Caracterización mineralógica y geoquímica de la Veta Rocío, distrito La Paloma, Macizo del Deseado, Santa Cruz, Argentina.

Abstract

The La Paloma district is located in the northeastern sector of the Deseado Massif, Patagonia, Argentina. It comprises a complex system of veins hosted in volcanic rocks of the Middle Jurassic Bajo Pobre Formation. The Sulfuro vein system consists of the Sulfuro, Esperanza, and Rocío veins, and the Princesa-Reyna vein system, both trending NNW to NW. The Duquesa vein system trends E-W, and the Arco Iris system trends NE. The El Molino fault separates the Sulfuro vein system from the other systems mentioned, which are located to the north and northwest of the district (Fernández, 2022). The district's mineral resources are primarily derived from the Sulfide Vein System, which has measured resources of 260,566 ounces of gold (Au) and 637,590 ounces of silver (Ag), with average grades of 5.55 g/t Au and 13.58 g/t Ag (Garrone, 2018). The Rocío Vein exhibits a sub-horizontal geometry of the mineralized ore shoots, with the thickest areas located in the central sector of the analyzed section. The high-grade ore shoots of the elements of economic interest (Au, Ag, Cu, Mo, Pb, Zn, and Sb/As) are most developed in the central sector of the vein. Au is present at shallow levels (120–150 m a.s.l.) in the central sector. In deep zones, the elements Ag, Cu, Mo, and Pb are present, and in shallow zones within this sector, the elements Au and Zn. In the northern sector, small, isolated subsurface spikes of Cu, Mo, Pb, and Zn are present, while in the southern sector, small spikes of Au were observed, while the remaining elements are absent or present in low concentrations. The high geochemical correlation between Ag, Sb, and Se and the identification of polybasite in pulse five could indicate selenium-rich polybasite. The presence of selenium incorporated in the silver sulfosalts could be related to deposits linked to andesitic-type volcanism.Two mineralizing pulses contribute the metals of the Rocío Vein. Pulse 3 contains Mo ± Cu (molybdenite ± chalcopyrite) that precipitated in a millimeter band at the beginning of vein filling. Pulse 5 contains Cu ± Zn ± Pb ± Fe ± Au ± Ag (coarse pyrite, chalcopyrite, sphalerite, galena, tennantite-tetrahedrite, polybasite, and gold microinclusions in pyrite). Based on its metal content, gangue mineralogy, and textures, the Rocío Vein is classified as a low-sulfidation epithermal Au-Ag deposit enriched in Zn + Pb and unusually rich in Mo. The quartz textures (coloform-crustiform banding, mosaic, and ghost sphere) indicate multiple opening and filling events, and the deposition of a silica gel would indicate cooling and silica supersaturation in the fluid. In distal zones of the vein, hydrothermal alteration minerals (chlorite, albite, epidote) could represent propylitic alteration. The phyllosilicates illite and smectite replace the previous minerals and are contemporaneous with the two mineralizing pulses in the vein.