Hydrocarbons and Gold Mineralization in the Hot-Spring Deposit at Cherry Hill, California

Abstract

Hydrothermal solns. carrying gold and org. matter formed the veinlets which comprise the Cherry Hill hot-spring deposit. Solid bitumen and/or primary fluid inclusions contg. oil occur in Stages I-II and VII-XI of 12 paragenetic stages. Stages during which gold was deposited in some cases trapped bitumen and/or oil (IX) and in other cases did not (V and VI). Homogenization temps. (Th) of primary oil inclusions in general decrease from Stage VIII to Stage XI, following the same trend as aq. inclusions. As Th drops, the transmitted light color of the oils changes from orange/brown to light yellow to colorless and the UV-excited fluorescence emission changes from yellow/orange to blue, whereas the blue light-excited emission changes from weak orange to intense yellow/green. Oils were analyzed by crushing 1-7 mg samples in the inlet of a capillary column gas chromatograph. The column was programmed from -20 to 300 DegC and a flame ionization detector was used. All samples are dominated by an "unresolved hump" in the carbon no. (Cn) range C15-28. Most samples have no n-alkanes and only traces of hydrocarbons in the range C1-14. Four samples of Stage IX bitumen (trapped during a gold-pptg. stage) and one sample of Stage I bitumen (trapped during a nongold-bearing stage) were hand-picked, dissolved in a polar solvent mixt., and both sol. and insol. fractions were analyzed for trace metals (including Au, As, Sb, S, Fe, and V) by neutron activation. Gold contents of the sol. fraction ranged from 4 to 120 ppb but were always below the solvent blank (280 ppb). The insol. fractions contain up to 47 ppm gold and consist of insol. org. matter, needles of valentinite (Sb2O3) and iron oxide. Evidence for direct involvement of org. matter in gold transport and deposition is difficult to identify. The high gold concns. in the bitumen suggest a genetic connection between the orgs. and the gold. However, hydrocarbons and gold are not well correlated paragenetically. The high bitumen gold values are not simply the result of high concns. of dissolved gold in a precursor crude oil. Instead, the gold in the bitumen may be the result of encapsulation of pre-existing gold by hydrocarbons, the entrainment of fine gold particles by petroleum, or the scavenging of gold from later hydrothermal solns. The latter possibility best fit the evidence.