| Abstract
|
:
|
The occurrence of gold has been studied in (1) seafloor sulfides from the N.E. Pacific and Mid-Atlantic, (2) metalliferous sediments, hydrothermal plumes, and vent waters from the East Pacific Rise and Explorer Ridge, (3) sub-seafloor stockwork mineralization from DSDP Hole 504B and Agrokipia B, Cyprus and (4) supergene sulfides and Fe-oxide gossans from the mid-Atlantic Ridge. A routine method for analysis of gold and associated elements by direct neutron activation with a low-flux reactor was developed for this study. Gold contents of seafloor sulfides range from <0.1 ppm Au up to 6.7 ppm Au (average 1 ppm Au, n = 170). High gold concentrations (>1 ppm Au) occur in low-temperature (<300C) pyritic sulfides together with Fe-poor sphalerite and commonly with fine-grained, Pb-As-Sb sulfosalts. Higher-temperature precipitates typically have low gold contents, with pyrrhotite-bearing assemblages being uniformly gold-poor (usd\lequsd0.2 ppm Au). Gold-rich sulfides contain high levels of Zn, Pb, Ag, As, and Sb, but are typically depleted in Cu, Co, Se, and Mo. Local, secondary enrichment of gold up to 23 ppm Au occurs in gossans formed by submarine weathering, and similar occurrences are described from ancient ochres in Cyprus. Quenched products from high-temperature vents (up to 350C) indicate gold concentrations of about 0.1 ppb Au in end-member hydrothermal fluids. Analyses of sub-seafloor stockwork mineralization suggest that little gold is lost from ascending hydrothermal fluids in the up-flow zone. Due to the high concentrations of H2S in vent fluids arriving at the seafloor, gold tends to remain in solution as aqueous Au(HS)2\sp- complexes below about 300C. Evidence from sulfide mineral equilibria and the composition of sphalerite in modern and ancient deposits indicates a close relationship between gold mineralization and the sulfidation state of the hydrothermal fluids. The oxidation of H2S during sustained mixing with seawater leads to the precipitation of gold from Au(HS)2\sp- in the late-stage, low-temperature assemblages. Gold in high-temperature vents is lost to diffuse hydrothermal plumes and deposited at low concentration in distal metalliferous sediment.
|