T65 Sources of Porphyry Copper Deposits: Magmas, Metals, and Fluids I

Monday, 17 October 2005
8:00 AM - 12:00 PM

SULFUR CONTENT OF THE ERTSBERG MAGMA OF WEST PAPUA, INDONESIA AND SULFUR BUDGET OF ASSOCIATED ORE DEPOSITS, DEDUCED FROM APATITE COMPOSITIONS

Paper number: T65-10
Program: Technical Sessions
Time: 10:20 AM – 10:35 AM

Authors

Abstract

Sulfate content of zoned apatites from the Ertsberg Intrusions that host the Ertsberg Stockwork Zone of West Papua, Indonesia, ranges from below detection limit of 650 ppm to 13500 ppm. Most apatites are zoned from sulfur-rich cores to sulfur-poor rims, and many are partially replaced by silica. Notwithstanding, the sulfur content of the apatites is not affected. Apatite saturation temperatures were estimated using the silica and phosphate content of the melt, and published distribution coefficients at those temperatures (936-989 °C) were used to calculate magmatic sulfate concentration based on apatite sulfate content. The calculations indicate that main body of magma contained about 1500 ppm sulfate at near-liquidous temperatures. Anhydrite crystallization may have caused sulfur zoning in the apatites; if anhydrite crystallized, the sulfate concentration in the magma may have dropped as low as 330 ppm. From consideration of the melt sulfate concentration the sulfur budget of the hydrothermal system can be determined.

The volume of magma required to supply the sulfur contained in ore deposits associated with the Ertsberg Intrusions is calculated to be approximately 60 km3. If sulfur contained in the nearby Grasberg porphyry copper-gold system was derived from the same deep magma chamber, the volume of that magma chamber may have exceeded 200 km3. This volume of magma is comparable to estimates of magma volumes needed to supply other components (fluid, metals, etc.) to other large magmatic-hydrothermal systems.