Report on Geology and Metallogeny of the “Sierras de Septentrionales de Córdoba” 1:250.000 Map Sheet Province of Córdoba

Abstract

Geology The Sierras septentrionales de Córdoba sheet area is located in the northern Sierra Grande and Sierra Chica. They form part of the southern Sierras Pampeanas which are part of a distinct morphotectonic province of tilt blocks of Early to Mid Paleozoic metamorphic and igneous basement. Recent work, as part of the Argentina-Australia Cooperative Project, has shown that basement in the sheet area is part of the Pampean domain intruded by Cambrian, Ordovician, and Devonian granitoids. The domain is comprised of a number of metamorphic complexes and formations, the Ascochinga Igneous Complex, and some major shear zones, most notably the Guamanes Shear Zone. The oldest basement rocks are principally metapelite, calc-silicate and marble, and minor amphibolite which reached amphibolite grade conditions (M1; high T, low P), with localised migmatisation, about 530 Ma. Localised melting produced a number of S-type granites which probably crystallised about 10 to 15 Ma later. East of the Carape Fault, in the Sierra Chica, the Early Cambrian Ascochinga Igneous Complex, consisting of tonalite, granodiorite, granite and minor enclaves of paragneiss, probably crystallised at about 515 Ma. Protolith sediments were deposited on the passive western margin of Gondwana which developed during the separation of Laurentia from Gondwana, and the opening of the Iapetus Ocean, at the beginning of the Cambrian about 540 Ma. Following intrusion of mafic dykes, progressive deformation (D1) and metamorphism (M1) produced the penetrative differentiated foliation formed during the westerly directed thrusting of the Pampean cycle. Compressive deformation, closely followed by extension, possibly as a continuos event, (D2) and lower amphibolite/upper greenschist metamorphism (M2) during the Early Ordovician (490-470 Ma) resulted in tight folding of D1 fabrics and development of gneissic fabrics in the Ascochinga Igneous Complex. Thrusting occurred on the Carape Fault and within the El Manzano Formation. First movement on the Guamanes Shear Zone probably occurred at this time. Phyric granites within the Guamanes Shear Zone are interpreted to have been emplaced during extension, around 470 Ma. The Ordovician deformation, the Famatinian cycle, resulted from the closure of the Iapetus Ocean and eastward subduction beneath the Gondwana continent. The resumption of convergence on the Gondwana margin in the Early Devonian, during the Achalian cycle, resulted in the reactivation of the Guamanes Shear Zone and local isoclinal folding of earlier fabrics (D3), greenschist facies metamorphism (M3), and the development of a magmatic arc which generated large volumes of felsic magma from partial melting of MgO depleted crust. Devonian granites in the sheet area were emplaced at this time and magnetics indicate that a number of plutons have not been uncovered, or are buried beneath Cainozoic cover. Small bodies south of the Capilla del Monte Granite are apophyses of a larger body. The Achalian cycle continued until the end of the Devonian, as evidenced by 40Ar-39Ar dates from the Guamanes Shear Zone. Peneplanation of the basement was followed by deposition of fluvio-lacustrine sediments of the Carbo-Permian Panganzo Group which are preserved in a graben in the north-west of the sheet area. Graben formation may have initiated at this time and continued (episodically?) until the Early Cretaceous, when extension accommodated on the Punilla and La Calera Faults formed half-grabens accompanied by the deposition of continental clastic material and extrusion of mafic lavas. Block tilting of basement during the Cainozoic Andean uplift gave rise to the present day topography. Uplift occurred on moderate to steeply dipping reverse faults. Economic Geology 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 Famatinian (early Ordovician) tectonic and magmatic cycle, and the third major period of mineralisation occurred during the Achalian (Devonian) tectonic cycle. The most important metallic mineral deposits in the Sierras septentrionales de Córdoba broadly correlate with the Achalian (Devonian) tectonic cycle which constitutes the third metallogenic phase in the southern Sierras Pampeanas. It 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. The deposit styles include mesothermal shear-related Au quartz vein deposits in the Candelaria, high-level Ag-Pb-Zn quartz veins in the El Guaico district and W-quartz vein deposits near Aguas de Ramón. Epigenetic W±Cu deposits hosted mainly by calc-silicate rocks in the Punilla region of the Sierras de Cór doba (e.g., El Zinqui) are proposed to have formed during the Achalian phase. Regional mapping and metallogenic modelling have allowed delineation of zones of potential for Au, Ag- Pb-Zn, Cu and W mineralisation in the Sierras septentrionales de Córdoba, focussed on Devonian structures and granites and suitable chemical and structural trap environments. New 40Ar-39Ar dating of white mica hydrothermal alteration associated with shearrelated Au±Cu, W vein and Ag-Pb-Zn vein mineralisation in the southern Sierras Pampeanas suggests mineralisation occurred from about 390 to 360 Ma. 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. Throughout the Sierras Pampeanas, these granites are fractionated, peraluminous to borderline-metaluminous, oxidised to weakly reduced, magnetic to non-magnetic S- and I-types, and form zoned ovoid plutons that were emplaced at relatively high crustal levels synchronous with compressive deformation. Oxygen and hydrogen isotope compositions of alteration and vein minerals are compatible with input of evolved meteoric fluids with or without a minor component of magmatic or metamorphic waters in the formation of the Au±Cu, W and Ag-Pb-Zn deposits. The Neogene metallogenic cycle is characterised by epithermal Au (-Ag-Pb-Zn) and porphyry-style Cu mineralisation formed in association with Miocene-Pliocene volcanism in the La Carolina - Sierra del Morro volcanic belt of San Luis Province. Volcanic rocks of similar composition and age occur in the Volcán Pocho region, south of the Sierras septentrionales de Córdoba area. Exploration for epithermal Au- Ag in this region has been relatively limited.