Geology and structural controls of the Ag–Sn–Zn Pirquitas deposit, northwestern Argentina

Abstract

Pirquitas is a polymetallic silver, tin, and zinc deposit, located in the Altiplano–Puna plateau, northwestern Argentina. The mineralization occurs in veins and hydrothermal breccia hosted in Ordovician meta–sedimentary rocks, but there is a lack of data on the structural geology of the deposit. In this paper we use detailed field mapping accompanied by structural analysis and mineralogy (petrography and XRD) to infer: (i) the mineralization structural controls, (ii) the influence of regional structures on the deposit formation, and (iii) characteristics of the Pirquitas deposit as compared with those from the Bolivian Tin–Silver Belt. The main stage of deformation observed at Pirquitas is associated with the Late Ordovician–Early Silurian Oclóyic orogeny. However, the ore deposition occurs as recurrent hydrothermal pulses related to the reactivation of Ordovician structures during the upper Miocene Quechua orogeny, which was partially contemporaneous with volcanic activity. The NW–SE and WNW–ESE ore–bearing veins of the Pirquitas deposit represent, respectively, second–order R– and T–fractures associated with major regional–scale, sinistral, brittle shear zones such as the Lípez and the Olacapato–El Toro faults. Similarities in mineralogy, structural control and association with magmatic activity indicate that Pirquitas is the southernmost deposit of the Bolivian Tin–Silver Belt. As such, subvolcanic intrusions associated with the Granada Volcano and the Coranzuli Caldera that are aligned along regional structures should be investigated by future exploratory works looking for porphyry to epithermal mineralization in the region.