quartz Ž qtz. cut by stage 2 dusty hematite Ž hm., which is in turn overprinted by chalcedony goethite Ž goet. veins and breccia of stage 3. Sample A95RS047, scale subdivisions are centimetres. Ž. e Ag Pb Zn vein, Rara Fortuna deposit, El Guaico district. Crustiform quartz Ž qtz. overgrows domains of galena sphalerite arsenopyrite Ž gal sph aspy.. Sample A95RS068B, scale divisions in centimetres. Ž f. Photomicrograph of euhedral growth-zoned quartz Ž qtz. and intergrown sphalerite Ž sph., and possibly later arsenopyrite pyrite Ž aspy py., Rara Fortuna Ag Pb Zn deposit. Sample A95RS069F, plane-polarised light. Ž. g Vein of colloform banded chalcedony and breccia of this vein material with chalcedony goethite Ž goet. matrix, representing late-stage infilling at the Rara Fortuna Ag Pb Zn deposit. Sample A95RS068E, scale divisions in centimetres. Ž. h Zoned W-bearing veins near Veta Santa Rita, Mina Esmeralda, Aguas de Ramon district: quartz muscovite tourmaline wolframite Ž qtz mv tour wolf. veins with alteration aureoles in the Esmeralda Granodiorite. Scale bar is 10 cm length, view looking east. Ž. i Loma Blanca W deposit, Sierra de Los Morillos: foliated amphibole phlogopite Ž amph phl. rock overprinted by epidote" fluorite" garnet associated with a vein of quartz feldspar epidote Ž qtz fs ep.. Scale divisions in centimetres. Ž j. Photomicrograph of sericitised quartzofelspathic gneiss, adjacent to Au-bearing quartz veins at the San Ignacio deposit. Feldspar is totally replaced by sericite Ž ser., and relict metamorphic muscovite Ž mv. and quartz Ž qtz. are deformed and partly sericitised. Sample A95RS008A, crossed polars.

12 50 ( ) R.G. Skirrow et al.rore Geology ReÕiews south-oriented shear zones. Some linear magnetic lows, where such are cropping out, correspond to hydrothermally altered and in places Au-mineralized shear, fault and fracture zones Ž Fig. 3.. Additionally, most of the larger exploited deposits of Au Cu Ag in the Sierra de Las Minas occur within a regional domain of low aeromagnetic response. The origin of this regional feature is enigmatic, and may represent demagnetisation produced by regional hydrothermal alteration. Au Cu Ag-bearing quartz vein systems ŽTable 2. exhibit two principal orientations: NW- and NEstriking; a few strike roughly E W. Mylonitic and S C shear fabrics are well-developed in places, overprinting the Ordovician foliation. These shear fabrics show consistent region-wide kinematic and geometrical relationships to quartz vein orientations, as shown schematically in Fig. 5. The orientations and morphologies of quartz veins are consistent with syntectonic formation in locally extensional domains within conjugate brittle ductile transcurrent shear zones Ž Fig. 5.. Recrystallization and later cataclastic deformation of the veins are viewed as part of this progressive deformation, although a separate and later brittle event has also been identified. A generalised alteration and ore mineral paragenetic sequence for Au" Cu" Ag deposits of the southern sierras of La Rioja Province is presented in Table 3. Electrum with fineness as low as 640 Ž JICA-MMAJ, 1993; Cravero et al., is associated with sulfides and sericite pyrite alteration of the earliest paragenetic stage Ž Fig. 4b.. These hypogene assemblages are overprinted during stage 2 by bands, seams and fractures containing microcrys- Fig. 5. Schematic relationships between conjugate transcurrent shearrfault zones, Au" Cu" Ag vein mineralization, and alteration zones of low magnetic susceptibility, Sierra de Las Minas Ž La Rioja Province.. The inferred component of east-west compression Ž broad arrows. may be part of a larger scale Devonian wrench fault system Ž not shown..

