G. Simon et al., Gold in porphyry copper deposits: Experimental determination of the distribution of gold in the Cu-Fe-S system at 400 degrees to 700 degrees C, ECON GEOL B, 95(2), 2000, pp. 259-270
Citations number
33
Categorie Soggetti
Earth Sciences
Journal title
ECONOMIC GEOLOGY AND THE BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS
Experiments in the system Au-Cu-Fe-S were carried out at temperatures of 40
0 degrees to 700 degrees C to determine how much gold could be accommodated
by bornite and chalcopyrite, the two most common ore minerals in porphyry
copper-gold deposits. Our results show that for all temperatures bornite co
ntains one order of magnitude more gold than chalcopyrite (or intermediate
solid solution (iss), its high-temperature equivalent). The range of gold c
oncentrations in bornite and chalcopyrite (or iss) decreases with decreasin
g temperature from 1,280 to 8,200 ppm Au in bornite and 100 to 125 ppm Au i
n iss at 600 degrees C, to 235 to 364 ppm Au in bornite and 5 to 16 ppm Au
in chalcopyrite (or iss) at 500 degrees C, and to 13 to 80 ppm Au in bornit
e and 2 to 4 ppm Au in chalcopyrite (or iss) at 400 degrees C. The amount o
f gold in bornite is also strongly dependent on the composition of bornite,
being highest in "stoichiometric" bornite compositions (Cu5FeS4),and decre
asing toward Cu-rich and Cu-poor compositions.
Phase equilibrium constraints for solutions with geologically reasonable re
duced sulfur contents indicate that high-temperature porphyry copper-gold d
eposits will contain bornite and magnetite, whereas lower temperature depos
its (whether primary or overprinted by phyllic alteration) will contain cha
lcopyrite and pyrite. If gold is present in the ore-forming solutions, more
of it will be deposited in high-temperature porphyry copper-gold deposits
where it will be closely associated with bornite. Coexisting magnetite in t
hese deposits should generate magnetic anomalies. Lower temperature deposit
s will contain less gold, which is hosted by pyrite as well as chalcopyrite
, and will lack magnetic anomalies. Comparison of the amount of gold hosted
by natural porphyry copper-gold ores to that hosted by bornite and chalcop
yrite in our experiments suggests that significant amounts of gold can be l
ost from these deposits into surrounding hydrothermal systems.