GEOLOGY AND STRUCTURAL CONTROL OF THE HUANCAVELICA MERCURY DISTRICT, CENTRAL PERU

Huancavelica, the most important mercury district in the Western Hemisphere, produced >1.5 million flasks over 450 years of mining. The district is NS-elongate, extending 10 km from Yanamina in the south to newly discovered Spanish mines north of Huancavelica. Major through-going NS vertical faults are the main control on mineralization. The Santa Bárbara fault (SBF) hosts the Santa Bárbara ore body, the largest in the district. Underground workings mapped by Don Pedro Subila in 1795 show that the elongate ore body lies parallel to and below the surface exposure of the SBF. The 20th century Chalatacana open pit, which overlies the underground workings, exposes the SBF in the east highwall of the pit; here horizontal striations have an indeterminate sense of slip. The main host rocks are quartz arenite of the Early Cretaceous Goyllarisquizga Fm and limestone of the overlying Chulec Fm. The SBF cuts a pre-mineral sequence of conglomerate, sandstone, fresh water limestone, and flows of basalt that yielded 40Ar/39Ar isochron ages of ~35 Ma. These units are unconformably overlain by a ~23 Ma sequence of conglomerate, lacustrine sediments, tuff, and lavas that are truncated by the SBF and rotated ~35 degrees SW. The SBF was intruded by a 16.6 Ma dacite stock and by a 3.3±0.3 Ma dacite dike that postdates faulting and mineralization.

Ore forms replacement bodies in breccia and open space fillings in EW- and NE-striking fractures. Cinnabar, the principal ore mineral, occurs with native mercury, realgar, orpiment, arsenopyrite, pyrite, marcasite, barite, silica, and hydrocarbons. Arsenic is 100-400+ ppm; Au is typically <1 ppb. Altered and weakly mineralized breccia pipes appear to have formed above stocks intruded into the faults. Adularia in the Au-bearing Balcompata breccia has 40Ar/39Ar dates of 7.09±0.11 and 7.63±0.08 Ma. Although these breccia bodies do not contain mercury, they may be related to the same general hydrothermal system that formed the mercury deposits.

The SBF had a protracted slip history, with initial reverse movement overprinted by strike-slip motion. The fault has a minimum throw of 2,000 m based on a fault slice of early Miocene conglomerate north of Huancavelica. Deeper portions of the mineralized SBF have not been explored, and remain one of the more intriguing polymetallic exploration targets in Perú.