13 ( ) R.G. Skirrow et al.rore Geology ReÕiews Table 3 Generalized paragenetic stages, alteration and textures for Au Ž Cu Ag. deposits, Sierras de Las Minas and Chepes Stage Vein mineral Wall rock alteration Vein Deformation assemblage relative to vein textures Ž. 1 Hypogene, Milky white quartz, Proximal Ž - 3 m.: Massive, strained, milky, Strike slip mesothermal minor carbonate, sericite pyrite anhedral to subhedral cavity- shearing; S C fabrics pyrite, chalcopyrite, Žsericitisation lining quartz; disseminated in altered wall rocks; galena, sphalerite, of feldspar. pyrite; cavity-filling crack-seal fibre quartz veins Au Ž electrum. Distal: chlorite" chalcopyrite and other sulfides; epidote Žchloritisation disseminated anhedral carbonate; of biotite, amphibole. electrum inclusions in pyrite Ž. 2 High-level, Hematite, carbonate, grey Proximal and distal: Anastomosing fractures and Brittle faulting, low-temperature quartz, recrystallized hematite bands of microcrystalline fracturing, brecciation hydrothermal quartz, chalcedony, Au Žmicrocrystalline, hematite chalcedony; veinlets of stage 1 quartz disseminated. of dusty hematite carbonate; networks of recrystallized and fine grey quartz in older quartz; coarse Au with hematite Ž. 3 Deep supergene Goethite, chalcedony, None Clear euhedral quartz lining No deformation to weathering clear quartz, Au, malachite, cavities; cavity infilling Ž rare brecciation. chrysocolla, covellite, by goethite chalcedony" tenorite, cuprite, hematite; coarse Au; fineanglesite, cerrusite grained replacements of silicates, sulfides, carbonates, oxides talline hematite " chalcedony and by hematite carbonate veins Ž Fig. 4c,d.. Coarse gold with fineness of greater than 950 is most closely associated with hematite within the veins. Supergene minerals replacing sulfides and infilling vughs include goethite, chalcedony, carbonates, clear euhedral quartz and possibly gold Ž stage 3, Fig. 4d.. Reconnaissance microthermometry on fluid inclusions in quartz probably correlating with our stages 1 and 2 indicates trapping of CO2-bearing aqueous fluids with moderate salinities ŽTable 2; Cravero et al., Liquid CO2-bearing inclusions were also documented widely in gold-bearing quartz veins of the Sierra de Las Minas by JICA-MMAJ Ž Our observations of fluid inclusions in stage 1 crack-seal quartz veins and in quartz intergrown with sulfides confirm the widespread presence of primary and pseudosecondary three-phase CO2-bearing inclusions Ž water, CO liquid, CO vapour. in deposits of the 2 2 Sierra de Las Minas. Although the maximum Au values reported for some deposits are relatively high, Au grades are extremely variable Ž Table 2, Fig. 6.. The average for 525 samples is 5.7 grt Au, which is clearly skewed by the sporadic high values. Most high grade Au assays are from samples of outcrop, shallow workings, and drill hole depths of less than m, and probably represent supergene enrichment rather than primaryrhypogene Au. Silver grades in higher grade Au zones are generally less than ppm; rare values up to 553 ppm were reported by JICA- MMAJ Ž Copper assays of veins are commonly in the range 0.1 3%, whereas Pb contents are generally in the range ppm in higher grade Au zones. These relatively low tonnage vein systems are most prospective for Au and possibly Ag, rather than Cu or Pb. Au Cu Ag mineralized quartz vein systems of the Sierras de Las Minas, Ulapes and Chepes have previously been interpreted as epi-hydrothermal ŽSarudiansky, 1988, 1990; Cangialosi and Baldis, 1995.; Ordovician to Carboniferous epithermal Ž JICA-MMAJ, 1993.; and shear-hosted deposits related to regional tectonic processes ŽCravero and

14 52 ( ) R.G. Skirrow et al.rore Geology ReÕiews Fig. 6. Gold assay distribution in vein deposits of the Sierra de Las Minas and Sierra de Chepes Ž La Rioja Province., grouped by district. Samples are from outcropping veins and mine workings, and including diamond drill core samples for the Callanas district. Upper and lower limits of boxes are 75th and 25th percentiles; whisker with caps are 90th and 10th percentiles with outliers shown as filled circles; median and mean values are shown as solid and dotted horizontal lines. Data sources: JICA-MMAJ Ž 1993.; YAMIRI Secretarıa de Estado de Minerıa, 1989 Ž J. Rıos Gomez, 1995, personal communication.; Marcos Ž 1988.; Cravero et al. Ž Rıos Gomez, 1988; Cravero et al., 1995; Rıos Gomez et al., Cangialosi and Baldis Ž suggested