Abstract

Many fresh silicic volcanic rocks have markedly lower initial gold contents than previously recognized. Of 129 carefully selected glassy silicic volcanic rocks analyzed, 113 contain <1.0 ppb Au, and many have only ≤0.1 to 0.3 ppb Au. Nonperalkaline rhyolites contain <0.1 to 0.7 ppb, with a mean of 0.22 ppb Au; of these, highly evolved, high-silica subalkalic and peraluminous rhyolites have the lowest Au contents. Peralkaline rhyolites have a mean about 1 ppb Au, showing that less polymerized melts with low f(O2) more readily accommodate gold. The mean of 23 relatively silicic intermediate rocks is 0.54 ppb Au, with tholeiitic andesites (icelandites) generally higher in gold than calc-alkalic types. There is little evidence that particular geologic regions are intrinsically richer in gold than others. Bulk composition, melt structure, and the amount and timing of separation of vapor, mineral, and sulfide and/or metal melt phases would appear to largely determine the gold contents of silicic magmas. The various lines of evidence for the removal of gold from magma by the separation of vapor, crystalline, and immiscible melt phases indicate that fresh volcanic rocks provide only minimum limits on magmatic gold concentrations. In most cases, magmatic values will be much higher. Elevated gold contents (0.3 to >1.0 ppm) of some porphyry deposits suggest the existence of salic magmas containing gold in concentrations appreciably greater than the 1-2+ ppb concentrations of basalts.

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