The Fresnillo district, one of the major silver producers in the world, contains a spectrum of hydrothermal mineralization ranging from skarn and Pb-Zn replacement orebodies to epithermal veins and paleohot spring deposits. The purpose of the K-Ar age study was to elucidate the temporal relationship between the magmatic and hydrothermal events.The oldest lithologic units in the area are Mesozoic marine sediments and mafic volcanics, cut by small intrusions of granodioritic-monzonitic composition and unconformably overlain by mid-Tertiary felsic volcanics. Adjacent to the latter are some of the more important orebodies of the Fresnillo district.The K-Ar measurements demonstrate an evolving pattern of initial felsic volcanism (38.4 + or - 0.8 m.y.) followed by igneous intrusion (32-33 m.y.), hydrothermal activity (29-31 m.y.), and finally postmineralization felsic volcanism (27-28 m.y.). These data are in agreement with the sequence of events that can be deduced from the local geologic relationships. The K-Ar ages also emphasize the fundamental association between intrusive magmatism and metallization in the Fresnillo district.

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