PRECIOUS-METAL MINERALOGY IN PORPHYRY, SKARN-TYPE, AND REPLACEMENT-TYPE ORE-DEPOSITS OF THE ERTSBERG (GUNUNG-BIJIH) DISTRICT - IRIAN-JAYA, INDONESIA

Petrographic and electron microprobe analyses of native Au and a varie ty of sulfide minerals indicate a generally high but widely varying na tive Au fineness and a Cu-Fe sulfide host for most Ag in the Gunung Bi jih Timur complex of Cu-Au skarn deposits, the Big Gossan and Dom Cu s karn deposits, and the Grasberg porphyry Cu-Au deposit. Native Au rims bornite in veins and forms inclusions in bornite and chalcopyrite in samples from Grasberg and from the Gunung Bijih Timur complex, as well as filling open space in quartz veins in the complex. Skarn and quart z veins in the Big Gossan area host native Au in association with pyri te, pyrrhotite, galena, chalcopyrite, sphalerite, and arsenopyrite. Na tive Au from the Gunung Bijih Timur complex has a wide but bimodal ran ge of fineness: 920 to 990 and 340 to 820 (avg = 860). Native Au from Grasberg samples is less than or equal to 930 fine (avg = 960). Finene ss of Big Gossan Au ranges from 540 to 960 (avg = 830), although compo sitional ranges are small within most individual samples. Native Au fr om Grasberg falls into two groups with respect to Pd: Pd-bearing (less than or equal to 3,500 ppm; avg = 1,800 ppm) and with no detectable P d. Fire assay of Grasberg ore reveals trace but detectable (10 ppb < [ Pd] < 500 ppb, avg = less than or equal to 50 ppb) concentrations of P d and no detectable Pt. Native Au from the Gunung Bijih Timur complex averages twice as much Cu as that from Grasberg (1.7 vs. 0.8 wt % Cu). Native Au from Big Gossan has uniformly low Cu contents (less than or equal to 1,200 ppm Cu; avg = 300 ppm). Mass-balance calculations and analytical data indicate that the bulk of the Ag in the Gunung Bijih T imur complex is contained by bornite, indicating a partitioning of Ag into bornite, relative to native Au or a discrete Ag mineral. This is supported by the consistent positive correlation of Cu and Ag througho ut the Gunung Bijih Timur complex, as well as in Grasberg and Dom. Ag- bearing galena occurs in the Big Gossan area, but is rare in the Grasb erg, the Gunung Bijih Timur complex, and the Dom orebodies. The high f ineness of native Au from the Gunung Bijih district represents the upp er end of the reported range for porphyry- and skarn-type Cu-Au deposi ts but is similar to values reported elsewhere in Indonesia. The finen ess of native Au in the Gunung Bijih Timur complex does not appear to be controlled by depth or distance from the local intrusive contact, m ineral assemblage, or paragenetic relationship. The fineness of native Au from the Big Gossan area shows a positive correlation with elevati on, suggestive of a lower temperature, more distal character than in t he other skarns of the district. Differences in Pd contents may be rel ated to acidity and redox conditions during native Au deposition. High Cu, high Pd, and low Ag compositions of Au grains in pan concentrates suggest a porphyry or Cu skarn source relative to a more distal sourc e. Compositional characteristics identified in this study may have app lications to exploration within the Ertsberg district and elsewhere.