Gold Occurrences in the Stockwork
Mineralization in the Ertsberg Diorite, West Papua, Indonesia
Cook,
E.,
Friehauf, K.,
Gibbins, S., and Titley, S., 2003, Gold
Occurrences
in the Stockwork Mineralization in the Ertsberg Diorite,
West Papua, Indonesia [abs.]: Geological Society of America
Annual
Meeting (November 2-5, 2003) Seattle, WA, Program with Abstracts, vol.
35, no. 6, p. 232.
SEM/EDS
analysis of samples from the gold-rich (>1 ppm Au) Ertsberg porphyry
Cu-Au deposit demonstrate that important amounts gold were deposited in
the stockwork ore zone by late stage sericitic fluids in addition to
that deposited by early fluids associated with potassic alteration.
Gold occurred in all three vein types studied: 1) early quartz-bornite
± chalcopyrite ± anhydrite veins, with potassic
alteration (secondary biotite and K-feldspar), 2) quartz-bornite
± chalcopyrite veins with green sericite halos, and 3)
late-stage pyrite ± chalcopyrite veins with broad, white
sericite halos. Of the eight gold grains observed in the samples of the
early-stage quartz-biotite-biotite veins (vein type 1), four grains
were Au-Ag tellurides, one was a Au telluride, and three were electrum.
Most gold mineral grains were between 1-3 microns, but one Au-Ag
telluride grain was 20 microns. All of the electrum grains occurred on
the margins of bornite grains, whereas the tellurides were associated
with either bornite or chalcopyrite. Seven of the nine gold mineral
grains observed in the green sericite veins (vein type 2), were
electrum. Only one Au-Ag telluride grain and one native gold grain were
observed in the green sericite vein samples. Gold mineral grains in the
green sericite veins ranged in size from 2 to 6 microns. High fineness
electrum grains (Au:Ag 20:1 to 40:1) were restricted on the margins of
bornite grains associated with quartz, whereas lower-fineness electrum
grains (Au:Ag 1:10 to 4:1) occurred exclusively as subequant blebs on
and within chalcopyrite associated with orthoclase. Gold in the
late-stage pyrite ± chalcopyrite-white sericite vein (vein type
3) occurred as electrum (2 grains) and one gold telluride. Electrum
occurred with pyrite or chalcopyrite overgrowing pyrite, but the gold
telluride grain was associated with the chalcopyrite overgrowths. All
three gold mineral grains were located on the margins of sulfide
grains, ranging from 5-20 microns.
