Abstract

Mid-Tertiary polymetallic quartz sulfide vein deposits of the San Francisco del Oro-Santa Barbara district are among the largest Pb-Zn-Cu-Ag deposits in Mexico. The veins of this district are hosted by the Cretaceous Parral shale, and since 1650 have produced over 13 billion g (440 million oz) of silver. Significant production has come from vein ores and from several vein-related massive sulfide replacement bodies.Vein formation occurred in at least four distinct stages that define an unusual paragenetic sequence when compared to similar systems. Early massive sulfide veins are cut by veins rich in calc-silicates, quartz, and late sulfides. This group is cut by two stages of postore quartz-fluorite-calcite veins with minor sulfides. Early sulfides and late calc-silicates represent a paragenetic sequence which is the reverse of that normally observed in systems containing calcsilicates and sulfides.Stage 1 veins are characterized by abundant sphalerite and galena, and minor quartz, with weak alteration halos of epidote, axinite, chlorite, quartz, and minor andradite. Stage 1 vein quartz was precipitated from 250 degrees to 320 degrees C solutions with salinities ranging from 2 to 14 equiv wt percent NaCl. Vein-related replacement bodies were also formed during this stage of hydrothermal activity. Stage 2 veins are characterized by an abundance of calc-silicate minerals within the veins, sulfides (dominantly chalcopyrite-pyrite assemblages), a small amount of gold mineralization, and a pervasive, calc-silicate-rich alteration assemblage containing manganoan hedenbergite, manganoan ilvaite, quartz, minor andradite, and hematite. Homogenization temperatures of fluid inclusions in late stage 2 quartz range from 180 degrees to 300 degrees C and salinities range from 1 to 14 equiv wt percent NaCl. Stage 3 and 4 veins are characterized by quartz, calcite, and fluorite. These veins were formed by 220 degrees to 260 degrees C fluids with salinities ranging from 1 to 3 equiv wt percent NaCl.Fluid inclusion studies reveal a highly complex ore-forming history. Ore-bearing fluids varied widely in temperature (from 180 degrees -320 degrees C) and chloride concentration (from <1-14 equiv wt % NaCl). Fluid inclusion petrographic evidence for boiling, however, is not prominent.Metal ratio studies within the Granadena mine show systematic south to north increases in the ratios of Pb/Zn (from <0.30->2.0), Ag/Pb (from <13.0->50), and Ag/Zn (from <10->70) within stage 1 veins.Dating of igneous rocks in the district indicates a major andesitic magmatic event ca 32 m.y. ago, and a rhyolitic magmatic event ca 26 m.y. ago. Geologic relations between ore deposits and volcanic rocks indicate that ore deposition predates and postdates rhyolite emplacement.The variable temperature and salinity found in fluid inclusions, the distinct mineralogic differences observed between vein stages 1 and 2, and the variable ge relations between veining and igneous activity suggest that the deposits of the San Francisco del Oro/Santa Barbara district were probably emplaced during several episodes and that these ores may represent the superposition of at least two distinct hydrothermal systems.

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