Gary M. McMurtry
Evidence is increasing that the primary mechanism of metallogenesis at oceanic spreading centers is the convective cooling of basaltic intrusives and extrusives by circulating seawater. This massive circulation of seawater leaches and alters the basalt in fractures extending below the seafloor, and the mobilized metals are deposited in two major reservoirs: (1) a subsurface sulfide reservoir with reducing, acidic, high temperature (~300°C) conditions and (2) a surface hydroxide reservoir with oxidizing, alkaline, low temperature (~2°C) conditions. Whereas the subsurface reservoir corresponds to a classical hydrothermal ore deposit, the surficial metalliferous sediments found along the crest of the East Pacific Rise and in the Bauer basin resemble iron-rich later te soils both in mode of formation and major element composition. Minor and trace element enrichments originate from varied sources. Uranium and phosphorous are adsorbed from seawater, whereas copper, nickel, and barium originate from biogenic, detrital, and hydrothermal sources. Hydrothermal sources appear to dominate the iron, manganese, and zinc enrichments. Accumulation of iron and manganese vary exponentially along the East Pacific Rise crest to over 27 times the normal authigenic rate near 20°S lat., decreasing to normal authigenic accumulation near the equatorial zone of high productivity and near the Antarctic convergence. The Bauer basin appears to be an independent source area supplying metalliferous sediment to adjacent basins. The highly variable surface metallogenesis d pends more on variations in bottom-current distribution and regional sediment thickness than on variations in heat flow, spreading rates, seismicity, or basalt chemistry. The implication is that since both of the metallogenetic reservoirs depend on passive seawater-circulation processes, the degree of ore emplacement at spreading centers ultimately will depend on marine environmental factors that control crustal permeability.
AAPG Search and Discovery Article #90962©1978 AAPG 2nd Circum-Pacific Energy and Minerals Resource Conference, Honolulu, Hawaii