formation during the Pampean cycle. Based on quartz deformation textures, the presence of crack-seal fibre quartz veins containing liquid CO2-bearing fluid inclusions, and the synchronicity of alteration and veining with brittle ductile shearing, the stage 1 assemblages are interpreted to have formed in an upper crustal mesothermal environment. In contrast, stages 2 and 3 are inferred to have developed at low temperatures and near-surface conditions. New timing constraints Ž see below. and kinematic indicators suggest that stage 1 alteration and associated mineralization are related to transcurrent shear and fault systems that developed during the Devonian. Stage 2 brittle faulting and oxidation are speculated to have occurred during Permo-Carboniferous tectonism represented by narrow grabens in the region ŽPieters et al., andror during Andean uplift in the Tertiary. 5. Ag Pb Zn vein deposits El Guaico district, Province of Cordoba The El Guaico Ag Pb Zn district covers an area of about 80 km 2, centred approximately 22 km SW of Villa de Soto Ž Fig. 2.. The geology of the district and its metallic deposits was described by Sureda Ž 1978., Lucero Michaut and Olascher Ž and

15 ( ) R.G. Skirrow et al.rore Geology ReÕiews Candiani Ž The 65 documented deposits were worked intermittently between the 1880s and the 1970s, and individually are relatively small with resources ranging up to approximately 100,000 t at 280 grt Ag, 1.7% Pb, and 2.6% Zn ŽRara Fortuna, Candiani et al., The district is also known for the occurrence of minor Au and secondary vanadium minerals associated with the veins, such as vanadinite, brackebuschite and descloizite Ž Sureda, The quartz-sulfide veins are hosted by medium- to high-grade Cambrian metamorphic rock types, including banded quartz plagioclase biotite muscovite schist of the Formacion Tuclame, massive to foliated gneiss and migmatite Ž Complejo Pichanas., and granodioritic bodies. The latter are interpreted as melt that was generated during the Cambrian metamorphism Ž Lyons et al., 1997; Rapela et al., Schist of the Formacion Tuclame occurs in a ;1 km wide NW-trending belt through the centre of the district, and is bounded to the NE by the Dos Pozos mylonite and cataclasite zone. Parts of this zone are hematite chlorite-altered, whereas in other areas, the schist gneiss contact zone is occupied by sheared pegmatite. The structure may represent a shear zone developed initially at upper greenschist or amphibolite facies conditions that was reactivated under lower grade conditions, possibly during mineralization. A set of NE-trending faults and lineaments, featuring prominently in Landsat TM imagery and in aerial photographs, cuts the metamorphic rocks and possibly the NW-trending Dos Pozos mylonite cataclasite zone. Other NW-trending faults and lineaments in the district strike subparallel to the regional metamorphic foliation. Strike slip components of offsets on the NE faults are at most a few metres to tens of metres. Both orientations of faults, particularly the NW set, are common throughout the southern Sierras Pampeanas and typically show low magnetic response. Non-magnetic granite ŽCoro de Mesa granite. crops out 11 km SSW of Rara Fortuna, and granite was interpreted from aeromagnetic data, at several hundred metres depth within the metamorphic rocks, beneath the central-north part of the El Guaico district Ž Lyons et al., As discussed below, these observed and inferred granites may have been significant as heat andror metal andror fluid sources. The Ag Pb Zn quartz vein deposits strike principally NE and, to a lesser extent, NW. Many of the NE-oriented veins lie within the regionally extensive lineaments and faults mentioned above. Several of the larger deposits occur adjacent to the contact between schist and gneissrmigmatite. The mineralized vein systems have strike lengths of m and consist of steeply dipping single or multiple veins and vein networks with widths of m and rarely reaching 2.4 m. Veins are generally tabular with minor pinch-and-swell and with high length-towidth ratios. These relationships suggest that the rheological contrast between schist and gneissr migmatite units may have been important in localising vein formation through the development of dilatant zones during movement on the Dos Pozos structure. The lack of significant post-depositional deformation or recrystallization and the excellent preservation of vein textures have allowed