Gold Mineralization at the Oriental Mine, Alleghany, California
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 500°C 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 350°C 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 -2°C). 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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abundant adjacent alteration zones altered rock amounts amphibolite ankerite biotite boulangerite calcite carbonate carbonatization chalcopyrite chlorite commonly composition concentrations containing country rocks Coz-rich phase crosscut daughter salt dawsonite entrapment euhedral fault feet Ferguson and Gannett filling temperatures fluid inclusions foliation fractures galena Geol gold content gold pockets grains Granite Stope graphite hangingwall hessite hydrogen gas Hydrothermal idiomorphic inclusion in quartz inferred inner alteration envelope inner zone intensely altered granite isochore level Carrolson Wein liquid CO2 located mafic mafic dikes magnetite main veins mariposite metamorphism mineralogy occurs oligoclase Oriental vein ounces of gold outer P-T conditions parallel pervasive zone pervasively altered plagioclase precipitation present primary fluid inclusions pyrite pyrite and arsenopyrite pyrrhotite quartz quartzite restricted rock types rutile samples sericite serpentinite serpentinite contact serpentinization silica ſluid Specimen sphalerite Stage suggests thick trends Type I arsenopyrite vein material vein minerals vein quartz veinlets vicinity vugs wall rocks water-rich