Gold in porphyry copper deposits: its abundance and fate

Abstract

Porphyry copper deposits are among the largest reservoirs of gold in the upper crust and are important potential sources for gold in lower temperature epithermal deposits. Whether gold remains in porphyry copper deposits is important both to their economic attractiveness and to the distribution of gold in the upper crust. Cu/Au atomic ratios of porphyry copper ore deposits form a continuous range from about 5000 to 5,000,000 with a median near 40,000, which separates gold-rich and gold-poor deposits. Gold is found in porphyry copper deposits in solid solution in Cu–Fe and Cu sulfides and as small grains of native gold, usually along boundaries of bornite. SIMS (ion probe) analyses of ore minerals from the gold-rich Batu Hijau, Kingking and Skouries porphyry copper deposits show that bornite contains about 1 ppm Au, whereas chalcopyrite contains about an order of magnitude less. Chalcocite and covellite contain 10–20 ppm Au, but are not abundant enough to account for a significant part of the gold endowment in many porphyry copper deposits. The amount of gold presently in solid solution in Cu–Fe sulfides is not adequate to account for all the gold in porphyry copper deposits, and the remainder is present as micron-scale grains of native gold. Experiments in the Cu–Fe–S–Au system show that bornite and chalcopyrite can contain about 1000 ppm gold at typical porphyry copper formation temperatures of 600–700°C, and indicate that bornite and chalcopyrite in porphyry copper deposits were saturated with respect to gold at temperatures of only 200–300°C. In contrast, Cu/Au ratios of bulk ore in porphyry copper deposits would require bornite and chalcopyrite to be saturated with respect to gold at temperatures similar to those at which primary (potassic) ore and alteration are thought to form. This indicates that the maximum gold endowment of porphyry copper deposits is probably fixed by the amount of gold that will go into solid solution in Cu–Fe sulfides when the deposit forms at high temperature, and that gold is not commonly added later from other sources, although it can be redistributed during cooling or later events. The experimental data also suggest that high-temperature (600–700°C) vapors can extract considerably more gold from porphyry copper systems than can low-temperature (300°C) alteration. Comparison to Cu/Au ratios of volcanic emissions suggests further that high-temperature processes remove copper (relative to gold) from porphyry copper systems, whereas low-temperature processes remove gold preferentially and that this can account for deposits with extremely low and high Cu/Au ratios, respectively. Deposits with Cu/Au ratios between about 20,000 and 100,000, however, probably reflect different degrees of removal of gold or copper by immiscible sulfides.