149 Economic Geology

Tuesday, 24 October 2006
1:30 PM - 5:30 PM

SOURCES AND SPATIAL AND TEMPORAL VARIATIONS OF S, O, AND H ISOTOPES IN THE ERTSBERG STOCKWORK ZONE, ERTSBERG MINING DISTRICT, WEST PAPUA, INDONESIA

Paper number: 149-4
Program: Technical Sessions
Time: 2:15 PM – 2:30 PM

Authors

Abstract

Samples from the three main stages of veining (III, V, and VI) in the Ertsberg Stockwork Zone (ESZ) and nearby ore deposits were analyzed for their S, O, and H isotope compositions. Sulfur isotope composition of sulfides in the ESZ ranges from -3.0 to 3.6‰ (n = 94), and sulfates cover the range from 4.2 to 16.6‰ (n = 47). Isotopic values of sulfides and sulfates from all vein stages overlap considerably. Using average sulfide mineral values for the three stages, it appears that there is a slight (~1‰) increase in δ34S through time, but when fluid δ34S values are calculated, it is clear that 34S was depleted in stage VI fluids by about 1‰ with respect to earlier fluids. Sulfide-sulfate equilibrium T calculated from 41 mineral pairs range from 409 to 697 ºC, with one clear outlier at 1340 ºC. Average equilibrium T for the three vein stages are identical within error: 551, 580, and 558 ºC (+/- 30 ºC) for minerals from stage III, V, and VI veins, respectively. These temperatures overlap with fluid inclusion Th from the adjacent and Gunung Bijih Timor (GBT) skarn system, which was formed by the same igneous system; calculated T from stage III probably reflect actual equilibrium T, but it is unlikely that stages V and VI veins formed at such high T. In all three vein stages analyzed, S isotopes show a decrease of about 1.5 – 2‰ from the 3300 m level to 3600 m. This probably reflects an increase in the rate of removal of 34S from the system, possibly owing to increased anhydrite precipitation.

Silicate (mostly sericite) and sulfate (anhydrite) minerals from the ESZ were also analyzed for O isotopes, and a subset of the silicates was analyzed for H isotopes. The δ18O of anhydrite ranges from 1.8 to 11.4‰ (n = 4), and δ18O of hydrothermal silicates ranges from 6.3 to 11.6‰ (n = 12). Silicate analyses and corrected fluid compositions all fall within or very near the range of normal magmatic waters showing limited interaction with meteoric waters; except for the highest value, all sulfate samples show evidence for a meteoric component (δ18O = -1.1 – 8.3‰). The S, O, and H isotopes show that most of the fluid and sulfur (and metals?) in the ESZ were derived from the magmas that host the deposit. There is evidence for limited input from meteoric water. Nearby deposits in the district (Grasberg, GBT, and Dom) yield similar isotopic results.