Report on Geology and Metallogeny of the "Sierras de San Luis y Comechingones" 1:250.000 Map Sheet Provinces of San Luis and Córdoba

Abstract

Geology The Sierras de San Luis y Comechingones map area covers 12 000 km2 extending from east of the Sierras de Comechingones to west of the Sierras de San Luis. The city of San Luis is located in the south west of the map area. The ranges form the southernmost part of the Sierras Pampeanas morphotectonic province of basement tilt blocks. Recent work, as part of the Geoscientific Mapping of the Sierras Pampeanas Argentine- Australian Cooperative Project, has shown that the basement is comprised of three lithostratigraphic domains, a Cambrian domain, a Cambro Ordovician domain and a younger Ordovician domain. These three domains have shared a common tectonic history since the Early Devonian. The Cambrain domain is comprised of the Monte Guazú, Conlara and Nogoli metamorphic complexes. These rocks consist of pelitic and psammitic gneiss and schist with subordinate orthogneiss. The meta-sedimentary protoliths are interpreted to have deposited on the a passive margin during the separation of Laurentia from Gondwana and the opening of the Iapetus Ocean at around 540 Ma. Deformation and upper amphibolite- to granulite-facies metamorphism of the Pampean cycle had commenced by about 530 Ma and occurred during convergence on the newly created margin of Gondwana. Widespread magmatism at the closing stages of the cycle produced granitoids within the Monte Guazú Metamorphic Complex and possibly the Nogoli Metamorphic Complex at around 515 Ma(?). The Cambro Ordovician domain consists of pelitic gneiss and schist of the Pringles Metamorphic Complex. The protoliths to these metasediments were probably deposited in a back-arc basin formed within the older basement, coincident with the commencement of subduction along the margin of Gondwana in the latest Cambrian. During the Early Ordovician widespread compressive deformation, metamorphism, and magmatism of the Famatinian cycle resulted in amalgamation of the back arc basin with the Cambrian basement. Mafic and ultramafic bodies of the Las Aguilas Group intruded the Pringles Metamorphic Complex at ~480 Ma and provided a significant heat source for high-grade metamorphism. Deformation at upper amphibolite- to granulite-facies was accompanied by the development of kilometre-scale ductile shear zones. Towards the close of the Famatinian cycle, discrete belts of extensional tectonism developed at upper greenschist-facies conditions accompanied by emplacement of S-type granite and pegmatite. New U-Pb monazite data suggests that the Famatinian tectonism had ceased and the terrain was cooling by approximately 450 Ma. The Ordovician domain comprises the San Luis Formation, which consists of phyllites, arenites and minor conglomerates. These low-grade metasediments were deposited during the Famatinian cycle and are intruded by granitoids that have been dated at about 470 Ma. The absence of Famatinian tectonic fabrics in these rocks suggests they were deposited in a basin formed during the late extensional phase. The resumption of convergence on the Gondwana margin in the Early Devonian, resulted in compressive deformation of the San Luis Formation, and older basement rocks. This important tectonic episode is termed the Achalian cycle. Partial melting of crust during this tectonism produced voluminous felsic magmas which intruded during and after shearing. The magmatism had commenced by about 404 Ma and may have continued until the Early Carboniferous. Deformation was at greenschist facies and produced westerly directed thrusting and regionally extensive ductile to brittle-ductile, conjugate, strike-slip shear-zones. New 40Ar-39Ar data shows the shearing had ceased by approximately 350 Ma. Subduction of the Nazca Plate resulted initially in Andesitic volcanism at the start of the Andean cycle in the Mid Miocene. The volcanism resulted in basement uplift with volcanic edifices and extensive pyroclastic aprons. Following cessation of volcanism in the Pliocene, continuing east-west compression has resulted in uplift of the basement along moderate to steeply dipping reverse faults. Metallogeny Three principal Paleozoic metallogenic cycles and one Neogene cycle are recognised in the southern Sierras Pampeanas, including the Sierras de San Luis and Comechingones. The first two metallogenic stages are closely related to the Early Ordovician Famatinian tectonic and magmatic cycle, and the third major period of mineralisation occurred during the Devonian Achalian cycle. The earliest metallogenic stage includes Ni-Cu-Co sulfide deposits with anomalous PGEAu hosted by mafic/ultramafic intrusions, in the Las Aguilas district. Sulfide mineralisation formed initially in magmatic cumulate zones, with partial remobilisation during deformation. New U-Pb (zircon) age dating indicates that differentiates of the tholeiitic parent magma crystallised during the early Ordovician at 478±6 Ma, approximately coeval with Famatinian high grade metamorphism and compressive deformation. A resource of 2.2 Mt at 0.51% Ni and 0.50% Cu has previously been estimated. Regional mapping and metallogenic modelling indicates potential for Ni-Cu mineralisation in several exposed and concealed zones elsewhere in the Sierras de San Luis. The second metallogenic phase is spatially and temporally related to extensional deformation of the final stages of the Famatinian cycle. Historically important deposits of Li, Be, Nb, Ta, Sn, and currently exploited industrial mineral resources (mica, feldspar, quartz), are associated with granites and voluminous pegmatites emplaced during this and the Achalian cycles. The third phase of metallogenic evolution in the southern Sierras Pampeanas is broadly correlated with the Achalian cycle, and is characterised by diverse deposits of Au, W, Ag, Pb, Zn, Cu, and a second period of pegmatite-related mineralisation including Be, Li, Nb, Ta, U, REE, Th and F. New 40Ar-39Ar dating of white mica hydrothermal alteration associated with shear-related Au±Cu, W vein and Ag-Pb-Zn vein mineralisation in the southern Sierras Pampeanas suggests mineralisation occurred from about 390 to 360 Ma, including at least some of the W mineralisation in the Sierra de Los Morillos. This metallogenic phase commenced during the period of Devonian felsic magmatism, which includes granites yielding U-Pb (zircon) crystallisation ages of about 403 to 382 Ma. Oxygen and hydrogen isotope compositions of alteration and veins minerals are compatible with input of evolved meteoric fluids with or without a minor component of magmatic or metamorphic waters in the formation of these Au±Cu, W and Ag-Pb-Zn deposits. Numerous tungsten deposits hosted in calc-silicate rocks in the Sierras del Morro, Yulto and Estanzuela are proposed to be epigenetic in origin and to have formed during the Devonian metallogenic phase. However, it is conceivable that minor tungsten accumulated in calc-silicate rocks prior to the Devonian and was remobilised during the Achalian cycle. Localisation of quartz-scheelite vein deposits and possibly some Au±W quartz vein deposits in the Sierras de San Luis in structures characteristic of the Achalian cycle suggests these deposits also formed during the Devonian metallogenic cycle. Structural studies and metallogenic modelling have been used to outline areas of potential for shear-related mesothermal Au in the Sierras de San Luis. The Neogene metallogenic cycle is characterised by Au (-Ag-Pb-Zn) and Cu mineralisation formed in association with Miocene-Pliocene volcanism in the La Carolina - Sierra del Morro volcanic belt. Alteration, geochemical, and geological characteristics of the Au mineralisation indicate metal deposition in the upper levels of low sulfidation epithermal systems, controlled in part by Tertiary and pre-Tertiary structures. Quartz-chalcopyrite vein stockworks and alteration in andesite breccia at Diente Verde suggest the existence of porphyry Cu style mineralisation in the La Carolina - Sierra del Morro volcanic belt. Despite the extent of this belt, exploration for Au-Ag and Cu in this region has been relatively limited.