three main paragenetic stages to be identified across the district. These observations are supported by the detailed mineralogical work of Sureda Ž Ž. 1 Coarse, milky, crustiform and comb-textured quartz deposition in millimeter-to centimeter-scale bands parallel to vein walls, indicative of open space filling Ž Fig. 4e.. Pyrite, arsenopyrite, sphalerite, galena and sulfosalts of Ag, Pb, Sb and Sn ŽSureda, formed in bands parallel to crustiform quartz and interstitial to euhedral quartz. In some deposits, coarse sulfides may have replaced wall rock fragments in quartz vein stockworks Že.g. San Carlos, Eufemia.. Sphalerite ranges from colorless to deep red-orange in color, suggesting low to moderate FeS contents. Ž. 2 Development of veinlet networks of clear to grey, subhedral to euhedral, growth-zoned quartz that cut the older banded quartz, with sulfides similar to those of stage 1 Ž Fig. 4f.. Reconnaissance fluid inclusion petrography indicates that three-phase CO -bearing inclusions Ž 2 water, CO 2 liquid, CO 2 vapour. and two-phase aqueous inclusions are present in stage 2 quartz and sphalerite. Ž. 3 Re-opening of vein structures and formation of vein-like zones of colloform-banded chalcedonic silica and microcrystalline hematite, mainly in the southwestern part of the El Guaico district Ž Fig. 4g.. The chalcedony veins cut stages 1 and 2 assem-

16 54 ( ) R.G. Skirrow et al.rore Geology ReÕiews blages, but chalcedony also forms rare intergrowths with stage 2 quartz. No sulfides were observed in chalcedony in the deposits investigated, although Sureda Ž included argentite and several Cu sulfides in an opal-forming supergene stage. Supergene vanadium minerals, carbonates of Cu and Pb, and oxides of manganese and iron formed during this stage Ž Sureda, Primary or hypogene calcite in crustiform bands and nodules was described at two deposits by Sureda Ž 1978., and minor supergene calcite is widespread. Silver occurs primarily in argentite and in sulfosalts of As, Sb and Pb, and rarely as the native metal Ž Sureda, At the Rara Fortuna mine, Ag distribution closely correlates with Pb ŽCandiani et al., Native gold was observed by Sureda Ž in several deposits, and Au values of up to 0.41 grt have been reported from mullock samples in the south-central part of the district ŽTorres and Leynaud, Visible hydrothermal alteration is restricted to within 1 2 m of veins and to wall rock fragments within veins, and consists of pervasive sericite" pyrite replacement of host rock and rare chloritisation Ž e.g. Asuncion.. Mass balance calculations indicate net gains of K, Si, Fe, S plus the ore metals, and losses of Na, Ca and Mg during sericite pyrite alteration at the Garibaldi deposit, whereas Al, Ti, Zr and HREE remained essentially immobile ŽLyons et al., Paragenetic stages 1 and 2, characterised by the assemblage sericite pyrite arsenopyrite with sphalerite of low to moderate FeS contents, are interpreted to have formed under conditions of intermediate oxygen fugacity Ži.e. between pyrrhotite and hematite stability. and acid ph from CO2-bearing aqueous fluids. The open space vein-filling textures, crustiform banding of coarse to fine comb quartz and virtual lack of recrystallization are characteristic of veins formed in hypabyssal to shallow mesothermal P T environments Ž Dowling and Morrison, The presence of sulfosalts of Pb, Sb, As, Ag and Sn is suggestive of relatively low temperatures of ore deposition. For example, stephanite ŽAg 5SbS 2; Sureda, 1978., has an upper stability limit of 1978C Ž Barton and Skinner, The third paragenetic stage of chalcedony" hematite deposition occurred during introduction of relatively cool, oxidized fluids, probably at shallower crustal levels than stage 1. Textures and mineralogy of stage 3 resemble those in some epithermal systems. 