J.M. Peter | USGS Science

USGS1998

The deep structure of a sea-floor hydrothermal deposit

Hydrothermal circulation at the crests of mid-ocean ridges plays an important role in transferring heat from the interior of the Earth. A consequence of this hydrothermal circulation is the formation of metallic ore bodies known as volcanic-associated massive sulphide deposits. Such deposits, preserved on land, were important sources of copper for ancient civilizations and continue to provide a significant source of base metals (for example, copper and zinc). Here we present results from Ocean Drilling Program Leg 169, which drilled through a massive sulphide deposit on the northern Juan de Fuca spreading centre and penetrated the hydrothermal feeder zone through which the metal-rich fluids reached the sea floor. We found that the style of feeder-zone mineralization changes with depth in response to changes in the pore pressure of the hydrothermal fluids and discovered a stratified zone of high-grade copper-rich replacement mineralization below the massive sulphide deposit. This copper-rich zone represents a type of mineralization not previously observed below sea-floor deposits, and may provide new targets for land-based mineral exploration.

Nature↗

USGS1992

Sulfur, carbon, and oxygen isotope variations in submarine hydrothermal deposits of Guaymas Basin, Gulf of California, USA

Sulfur, carbon, and oxygen isotope values were measured in sulfide, sulfate, and carbonate from hydrothermal chimney, spire, and mound samples in the southern trough of Guaymas Basin, Gulf of California, USA. δ 34 S values of sulfides range from −3.7 to 4.5%. and indicate that sulfur originated from several sources: 1. (1) dissolution of 0‰ sulfide contained within basaltic rocks, 2. (2) thermal reduction of seawater sulfate during sediment alteration reactions in feeder zones to give sulfide with positive δ 34 S , and 3. (3) entrainment or leaching of isotopically light (negative- δ 34 S ) bacteriogenic sulfide from sediments underlying the deposits. δ 34 S of barite and anhydrite indicate sulfur derivation mainly from unfractionated seawater sulfate, although some samples show evidence of sulfate reduction and sulfide oxidation reactions during mixing within chimneys. Oxygen isotope temperatures calculated for chimney calcites are in reasonable agreement with measured vent fluid temperatures and fluid inclusion trapping temperatures. Hydrothermal fluids that formed calcite-rich chimneys in the southern trough of Guaymas Basin were enriched in 18 O with respect to seawater by about 2.4‰ due to isotopic exchange with sedimentary and/or basaltic rocks. Carbon isotope values of calcite range from −9.6 to −14.0‰ δ 34 C pDB , indicating that carbon was derived in approximately equal quantities from the dissolution of marine carbonate minerals and the oxidation of organic matter during migration of hydrothermal fluid through the underlying sediment column. Statistically significant positive, linear correlations of δ 34 S , δ 34 C , and δ 18 O of sulfides and calcites with geographic location within the southern trough of Guaymas Basin are best explained by variations in water/rock ( w r "> wr ) ratios or sediment reactivity within subsurface alteration zones. Low w r "> wr ratios and the leaching of detrital carbonates and bacteriogenic sulfides at the southern vent sites result in relatively high δ 13 C and low δ 34 S in chimney carbonates and sulfides, respectively. In the north, where the depletion of alkalis in vent fluids indicates higher w r "> wr ratios, positive δ 34 S and more negative δ 13 c are due to increased contributions from organic matter oxidation and sulfate reduction reactions.

Geochimica et Cosmochimica Acta↗