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.