6. Tungsten mineralization Three principal styles of W mineralization occur in the southern Sierras Pampeanas: Ž. i wolframite with minor sulfides in large quartz veins; Ž ii. scheelite associated with calc-silicate rocks; and Ž iii. scheelite associated with quartz veinlets in generally low grade metasedimentary sequences Žde Brodtkorb and Brodtkorb, Minor wolframite and scheelite also occur in pegmatites. Tungsten mineralization is very widespread, with more than 250 deposits and occurrences in the San Luis study area alone. Wolframite-quartz veins of style Ž. i include the Aguas de Ramon district in the Province of Cordoba, described below, and the San Roman district in the Sierra de San Luis. Style Ž ii. calcsilicate-associated scheelite deposits have been the largest producers of tungsten in the region, and include those of the Sierras del Morro and Los Morillos Ž described below.. Tungsten deposits of style Ž iii. occur in the La Florida Paso del Rey Santo Domingo W belt and Pancanta district of the Sierra de San Luis Žde Brodtkorb et al., Near Santo Domingo, some of the W deposits contain significant Au Že.g. El Duraznito; Leveratto and Malvicini, The W mineralization has been described in detail elsewhere Že.g. Lyons et al., 1997; Sims et al., 1997., and we focus here on two representative districts within the study areas for which 40 Ar 39 Ar and stable isotope data have been obtained Aguas de Ramon district, ProÕince of Cordoba The Aguas de Ramon W district is situated 50 km ESE of Villa de Soto, northwestern Sierras de Cordoba Ž Fig. 2.. Three zones of W mineralization are distributed over an area measuring 3 km from north to south, with mine workings on more than 20 main veins and numerous subsidiary veins ŽLapidus and Rossi, 1959; Lucero Michaut and Olascher, Resources include 22,876 t at 0.94% WO 3 Ž Mina Esmeralda. and 10,071 t at 2.15% WO Ž 3 Mina El Carmen; Lapidus and Rossi, Cambrian metamorphic rock types in the Aguas de Ramon district comprise quartz plagioclase biotite

17 ( ) R.G. Skirrow et al.rore Geology ReÕiews muscovite banded migmatite, which hosts the northermost veins, and minor amphibolite, marble, pegmatite and aplite. The metamorphic rocks were intruded by lamprophyre dykes and by the Devonian Ž.? Granodiorita Esmeralda pluton, which hosts the central and southern veins and contains a tectonic foliation that is parallel to the regional NNE trending foliation. Two types of W mineralization were distinguished by Lapidus and Rossi Ž 1959.: Ž i. quartz muscovite tourmaline veins containing wolframite, scheelite and sulfides; and Ž ii. minor metasomatic replacement mineralization consisting of disseminated scheelite in small amphibolite marble bodies Ž -60 m long.. Vein sets of type Ž i. mineralization strike predominantly E W; those in the northern zone dip S, whereas in the central and southern zones, the dips are steeper. The vein sets are gently curved in plan view, with en echelon geometries in places. Exploited veins are -800 m long and are typically m wide. The veins are interpreted to have formed in a locally extensional regime with s3 lying roughly in the plane of the regional foliation. Mineralized veins, including Veta Santa Rita at Mina Esmeralda that was sampled for 40 Ar 39 Ar and stable isotope analysis, are typically zoned from coarse-grained muscovite at the margins through coarse, milky white, recrystallized quartz with disseminated sulfides and wolframite in places, to a central fine-grained tourmaline zone Ž Fig. 4h.. Several generations of veins are present, each with similar internal zonation, or containing only some of the main vein minerals. Rare chalcedonic quartz is present with the late tourmaline in Veta Santa Rita at Mina Esmeralda. Other vein minerals include scheelite Ž replacing wolframite., pyrite, chalcopyrite, sphalerite, calcite, bismuthinite, native bismuth, fluorite, apatite, ferrocalcite, molybdenite, and trace pyrrhotite inclusions in pyrite and sphalerite. Lapidus and Rossi Ž suggested the sulfides postdated quartz tourmaline wolframite. Well developed hydrothermal alteration occurs adjacent to veins, particularly in the granodiorite, and consists of bleached, pervasively sericitised and silicified aureoles up to 0.5 m wide. A variety of Fe, W and Bi oxides and carbonates of Cu developed during supergene oxidation of the mineralization W mineralization in the Sierra de Los Morillos, San Luis ProÕince The W deposits of the southmost Sierras Pampeanas are distributed in three main districts: the Sierra de Los Morillos, which includes the most productive mines such as El Morro and Loma Blanca; the Sierra del Morro and the Sierra de Yulto, which lie to the east and south, respectively, of the Sierra de Los Morillos Ž Fig. 1.. The region forms part of the late Neoproterozoic to early Cambrian Conlara Metamorphic Complex Ž Sims et al., and comprises mainly metapelitic, metapsammitic and metacarbonate rocks and associated amphibolitic rocks that were polymetamorphosed up to amphibolite facies and deformed during the early Paleozoic Ž Sims et al., The