Textures, fluid inclusions and stable isotopes in quartz-auriferous tensional veins of Paleozoic shear zones, Eastern Sierras Pampeanas

Abstract

In Sierras Pampeanas, conspicuous Famatinian Ordovician - Silurian shear zones crop out, with brittle to brittle - ductile deformations. In the Devonian - lower Carboniferous, these shears were reactivated by the superimposing achalian orogeny in a brittle - ductile to brittle regimen, transitional to brittle in the last evolution stage. Tensional veins synchronically developed in the shear systems, after the achalian Devonian magmatism, were filled with white1, white to transparent grey₂,grey₃ and chalcedonic₄ quartz, bearing gold, W - Fe - Bi oxides and base metal sulphides. The deposition of gold in the quartz vein systems and the precipitation of base metal sulphides (< 15 % per volume) was the most important process in stage₃; these metals arranged in chains and disseminations in the microcrystalline grey quartz. The stage₃ quartz appears in vein lets and bands, related to strain sites in the whitemilky and transparent grey quartz's, and determines mainly laminated, ribbon and stockwork textures. We emphasise in the grey₃ quartz, through macro and microstructurals, termobarometric, fluid inclusions and δ¹⁸O isotopes studies, with complementary information about massive white1 and chalcedonic₄ quartz's, and metamorphic and igneous basement rocks. The results point out the existence of double and triple phases in the fluid inclusions, with homogenisation temperatures around ~ 278 °C, lower salinities (< 5,22Na Cleq) and fluid pressures (Pint < Pconf), and hydrothermal fluid signature derived from magmatic and/or metamorphic sources. Moreover, the stage₃ is related with deformational process, neomineralization and tensional fracture filling in rheologic brittle conditions, and interaction between hydrothermal fluid and basement rocks, during the achalian orogeny final phase. Furthermore, the results show that meteoric water participated in the final stage₄ due to the Sierras Pampeanas uplift since the Devonian.