|Porphyry-Style Mineralisation in the Ertsberg Diorite, Gunung Bijih
(Ertsberg/Grasberg) District, West Papua, Indonesia.
Kurt C Friehauf Kutztown University, Kutztown, USA, Spencer R Titley, Stacie L Gibbins University of Arizona, Tucson, USA
in - Porter, T.M. (Ed), 2005 - Super Porphyry Copper & Gold Deposits - A Global Perspective; PGC Publishing, Adelaide,
v. 2, pp 357-366.
Newly-recognised porphyry-style mineralisation within the Ertsberg intrusion displays significant differences from porphyry mineralisation at the Grasberg porphyry Cu-Au deposit. Stockwork mineralisation in the Ertsberg occurs near the giant East Ertsberg Skarn System close to the northern margin of the intrusion. Stockwork mineralisation in the diorite is spatially associated with 5 to 15 m wide east-striking dykes of porphyritic hornblende monzonite that cut equigranular Ertsberg diorite. The porphyry dykes strike parallel to major district structures and occur where those structures project into the Ertsberg intrusion. Hornblende abundance greater than biotite, the much greater abundance of sphene, a paucity of broken phenocrysts, and the aplitic groundmass distinguish the porphyry dykes in the Ertsberg Stockwork Zone from the finer-grained groundmass Kali dykes of the Grasberg deposit.
Hydrothermal alteration in the Ertsberg Stockwork Zone proceeded through three main stages with little repetition: i) early feldspar-stable alteration, ii) transitional stage characterised by green sericite veins and endoskarn development, and iii) late stage quartz-sericite-pyrite alteration. Feldspar-stable mineralisation pre-dates the Ertsberg porphyry dykes, but feldspar-destructive alteration post-dates the porphyry. Although economically-important, early, hairline bornite veinlets and zones of pervasive shreddy biotite alteration of hornblende cut equigranular diorite and not adjacent porphyry, while garnet-bearing endoskarn and sericitic alteration types both cut the porphyry dykes, suggesting the dykes intruded during mineralisation. Transitional stage bornite-chalcopyrite and chalcopyrite-pyrite bearing green sericite veinlets which are important contributors to both copper and gold grades, have magnetite-bearing halos where they cut endoskarn, and quartz-green sericite selvages in diorite. Late-stage veins contribute little to grade, contain quartz-sericite-pyrite ± chalcopyrite filling and have white sericite selvages.
Compared with rocks from the nearby Grasberg deposit, the Ertsberg porphyry mineralisation has a coarser grained groundmass in the porphyritic phases, a much weaker development of hydrolytic alteration styles, an absence of very high sulphidation state mineralisation / advanced argillic alteration, and an absence of breccias in igneous rocks, suggesting the physiochemical conditions of mineralisation for the two deposits differed significantly.
The calcic nature of the endoskarn veins, their spatial relationship with regional structures and the porphyry dykes, and their coincidence with a change in mineralisation style from feldspar-stable to feldspar-destructive hydrolytic assemblages suggests porphyry dyke and endoskarn formation correspond to a major shift in mineralising conditions in the Ertsberg Stockwork Zone.