Friehauf, Kurt C., Dept. of Physical Sciences, Kutztown University,
Kutztown PA 19530, Soebari, Lasito, PT Freeport Indonesia, P.O. Box
51777, New Orleans, LA 70151-1777, and
Titley, Spencer, Dept. of Geosciences, University of Arizona, Tucson AZ
85721.
Geological Society of America Abstracts with Programs, Reno, NV,
November 14-16, 2000, v. 32, no. x, p. Axx.
Newly-recognized porphyry-style mineralization within the Ertsberg diorite displays significant differences from porphyry mineralization at Grasberg. Ertsberg porphyry-style mineralization is spatially associated with 5-15 m wide E-striking dikes of porphyritic diorite that cut equigranular Ertsberg diorite. The porphyry contains phenocrysts of 1.5-3 mm plagioclase, 1-3 mm hornblende, and 1.5 mm biotite set in an aplitic groundmass. Hornblende abundance greater than biotite, the much greater abundance of sphene, a paucity of broken phenocrysts, and the aplitic groundmass distinguish the porphyry from the finer-grained groundmass Kali dikes of the Grasberg deposit.
Feldspar-stable mineralization pre-dates the Ertsberg porphyry dikes, but feldspar-destructive alteration post-dates the porphyry. Hairline bornite veinlets and zones of pervasive shreddy biotite alteration of hornblende cut equigranular diorite, but not adjacent porphyry. The margins of the porphyry dikes are commonly altered to garnet-bearing endoskarn. Bornite-anhydrite-quartz veinlets have magnetite-bearing halos where they cut endoskarn and quartz-green sericite selvages in diorite. Quartz-sericite-pyrite ± chalcopyrite veins follow a similar trend. Very late barren anhydrite veins crosscut all rock types.
Compared with rocks from the nearby Grasberg deposit, the Ertsberg porphyry deposit's coarser grained groundmass of porphyritic phases, much weaker development of hydrolytic alteration styles, absence of high sulfidation state mineralization / advanced argillic alteration, and absence of breccias suggest the Ertsberg porphyry deposit may have formed at a greater depth than the Grasberg deposit. The calcic nature of the vein endoskarn in the porphyry suggests that vertical fluid flow interacting with calcic limestones characteristic of the upper part of the section may have dominated, rather than lateral fluid flow from the nearby dolomitic carbonate wall rocks to the north where the highly magnesian forsterite-monticellite skarns of the GBT/IOZ/DOZ deposit occur.
November 14-16, 2000 Kurt C. Friehauf