GEOLOGY, MINERALOGY AND STRUCTURAL CONTROL OF DEPOSITS OF MERCURY AND ANTIMONY OF THE SIERRA GORDA OF QUERETARO, MEXICO

The deposits of mercury and antimony in the Sierra Gorda of Queretaro are known and have been exploited since pre-Columbian times, but intensively take industrial grade exploitation since the early twentieth century. Despite, its comprehensive study has not been conducted with modern standards, low metal prices have contributed to this, but in recent years (2010-2016) prices mercury and antimony (with associated gold) have shown an increase by which it has started its operation again by the residents of this area at the level of small-scale mining.

The mercury deposits extend from the southern part of the town of San Joaquín following the NW to the villages of La Plazuela, Peña Blanca and further north around Camargo, Qro., are linked with Cretaceous limestones rocks, mainly with Formation El Doctor and with the shales and sandstones of La Negra Facies of Coniaciano-Santonian age.

The mineralogy of Mercury is made up of cinnabar and calcite; Antimony is presented as stibnite and antimony oxides as cervantite and valentinite accompanied by spatic calcite. The temperature of Hg veins is low, of no more than 150 °C mesured by homogenization of fluid inclusions in calcite.

Mineralization is controlled by stratigraphy, by large regional normal faults of post-Laramide age and by its proximity to andesitic dykes and intrusives of Miocene age (K-Ar age 15.2 Ma). The mineralization has 1-2 to 10% of content of Hg or Sb, the structures are narrow veins of not more than 1 m and of 1-5 km long, vertical distance is 200 m at least known. The Sb veins at the south of Maguey Verde village are related with the limestones of El Doctor Formation and are deeper than the veins of Hg.