Abstract

In the vicinity of serpentinite wall rocks, the three fissure veins at the Oriental mine, Alleghany district, California, contain small high-grade ore shoots carrying hundreds of troy ounces of gold per ton. In addition minable zones of albitized wall-rock granite ore contain an average of 0.2 ounces of gold per ton. In the veins most native gold is associated with arsenopyrite, but in the granite it occurs as microscopic inclusions within the pyrite. Various geothermometers and paragenetic relations suggest that vein gold deposition occurred during the end stages of mineralization at temperatures between approximately 200 degrees and 300 degrees C, at pressures between 670 and 2,500 bars, and at depths exceeding 2.5 km. During the early stages of mineralization, when most hydrothermal alteration and gold deposition in granite probably occurred, temperatures were above 340 degrees C and possibly as high as 491 degrees C.Muscovite, paragonite, calcite, dolomite, dawsonite, and albite fluid inclusion daughter minerals have been identified in situ by refractive index measurements and by X-ray powder diffraction. The near neutral to alkaline character of the mineralizing fluids, implied by the presence of 0.34 M dissolved dawsonite, and the low equivalent NaCl contents of fluid inclusions suggest that gold may have been complexed with sulfur or arsenic, rather than occurring as chloride complexes.Wall-rock alteration effects include envelopes of intense carbonatization and desilication, surrounding the principal gold quartz veins but also occurring along splits and fractures emanating from the main vein structures. For example, in granite the main alteration effects, dissolution of original quartz and formation of albite, occur along nearly flat-lying fractures, chiefly in the footwall of the Oriental vein. Chemical analyses suggest that desilication of granite and removal of silica during ankeritization of mafic minerals in the other rock types liberated sufficient amounts of silica to form the vein quartz at the Oriental mine. Beyond the fringes of the main alteration envelopes is an outer essentially pervasive alteration zone, characterized by sericitization and chloritization, which grades into greenschist-facies country rocks and may predate mineralization. Gold in high-grade ore shoots may have been precipitated by reactions involving reducing fluids derived from the wall-rock serpentinite near which all high-grade vein ores occur. Limited fluid inclusion data for ore shoot quartz, however, seem to be consistent with the alternative possibility of temperature controls for ore deposition.

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