METAL ENRICHMENTS IN HYDROTHERMAL PETROLEUM FROM SEDIMENT-COVERED SPREADING CENTERS, NORTHEAST PACIFIC OCEAN

Petroleum deposits are byproducts of magmatic and hydrothermal processes at sediment-covered oceanic spreading centers in the northeast Pacific Ocean. In these turbidite-filled rift basins, solid, liquid, and gas hydrocarbons form during hydrous pyrolysis of immature organic matter by circulating hydrothermal fluids. In Escanaba Trough (ET), located offshore northern California, small (<1 m diameter) lobate deposits of asphalt occur on the sea-floor in close proximity to polymetallic massive sulfide mounds. In the Southern Trough of Guaymas Basin (GB), Gulf of California, oil occurs in heated surface sediment adjacent to carbonate-sulfate-sulfide mounds venting high-temperature (~300°C) fluids, and in the pore space of lower-temperature, barite-rich chimneys.

High formation temperatures and low maturity levels for ET asphalt samples are indicated by their biomarker characteristics (abundant moretane, nonequilibrium isomerization of hopanes and steranes) and the dominance of nonalkylated, high-molecular-weight aromatic hydrocarbons (e.g., pyrene, phenanthrene, and fluoranthene). In contrast, the equilibrium ratios of hopane and sterane stereoisomers and dominance of alkylated aromatic compounds (e.g., naphthalenes and phenanthrenes) indicate greater maturity and somewhat lower temperatures of formation for GB oils. The distribution of n-alkanes (maximum at C₁₅) and low levels of biomarkers suggest a condensate origin for some GB oil samples.

Trace-element analyses of filtered petroleum extracts (<0.2 μm, dissolved in dichloromethane) from ET and GB by ICP-MS reveal that GB oils have higher total metal contents with Fe, Ni, Sb, V, and Zn in the range 10-150 ppm, and Ag, As, Cr, Cu, Hg, Mn, Mo, Pb, and U in the range 1-10 ppm. ET asphalts contain As, Cr, Cu, Fe, Mn, Ni, Pb, and Zn in the range 1-10 ppm. Au and Pd are present in the range 1-10 ppb in both ET and GB samples. Compared to high-temperature hydrothermal fluids discharging from mounds and chimneys in GB, the oils are considerably enriched in Fe (~10X), Cu (~25X), Zn (~10X), and Pb (~30X). The metal contents of solid and liquid hydrocarbons from ET and GB indicate that petroleum plays a potentially significant role in metal distribution within sedimentary basins influenced by hydrothermal activity.