The Marianas Epithermal Au-Ag Vein System in the Cerro Negro District, Deseado Massif, Patagonia, Argentina

PERMUY VIDAL, C.; GUIDO, D.; GASSAWAY, B.

Integrated Exploration and Ore Deposits

SEG

Año: 2012; p. 300 - 301

The Marianas epithermal vein system is located in the Cerro Negro district, in the Deseado Massif province, Patagonia, Argentina (Fig.1). It comprises two main structures: Mariana Central with NW-SE strike and Mariana Norte with WNW-ESE strike, both hosted in the Late Jurassic Bahía Laura volcanic complex of andesitic to rhyolitic composition. Eight mineralization stages were described and grouped into four main episodes in these veins. The first episode (E1) is characterized by crustiform-colloform quartz (after chalcedony), clay minerals (mainly smectites), adularia, zeolite group minerals (mordenite), and minor truscottite. Ore minerals are present in fine bands or spots of dark, rich “ginguro” sulfides, and/or disseminated in quartz bands. This episode is represented mostly in Mariana Central and restricted in Mariana Norte. The second episode (E2) is the most voluminous in both structures and comprises massive quartz, breccias, and veinlets (fine comb quartz and amethyst variety and carbonate pseudomorphs) with minor amounts of adularia and zeolites (stilbite). The third episode (E3) appears only in the Mariana Norte vein, and it is composed of a quartz (after chalcedony and amethyst variety) colloform-crustiform banding with disseminated sulfides. E3 comprises clay-rich matrix milled breccia with previous-stage rounded clasts and locally presents dark Ag sulfosalts-rich matrix with cataclastic texture. Finally, episode 4 (E4) is widely developed on both structures and it is characterized by late calcite as veinlets/breccias or filling vugs. The mineralogy and textures described at the Marianas vein system from Cerro Negro project can be compared with the Hishikari (Japan), Waihi (New Zealand), and Sleeper (USA) deposits, evidencing a shallow nature of the veins. This is also supported by hot spring deposits in the area and the possible presence of a hydrothermal eruption breccia deposit that capped both veins.