The Fate of Chalcophile Elements During Magma Evolution

The formation of magmatic sulphide ore deposits has been a field of considerably deep research and study due to the fact that the process under which these mineral deposits were formed is still not fully understood. Large ore bodies of economic interest such as the PGE (Platinum Group Element) sulphide deposits have been target of research in order to develop a solid hypothesis that explains the genesis of these types of mineral accumulations. A well-known example of a major ore body formed from mafic magmas includes the Merensky Reef in the Bushveld Complex, South Africa (Robb, 2005) that contains an estimated 22% of the world's PGE deposits. Despite the extreme conditions at which these processes occur, it has been possible to try and replicate these scenarios on laboratories suited with high temperature and high pressure equipment.

One of the widely accepted theories for the ore-forming process is based on the formation of two immiscible liquids in the mafic magma during its evolution. Particularly, the silicate - sulphide immiscibility is used for the explanation of the PGE sulphide deposits. Once the silicate melt becomes sulphur saturated, then the second phase (sulphide or sulphate) is formed and separates. According to Buchanan & Nolan (1979), studies by Katsuda & Nagashima (1974) have shown that for fixed partial pressures of sulphur in the gas phase sulphate increases as oxygen pressure increases and sulphide sulphur decreases. Focus is given to the range of oxygen fugacity (-log 8.50, -log 11.50) in which sulphide is the predominant species given that the latter is the most abundant in the Bushveld complex (Buchanan & Nolan, 1979). This physical separation of immiscible sulphide phases (separation of sulphide melts from silicate host magma) due to differences in density and thus negatively buoyant (Holzheid, 2010) is believed to be the process by which these ore deposits are formed. The separation between phases results in the formation of immiscible sulphide droplets that migrate downward in the silicate melt, scavenging the metals from it. It is believed also, that the earlier this sulphide saturation happens in the crystallization history of the magma, the higher the potential for a rich metal-content ore deposit to form (Robb, 2005). The theory states that ore deposits linked with mafic igneous rocks usually contain siderophile ("iron-loving") and chalcophile ("sulphur loving") metals including the PGE and the Re-Os isotopic system (Peach & Mathez, 1993). The processes that promote sulphide saturation as well as the concentration of the different metals in the sulphide droplets will be discussed below.

Sulphide Solubility Controls

In order to understand the process by which sulphide ore deposits are formed, it is necessary to comprehend the chemistry behind it. The process is controlled by a series of chemical reactions that help explain the different conditions that affect sulphur solubility and thus the phase separation. Sulphide solubility, or the amount of sulphide dissolved in the magma, is controlled by various factors such as temperature, ferrous oxide (FeO) content, pressure, silicate (SiO2) content, oxygen fugacity and sulphur fugacity (Buchanan & Nolan, 1979).

There are three fundamental steps that lead to the formation of sulphide magmatic ore deposits (Robb, 2005). First, the creation of a significant fraction of immiscible sulphide fluid on the silicate melt. Next, the creation of the necessary conditions that permits the sulphide globules to equilibrate with a large volume of silicate magma and finally the effective accumulation of these globes into a single body. As can be seen, there is no chance of an ore body formation if an immiscible fraction does not exist. So as mentioned before, studies have been focused on identifying the conditions and controls that make possible the formation of this second phase (i.e. the factors that reduce sulphide solubility).

Sulphur is dissolved in magmas by displacing the oxygen bonded to ferrous iron (Fe+2) and forming ferrous sulphide (FeS). Only when the silicate melt becomes sulphur saturated, immiscible globules of sulphide melt will form and then crystallize along with the silicate magma as it evolves. Therefore, the amount of sulphur required to achieve this sulphide saturation is one of the basic controls of sulphide solubility, which at the same time decreases with increasing oxygen content. The amount of sulphur will increase in direct proportion to the sulphur fugacity if the silicate liquid is under saturated with sulphur. This will happen until the saturation point is reached and the droplets of sulphide form. The controlling chemical reactions are seen below in order to give a clearer picture of what is going on as the Sulphur enters in contact with the oxygen in the silicate melt (Buchanan & Nolan, 1979).

1/2 S_2+FeO↔1/2 O_2+FeS (1)

1/2 O_2+2FeO↔〖Fe〗_2 O_(3 ) (2)

Equations 1 and 2 can be combined to represent the overall sulphidation reaction.

1/2 S_2+3FeO↔FeS+〖Fe〗_2 O_3 (3)

It can be seen from the previous reactions, that increasing the content of FeO in the silicate melt enables it to dissolve more sulphur and thus increases sulphide solubility. This is why; extracting olivine from the magma would decrease the FeO content and result in sulphide saturation (Buchanan & Nolan, 1979). On the other hand, it is also controlled by the amount of oxygen (measured by oxygen fugacity) because as there is more oxygen, FeO reacts on the competing reaction reducing the amount of sulphide held in the melt. Despite the fact that silicate content (SiO2) also influences the solubility of sulphide, studies by Buchanan & Nolan (1979) have shown that this is masked by the dominant FeO control

Another factor that affects sulphide solubility is temperature (Li & Ripley, 2005). Experiments have shown that if oxygen fugacity is held constant and sulphide saturation is a function of temperature and sulphur fugacity, sulphide solubility decreases with decreasing temperature (Buchanan & Nolan, 1979). This is why sulphide saturation is achieved as the magma ascends, quenches and crystallizes giving a bigger chance of the formation of an ore body.

Regarding the effect of pressure on sulphide solubility, findings by Li & Ripley (2005) reveal that there has not been a final consensus on whether there exists a positive or negative dependency between the sulphur content at sulphur saturation and total pressure (Li & Ripley, 2005). Nevertheless, empirical equations for estimating sulphur contents in silicate melts have been developed taking into account the