METALLOGENY OF THE SIERRAS PAMPEANAS OF SAN LUIS, ARGENTINA Urbina Nilda E., Ulacco Humberto J., Sosa Graciela M., Ramos Gabriel A., Lacreu Hector L., Oggier Fabian P., Aguilera David And Guerstein Pablo G. Universidad Nacional de San Luis. Ejército de los Andes 950 (5700) San Luis. Argentina
[email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] ABSTRACT An updated review of the ore deposits of the Sierras Pampeanas of San Luis, Argentina, is presented by differentiating 5 main metallogenic epochs: (1) Late Proterozoic to Late Cambrian deformed W and Au-Sb deposits; (2) Early Ordovician to Early Silurian Cu-Ni-PGE-deposits, and Be-Li-Nb-Ta-Sn-bearing granitic pegmatites, related to the main deformational phase; (3) Middle Silurian to Late Devonian post-orogenic Wbearing, and Au-quartz deposits; (4) Early Carboniferous to Early Permian W, Pb-Zn-Cu-Ag, REE-U-Th, and fluorite deposits; and (5) Late Miocene to Early Pliocene Au-Cu-porphyry, Au-epithermal, and carbonate-epithermal deposits associated with the Andean orogeny. INTRODUCTION San Luis Province has a large and extensive mining tradition in which gold and tungsten, as well as pegmatites and calcareous onyx have been historically exploited. Numerous papers about individual ore deposits of San Luis have been published, but only recently the deposits were described from a regional perspective, e.g. Malvicini et al. (1991) described the Precambrian-Paleozoic metallogeny and Urbina et al. (1997) described Au-Cu bearing deposits related to the Mio-Pliocene volcanism. This paper presents the results of Project 348903, supported by Universidad Nacional de San Luis, Argentina. The studies were carried out to characterize the mineral deposits and develop metallogenic models for the Sierra de San Luis, which may serve as a guide for ore prospecting. GEOLOGICAL SETTING The Sierras Pampeanas are an extensive geological province of western Argentina that comprise a series of subparallel, north-trending, fault-bounded mountain ranges of Upper Proterozoic to Lower Paleozoic basement rocks. This structural style is a result from uplift on reactivated reverse faults during the Andean orogeny showing an asymmetrical topography with a steep western hillslope. Caminos (1979) divided these ranges into the Western and Eastern Sierras Pampeanas on the basis of distinct geological characteristics. The Sierra de San Luis (Fig.1) represents the southern extreme of the Eastern Sierras Pampeanas where the igneous-metamorphic basement consists of metamorphic units, granitoids, and mafic-ultramafic rocks. The metamorphic rocks include metavolcanics, slates, metaconglomerates, quartzites, and phyllites of greenschist facies, and biotite-quartzoligoclase schists of intermediate grade. Gneisses, amphibolites, and migmatites associated with mafic-ultramafic bodies represent the highest metamorphic grade recognized (Ortiz Suárez et al., 1992). The granitoids were grouped into pre-, syn-, and post-kinematic intrusions with respect to the main deformational event of Ordovician age (Ortiz Suárez et al., 1992; Ulacco, 1992; Brogioni et al., 1994; among others). The pre-orogenic granitoids (Bemberg, Las Verbenas, Tamboreo, Gasparillo, San Miguel, El Realito) are deformed tonalites and granites of Late Cambrian to Early Ordovician age. The early- to syn-orogenic intrusions (La Florida, Paso del Rey, Cruz de Caña, Río de La Carpa, Cerros Largos, La Ciénaga, La Represa, La Tapera) are granodiorites and granites of Ordovician age. Abundant pegmatites are genetically related to these granitoids.The post-orogenic intrusions (Las Chacras, Renca, La Totora, El Morro, El Telarillo, El Hornito, Los Alanices, El Peñón, Rodeo Viejo, El Salado, Las Cienaguitas, La Escalerilla) are granites and minor tonalites of Devonian to Early Carboniferous age. On the basis of geochemical features, ages, and geotectonic implications, Dalla Salda et al. (1998) and Llambías et al. (1998) consider that: (1) the pre-orogenic granitoids belong to a subduction-related magmatic arc, (2) the early- to synorogenic granitoids were emplaced in association with crustal thickening during the main orogenic phase and are similar to collisional types, (3) the post-orogenic granitoids would have collisional to within-plate characteristics and were emplaced associated with the crustal uplift. The mafic-ultramafic bodies extend along a 50-km NNEtrending belt associated with high-grade metamorphic rocks. These bodies are composed of pyroxenites and melanorites with minor melagabbronorites, leuconorites and peridotites (Malvicini and Brogioni, 1996 and references therein) that were intruded contemporaneously with regional deformation during early Ordovician time (Sims et al., 1998; von Gosen and Prozzi, 1998). In the Cretaceous, a small volume of alkaline basaltic volcanism occurred as a local response to continental break-up and the opening of the South Atlantic Ocean farther east (Llambías and Brogioni, 1981). Tertiary volcanic rocks outcrop along an 80-km WNW-ESE-trending belt from La Carolina in the west to El Morro in the east. Volcanics are hosted in the igneous-metamorphic basement and formed during a volcanism occurred between 9.5 Ma and 1.9 Ma (Sruoga et al., 1996 and references therein) becoming progressively younger towards the eastern end of the belt. Andesitic and trachytic magmas were emplaced as domes, lava flows, and volcaniclastics. The magmatism belongs to normal to high-K calc-alkaline and shoshonitic magma types and displays a K enrichment as function of time (Brogioni, 1987; Urbina et al., 1997). Located 600 km from the Peru-Chile trench and lying almost perpendicular to the main reverse faults of the western boundary of the ranges, the volcanic belt represents the easternmost and youngest magmatic manifestation associated with the shallowing of the Nazca plate in the flat-slab Andean segment extending from 28º to 33ºS (Kay et al., 1988).
