Sulphide solubility in magma

Abstract

This essay aims to define the controls on sulphide solubility in magma, such as pressure, temperature and element activity. Chalcophile elements, elements of low abundance, form sulphide and arsenide minerals but are not stable at high temperatures of igneous crystallization. The relation of the processes of sulphide solubility against the relative outcomes for chalcophile elements will be looked into.

Introduction

Sulphur (or sulfur) is a natural substance; its elementary state is a crystalline solid of yellow colour. Sulphur is found in various forms, as mineral sulphides and sulphates, such as galena and cinnabar. Magmatic sulphide deposits with Nickel-Copper-Platinum-group elements are a result of “segregation and concentration” of liquid sulphide from mafic or ultramafic magma, and the screening of chalcophile elements into these deposits from the silicate magma. (A. J. Naldrett 2004)

Due to the variation of sulphide deposits, it is convenient to relate the magmatic sulphide deposits to the bodies of mafic or ultramafic rock in terms of the composition of the magma associated with these rocks. It is then reasonable to investigate the geochemical and geophysical setting and the processes in which these deposits are formed. According to A. J. Naldrett, 2004, the key aspects in the creation of magmatic sulphide deposits are that the magma saturation in sulphide and segregation of the immiscible sulphide has to be concentrated locally with a sufficient amount of magma concentrating chalcophile elements to comprise ore of economic value.

In conditions of normal mafic magma cooling and crystallization, no early segregation of liquid sulphides are noticed. This implies that the magma would have to be saturated in sulphide in the igneous body state. The assumption is that there is a process prior to the magma’s extrusion from the ground, and final emplacement, where sulphide saturation is caused. The above assumption is reinforced due to the fact that most basaltic magmas, other than Mid Ocean Rig Basalts, have high contents of Platinum Group elements.

The Controls

The mantle has an estimated concentration of 300-1,000 parts per million (Sun 1982). This is believed to be the primary source of the sulphur carried in basaltic magmas. The solubility of sulphur is controlled by temperature, pressure, contents iron oxide and titanium oxide and their activity in the melt, oxygen and sulphur fugacity, the oxidation state of the melt and the mafic versus the felsic components in the melt. (Fincham & Richardson 1954, Haughton et a. 1974, Shima & Naldrett 1975, Buchanan & Nolan 1979, Buchanan et al. 1983) These factors, or otherwise conditions, of sulphide solubility in the melt will be explored in order to record the effects of the controls of sulphur solubility against the deposit’s composition.

Leaving the mantle, and entering the crust of the earth, the melts can either be intrusive or extrusive, yet the fundamental pressure temperature relation for sulphide solubility, from mantle to the crust in liquid state, remains the same.

Pressure and temperature increase with depth; according to Marvrogenes and O’Neil (1999), increased pressure presents a negative effect on a silicate melt, dissolving less sulphide. As pressure decreases and the melt reaches the surface, it ability to dissolve iron sulphide increases. Considering that the majority of melts leave the mantle unsaturated in sulphide, as they reach lower depths, while the pressure and temperature decrease, the sulphide saturation is not achieved; furthermore the temperature decrease can offset this relation, causing further decrease in sulphur solubility. (Buchanan and Nolan 1979)

Exiting the mantle, sulphide segregation will occur after the silicates initiate crystallization. Therefore, the sulphides will be assorted with the silicate grain. The simultaneous crystallization, as a function of temperature and pressure conditions along with the composition of the liquid sulphide would produce a sulphide rich deposit (for nickel and copper dominant magmatic deposits against platinum group element magmatic deposits) only if an external factor could intervene. This factor, or condition, would reinforce the sulphide segregation, without further enhancing silicate crystallization.

At isothermal conditions of 1200oC, Buchanan 1988, sulphur content in the silicate melt decreases with the increase of oxygen fugacity (fO2) at constant sulphur fugacity (fS­2). Furthermore, the study displays a correlation between sulphur (wt % S2) content and iron oxide (FeO %), with a logarithmic increase on the field of saturation.

On the other hand, the compositions of the melt in terms of the variation of content of iron oxides (FeO) or titanium dioxide (TiO2) diversify the ability of the melt to dissolve sulphide. The increase or decrease of oxide content in the melt is correlated to the sulphide solubility in the mixture. Oxidation is capable of causing the formation of an insoluble sulphide in the melt as a result of a reaction within the liquid between the soluble substances, usually without causing silicates crystallization. Oxygen and sulphur fugacity, and their relation to the pressure temperature setting, along with their ratio, are important factors for sulphide solubility. Induction of sulphur from an external source, and the felsification of a mafic magma are important causes of sulphide segregation. Silicate magma reacts with the sulphide liquid, resulting in the formation of the magmatic sulphide deposit. The ratio of reaction and the composition of both the sulphide liquid and the silicate magma, along with the controls on which the elements react are responsible for the final outcome.

