CONICET | Buscador de Institutos y Recursos Humanos

Arroyo Rojo deposit is the largest polymetallic massive sulphide from the Tierra del Fuego rhyolitic belt, displaying two main mineralized lenses and high aspect ratio. The host rocks of the deposit are felsic volcanic and volcaniclastic rocks. Mineralization consists of pyrite, sphalerite and lesser amounts of galena and chalcopyrite and rare pyrrhotite and sulphosalts, and shows no vertical either lateral compositional change. Many textures of the early stages of crystallization are still preserved as a result of relatively minor subsequent deformation and metamorphism; acid etching revealed the existence of framboids and poliframboids, and also delicate colloform and radial overgrowths. Near the top of the orebody centimeter scale pyritic laminae alternate with sphalerite-(galena) rich laminae. The lamination is precleavage and it mat resulted from sedimentation and winnowing. Likewise, lack of evidence for ore deposition by replacement, presence of albite, lack of barite and Fe-oxides occurrences and no significant change in FeS content of sphalerite in passing from the massive sulphide lenses, also suggest deposition in a brine pool. Additionally, no feeder conduits or chimney fragments such as seen at Hellyer (Solomon et al., 2004), have been found. Based on our field observations, the fluid sources possibly lie outside the mineralized zone, within the so-called Lago Guanaco prospect that is stratigraphically lower than Arroyo Rojo deposit and can be interpreted as a feeder zone since it consists of quartz-sulphide stringer veins, which may channel fluids that precipitated massive sulphides. Preliminary in situ d34S values in sulphides range from slightly more positive to very negative d34S values (-27,5 to + 2,3 %0), but most of them are in the -20 to +1 per mil range. There is no vertical variation in d34S values through Arroyo Rojo deposit. Sulphur isotopic data indicate a contribution of both biologically reduced sulphur production from seawater sulphate and deep-seated sulphur what also fits well a brine pool model.