Abstract

The Zacatecas mining district is located in north-central Mexico, at the transition of the Sierra Madre Occidental and Mesa Central physiographic provinces. The existence of a deeply eroded caldera is evidenced by concentric and radial fracture systems, a ring fracture zone, a thick pile of ash-flow tuffs, several subvolcanic plugs, a lava-flow dome, two partially superimposed asymmetric collapse features, and a resurgent dome. The prevolcanic floor of the caldera is represented by a Late Cretaceous microdiorite laccolith which intruded metasediments of possible Triassic age, all overlain by a continental red conglomerate of Paleocene age.The volcanic center is spatially associated with economic mineralization and exerted strong structural control on the formation of the deposits. Three groups of veins are recognized, varying in ore assemblage and structural position: (1) the most important and extensive is the Ag-(Pb, Zn, Cu, Au) system, occurring in concentric and tangential faults and fractures that cut precaldera rocks; (2) the Au-Ag(Se) system, located in radial fractures that were emplaced in the caldera-fill host rocks; and (3) the F system, subparallel to the other margin of the caldera and also located in caldera-fill lithologies. Silver, the dominant precious metal, occurs mainly as acanthite, silver sulfantimonides, such as freibergite, and native silver. Free gold and electrum were identified optically, and aguilarite (Ag 2 SeS) was identified for the first time in the district. Pyrite, sphalerite, galena, and chalcopyrite are also present. Gangue minerals are mainly quartz and calcite.Preliminary fluid inclusion data on samples from the Cantera vein system, which contains precious and base metals, fall in the temperature range of 300 degrees to 160 degrees C. The early Tertiary mineralization in the district is epithermal and controlled by volcanotectonic features, modified by erosion, oxidation, and supergene enrichment.

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