Microscopic study of a large suite of specimens from the famous Mexican silver camp of Zacatecas shows that the prevailing order of deposition among the hypogene minerals was (1) pyrite, (2) sphalerite, (3) quartz, (4) nearly simultaneous deposition of tetrahedrite, galena, chalcopyrite and argentite, (5) sulphantimonides of silver, (6) calcite. Individual specimens show departures from this general sequence. The silver minerals of the rich ores are interpreted as in the main late hypogene. There seems to be no evidence of more than a single period of hypogene mineralization. Native silver and some argentite occur as supergene minerals in the zones of oxidation and enrichment. A little cerargyrite occurs in oxidized ores.

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