principal metamorphic rock types in the W districts are biotite muscovite" sillimanite " garnet banded gneiss, schist and migmatite Ž Llambıas and Malvicini, Leucotonalitic pegmatites and aplite occur subparallel with the metamorphic foliation. Mylonite and other shear fabrics are common, and generally trend N S. Zones of dolomitic and calcitic marble up to 5 m thick and amphibolite up to 4 m thick occur in the three abovementioned W districts where, in association with calcsilicate rocks, they host scheelite mineralization. Intense multiphase deformation has resulted in complex distributions of the marble amphibolite zones ŽSmith and Gonzales, 1947; Delakowitz et al., The San Jose del Morro granite was emplaced in the central part of the region, and yielded K Ar ages of 365"15 and 380"20 Ma Ž biotite Lema, Numerous granitic pegmatites with accessory beryl, apatite, fluorite and tourmaline cut the metamorphic foliation, and are themselves intruded by the San Jose del Morro granite ŽLlambıas and Malvicini, Three types of W mineralization have been recognized in the Sierra de Los Morillos, Sierra del Morro and the Sierra de Yulto: Ž. a disseminated scheelite hosted by calcsilicate rocks, constituting the major W resources; Ž. b wolframite and scheelite in quartz veins; and Ž. c non-economic scheelite and minor wolframite in granitic pegmatites where they cross cut calcsilicate zones Žde Brodtkorb and Brodtkorb, 1977; Llambıas and Malvicini, 1982; Delakowitz et al., The Loma Blanca deposit in the Sierra de

18 56 ( ) R.G. Skirrow et al.rore Geology ReÕiews Los Morillos, sampled for 40 Ar 39 Ar dating and stable isotope studies, is representative of the larger deposits and contains all three mineralization types. Calcsilicate rocks in the Sierra de Los Morillos W district occur as zones within marble and amphibolitic rocks or at their contacts with gneiss, and are the principal host rocks for scheelite mineralization. They contain assemblages typical of skarns ŽMeinert, and consist of combinations of tremolite actinolite, biotiterphlogopite, hornblende, epidote, clinozoisite, feldspar, calcite, dolomite, muscovite, and accessory quartz, titanite, magnetite, pyrite, pyrrhotite, fluorite, apatite, chlorite, ilmenite and beryl ŽLlambıas and Malvicini, 1982; Delakowitz et al., Chalcopyrite, sphalerite, molybdenite, aikinite, bismuthinite and sulfosalts are present in some areas. At least some of the scheelites in ore zones in the Loma Blanca deposit are associated with undeformed alteration patches containing garnet epidote" fluorite that overprint metamorphicbanded amphibole phlogopiterbiotite rock. Some of these alterations are spatially related to quartz feldspar pegmatitic veins and quartz veins Ž Fig. 4i.. Disseminated diopside and vesuvianite and veins of wollastonite occur in marble at Loma Blanca, and wollastonite fluorite scheelite veins cut marble at the Don Jose deposit. Unoriented talc and tremolite replace diopside, whereas epidote clinozoisite and chlorite replace wollastonite in veins at Loma Blanca. Although quartz vein W mineralization represents only 5 7% of total W production for the region, it is widespread in the Sierra de Los Morillos. The veins consist of quartz, muscovite, tourmaline, fluorite, apatite, epidote and beryl, with accessory scheelite, wolframite, titanite, pyrite, chalcopyrite, sphalerite, bismuthinite, cassiterite, magnetite, hematite, rutile and ilmenite Ž Llambıas and Malvicini, Tourmaline is ubiquitous in the veins except at the Loma Blanca deposit, where quartz veins cutting calcsilicate-bearing marble contain abundant coarse muscovite, fluorite, apatite and scheelite. This hydrothermal muscovite was sampled for 40 Ar 39 Ar dating and stable isotope geochemistry. Although the temporal and genetic links between W-bearing veins and disseminated scheelite in calcsilicate zones are yet to be proved, the relationships outlined above point to scheelite growth postdating high grade regional metamorphism in all three W mineralization types. Our results support the model of Llambıas and Malvicini Ž advocating that major W introduction was broadly synchronous with the granitic pegmatites and ensuing high-to-moderate temperature hydrothermal activity. We cannot, however, rule out the possibility that some W deposition was pre-devonian or even pre-ordovician and was remobilized into later vein- and pegmatite-hosted W mineralization Ž cf. de Brodtkorb and Brodtkorb, Ar 