Aportes a la metalogénesis del basamento proterozoico superior-cámbrico de las Sierras Pampeanas Orientales: mineralogía, termometría e isótopos de azufre del skarn Malagueño (Cu- Fe±Zn), Sierra Chica, Córdoba, Argentina

Abstract

On-going mining operations in a marble quarry (Cantera Centro) from Malagueño, Sierra Chica de Córdoba, Argentina, have unearthed veins, veinlets and lenses of sulfides (pyrrhotite>pyrite≅
 chalcopyrite>>sphalerite). These veins and lenses are up to 0.3 m thick and 2-3 m long, although intermittently can extend about a hundred meters. They are associated with skarns. The metasedimentary host sequence, largely composed of gneisses, amphibolites and marbles, was intruded by amphibolic metagabbro and metadiorite dykes, metatonalite plutons and alkali-feldspar metagranites; the whole complex was metamorphosed into medium to high amphibolite facies and strongly deformed as a result of the a regional event M2-D2/D3 that affected the Neoproterozoic-Cambrian basement during the Pampean orogeny. Except for gneisses, all the other metamorphic lithologies register evidence of differential alteration into skarn, although the process was preferentially developed on marbles, amphibolites and metagranite, and to a lesser exten on mafic and mesosilicic dykes. The metasomatic rocks are characterized by a garnet>>pyroxene skarn (Grs27-48Adr22-34Alm15-27Sps9-21), formed after the replacement of the metagranite; a pyroxene-rich skarn (Hd42-63Di32-50Jo3-5) developed after para-amphibolite, and a garnet (Adr54-71Grs22-40Alm4-7Sps1-2) (±wollastonite) skarn that replaced a calcic marble. The dykes show poor metasomatic replacement and lack sulfides of metasomatichydrothermal origin. Retrograde mineral associations include hastingsite, ferroactinolite, epidote, clinozoisite, sericite, plagioclase (An18), chamosite and calcite. The sulfide mineralization is paragenetically associated with late-stage, infilling skarn-hydrothermal minerals that were sequentially deposited as: calcite→clinochlore→quartz→pyrite→pyrrhotite→chalcopyrite+sphalerite; these phases occur as veins and veinlets within the garnetpyroxene skarn, and as massive pyrrhotite lenses in the piroxene-rich skarn. Microthermometric data from fluid inclusions in sulfide related calcite, together with the geothermometric data from the Fe/Mg ratios in clinochlore and the phase equilibria data from intergrown high and low T ºC pyrrhotite phases, all constrain the infilling gangue phases and sulfide crystallization temperature within the ~360 °C to 250 °C range; the gradual termal decrease is in agreement with the temporal depositional sequence of the infilling phases. Fluid inclusion petrographic data and salinity estimations suggest that sulfide precipitation was triggered by boiling, from a fluid of moderate to high salinity (~14.5 to 33.5 wt% eq. NaCl). Evidence that the fluid evolved under dominantly reducing conditions are the high Fe+2/(Fe+2+Fe+3) ratios and molar proportions of subcalcic garnet (Alm+Sps) in garnet-pyroxene skarn, the presence of fluid inclusion sulfide daughter crystals in calcite, the high H2S/SO42- ratios in the sulfide-bearing fluid and the presence of pyrrhotite among the sulfide phases. Disseminated primary chalcopyrite and pyrite in metagabbro-diorite dykes (Cu ~300 ppm) and the high contents of Cu in amphibolite (~900 ppm) suggest that these protolithic lithologies were the probable sources of metals (Cu>>Zn±Ag). Values of δ34SΣfluid between ~-4 to +1.4‰ obtained from the fractionations factors of pyrite, pyrrhotite and chalcopyrite within the thermal range 350-150 ºC, indicate a magmatic source for sulfur, likely provided by leaching or desulfidation of primary sulfides of the metagabbro-diorite dykes. Metals and sulfur supplied by these dykes and amphibolite would have been redistributed in the skarn after the circulation of the metasomatic-hydrothermal fluids. Skarn bodies would have formed by infiltration of deep metasomatic fluids and fluid-rock interaction which affected lithologies of the Upper Proterozoic-Cambrian metamorphic basement. Fluids could have derived from hidden Cambrian intrusives, or from the surrounding regional migmatization, channeled along lithological contacts and faults/fractures. Field setting, textural and mineralogical evidence of sulfide mineralization in the skarn assemblage of Cantera Centro, suggest a Cambrian age associated to the post-deformational stage of the Pampean Orogeny.