Age of Epithermal Mineralization, Fresnillo District, Zacatecas, Mexico

Precise 40Ar/39Ar dating of sanidine and adularia from Fresnillo mining district provide the beginnings of a reliable temporal framework that links the magmatic and mineralization history. This important district, discovered in 1554 currently produces silver from a system of low sulphidation epithermal veins. In the last five years, four new veins have been discovered 3 km to the west and 7 km to the south, considerably expanding the size of the district. Despite some metal zoning indicated by slight variations in gold grades, these recently discovered veins are mineralogically and texturally similar to the previously described veins, consisting of crustiform banding of quartz and calcite with base metal sulfides, silver sulphosalts, electrum and gold. These similarities suggest a genetic link between old and new discoveries. A better understanding of the district as a whole is necessary and timing relationships between mineralization and magmatic activity across the district are badly needed. Based on new data, the previous K-Ar study of Lang. et. al. (1988, Economic Geology, vol.83, pp. 1680 – 1646) using whole rock samples from volcanic and intrusive rocks and vein material from advanced argillic alteration are not accurate.

New 40Ar/39Ar ages of sanidines from two tuff units from the base of the volcanic sequence and one dome, as well as two adularias within the recently discovered Jarillas vein accurately define eruption and mineralization history. Tuff deposition occurred at 44.73 +/- 0.06 Ma and 40.88 +/-0.08 Ma, with dome emplacement at 31.57 +/- 0.05 Ma whereas adularia dates indicate that mineralization occurred at 29.7 +/-0.2 Ma.