LAUSR

Abstract Vein-type gold occurrences are found along the western margin of the Neoproterozoic Dom Feliciano Belt (Uruguay) and its Archean to Proterozoic basement. The “El Taruman” gold prospect is related to quartz veins hosted in dolomitic marble, which is part of the Cebollati Complex, metamorphosed in amphibolite facies. The quartz veins formed in fold hinges and sheared marble bands in a transpressive regime. Their orientation coincides with the Brasiliano structural grain, striking mostly NNE-SSW. Pyrite is the most important ore mineral, besides minor galena, chalcopyrite, and sphalerite, and accessory enargite, chalcocite and tennantite. The oxidation of sulfides produced free gold besides malachite. In our study, we combined fluid inclusion data with microstructures, which represent successive alteration stages after hydrothermal quartz mineralization and are made visible by cathodoluminescence (CL)-microscopy. In this way, the fluid evolution during the main mineralization and subsequent cooling could be constrained. The vein mineralization was associated with post-collisional magmatism and hydrothermal fluid circulation at ca. 500–550 °C/3 kbar, postdating the peak of metamorphism. The mineralizing fluid with an estimated composition of H2O(55) CO2(40) NaCl(5) was trapped as primary fluid inclusions in non-deformed quartz (T0) showing blue CL. Alteration structures (T1-T4) are marked by quartz showing brown and reddish CL hosting different types of fluid inclusions, and are characterized by (T1) a diffusive intragranular mosaic structure (subgrains), most likely related to minor ductile shearing, (T2) grain boundaries which formed as a result of dynamic recrystallization (secondary grain growth), (T3) shear bands of fine-grained quartz, and (T4) healed microfractures. The fluid inclusions trapped in these structures represent retrograde fluids characterized by a systematic loss of CO2 because of carbonation, slightly increasing salinity and the introduction of CaCl2. The latter solutes were mainly trapped around the quartz brittle-ductile transition zone between 350 and 300 °C/2.5–3 kbar (T1), and 300-260 °C/2–2.5 kbar (T2), together with the precipitation of pyrite. This shows that the sulfides formed by the interaction of cooling hydrothermal fluids and the host rock. The CaCl2/NaCl ratio for the inclusions in the T2-structures is ca. 0.65 at maximum. Late shear bands of fine-grained quartz (T3) are free of fluid inclusions. The latest fluids preserved in the quartz veins were trapped in healed microfractures below ca. 280 °C and represent the influx of low salinity (6–7 wt %) NaCl-bearing solutions (T4). The structural data and high fluid trapping temperatures indicate that the veins host an orogenic gold mineralization associated with magmatic hydrothermal fluids. The finding of an Ediacaran K–Ar cooling age of 592.8 ± 8.7 Ma for muscovite in the veins is consistent with regional sinistral shearing and strong basement reworking during the Neoproterozoic Brasiliano Orogeny.