Geology of the Bailadores, Venezuela, massive sulfide deposit

Abstract

The Bailadores zinc-lead-copper sulfide deposit, located near the town of Bailadores in the southern Venezuelan Andes, represents a potentially large base metal occurrence. Originally it was perceived as a vein deposit, but the recent recognition of associated siliceous pyroclastic rocks and the stratiform nature of the sulfide mineralization suggest that it is a volcanogenic massive sulfide.The sulfides occur in the lower part of the Upper Paleozoic Mucuchachi Formation along the contact between an underlying basal pyroclastic unit, which unconformably overlies the Precambrian basement rocks, and overlying black phyllite, which is the dominant rock type of the Mucuchachi Formation. The volcanic rocks appear to form a local pile which interfingers with the overlying phyllite along its flanks.Associated with the mineralization are a chloritic quartzite and a chlorite-biotite-margarite schist, both of which are located in the footwall. The chloritic quartzite was possibly originally a pyroclastic rock which has been altered by silicification and sericitization and is similar to chlorite alteration pipes observed, for example, with some Archean massive sulfide deposits. The schist was probably a pelitic sediment, enriched in Mg, Ca, and Fe.The massive sulfide zone consists of sphalerite, pyrrhotite, galena, chalcopyrite, and minor arsenopyrite and pyrite. Gangue minerals include quartz, chlorite, and sericite. Disseminated mineralization associated with the massive sulfide zone includes pyrrhotite-sphalerite-galena with minor chalcopyrite in siliceous phyllitic rocks, usually in the immediate hanging wall or footwall, and pyrrhotite-chalcopyrite with minor sphalerite and galena in the footwall chloritic quartzite. Typical zoning is from lead-zinc in the upper massive zone to copper in the footwall.Rocks in the vicinity of the deposit have been sheared and metamorphosed. The Mucuchachi Formation has undergone general low-grade regional metamorphism, whereas the Bailadores area has been subjected to somewhat higher grade metamorphism with the development of garnet and the formation of pyrrhotite from the pyrite which is more typical regionally.The Bailadores deposit is interpreted to be a stratiform volcanogenic massive sulfide deposit based on the following criteria: (1) Sulfide mineralization is stratigraphically controlled. (2) Alteration within the pyroclastics and stratigraphically below the massive sulfides suggests that mineralization resulted from hydrothermal activity related to late-stage volcanism. (3) Fine interbanding of sulfides and pelitic sediments adjacent to the massive sulfide zone indicates contemporaneous emplacement in a marine environment. (4) Zoning of metals from lead and zinc in the massive hanging-wall zone to copper in the disseminated footwall zone is typical of many deposits of this type.