Report on 1:250.000 Scale Geological and Metallogenic Maps Sierras de Las Minas, Chepes and Los Llanos Provincia de La Rioja

Abstract

Geology The map area covers most of the Sierras de Chepes, de Los Llanos and de Las Minas. The oldest rocks exposed are metasediments and meta-igneous rocks of the Olta Metamorphic Complex. The metasediments are interpreted as being deposited on the passive margin of western Gondwana, developed during intracontinental rifting and break-up of Laurentia from Gondwana and opening of the Iapetus Ocean in the Early Cambrian. Based on U-Pb dating of zircons, the minimum provenance age of the metasediments is about 540 Ma, and the minimum age limit of sedimentation is about 530 Ma. Following sedimentation and minor magmatic activity, the western margin of Gondwana continent was subjected to compressive deformation (D1) and regional metamorphism (M1) of the Pampean cycle. The age of the Pampean cycle is about 530 Ma (Early Cambrian). In the early Ordovician, closure of the Iapetus Ocean and eastward subduction beneath the western margin of Gondwana was accompanied by the formation of a large continental margin magmatic arc. During this early phase of the Famatinian cycle the dominantly calc-alkaline granitoids and minor intermediate and mafic plutonic rocks of the Chepes Igneous Complex were emplaced. These rocks represent the infrastructure of the magmatic arc, and because of the high heatflow, the country rock of the Olta Metamorphic Complex was subjected to low pressure/high temperature metamorphism and migmatisation (M2) overprinting the earlier phase of regional metamorphism (M1). U-Pb dating of zircons of the granitoids of the Chepes Igneous Complex yield crystallizaton ages ranging from ~491 to ~477 Ma (early Ordovician). During the late Ordovician the Cambrain basement and continental margin magmatic arc underwent east-west, non-coaxial compressive deformation (D2) at regional greenschist-facies metamorphic conditions (M3). A weakly to strongly penetrative north to north-northwest trending foliation has affected the rocks of both the Olta Metamorphic and Chepes Igneous Complexes, and retrogressive metamorphism was widespread. In zones of high strain, up to 1 km wide, mylonitic shear zones were formed within and bounding the sierras, but also, as indicated by airborne geophysics, in the basement rocks underlying the plains. The ductile shear zones are mostly east-dipping and kinematic indicators show orthogonal, westerly directed thrust movement. 40Ar-39Ar dating of muscovite from mylonitised granite exposed in the southeast part of the Sierra de Chepes has produced an age of about 455 Ma for the shearing. Peraluminous to slightly peralkaline and zoned granite plutons of the Devonian Achalian cycle are widespread east of the project area in the Córdoba and San Luis project areas. The granite bodies were emplaced in country rock during and following east-west compressive deformation. In the map area there are no outcrops evident of felsic magmatism and deformation at this stage, but airborne magnetics suggest the presence of zoned granite bodies beneath Cainozoic sediments in the plain west of the Sierra de las Minas, and southeast of the Sierra de Los Llanos. During the later stage of the Achalian cycle, east-west compression produced a regionally widespread conjugate system of rectilinear brittle-ductile, vertical, northwest- and northeast-trending strike-slip faults and fractures. The orientation and conjugate relationship of the faults indicate a continuation of the east-west compressive tectonic regime. In the Sierra de Las Minas this fault system is particularly well developed, and is associated with sericite - pyrite ± chlorite alteration and quartz vein-type Au ± Cu mineralisation, resulting from mesothermal hydrothermal activity. 40Ar-39Ar isotopic dating of sericite associated with the hydrothermal alteration has produced a crystallization age of about 390 Ma. Following peneplanation of the Cambrian to Devonian basement, continental sediments with rare marine incursions (from the west) were deposited in the Carboniferous to Triassic Paganzo Basin, a large cratonic basin which covered the west and central areas of Argentina. In the project area remnants of late Carboniferous and Early Permian sediments of the Paganzo Group are preserved in graben structures. During the Cainozoic, the peneplained Paleozoic basement and preserved overlying sediments of the Sierras Pampeanas were deformed into north-south oriented, elongate fault blocks forming the present characteristic topography of rugged sierras separated by broad intermontane basins. The Sierras de Las Minas, de Chepes and de Los Llanos were uplifted and tilted by reverse faults which in places have reactivated Paleozoic mylonitic shear zones. Locally these sierras are traversed by graben structures parallel and transverse to the regional north-south structural grain. Metallogeny Three principal Paleozoic metallogenic cycles are recognised in the southern Sierras Pampeanas. 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. Only the latter cycle is significant in the Sierras de Las Minas and Chepes. Metallic mineral deposits broadly correlated with the Achalian (Devonian) tectonic cycle constitute 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 Achalian metallogenic cycle is represented in the Sierras de Las Minas and Chepes by mesothermal shear-related Au±Cu. New 40Ar-39Ar dating of white mica hydrothermal alteration associated with shear-related Au±Cu suggests mineralisation occurred from ~390 to ~360 Ma. This metallogenic phase commenced during the period of Devonian felsic magmatism. 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. Granite data are taken from elsewhere in the Sierras Pampeanas as no Devonian granites crop out in the sheet area, although magnetics indicate they exist beneath Cainozoic cover units. 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. Regional mapping and metallogenic modelling have defined zones of potential for Au±Cu mineralisation in the sierras de Las Minas and Chepes. The zones of Au±Cu potential are focussed on a conjugate set of northwest and northeast trending Devonian shear zones characterised by sericite-pyrite and hematite alteration. These are evident in the aeromagnetic imagery as magnetic ‘lows’, within a regional domain of demagnetisation(?) of Ordovician granitoid rocks, and extend to the west of the Sierra de Las Minas beneath Cainozoic cover. Sediment hosted uranium mineralisation in Carboniferous and/or Permian clastic sedimentary rocks in the sierras de Las Minas and Chepes region has not been investigated extensively. This mineralisation style may be part of a separate metallogenic cycle of Carboniferous or younger age. Potential areas for U have been outlined in a metallogenic model.