Buchanan 1988, determined the solubility of sulphur as a function of sulphur fugacity (fS2) in a basaltic melt containing 17 wt% iron monoxide (FeO) at a range of 1000 to 1400oC. Although the fugacity of oxygen and sulphur remain constant, the rate of increase of sulphur solubility drops from a factor of 10 times per 100oC at 1100oC, reduced to 3 times at 1400oC, implying that there is a threshold of maxima in sulphur saturation with temperature increase; the saturation is achieved in 1450oC. Hence, the actual sulphur content increases with temperature but decreases in rate, although with higher sulphur fugacity saturation is achieved at lower temperatures. The increase in sulphur content with increase temperature is reinforced by the experiments of Haughton et al 1974 and Shima and Naldrett 1975, for which, although the figures are of different nature for direct comparison, the fact remains that the sulphur content actually increases with temperature, making this a control of sulphur solubility in the magmatic melt.

The studies and experimental conditions on which pressure is investigated as a control in sulphur solubility are vast and vary on their conditions. The outcome of different studies such as Haung and Williams 1980 and Wendlandt 1982, indicate that under natural conditions increase in pressure has a negative effect on sulphur content. In contrast, the increase of FeO levels in the melt increase the sulphur solubility and this is confirmed by Mavrogenes and O’Neil 1999, where the study of basaltic melts with 6-14 wt% FeO on pressures varying from 5-90 kilo bars and temperatures of 1400oC and 1800oC shows increase sulphur content at sulphide saturation with pressure.

Sulphur can be found dissolved in an aqueous fluid. The sulphur content of the fluid is determined by its sulphur dioxide against hydrogen sulphide ratio (SO2:H2S). The ratio (Misra K.C. 1999) increases with increasing oxygen fugacity (fO2) of the initial magma before the start of second melting. Aqueous fluids originating from high oxygen fugacity magmas (I-type; high fO2) may contain large quantities of sulphur dioxide (SO2) as well as hydrogen sulphide (H2S). “At lower temperatures on cooling hydrolysis of the SO2 (4SO2 + 4H2O = H2S + 3H2SO4) or its reaction with Fe2+ bearing minerals of the wallrocks (SO2 + 6″FeO” + H2O = H2S +3″Fe2O3″) increases the activity of H2S, causing precipitation of sulphide ore minerals from the metal-chloride complexes in the aqueous solution.” On the other hand, aqueous fluids originating from low oxygen fugacity magmas (S-type; low fO2) may contain as much H2S as those derived from high oxygen fugacity, but because of lower oxygen fugacity they contain less sulphur dioxide so the total amount of sulphur is smaller. “Thus, aqueous fluids that separate from I-type magmas tend to produce Cu-Mo-Zn-Fe sulphide deposits, whereas fluids from S-type magmas generally precipitate smaller quantities of sulphides, mainly pyrrhotite, and correspondingly larger quantities of oxides, such as cassiterite (Burnham & Ohmoto 1980). In either case, the precipitation of sulphides form metal-chloride complexes is accompanied by generation of HCl. The HCl and the H2SO4 produced by SO2 hydrolysis are consumed by “acid” alternation of aluminosilicate minerals in the wallrocks.” (Misra K.C. 1999)

Chalcophile Elements

The partitioning of Chalcophile metal elements between sulphides and silicate metals are referred to as metals with low concentration, such as Nickel (Ni), Copper (Cu) and Colbat (Co) are exchanged with elements of higher concentration, like Iron (Fe). The Nernst coefficient of partitioning, arranged for iron substitution is the ratio of the products of the % (per cent) weights of the elements substituted.

References

1. Buchanan D. L. (1988). Development in Economic Geology – Platinum-Group Element Exploration. Elsevier. ISBN 0444429581
2. Naldrett A. J. (2004). Magmatic Sulfide Deposits : Geology, Geochemistry and Exploration. Springer. ISBN 3540223177
3. Mungall J. E. (2005). Exploration for Platinum-Group Elements Deposits. Mineral Association of Canada. ISBN 0921294352
4. Misra K. C. (1999). Understanding Mineral Deposites. Kluwer Academic Publishers. ISBN 0045530092
5. Whitney J.A. (1989). Ore Deposition Associated with magmas. Society of Economic Geologists. ISBN 0961307439
6. Vaughan D. J. (1977) Mineral Chemistry of Metal Sulfides. Cambridge University Press. ISBN 0521214890