39 Ar dating of hydrothermal alteration 7.1. Methods Muscovite and sericite concentrates for the present study were prepared from crushed and washed samples. Samples were screened to produce y250 q 180 and y180q 125 mm size fractions. Initial concentration of the mica was made by heavy liquid separations at 2.75 and 2.96 s.g., followed by use of a Frantz Isodynamic Separator, and finally heavy liquid separations in a density gradient column in the range s.g. In samples containing both sericite and muscovite Ž 8a, 21c, 28b, 47a., separation of the two types of white mica was achieved by virtue of the generally platy nature of muscovite. High purity Ž ; 99%. sericite and muscovite separates were obtained for all samples with around 1% quartzropaque contaminants in samples 21c, 33b and 68c, and traces of tourmaline and carbonate contaminants in sample 79c. Total fusion 40 Ar 39 Ar ages were obtained for sericite andror muscovite in all samples, whereas step heating 40 Ar 39 Ar analysis was carried out only on the hydrothermal white micas Žsericite, and muscovite in samples 79b and 101a., and not on muscovite interpreted as relic metamorphic in origin. Irradiation of mica separates for 40 Ar 39 Ar analysis was carried out in the HIFAR reactor of the Australian Nuclear Science and Technology Organisation. All mineral samples were irradiated for 336 h, with the biotite standard GA 1550 Žage of 97.9 Ma; McDougall and Roksandic, in the central position as described by McDougall and Harrison Ž Cadmium sleeves, 0.2 mm thick, were placed inside the sample cans to minimise interference from

19 ( ) R.G. Skirrow et al.rore Geology ReÕiews thermal neutrons. Sample cans were rotated 1808 three times through the irradiation in order to minimise the effects of the neutron flux gradient in HIFAR. The micas were analysed for isotopic argon concentrations at the Research School of Earth Sciences Ar Ar facility, Australian National University. The technique involved progressively increasing the temperature from 5008C to 14508C, with the gas from each extraction step exposed to Zr Al getters to remove active gases. Argon was subsequently analysed using a VG Isotech MM1200 gas source mass spectrometer operating in static mode. Corrections for argon produced by neutron interactions with calcium and potassium were made using the factors 40 determined by Tetley et al. Ž The K abundance and decay constants recommended by the IUGS Subcommission on Geochronology were used Ž Steiger and Jager, Ar 39 Ar results Results of 40 Ar 39 Ar step heating and total fusion analysis are summarized in Table 4, and 40 Ar 39 Ar spectra are presented in Fig. 7a h. Step heating data are given in Appendices A.1 A.8. Three step heating age spectra Ž samples 68c, 79b and 101a. display overall flat spectra, whereas the remaining samples yielded disturbed spectra with either continuously rising apparent age steps or saddle-shaped form Ž sample 33b.. Data from the step heating analyses of white mica show that the first and last few steps generally have CarK ratios considerably higher than the remainder of the steps Ž Appendices A.1 A.8.. These data are considered anomalous and the derived ages for these steps are believed not to have major geological significance. Consequently, these ages are not incorporated when calculating the mean age. All errors quoted are at the 1s level of the mean age unless stated otherwise W-bearing Õeins Hydrothermal muscovite in a wolfamite-bearing quartz tourmaline vein Žsample 79b; Aguas de Ramon. and in a scheelite-bearing quartz fluorite vein Ž sample 101a; Loma Blanca. display overall flat spectra, with apparent ages of 366"1 and 363"2 Ma, respectively Ž Fig. 7g,h.. The apparent ages are regarded as approximate dates of ore vein formation, based on: Ž. i the absence of textural evidence for a significant post-crystallization overprint at temperatures above the muscovite closure temperature; Ž ii. the proposed formation temperatures for muscovite of ; C; and Ž iii. the flat shape of the age spectra Au Õein deposits Sericite total fusion 40 Ar 39 Ar ages from the San Ignacio Ž 8a., Puigari Ž 21c., La Bragada Ž 28b. and Vallecito Ž 47a. Au vein deposits in the provinces of Cordoba and La Rioja range from ;350 to ;375 Ma Ž Table 4.. Muscovites from the same samples yield total fusion 40 Ar 39 Ar ages that are between ; 40 and ; 70 Ma older than the sericite ages, consistent with the textural observations