Summary:
The Pirquitas deposit is an example of the Ag Sn sub-group of the epithermal class of mineral deposits (Panteleyev, 1996). Also known as Bolivian-type polymetallic deposits, examples of this deposit type are numerous in the Bolivian Silver -Tin Belt that extends between the San Rafael Sn(-Cu) deposit in southern Peru and the Pirquitas deposit in northwestern Argentina.
Bolivian-type Ag-Sn deposits generally consist of sulphide and quartz-sulphide vein systems typically containing cassiterite and a diverse suite of base and trace metals, including Ag in a complex assemblage of sulphide and sulfosalt minerals. The vein systems are generally spatially and likely genetically associated with epizonal (subvolcanic) quartz-bearing peraluminous intrusions one to two kilometres in diameter, although the mineralization may be entirely hosted by the country rocks into which the intrusive stocks were emplaced.
The suite of principal and subordinate ore minerals that characterize this deposit type includes, but is not limited to, pyrite, cassiterite, pyrrhotite, marcasite;,sphalerite, galena, chalcopyrite, stannite, arsenopyrite, tetrahedrite, scheelite, wolframite, andorite, jamesonite, boulangerite, ruby silver (pyrargyrite), stibnite, bismuthinite, native bismuth, molybdenite, argentite, gold and a variety of complex sulfosalt minerals. Metal zoning from depth to surface and from centers outward shows: Sn + W, Cu + Zn, Pb + Zn, Pb + Ag, and Ag ± Au; commonly there is considerable ‘telescoping’ of zones. Oxidized zones may have secondary silver minerals such as Ag halides.
Quartz and sericite are the main gangue minerals, with tourmaline appearing at deeper levels and kaolinite + chalcedony commonly present close to surface. Quartz-sericite pyrite alteration is characteristic of these deposits; it is pervasively developed in certain lithologies and restricted to narrow vein halos in rocks that are less susceptible to reactions with the hydrothermal fluids.
Sn-Ag vein deposits are believed to be the source of cassiterite that has been mined from placer deposits around the world, and the lodes themselves are extensively mined in South America, particularly Bolivia. At present, the San Rafael deposit in southern Peru is the world’s largest and richest underground tin mine, while the Pirquitas deposit is Argentina’s largest silver mine.
The fracture and breccia-hosted mineralization at the Pirquitas Mine consists of Fe and Zn sulphides with accessory cassiterite (Sn oxide) and a large variety of Ag-Sn-Zn (-Pb-Sb-As-Cu Bi) sulphides and sulfosalts. Crystalline quartz, along with chalcedony in the upper levels of the system, and kaolinite are the main gangue minerals in the veins and mineralized breccias. The main sulphides, specifically pyrite, pyrrhotite, sphalerite and wurtzite, form colloform bands parallel to vein margins, which together with crustiform and drusy vein textures suggest that the mineralization is epithermal in origin.