REGIONAL METALLOGENY In the Sierra de San Luis (Fig.1) five preeminent metallogenic epochs may be defined: (1) Late Proterozoic to Late Cambrian; (2) Early Ordovician to Early Silurian; (3) Middle Silurian to Late Devonian; (4) Early Carboniferous to Early Permian; and (5) Late Miocene to Early Pliocene.
Fig. 1. Simplified map of the Sierra de San Luis showing the location of the main mineralized districts. LATE PROTEROZOIC TO LATE CAMBRIAN METALLOGENY Tungsten and gold-antimony deposits are located in the southern part of the Sierra de San Luis. They are hydrothermal mineralizations associated with acid volcanism in a basin of B, Mn and W-rich pelitic and psammitic sediments (Fernandez et al., 1994; Ramos et al., in press). These deposits have been remobilized during the main Ordovician orogenic phase ( Malvicini et al., 1991). Tungsten deposits are hosted by low-grade metamorphic rocks in the La Florida-Santo Domingo Belt. Three different types of deposits have been found in this geological setting: (a) disseminated porphyroblastic scheelite and scheelite-bearing quartz veins hosted by quartzite layers and metavolcanites (Brodtkorb et al., 1989; Ramos, 1992), (b) folded NE-trending discordant scheelite-bearing pegmatitic veins (Ramos, 1990), and (c) folded tourmaline-scheelite-bearing quartz veins (Ramos, 1992). The two latter types are hosted by schists. Gold and Antimony deposits are confined to scattered occurrences juxtaposed with the above W-bearing district. The deposits are Au-Sb-bearing quartz-veins hosted by acid metavolcanites and phyllites. EARLY ORDOVICIAN TO EARLY SILURIAN METALLOGENY Only two principal types of deposits have been defined in relation to the main deformational phase: Cu-Ni-PGE deposits along narrow shear zones in the southern part of the Sierra de San Luis and Be-Li-Nb-Ta-Sn-bearing granitic pegmatites in two parallel belts.
Copper-Nickel and PGE deposits (Malvicini and Brogioni, 1996 and references therein) at Virorco and Las Aguilas are present in partially metamorphosed mafic and ultramafic bodies. The ore minerals appear in the cores of both antiform and synform structures with replacement textures after silicates that grade towards the most deformed ultramafic rocks through disseminated and matrix to massive sulfides. The mineralogy consists of pyrrhotite, chalcopyrite, pentlandite, chromite, mackinawite, gold, electrum, melonite, platinum-group minerals, cubanite, cobaltite, and graphite. These deposits are interpreted to have been generated by hydrothermal activity after the main shear episode during the Ordovician age. Gervilla et al. (1997 and references therein) complete the mineralogy with sperrylite, merenskyite, michenerite, moncheite and Rh-Ir-sulphoarsenides. Be-Li-Nb-Ta-Sn-bearing granitic pegmatites occur in two parallel belts, striking NNE-SSW over more than hundred kilometers (Sosa, 1990). Nb-Ta-bearing pegmatites outcrop in the eastern belt while Sn-bearing pegmatites outcrop in the western belt. The pegmatites are concordantly emplaced in quartz-mica schist with N-S general strike, and vertical dip. Contacts between pegmatite and country rock are sharp and parallel to the metamorphic foliation. Except for local tourmaline perpendicular to the contacts, there is no evidence of metasomatism, suggesting a late emplacement of the pegmatites. The pegmatitic bodies are related to the Ordovician, early to syn-orogenic, meta to peraluminous, S-type granites (Ortíz Suarez and Sosa, 1991). The 2 granitic bodies are small (
