Precious metal mineralogy in porphyry-, skarn-, and replacement-type ore deposits of the Ertsberg (Gunung Bijih) District, Irian Jaya, Indonesia

Abstract

Petrographic and electron microprobe analyses of native Au and a variety of sulfide minerals indicate a generally high but widely varying native Au fineness and a Cu-Fe sulfide host for most Ag in the Gunung Bijih Timur complex of Cu-Au skarn deposits, the Big Gossan and Dom Cu skarn 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 quartz veins in the Big Gossan area host native Au in association with pyrite, pyrrhotite, galena, chalcopyrite, sphalerite, and arsenopyrite.Native Au from the Gunung Bijih Timur complex has a wide but bimodal range of fineness: 920 to 990 and 340 to 820 (avg = 860). Native Au from Grasberg samples is < or = 930 fine (avg = 960). Fineness of Big Gossan Au ranges from 540 to 960 (avg = 830), although compositional ranges are small within most individual samples. Native Au from Grasberg falls into two groups with respect to Pd: Pd-bearing (< or = 3,500 ppm; avg = 1,800 ppm) and with no detectable Pd. Fire assay of Grasberg ore reveals trace but detectable (10 ppb < [Pd] < 500 ppb, avg = < or = 50 ppb) concentrations of Pd 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 (< or = 1,200 ppm Cu; avg = 300 ppm).Mass-balance calculations and analytical data indicate that the bulk of the Ag in the Gunung Bijih Timur 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 throughout 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 Grasberg, the Gunung Bijih Timur complex, and the Dom orebodies.The high fineness of native Au from the Gunung Bijih district represents the upper end of the reported range for porphyry- and skarn-type Cu-Au deposits but is similar to values reported elsewhere in Indonesia. The fineness of native Au in the Gunung Bijih Timur complex does not appear to be controlled by depth or distance from the local intrusive contact, mineral assemblage, or paragenetic relationship. The fineness of native Au from the Big Gossan area shows a positive correlation with elevation, suggestive of a lower temperature, more distal character than in the other skarns of the district. Differences in Pd contents may be related 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 source. Compositional characteristics identified in this study may have applications to exploration within the Ertsberg district and elsewhere.