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The San Dimas district is one of the major silver and gold producers in the world. The high-grade silver-gold deposits are of the low sulfidation, adularia-sericite epithermal type, formed during the final stages of igneous and hydrothermal activity of small Eocene quartz monzonitic and andesitic intrusions. The veins are hosted by tuffs, flows, and agglomerates of Eocene age, belonging to the Lower Volcanic Group, and are unconformably overlain by the younger Miocene Upper Volcanic Sequence. K-Ar age studies were done with the purpose of investigating the relationship between the magmatic and hydrothermal events within the district.

Three episodes of intrusion took place in the district. K-Ar ages of the Piaxtla intrusions suggest that the batholithic complex was emplaced about 45.1 Ma. The second event was the Intrusive andesite, giving ages ranging from 39.9 to 37.9 Ma. The last intrusion event was the Arana quartz monzonite. K-Ar ages of these rocks range from 38.1 to 36.6 Ma. Ages obtained for the vein stage of economic mineralization ranged from 39.1 to 31.9 Ma throughout the district. Alteration and mineralization occurred within a 0.3 to 3.4 Ma time span after the emplacement of the Intrusive andesite and the Arana quartz monzonite. These last two intrusive events are genetically related to ore mineralization.

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