Entrapment and re-equilibration of fluid inclusions in hydrothermal veins related to transtensional shear zones

MAFFINI, MARÍA NATALIA; SOSA, GRACIELA MIRIAM; VAN DEN KERKHOF, ALFONS M; CONIGLIO, JORGE ENRIQUE; DEMARTIS, MANUEL; WEMMER, KLAUS

Reykjavík

Congreso; European current research on fluid and melt inclusions (ECROFI 2021); 2021

GeoRessources and CRPG (Université de Lorraine ? CNRS) in partnership with the University of Iceland, the Università Degli Studi di Torino, and the Eötvös Loránd University of Budapest

Polymetallic quartz veins (Pb-Zn-Cu-Ag ±Au) outcrop in the southern Sierras Pampeanas de Córdoba, Argentina. They are structurally controlled by NNW-striking brittle-ductile transtensional shear zones. Two main episodes of mineralization can be recognized: a first barren episode of milky quartz (Qtz0 and Qtz1), and a second ore-rich episode of grayish quartz (Qtz2), sulfides (pyrite, galena, sphalerite, chalcopyrite and accessory tetrahedrite), Ag-Au tellurides and hematite. Illite from alteration zones reveals K/Ar ages of 329-315 Ma.The primary quartz (Qtz0) shows blue cathodoluminescence (CL), which correlates with high Al contents (up to 3000 ppm) as revealed by EPMA data. Qtz0 is altered and partly replaced by Qtz1 with brown-reddish CL (Fig. 1a) and lower Al (<60 ppm). An extreme brittle fissuring is produced by the injection of Qtz2 with bright red CL and up to 145 ppm of Al. Other trace elements (Fe, Ti and Mn) did not show correlation with CL.Primary aqueous-carbonic and aqueous fluid inclusions (FIs) were found in relict Qtz0 (Fig. 1c). They show water vol. fracs. of 0.7-0.9, salinities of 5-10% NaCl and homogenization temperatures (Th) between 250-290 °C (aqueous-carbonic) and 150-260 °C (aqueous). Qtz1 contains abundant large pseudo-secondary FIs (Fig. 1c), which comprise aqueous, aqueous-carbonic, and pure-CO2 compositions with highly variable water vol. fracs., salinities (0.2-10% NaCl) and Th (130-450 °C). The large spread of Th can be explained by re-equilibration mechanisms, evidenced by modification textures (shape distortions), necking down, partial water leakage, and precipitation of daughter minerals (muscovite). Raman analysis shows a slight increase of N2 in the modified inclusions. On the other hand aqueous inclusions in Qtz2 show relatively constant water vol. fracs. of 0.8-0.9, salinities of 2-8% NaCl and Th between 120-235 °C.Isochore calculations, combined with regional data constraints, indicate that primary FIs in Qtz0 trapped at 320-400 °C / 1.5-2 kbar, representing the fluid system before ore deposition. Pseudo-secondary FIs in Qtz1 were trapped along crackling microstructures at almost the same P-T conditions. The fluids associated with the ores (Qtz2) were more diluted and relatively cooler, and must have been trapped at 250-330 °C / 1.3-1.8 kbar.