Mercury Deposits in Mexico

Abstract

The mercury deposits in Mexico may be divided into two major groups of distribution, one in the Sierra Madre Occidental district, and the other in the Mesa del Norte. Most of the ore deposits are found in the opening of pre-existing fault fractures and fractured zone, sometimes with many bonanzas in the flatter portion of reversed faults or thrust faults. This indicates that the formation of the ore deposits is largely controlled by the geological structure. The scale of the ore deposits is usually large when its host rock consists of Cretaceous sedimentary rocks, and generally small when of Tertiary igneous rocks.
Most of the sedimentary rocks are weakly-silicified, or argilitized, except some limestones which are dolomitized. The igneous rocks are silicified, kaolinized, and sometimes altered to clay. In most cases alteration in the deposits is not so remarkable, generally accompanied by mineralization. This also leads to a conclusion that localization of the deposits is much more closely related with the geological structure.
The principal ore mineral is cinnabar with an eminent exception of livingstonite in Huitzuco. Native mercury and metacinnabar are also produced in small amounts, followed by mercury minerals such as guadalcazarite, montroydite, tiemannite, onofrite, terlinguaite, barcenite, egalestonite and calomel.
In the process of formation of the mercury deposits, at first, the earliest geologic activity which affected the scale and localization of the ore took place, forming the many sheared and fractured zones to be mineralized. The mineralization was performed in three different stages, namely, 1. silicate stage (opal, chalcedony), 2. sulphide stage (pyrite, marcasite, cinnabar and livingstonite), and 3. carbonate stage (calcite). In one place, another source brought about acidic hydrothermal action in the last stage of mineralization.