Gold enrichment and the Bi-Au association in pyrrhotite-rich massive sulfide deposits, Escanaba Trough, southern Gorda Ridge

High gold contents (to 10.1 ppm, avg 1.4 ppm, n = 34) occur in pyrrhotite-rich massive sulphide samples from the sediment-covered floor of the Escanaba trough, the slow-spreading, southernmost segment of Gorda Ridge. These concentrations reflect the presence of primary gold, formed during high-T hydrothermal activity in mounds and chimneys, and secondary gold deposited during sea-floor weathering of massive sulphide. Primary gold occurs as fine-grained (<2mu m) gold, electrum, maldonite (Au (sub 2) Bi), and a rare unnamed mineral. Larger (>2/mu m) secondary gold grains have a porous, flaky morphology and occur in samples in which pyrrhotite is oxidized and replaced by Fe oxyhydroxides, Fe sulphate, and sulphur. Mounds and chimneys dominated by pyrrhotite and containing lesser amounts of isocubanite, chalcopyrite, and Fe-rich sphalerite were formed by high-T (estimated range 325-275 degrees C), reduced, low-sulphur vent fluids. The mineral and fluid compositions during this main stage of hydrothermal venting reflect subsurface interaction between circulating hydrothermal fluids and turbiditic sediment containing as much as 1.1 percent organic carbon. As the deposition of pyrrhotite, Cu-Fe sulphides, and sphalerite waned, a volumetrically minor suite of sulpharsenide, arsenide, Bi, and Au minerals was deposited from highly reduced, late main-stage fluids diffusing through mounds and chimneys. The low solubility of Au as a bisulphide complex and the absence of fluid mixing during this stage of hydrothermal activity apparently inhibited the precipitation of gold directly from solution. Instead, gold precipitation is thought to be linked to elevated concentrations of Bi in the late main-stage fluids.