The Zacatecas Mining District covers an area of over 700 square kilometres in north central Mexico known for its rich epithermal and mesothermal vein deposits containing silver, gold with varying amounts of copper, lead and zinc. The dominant features responsible for the localization of precious and base metals are believed to be Tertiary age and are likely related to a volcanic caldera complex and to a set of northerly trending basin and range fault structures. The following description was obtained from Price (2008) and further condensed from Ponce and Clark (1988). “The district is located at the transition of the eastern flank of the southern Sierra Madre Occidental province and the north-western limit of the Mesa Central physiographic province. The Sierra Madre Occidental province, one of the most extensive volcanic fields of the world, is a massive pile of nearly horizontal volcanic rocks that underlies a vast plateau, composed largely of siliceous volcanic rocks of the upper volcanic series that rest discordantly either on the lower volcanic series, which are composed mainly of intermediate lavas, or on metamorphic rocks of Precambrian or Palaeozoic age and igneous or sedimentary rocks of the Mesozoic era (the Pimienta series).
Local Geology and Mineralization – San Acacio area (on the main Veta Grande) The San Acacio concession is underlain by the early Cretaceous Chilitos Formation. Rock types include a variety of different submarine andesitic volcanics with textures commonly pillowed, massive, porphyritic or brecciated in nature. Minor clastic sediments are intercalated with this dominant volcanic package.
Regional Mineralized Vein Systems – Zacatecas Area Precious and base metal veins in the Zacatecas area can be divided into three distinct vein systems depending on mineral assemblage, age and general orientation. These are briefly described below in sequence of importance, partially condensed after Price (2008). A detailed account is discussed in Ponce and Clark (1988).
- Ag-(Pb,Zn,Cu,Au) system is by far the most dominant and economically important vein type in the district. These have been exploited for years with the more important structures being the Panuco, Guadalupe, Veta Grande, San Juan, Mala Noche, San Roberto and Cantera among others. The San Acacio vein is part of the larger Veta Grande structure. The veins are hosted in the Chilitos Formation (San Acacio), Zacatecas Micro Diorite and to a lesser degree in the Pimienta Metasediments and Red Conglomerate and parts of the Guadalupe Formation rhyolitic bodies (Cantera vein). Historical grades have commonly been in excess of 1000 grams per tonne silver.
- Au-Ag (Se) system is a series of smaller veins structures trending on azimuth 0° to 340° with dips varying from vertical to 60° East and West. They include the El Pistol, Orion and Tres Vetas zones. The average length of continuous outcrop is 600 metres. They are commonly ‘fan’ shaped and open to the northwest and generally occur in radial form with respect to the caldera structure mapped to the south of Zacatecas by the Mexican Geological Survey or CRM (Vargas, 1992).
- Fluorite bearing vein system is the least developed in the Zacatecas District. They are intermittently exposed 1.5 kilometres southwest of the town of Guadalupe and may extend through talus cover for over 3 kilometres.