Lateral Zonation of Gold Mineralogy in
the Grasberg Porphyry Cu-Au Deposit, Indonesia
Rock, Marlena; Mason, Kelsey; Friehauf,
Kurt, 2013, Lateral Zonation of Gold
Mineralogy in the Grasberg Porphyry Cu-Au Deposit, Indonesia:
Geological Society of America National Meeting, Denver, Colorado
(27–30 October, 2013).
Gold grades in the Grasberg
porphyry deposit (West Papua) decline sharply outward from a central
high grade zone, in contrast to a more gradual outward decrease in
Cu grades. Using an ESEM in gaseous mode, we examined uncoated,
polished thin sections of veined samples from core drilled
horizontally from the center of the deposit. We analyzed the
compositions of 109 Au-bearing grains with EDS and examined all
samples with a reflected and transmitted light petrographic
microscope.
In the central high grade zone (78-110 m from the deposit center),
Au only occurred as gold (x̅ = 8.9 μm) in bn-cpy-mt-qtz-Kfsp-bio
veins. Coexisting hypersaline fluid inclusions with multiple
daughter crystals and vapor-dominated inclusions characterized this
quartz. Gold occurred most commonly in bornite (62%), but also in
mt, qtz, Kfsp (33%), and chalcopyrite (5%).
In the intermediate zone (123-205 m), Au occurred as both gold and
electrum in bn-cpy-anhy-mt-qtz-Kfsp-bio veins. Gold (x̅ = 3.5 μm)
occurred in bornite (45%), Kfsp, anhy, qtz (36%), and chalcopyrite
(19%). Electrum accounted for 10-18% of Au-bearing grains (x̅ = 2.5
μm, 33 wt% Ag) and occurred more frequently in chalcopyrite (57%)
than bornite (43%).
In the distal zone (229 m), where pyrite and 30% sericitic
alteration of feldspars overprinted early
cpy>>bn-qtz-anhy-plag-bio veins, Au occurred as gold,
electrum, and telluride. Gold (x̅ = 4.0 μm) accounted for 95% of Au
in this zone, primarily with bornite (69%), but also in chalcopyrite
(25%), and Kfsp, anhy, qtz (6%). This sample contained one electrum
grain (1.4 μm, 26 wt% Ag) and one Au telluride (2.4 μm), both in
bornite.
Overall, 54% of gold grains occurred in bornite. Calculated relative
masses of gold and enclosing bornite ranged from 3,000-18,000 ppm.
Eight of the 45 grain pairs exceeded the experiment solubility of Au
in bornite (Simon et al., 2000), suggesting at least some
bornite-associated gold precipitated from gold-saturated solutions
rather than in a bornite solid solution.
The steep decline in Au grade probably reflects a limit of fractures
mineralized by hypersaline brines, whereas the gradual decrease in
Cu grades reflects a lateral transition to abundant
pyrite-chalcopyrite veins with sericitic alteration. Gold in the
distal, low grade zone was mineralogically similar to the high grade
zone, but veins were sparser..
Lateral Zonation of Gold Mineralogy in
the Grasberg Porphyry Cu-Au Deposit, Indonesia
Gold grades in the Grasberg
porphyry deposit (West Papua) decline sharply outward from a central
high grade zone, in contrast to a more gradual outward decrease in
Cu grades. Using an ESEM in gaseous mode, we examined uncoated,
polished thin sections of veined samples from core drilled
horizontally from the center of the deposit. We analyzed the
compositions of 109 Au-bearing grains with EDS and examined all
samples with a reflected and transmitted light petrographic
microscope.