of overprinting of this relic metamorphic muscovite by hydrothermal sericite Ž Fig. 4j.. Step-heating experiments on the sericites produced monotonically rising age spectra Ž Fig. 7a e. and may reflect the timing in which the different white mica domains closed isotopically during cooling. The sericites analysed contain grain sizes ranging over an order of magnitude or more. For this reason, the smaller grains Žsmall domains. may have a lower closure temperature than the larger domains or crystals Že.g. McDougall and Harrison, The oldest apparent age may be interpreted to represent the approximate age for the time of initial closure, as the temperatures of vein formation Ž C; see above. are close to or below the closure temperature for Ar diffusion from sericite Ž ; 3208C; McDougall and Harrison, Alternatively, the shape of the spectra may reflect partial Ar loss during a subsequent thermal perturbation. However, this possibility is considered by us to be unlikely because there is no evidence from mineral assemblages or fluid inclusions that temperatures exceeded 3208C after the main stage of quartz vein formation and sericitic alteration. Overall, the most probable age of sericite crystallization corresponds to the older limits of these continuously rising spectra, i.e. ; Ma. The younger parts of the spectra may represent closure of progressively finer grained sericite during cooling of the hydrothermal systems. It is unlikely that the monotonically rising age spectra represent mixtures between an older relic muscovite component from

20 58 ( ) R.G. Skirrow et al.rore Geology ReÕiews Table 4 Summary of 40 Ar 39 Ar geochronology samples and results Sample Deposit and type, district, province Latitide 8S, Longitude 8W Rock type and texture A95RS 8a San Ignacio Au, San Ignacio, Cordoba , Quartz Ž pyrite. vein with sericite-altered 21c Puigari Au, Rıo Candelaria, Cordoba , gneissic wall rock inclusions Relict metamorphic mica Quartz Ž pyrite. vein with sericite-altered gneissic wall rock inclusions Relict? metamorphic mica 28b La Bragada Au, Rıo Candelaria, Cordoba , Sericite-altered quartzo-feldspathic gneiss; wall rock to quartz veins, now quartz sericite muscovite Relict metamorphic mica 33b 47a Las Callanas VI Au, Sierra de Las Minas, La Rioja Vallecito Au, Sierra de Las Minas, La Rioja , , Sericite-altered, quartz veined and sheared granite, now quartz sericite; late chalcedony hematite Quartz Ž pyrite. vein with sericite-altered granite wall rock inclusions; late chalcedony hematite goethite veinlets Relict igneousrmetamorphic mica 68c Rara Fortuna Ag Pb Zn, El Guaico, Cordoba , Sericite-altered fine grained quartzofeldspathic gneiss; quartz veined 79b Santa Rita W, Aguas de Ramon, Cordoba , Quartz muscovite tourmaline wolframite sulfide vein; coarse grained muscovite 101a Loma Blanca W, San Luis Quartz muscovite fluorite scheelite vein; coarse grained muscovite the host gneisses and a younger sericite component associated with hydrothermal alteration because the oldest parts of the sericite spectra are tens of millions of years younger than the total fusion age of the muscovite. The saddle-shaped spectrum for sericite from the Las Callanas VI Au vein deposit contains a short flat segment at its trough corresponding to ;40% of the released 39 Ar. The apparent ages of the minima of such saddle-like spectra are generally taken as the best estimate of the age of closure to Ar. In sample 33b, this flat segment indicates a probable minimum age of ; 390 Ma for gold-associated sericitic alteration, which is similar to the approximate age of initial closure of ; 390 Ma for the Vallecito Au deposit Ž Fig. 7e.. Therefore, the timing of hydrothermal Au"Ag"Cu mineralization in the Sierra de Las Minas region is tentatively proposed for around ;390 Ma Ag Pb Zn Õeins Sericite from alteration at the Rara Fortuna Ag Pb Zn vein deposit Ž sample 68c. exhibits a relatively flat age spectrum with the ages decreasing slightly with increasing temperature in the step heating. If the first three steps and last step are excluded, the remainder of the gas yields a mean age of 387"3 Ma. Both the total fusion and step heating 40 Ar 39 Ar ages of sericite are significantly older than the K Ar age of 340"10 Ma for sericite at Rara Fortuna reported by Sureda Ž Oxygen, hydrogen and sulfur isotope geochemistry 8.1. Methods Quartz and sulfides were extracted from samples using a dental drill and handpicking from crushed