GEOCHEMISTRY AND MINERALOGY OF GOLD-RICH HYDROTHERMAL PRECIPITATES FROM THE EASTERN MANUS BASIN, PAPUA NEW GUINEA

Abstract

Three of the five hydrothermal vent fields currently known in the Manus back-arc basin of Papua New Guinea are enriched in gold. Samples of precipitates from the PACMANUS and Susu Knolls hydrothermal fields in eastern Manus Basin contain up to 56.8 ppm gold, and average 11.3 ± 13 (1� ) ppm (n = 103). Chimney samples dominated by sphalerite, barite and chalcopyrite have significantly higher contents of gold than sulfide-poor samples. Gold in the sulfide chimneys occurs as micrometric grains of silver-poor (1.4 to 4.2 wt.% Ag) native gold in three distinct associations. In copper-rich chimney samples, gold occurs as inclusions in chalcopyrite associated with tennantite, bornite and covellite. In zinc-rich chimney samples, gold is present mai nly as inclusions in tennantite associated with chalcopyrite and sphalerite. A high activity of sulfur is indicated for the gold-ri ch eastern Manus Basin precipitates by the sulfide assemblages, the low iron content of sphalerite (average = 2.3 ± 3 mole % FeS), and the silver-poor nature of gold grains. Evaluation of possible complexes of gold indicates that AuHS ° is the dominant complex, which is consistent with the high activity of sulfur in the hydrothermal system. According to available experimental data , AuHS° is the dominant complex up to 325°C. This complex thus is the only one required for gold enrichment in both the Zn–Au and the Cu–Au associations. Gold transport as the AuHS° complex is likely to be equally important in other seafloor hydrothermal systems with acidic end-member hydrothermal fluids.