New insights into hydrothermal alteration in the Ag-Pb-Zn Chinchillas deposit, Northwestern Argentina

Abstract

Hydrothermal alteration and ore deposition at Chinchillas (northern Puna, Argentina) occurred in pyroclastic deposits that fill an explosive diatreme, as well as in monomictic breccias composed of metasedimentary lithoclasts from the basement that bound the diatreme. Highly porous and permeable rocks acted as reservoir for hydrothermal fluids responsible of alteration and mineralization. Sequential mineral associations can be grouped in three pre- and two syn-to post-mineralization stages, which are frequently superimposed in space. The first stage (adularia-rich) is widespread and develops in a context of alkaline solutions, followed by upward circulation of relatively acidic and reducing fluids (second stage) with B availability (tourmaline ± pyrite ± quartz), limited to deep sectors. A third alteration stage is typical of the argillic alteration association and is given by kaolinite replaced by later illite + illite-smectite mixed-layers + quartz ± anatase ± pyrite ± alumino-phosfate-sulfates, which indicates the progressive neutralization of fluids during continuous interaction with rocks. The crystal structure characteristics and zonation of clay minerals with depth suggest temperatures between 200° and 170 °C and a vertical thermal gradient during illitization. These stages seem to have had minor effects on the reservoir quality of the diatreme, as the Ag-Pb-Zn ore + siderite (mineralization stage) was deposited in two relatively large areas of the diatreme, and late siderite (post-mineralization stage) is ubiquitous in both mineralized and barren areas.