Pollard, Peter J., and Taylor, Roger G. (2002) Paragenesis of the Grasberg Cu-Au deposit, Irian Jaya, Indonesia: results from logging section 13. Mineralium Deposita, 37 (1). pp. 117-136.

PDF (Published Version) Restricted to Repository staff only

 

36

2

Abstract

The Grasberg Cu-Au deposit is hosted within the Grasberg Igneous Complex (GIC), a Pliocene volcanic and intrusive complex situated in the highlands of Irian Jaya, Indonesia. The GIC is composed of intrusive and volcanic rocks that were disrupted by formation of the Dalam Diatreme and intruded by later, multistaged Grasberg and Kali intrusions. Each intrusive phase is overprinted by extensive hydrothermal infill and alteration. Based on drillcore logging on section 13, 35 separate stages of alteration and infill have been recognized, and their spatial distribution mapped in 14 drillholes that represent approximately 1.8 km of vertical section. Using intrusions as timelines, the hydrothermal stages can be timed as post-Dalam-pre-MGI (Main Grasberg Intrusion), post-MGI-pre-Kali, and post-Kali, and linked into seven groups that are interpreted as separate hydrothermal systems. Pre-Kali systems include ten of the recognized stages, and are mostly high-temperature alteration (K-feldspar and/or biotite) devoid of sulfide mineralization. Sulfides are restricted to post-Kali time and, excluding early quartz-anhydrite-sulfide and molybdenite veins, can be grouped into three main stages: (1) Heavy Sulfide Zone (HSZ) mineralization, (2) Grasberg copper-gold stage, and (3) late copper mineralization (mixed copper sulfides, covellite-enargite-pyrite and pyrite-covellite-marcasite). The HSZ is dominated by fine-grained replacement pyrite and distributed mainly towards the periphery of the GIC, with only minor occurrences towards the central zones. It is suspected that a high proportion of the copper and gold content of the HSZ is due to overprinting by the Grasberg copper-gold stage and late copper mineralization.

The Grasberg copper-gold stage is a major chalcopyrite-bornite-pyrite-gold-hematite event occurring as a centrally focused fracture system that has been traced from the surface to the limits of drilling (>1,800 m). It is considered to also have a minor focus in peripheral zones, where it overprints the HSZ. The Grasberg copper-gold stage exhibits no obvious vertical or lateral changes in mineralogy. Late copper mineralization comprises several stages and is dominantly disseminated in character. The early stages are dominated by chalcopyrite, bornite, digenite-chalcocite, covellite-nukundamite, and colusite, with the later stages containing pyrite, marcasite, covellite, and enargite-minor chalcopyrite. Late copper mineralization is essentially a high sulfidation system, and is associated with zones of mild acid leaching, development of small-scale vugs, andalusite alteration, and abundant intermediate argillic alteration (illite, kaolinite). Rare pyrophyllite is reported. Hydrothermal alteration/infill stages within the GIC are controlled dominantly by oriented fracture arrays and major brittle-ductile fracture systems. These are focused around pre-existing igneous or igneous-sediment contacts, with the margins of the Kali intrusions being a prime focus of fracturing linked to the Grasberg copper-gold stage. The pattern of repetitive introduction of fluid and magma from deeper levels is compatible with the presence of an evolving magma chamber at depth. No significant quantities of sulfur-bearing minerals were precipitated until formation of the purple anhydrite-quartz veins that preceded the major sulfide stages. This suggests that the early hydrothermal fluids had temperature and oxygen fugacity characteristics that precluded precipitation of sulfur-bearing phases, and/or that a model involving late-stage addition of sulfur to a deeper level magma chamber, perhaps by intrusion of more primitive magma, may be applicable to Grasberg.

Actions (Repository Staff Only)

Item Control Page