Geology of the Huancavelica quicksilver district, Peru

The Huancavelica quicksilver district, the world's largest producer of quicksilver for over a century and a half, is in the Cordillera Occidental of south-central Peru. All the important mines of the district are in a north-trending belt about 1 mile (2 kilometers) wide and 5 miles (8 kilometers) long, but a few small mines and prospects are in north and south extensions of this main belt. During the Spanish colonial period the mines produced over 1,400,000 flasks of quicksilver, most of this coming from one mine, the Santa Barbara. During the last hundred years little quicksilver has been produced, and in 1946 only one small mine was actively producing. It is not likely that the district will again become an important quicksilver-mining center unless new ore bodies are discovered or unless there prove to be unexhausted ore bodies in the inaccessible caved workings of the Santa Barbara mine. The Cordillera Occidental in central Peru is composed of Paleozoic, Mesozoic, and Tertiary sedimentary and volcanic rocks that have been folded, faulted, and intruded by various kinds of igneous rocks. In the Huancavelica district Jurassic limestones, Cretaceous sandstones, limestones, shales, and volcanic rocks, and Tertiary conglomerates, tuffs, and lavas constitute the sedimentary and volcanic rocks. Intruded into these are dacites and volcanic necks filled with pyroclastic material. The dominant structural feature is a north-trending anticline, which has a synclinal core bounded by high-angle reverse faults. Faulting accompanied and followed folding, and was itself followed by igneous intrusion and extrusion. The quicksilver deposits are classified into three types: (1) deposits occurring in sandstone, (2) deposits occurring in limestone, and (3) deposits occurring in igneous rocks. Cinnabar is the principal ore mineral and occurs mainly as a filling between sand grains in the sandstone, in fractures and porous marly bedsin the limestone, and as a filling in fractures in the igneous rocks. Other sulfide minerals are pyrite, arsenopyrite, realgar, and minor amounts of galena, sphalerite, and stibnite. Nonmetallic gangue minerals include quartz, calcite, barite, and hydrocarbons, none of which are abundant. The distribution of the ore bodies was controlled by the distribution of the more permeable sedimentary strata and of fracture openings. The cinnabar deposits are younger than the Tertiary volcanic rocks