Gold Mineralization at the Oriental Mine, Alleghany, California

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University of Michigan, 1972 - Gold mines and mining - 198 pages
At the Oriental mine two productive gold-quartz veins crosscut folded quartzites, amphibolites, granite and serpentinite. Ore occurs in small high-grade shoots close to the serpentinite wall rock. Combined gold and silver contents of the ore shoots are valued between $10,000 and $100,000 per ton. The chief metallic mineral accompanying the gold is arsenopyrite. Graphite is a constant but less abundant associate of the ore. Alteration has accompanied ore deposition and can be divided into two distinct zones: an outer pervasive zone of sericitization and albitization (steatization in the serpentinite) and an inner, more restricted envelope of carbonatization and silica removal. The serpentinite in the mine contains awaruite (FeNi2) whose occurrence is probably related to highly reducing conditions accompanying serpentinization. Mass spectrometric analyses indicate the presence of H2 gas in the serpentinite. It is postulated that H2 from the serpentinite reacted with gold complexes present in the ore fluids to precipitate the gold in the vicinity of that wall rock. Reaction between this gas and the carbonatized wall rock is believed to be responsible for the formation of hydrothermal graphite. Textural studies suggest four intergradational stages of hypogene mineralization: (1) Early stage -- quartz, arsenopyrite and pyrite, (2) Middle Stage -- quartz, carbonate, base metal sulfides and sulfosalts, (3) Ore Stage -- quartz, carbonate, gold and trace tellurides, (4) Late Stage -- pyrite and calcite. Compositions of arsenopyrite formed in the Early Stage are consistent with growth temperatures between 300 and 500C and total pressures of 0 to 2000 bars. Studies of primary fluid inclusions in quartz and oligoclase, both vein minerals of the Middle and Ore Stages, indicate formation temperatures between 220 and 350C at total pressures between 800 to 2000 bars. Fluids trapped in Middle and Ore Stage vein and alteration minerals contain approximately 23% CO2. They also contain significant quantities of Na, Al, Mg, Ca and Fe, as indicated by the presence of dawsonite, ankerite and calcite as daughter salts. Freezing temperatures of these fluids are relatively high (above -2C). It is inferred from field evidence that neither the granite exposed in the mine nor any closely related intrusive could be responsible for the origin of the ore bodies. Estimates of the amounts of material (such as silica) removed from or added to various parts of the vein-alteration system allow for most of the major components to be derived from the country rocks. However, crude calculations based upon assumed original gold contents in the wall rocks are inconclusive as to the source of the gold.

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