264 Economic Geology

Wednesday, 12 October 2011
1:30 PM - 5:30 PM
Room 101H-J

ISOTOPIC STRATIGRAPHY AND FLUID TRACING IN THE GUNUNG BIJIH SKARN, ERTSBERG DISTRICT, PAPUA, INDONESIA

Paper number: 264-2
Program: Technical Sessions
Time: 3:00 PM – 3:15 PM

Authors

Abstract

The Ertsberg (Gunung Bijih) Skarn is a roof pendant of variably replaced and metamorphosed Cenozoic New Guinea Limestone within the 2.6-Ma Ertsberg Intrusive Complex. Highest Cu-Au ore grades are hosted in magnetite skarn which generally includes pyroxene with chalcopyrite and bornite. Magnetite skarn is enveloped by and grades into calc-silicate assemblages. Most calc-silicate skarn is replaced carbonate strata; however, a small portion of adjacent igneous material is variably replaced by calc-silicate minerals. When present, this endoskarn is invariably adjacent to exoskarn, but contacts between unaltered igneous material and replaced strata are also observed.

A pilot strontium isotope study of four marbles adjacent to the Ertsberg Skarn indicates 87Sr/86Sr values between 0.7078 and 0.7081, allowing comparison with those of the sedimentary units exposed elsewhere in the district and with the Cenozoic seawater strontium isotope curve (87Sr/86Sr values between 0.7075 to 0.7091). These values for marbles are consistent with a limestone protolith deposited at 30 to 35 Ma. An analysis of the strontium isotope composition of a monticellite-dominant calc-silicate assemblage replacing sedimentary units (87Sr/86Sr value of 0.7071) within the pendant suggests interaction with fluids exsolved from a magmatic source. Two previous analyses determined that the Ertsberg Igneous Complex has 87Sr/86Sr values of 0.7063 and 0.7065.