Hj. Mertig et al., SKARN CU-AU OREBODIES OF THE GUNUNG-BIJIH (ERTSBERG) DISTRICT, IRIAN-JAYA, INDONESIA, Journal of geochemical exploration, 50(1-3), 1994, pp. 179-202
The major Cu-Au skarn deposits of the Gunung Bijih (Ertsberg) district
in central Irian Jaya are products of hydrothermal systems that devel
oped in association with Pliocene magma emplacement in an active conti
nental margin. The Cu-Au skarn orebodies occur within a Cretaceous to
Tertiary sedimentary sequence that was deformed as the northern Austra
lian continental margin entered a north-dipping subduction zone at app
roximately 12 Ma. The intermediate-composition intrusions consist of f
ine-grained porphyritic stocks, dikes, and sills that have K-Ar ages r
anging from 2.7 to 4.4 Ma. Most intrusions are slightly potassic, but
these data could be affected by alteration. The skarn orebodies in the
Ertsberg district are hosted in deformed lower Tertiary New Guinea Gr
oup carbonate strata along the periphery of the Pliocene Ertsberg intr
usion. Major skarn orebodies include the Ertsberg (GB), the Ertsberg E
ast (GBT) complex, including the GBT, the Intermediate Ore Zone (IOZ)
and the Deep Ore Zone (DOZ), and the Dom. Chalcopyrite is the dominant
ore mineral in the GB and Dom orebodies, whereas bornite dominates in
the GBT complex. Native Au occurs within bornite and chalcopyrite in
GB and GBT ores. The district calc-silicate alteration assemblages are
characterized by high-temperature skarn minerals, including forsterit
e, monticellite, and minor melilite. Diopsidic clinopyroxene is common
, particularly in GBT. Anhydrite and phlogopite are abundant in the GB
T complex, and the anhydrite: calcite ratio increases with depth from
GBT to DOZ where anhydrite is ubiquitous and calcite rare. At least th
ree types of garnets have been identified at the Dom and show a progre
ssive increase in ferric iron content. Garnet decreases with depth in
the GBT complex. Talc, serpentine, tremolite-actinolite, and chlorite
are common retrograde minerals. Copper sulfide mineralization is textu
rally associated with early retrograde alteration. Differences among t
he skarn orebodies are related in part to variable protolith compositi
on that affected skarn development within different stratigraphic posi
tions. Distinctive fossil replacement textures preserved within skarn
indicate that the Oligocene-Miocene Ainod Formation is the most likely
protolith for the GB and Dom orebodies. The GBT and upper IOZ orebodi
es probably are hosted by the Eocene Faumai Formation. The DOZ and low
er IOZ orebodies, dominated by magnesian skarn alteration, appear to b
e developed in a dolomitic unit within the lower New Guinea Limestone
Group, which probably is equivalent to the Paleocene Waripi Formation.