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

Hydrothermal solutions carrying gold and organic matter formed the veinlets which comprise the Cherry Hill hot-spring deposit. Solid bitumen and/or primary fluid inclusions containing 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 temperatures (T_h) of primary oil inclusions in general de-crease from Stage VIII to Stage XI, following the same trend as aqueous 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°C and a flame ionization detector was used. All samples are dominated by an “unresolved hump” in the carbon number (C_n) range from about C₁₅ to C₂₈. Most samples have no n-alkanes and only traces of hydrocarbons in the range C₁–C₁₄. Four samples of Stage IX bitumen (trapped during a gold-precipitating stage) and one sample of Stage I bitumen (trapped during a nongold-bearing stage) were hand-picked, dissolved in a polar solvent mixture, and both soluble and insoluble fractions were analyzed for trace metals (including Au, As, Sb, S, Fe, and V) by neutron activation. Gold contents of the soluble fraction ranged from 4 to 120 ppb but were always below the solvent blank (280 ppb). The insoluble fractions contain up to 47 ppm gold and consist of insoluble organic matter, needles of valentinite (Sb₂O₃) and iron oxide.Fig. 1.Location of the Cherry Hill gold deposit