CHEMICAL MOBILITY OF GOLD IN THE PORPHYRY-EPITHERMAL ENVIRONMENT

Using recently published experimental data, we have calculated the sol ubility of gold for simplified magmatic fluids that cool between 500 d egrees and 300 degrees C. The starting fluid has the following charact eristics: P = 1 kbar, Sigma Cl = 2.0 m, Sigma KCl/Sigma NaCl = 0.25, p H fixed by muscovite - K feldspar + quartz, f(O2) fixed by SO2H2S, and a(H2S) fixed by magnetite + pyrite. Parallel calculations were perfor med assuming no drop in pressure during cooling (isobaric model) or an instantaneous drop in pressure to 500 bars, resulting; in separation of a dense brine and a low-salinity vapor (boiling model). The isobari c model applies to magmas emplaced at hypozonal or mesozonal depths, w hereas the boiling model is more appropriate for shallow porphyry depo sits. In the isobaric model, gold solubility is initially dominated by AuCl2- at 500 degrees C, 1 kbar. If H2S levels are high (pyrite stabl e, the dominant complete shifts to Au(HS)(2)(-) upon cooling below sim ilar to 450 degrees C, and solubilities remain elevated (>100 ppb) ove r tile entire temperature range. If H2S levels are low (magnetite stab le), gold solubility decreases steadily to 300 degrees C, with AuCl2- the dominant complex throughout. Thus, gold dissolved in H2S-rich flui ds will tend to be carried away from the parent magma, whereas gold in H2S-poor fluids will tend to precipitate closer to the source. At 500 degrees C gold solubility as AuCl2- is highest for fluids that are ox idized (SO2/H2S > 1), acidic, highly saline, and potassium rich. Gold may precipitate in response to a number of mechanisms, including cooli ng, pH increase, and dilution. Magmatic fluids that evolve from shallo w porphyry bodies are apt to boil shortly after leaving the melt, at w hich point most of the dissolved gold will partition along with chlori de into the brine phase. This metal-rich fluid, because of its high de nsity, will tend to sink or reflux; near the parent intrusion, possibl y forming an Au-rich porphyry Cu deposit. Mass balance calculations su ggest that magmatic brines will initially be undersaturated with respe ct to metallic gold, although the metal may still precipitate as Au-ri ch copper sulfide minerals (iss, bornite). During boiling, most of the H2O and H2S will partition into the coexisting vapor phase. As this v apor cools, it may recondense into a low-salinity, H2S-rich water of m ixed magmatic-meteoric heritage that has a high potential for dissolvi ng and remobilization significant quantities of gold as Au(HS)(2)(-). Migration of this fluid to shallower levels may eventually form epithe rmal deposits of low- or high-sulfidation affinity, depending on the p H-buffering capacity of the wall rocks, and the extent of direct magma tic involvement. Lack of contact with retrograde, H2S-rich magmatic-me teoric waters may be a prerequisite for the preservation of early Au-l ich porphyry sh;le mineralization and may also explain the observed as sociation between gold and hypogene iron oxide alteration in many porp hyry deposits. Whether or not there is a direct temporal link between ore-forming processes in tile porphyry and epithermal regimes, an intr usive event may be important as a means of introducing a large quantit y of low-grade gold which is then available for Later remobilization a nd concentration by circulating fluids of nonmagmatic origin. Moreover , an early porphyry event may cause widespread sulfidation of surround ing rocks. Later fluids of meteoric origin circulating through this py rite-rich wall rock will have H2S concentrations that remain elevated during cooling and ascent, increasing the chances of forming a large, high-grade epithermal gold deposit.