Pliocene Intrusions in the Gunung Bijih (Ertsberg) Mining District, Irian Jaya, Indonesia: Major- and Trace-Element Chemistry

Abstract

A group of small, hypabyssal Pliocene intrusions are associated with Cu-Au ore deposits in the Gunung Bijih (Ertsberg) Mining District of Irian Jaya (western New Guinea), Indonesia. Several skarn orebodies (GB, GBT, Dom, and Big Gossan) are located around the margins of the Ertsberg Intrusion. All but the Big Gossan deposit are related genetically to the Ertsberg Intrusion, at least in terms of calc-silicate formation. The supergiant Grasberg porphyry copper deposit is hosted in and related to the first two stages of intrusions in the Grasberg Complex.

The intrusions are intermediate in composition, and can be divided into a high-K suite of latite to trachydacite and minor trachyte, and a low-K suite of andesite and dacite. All intrusions are enriched in large-ion lithophile elements (LILE) and have Nb and Ti depletions. The low-K suite tends to have higher La/Th and Ba/Th ratios than the high-K suite. Chemical variation in the Ertsberg Intrusion is largely the result of in situ fractional crystallization. Chemical variation in the rest of the high-K suite and in the low-K suite is the product of combined fractionation, assimilation, and recharge prior to emplacement in the shallow crust.

Magmas that formed both the low- and high-K intrusions were derived from the same lower crustal magma chamber, with the low-K intrusions representing smaller volumes of melt that were more affected by assimilation than were the high-K magmas. Differences in La/Th and Ba/Th ratios result from most low-K intrusions being emplaced prior to recharge of the lower crustal magma chamber with melt from a slightly different mantle source.