The ore deposits of Fresnillo have produced silver for more than 450 years and contain resources of approximately 1 Goz. Mineralization in Fresnillo occurs in epithermal veins hosted by a Middle Jurassic to Late Cretaceous volcanosedimentary sequence (Guerrero terrane). Late Paleocene conglomerates and Eocene to Oligocene tuffs and flows of trachyte, rhyolite, and rhyodacite lie unconformably on top of the Guerrero terrane. Steam-heated acid sulfate alteration containing alunite veins occurs in Eocene tuffs above some of the mineralized veins. Oligocene rhyolitic domes and a quartz-monzonite stock also occur in the area, whereas sporadic Miocene basaltic flows and dikes are the youngest igneous rocks in the district.

Spatial association of Tertiary volcanic rocks, intrusions, acid sulfate alteration, and epithermal veins makes the district ideal for evaluating the temporal relations between magmatism and hydrothermal activity using precise 40Ar/39Ar geochronology. Sanidine single crystal laser fusion analyses reveal discrete volcanic eruptions at 44.73 ± 0.04, 40.92 ± 0.04, 40.83 ± 0.04, 31.55 ± 0.03, and 31.04 ± 0.05 Ma. Dating of orthoclase from the Fortuna quartz-monzonite was problematic due to excess argon contamination, but a preferred age of 32.65 ± 0.05 Ma is suggested. Two adularia samples from the Jarillas vein give statistically identical ages of 29.75 ± 0.12 and 29.68 ± 0.10 Ma. Adularia from Valdecañas vein shows a climbing age spectrum related to hydrothermal alteration; however, a terminal step with an age of 29.74 ± 0.07 Ma suggests that the two veins may be coeval. Alunite recorded two distinct ages at 30.56 ± 0.04 and 31.03 ± 0.05 Ma. Alunite and adularia ages suggest that acid sulfate alteration and mineralization occurred separately well after emplacement of the dated phase of the Fortuna stock and, consequently, it cannot be the source of heat for the hydrothermal system that formed those. Finally, the ~3 m.y. difference in age between epithermal mineralization and the Fortuna stock is indicative of a long-lived but episodic hydrothermal system which could have been driven by multiple pulses of small and shallow intrusions.

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