GOLD-RICH POLYMETALLIC SULFIDES FROM THE LAU BACK-ARC AND IMPLICATIONS FOR THE GEOCHEMISTRY OF GOLD IN SEA-FLOOR HYDROTHERMAL SYSTEMS OF THE SOUTHWEST PACIFIC

Polymetallic sulfides recovered from the Valu Fa Ridge in the southern Lau basin (southwest Pacific) have gold contents of up to 28.7 ppm Au (about 1 oz per ton) with an average of 3.1 ppm Au (n = 75). These sa mples are among the most gold-rich hydrothermal precipitates yet repor ted from the modern sea floor. More than 200 grains of native gold hav e been identified in two of the most gold-rich chimneys. This is the f irst documented occurrence of native gold in sea-floor sulfides. The g old-bearing assemblages are dominated (40-85%) by Fe-poor sphalerite w ith less barite, and minor chalcopyrite, bornite, tennantite, galena, pyrite, Pb-As sulfosalts, and opaline silica. High gold concentrations are associated with elevated As (1,100-6,600 ppm), Sb (85-280 ppm), a nd Pb (0.01-2.1 wt %), but the correlation between these elements and gold is imperfect. About 85 percent of the gold is found as codepositi onal inclusions in massive sphalerite; the remainder occurs along grai n boundaries or within chalcopyrite and tennantite, and rarely as incl usions in barite. Some gold occurs in late fractures within massive sp halerite. Individual gold grains reach up to 18 mum in diameter, but m ost are between 1 and 5 mum. The gold is generally of high purity, con taining less than 10 wt percent Ag. The occurrence of visible gold in sulfide chimneys from the Lau basin provides a unique opportunity to s pecify in detail the timing and conditions of gold precipitation. In o ne white smoker from the Vai Lili vent field, gold contents average 30 ppm Au in a central sphalerite-rich zone but decrease sharply to less than 0.02 ppm Au in a gold-depleted, barite-rich margin. Analyses of fluid inclusions in sphalerite and barite indicate that rapid precipit ation of gold occurred at about 230-degrees-C. Infiltration of cold se awater into the porous walls of the chimney and conductive heat loss i s responsible for cooling of the vent fluids from about 280-degrees to 300-degrees-C in the center of the chimney to about 200-degrees-C at its outer margin. Freezing measurements indicate fluid salinities clos e to that of ambient seawater (5 wt % NaCl equiv). The iron content of sphalerite across the chimney wall is uniformly low (0.3-1.5 mole % F eS) and indicates high a(s)2 conditions at the time of gold deposition . Application of solubility models suggests that gold was precipitated largely from aqueous sulfur complexes [Au(HS)2-] as a result of the c ombined effects of conductive cooling, mixing with seawater, and oxida tion of H2S. Earlier, subsea-floor boiling and precipitation of sulfid es may have played an important role in separating gold from base meta ls in the ascending hydrothermal fluids.Sulfide deposits iii the Lau b asin and elsewhere in the southwest Pacific are strikingly similar to some gold-rich massive sulfides on land and may be better analogues fo r many ancient ore-forming systems than deposits found on midocean rid ges. The evolution of back-arc rifts and the petrogenesis of associate d calc-alkaline volcanics have important implications for the composit ion of hydrothermal fluids and associated sulfide deposits. The abunda nce of Zn, Ba, Pb, As, Sb, and Au in hydrothermal precipitates collect ed from young intraoceanic and intracontinental back-arc rifts is cons istently higher than in sulfides from the midocean ridges. Possible ex planations for the enrichment of gold are discussed with reference to recognized controls on its transport and deposition in modern sea-floo r vents.