Mineralogical, IR-spectral and geochemical monitoring of hydrothermal alteration in a deformed and metamorphosed Jurassic VMS deposit at Arroyo Rojo, Tierra del Fuego, Argentina

Abstract

The Arroyo Rojo Zn-Pb-Cu volcanogenic massive sulfide deposit is the main deposit of the Fin del Mundo District in the Fuegian Andes, Argentina. This deposit is hosted by a Middle Jurassic volcanic and volcanoclastic sequence forming the Lemaire Formation. The latter consists, from the base up, of the following: rhyolitic and dacitic porphyritic rocks, ignimbrite, tuff, and flow. It is underlain by a pre-Jurassic basement and overlain by the hyaloclastic andesites of the Yahgán Formation. The Arroyo Rojo consists of stacked lenticular lenses that are associated with disseminated mineralization in both the footwall and the hanging wall. The internal structure of the ore lenses is marked by the occurrence of massive, semimassive and banded facies, along with stringer and brecciated zones and minor ore disseminations. The mineral assemblage comprises mainly pyrite and sphalerite, with minor amounts of galena and chalcopyrite and rare pyrrhotite, arsenopyrite, tetrahedrite and bournonite. The ores and the volcanic host-rocks have metamorphosed to greenschist facies and were overprinted by a penetrative tectonic foliation, which led to the development of mylonitic and cataclastic textures, recrystallization and remobilization. Primary depositional characteristics and regional and hydrothermal alteration patterns were preserved despite deformation and metamorphism. Therefore, primary banding was preserved between facies boundaries. In addition, some remnants of magmatic origin are recognizable in preserved phenocrysts and volcaniclastic phenoclasts. Most of the volcanic and volcaniclastic rocks of the host sequence show a rhyolitic to rhyo-dacitic composition and a subalkaline, tholeitic to intermediate geochemical magmatic signature. Regional seafloor alteration, characterized by the presence of clinozoisite, Fe-chlorite and titanite, along with quartz and albite, is partially obliterated by hydrothermal alteration. The hydrothermal alteration is stratabound with the following assemblages, which developed from the base to top: (1) Quartz-Chlorite ± Sericite, (2) Quartz-Chlorite, (3) Chlorite ± Quartz- Sericite-Calcite, (4) Quartz-Chlorite ± Calcite and (5) Sericite + Quartz ± Chlorite ± Calcite. Magnesium-chlorite and phengitic white mica typically occur in the vicinity of the Arroyo Rojo ore lenses. Mass balance calculations show a high mobility of major oxides, mainly associated with chloritic alteration, with strong enrichment in MgO, Fe2O3, Cu and S. An increase in MgO content has also been associated with the sericite alteration in conjunction with Na2O and As mass losses. To provide field criteria for exploration vectoring, the chemical composition of chlorite and the phengitic and paragonitic content of the white mica were determined and correlated with PIMA Fe-OH and Al-OH absorption wavelengths, respectively, relative to their proximity to the mineralized lenses. The results of this study can be used to help identify (1) felsic proximal facies associations, (2) ore horizons and (3) favorable hydrothermal alteration zones in other parts of the Fin del Mundo district.