Empirical equations to predict the sulfur content of mafic magmas at sulfide saturation and applications to magmatic sulfide deposits

Abstract

Empirical equations to predict the sulfur content of a mafic magma at the time of sulfide saturation have been developed based on several sets of published experimental data. The S content at sulfide saturation (SCSS) can be expressed as: $$\ln \,X_{\text S} = 1.229 - 0.74(10^4/T) - 0.021(P) - 0.311\,\ln \,X_{{\text{FeO}}} - 6.166X_{{\text{SiO}}_{\text{2}}} $$ $$ - 9.153X_{{\text{Na}}_{\text{2}} {\text{O + K}}_{\text{2}} {\text{O}}} - 1.914X_{{\text{MgO}}} + 6.594X_{{\text{FeO}}} $$ where T is in degrees Kelvin, X is mole fraction and P is in kbar. The squared multiple correlation coefficient (r 2) for the equation is 0.88. Application of the equation to data from sulfide-saturated mid-ocean ridge basalts (MORB) samples show that the SCSS is closely predicted for primitive MORBs, but that accuracy decreases for lower T (<1,130°C) and more evolved MORB samples. This suggests that because the calibrations are based on anhydrous experimental runs done at temperatures of 1,200°C and above, it is not possible to extrapolate them to significantly lower temperatures and hydrous conditions. Because the SCSS of a primitive MORB magma increases with decreasing P, sulfide saturation in MORB appears to be a function of the degree of en route assimilation of S from country rocks as well as the degree of fractional crystallization in shallow staging chambers. Application of the equation to the high-T impact melt sheet that produced the Sudbury Igneous Complex and associated Ni–Cu sulfide ores indicates that sulfide-saturation was reached at ~1,500°C, well above the start of orthopyroxene crystallization at ~1,190°C. This would permit ample time for the gravitational settling and collection of immiscible sulfide liquid that produced the high-grade ore bodies. The development of a platinum group element (PGE)-enriched layer in the Sonju Lake Intrusion of the Duluth Complex is thought to be due to the attainment of sulfide saturation in the magma after a period of fractional crystallization. Using the composition of the parent magma of the Sonju Lake Intrusion the presented equation indicates that sulfide saturation would have been reached at ~60% crystallization, when iron oxide was a liquidus mineral; the prediction is in agreement with field evidence which indicates that PGE-enrichment occurs in the oxide-rich gabbro zone. Contamination and mixing processes that may be related to the attainment of sulfide saturation in mafic magmas can also be evaluated. Mixing of a siliceous melt and a liquid of olivine tholeiite composition, similar to that thought to be a reasonable parental composition for many Duluth Complex intrusions, can induce sulfide saturation at mixing ratios in excess of ~0.1. If the contaminant contains low quantities of sulfur the mixing ratio required to promote saturation is reduced. Mixing of mafic magmas at various stages of fractionation is evaluated using magma compositions that are thought to be appropriate for the generation of the Merensky Reef in the Bushveld Complex. Magma mixing is shown to be an effective process for the attainment of sulfide saturation, depending strongly on the sulfur concentrations of the end-member magmas.