Chemical mobility of gold in the porphyry-epithermal environment

Abstract

Using recently published experimental data, we have calculated the solubility of gold for simplified magmatic fluids that cool between 500 degrees and 300 degrees C. The starting fluid has the following characteristics: P = 1 kbar, Sigma Cl = 2.0 m, Sigma KCl/Sigma NaCl = 0.25, pH fixed by muscovite + K feldspar + quartz, f (sub O 2 ) fixed by SO 2 /H 2 S, and a (sub H 2 S) fixed by magnetite + pyrite. Parallel calculations were performed asstuning 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 isobaric model applies to magmas emplaeed at hypozonal or mesozonal depths, whereas the boiling model is more appropriate for shallow porphyry deposits.In the isobaric model, gold solubility is initially dominated by AuCl 2 (super -) at 500 degrees C, 1 kbar. If H 2 S levels are high (pyrite stable), the dominant complex shifts to Au(HS) 2 (super -) upon cooling below approximately 450 degrees C, and solubilities remain elevated (> 100 ppb) over the entire temperature range. If H 2 S levels are low (magnetite stable), gold solubility decreases steadily to 300 degrees C, with AuCl 2 (super -) the dominant complex throughout. Thus, gold dissolved in H 2 S-rich fluids will tend to be carried away from the parent magma, whereas gold in H 2 S-poor fluids will tend to precipitate closer to the source. At 500 degrees C, gold solubility as AuCl 2 (super -) is highest for fluids that are oxidized (SO 2 /H 2 S > 1), acidic. highly saline, and potassium rich. Gold may precipitate in response to a number of mechanisms, including cooling, pH increase, and dilution.Magmatic fluids that evolve from shallow porphyry, bodies are apt to boil shortly after leaving the melt, at which point most of the dissolved gold will partition along with chloride into the brine phase. This metal-rich fluid, because of its high density, will tend to sink or reflux near the parent intrusion, possibly forming an Au-rich porphyry Cu deposit. Mass balance calculations suggest that magmatie brines will initially be undersaturated with respect to metallic gold, although the metal may still precipitate as Au-rich copper sulfide minerals (iss, bornite). During boiling, most of the H 2 O and H 2 S will partition into the coexisting vapor phase. As this vapor cools, it may recondense into a low-salinity, H 2 S-rich xvater of mixed magmatie-meteoric heritage that has a high potential for dissolving and remobilizing significant quantities of gold as Au(HS) 2 (super -) . Migration of this fluid to shallower levels may eventually form epithermal deposits of low- or high-sulfidation affinity, depending on the pH-buffering capacity of the wall rocks, and the extent of direct magmatie involvement. Lack of contact with retrograde, H 2 S-rich magmatic-meteoric waters may be a prerequisite for the preservation of early Au-rich porphyry-style mineralization and may also explain the observed association between gold and hypogene iron oxide alteration in many porphyry deposits.Whether or not there is a direct temporal link betxveen ore-forming processes in the porphyry and epithermal regimes, an intrusive event may be important as a means of introducing a large quantity of low-grade gold which is then available for later remobilization and concentration by circulating fluids of nonmagmatic origin. Moreover, an early porphyry event may cause widespread sulfidation of surrounding rocks. Later fluids of meteoric origin circulating through this pyrite-rich wall rock will have H 2 S concentrations that remain elevated during cooling and ascent, increasing the chances of forming a large, high-grade epithermal gold deposit.