Jn. Rubin et Jr. Kyle, PRECIOUS-METAL MINERALOGY IN PORPHYRY, SKARN-TYPE, AND REPLACEMENT-TYPE ORE-DEPOSITS OF THE ERTSBERG (GUNUNG-BIJIH) DISTRICT - IRIAN-JAYA, INDONESIA, Economic geology and the bulletin of the Society of Economic Geologists, 92(5), 1997, pp. 535-550
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.