METALLOGENY OF THE BATOPILAS NATIVE SILVER DISTRICT, CHIHUAHUA, MEXICO

The Batopilas District in SW Chihuahua produced an estimated 300 million ounces of silver from the 1500s to early 1900s. Recent exploration allows reexamination of the nature and origin of this enigmatic district. Mineralization is hosted principally in Jurassic interbedded marine siliciclastic and andesitic volcanic strata dated locally at 149 Ma (U-Pb) that appear to be part of a basin documented from northern Sonora into western Durango. The Jurassic section is overlain by subaerial andesite flows and breccias of the Late Cretaceous Tarahumara Formation. The Mesozoic sequence is cut by numerous stocks and dikes with the oldest group dated at 85 Ma (U-Pb). Eocene rhyodacite flows dated at 46 Ma (U-Pb) crop out at the base of the Tertiary section and are capped by Oligocene to Miocene ash flow tuffs of the Sierra Madre Occidental.

Numerous styles and types of mineral occurrences are present in the Batopilas District. Stratabound polymetallic sulfide concentrations occur in Jurassic strata. An E-trending system of quartz-molybdenum veins dated at 84 Ma (Re-Os) is associated with the 85-Ma stocks. The 85-Ma dike system follows a radial distribution near the center of the district, and the calcite-native silver veins commonly follow the walls of these dikes. Veins range from simple fracture-fill to complex multi-stage veins; veins are locally tectonized, with the ore zone consisting of fault gouge. Sphalerite, galena, and barite, with or without silver, are locally abundant in calcite veins, and barren calcite veins are common. Calcite-sulfide veins cut the quartz-molybdenite veins. Minor native silver veins appear to cut a 55-Ma (U-Pb) Eocene diorite stock, although the major veins appear to follow the 85-Ma dikes in the same radial pattern and end at the Eocene diorite contact.

Research is in progress to constrain the timing and mineralization models for the native silver veins, testing magmatic hydrothermal and formation water mineralization system endmembers.