Magmatic sulfide deposits are critical sources of nickel (Ni), copper (Cu), and platinum group elements (PGE) globally. These deposits form from the segregation of a sulfide liquid from a silicate liquid, similar to oil separating from water, due to the sulfide saturation of a mantle-derived (i.e., mafic or ultramafic) silicate melt. While igneous fractionation can cause sulfide saturation in a mafic-ultramafic melt, it often produces minor amounts of sulfide mineralization. To form significant quantities of sulfide mineralization requires addition of external sulfur (and/or silica) to the mafic-ultramafic magma, typically via crustal contamination or crustal devolatilization. If sulfide saturation of the mafic-ultramafic magma occurs there is a partitioning of elements between the silicate magma and the segregating sulfide liquid. In particular, the chalcophile elements, such as Ni, Cu, and the PGE, are scavenged from the silicate liquid and partition into the sulfide liquid, thereby creating a sulfide liquid enriched in chalcophile elements and a silicate liquid depleted in chalcophile elements (e.g., Naldrett, 2010 and references therein). The final concentration of the chalcophile metals in the sulfide liquid is dependent on a number of factors including the initial concentration of chalcophile metals in the parental silicate liquid, how readily the element partitions into the sulfide liquid (i.e., the partition coefficient), and the mass of silicate liquid to sulfide liquid (e.g., Campbell and Naldrett, 1979; Barnes et al., 1997, Lesher and Burnham, 2001).
In the late 1970s, while researchers understood that Ni, Cu, and PGE had high partition coefficients and preferentially partitioned into sulfide liquids (e.g., Rajamani and Naldrett, 1978), there was still a problem yet to be solved: how could a sulfide melt become enriched to percent levels in Ni and Cu when the parental silicate magma contained only par parts per million or parts per billion chalcophile elements? This problem was even more significant for the PGE, which were present in parts per billion levels in parental silicate magmas. The paper by Campbell and Naldrett (1979) was a critical paper in addressing this metal enrichment problem and introduced the term the R-factor.
The R-factor represents the mass of silicate magma that a segregated sulfide liquid has equilibrated with. In essence, if we have a litre of sulfide liquid and an R-factor of 10 it means that the sulfide liquid equilibrated with 10 litres of silicate magma. My colleague Michael Lesher provides an outstanding analogy for the R-factor by comparing it to wearing a sweater (sulfide liquid) while running through a forest (silicate liquid). As you run through the forest burrs from the trees (burrs = Ni, Cu, or PGE) get stuck to your sweater. If you run through the forest once (low R-factor) only a few burrs will stick to your sweater (i.e., low grade mineralization). In contrast, as you run through the woods more and more (high R-factor), more and more burrs will stick to the sweater (i.e., high grade mineralization). It’s great analogy for teaching students the concept of the R-factor and how it can control metal grades in magmatic sulfides.
Campbell and Naldrett (1979) provided a mathematical formulation of the above illustrating that the grade (tenor) of a sulfide liquid (Cl) is dependent on the initial concentration of metal in the silicate magma (Co), the degree to which an element partitions into the sulfide liquid (i.e., the partition coefficient given by Di = [i]sulfide liquid/[i]silicate liquid, where i = element of interest, such as Ni, Cu, PGE), and the R-factor:
Cl = [CoDi(R+1)]/(R+Di)…….(1)
(see also Barnes et al., 1997, Lesher and Burnham, 2001, and Naldrett, 2010 for further reviews). It is obvious from equation (1) that with an increase in Co there will be an increase in Cl, all other things being equal (Figures 1). Therefore, the grade of Ni-Cu-PGE in a sulfide liquid (and deposit) will be proportional the amount of metal in the starting liquid. However, if we look at R=1 to 100, even with a metal-rich magma we cannot create sulfide liquids that have ore grades (Figure 2). This is where the R-factor comes in. In order to achieve ore grade sulfide mineralization a very high R-factor is required: the sulfide liquid must equilibrate with significant quantities of metal-bearing silicate magma (Figures 1 and 2). In Figure 2, an example of Ni, Cu, and Pt is used with rough ore grade values shown. In the case of Ni and Cu, they are in sufficient abundance in mantle-derived magmas that they only require R-factors of 100s to 1000s to yield ore grade mineralization (Figure 2). In contrast, low concentration elements like Pt (and other PGE) require significantly higher R-factors, ~10000 or higher, to achieve ore grade (Figure 2).
Figure 1. Concentration of Ni in a sulfide liquid in equilibrium with a silicate magma as a function of R-factor and initial concentration (Co); DNi = 500. Notably, at a given R-factor the initial concentration of Ni the silicate melt will control the tenor (grade) of the sulfide liquid. It is also notable, that with increasing R-factor there is an increase in the grade of Ni in the sulfide liquid.
Figure 2. R-factor models for various Ni, Cu, and Pt (DNi = 500 , DCu = 1500, and DPt = 10,000). Also shown is an approximate ore grade for the various commodities. This diagram illustrates the importance of both the starting concentration of an element (Co) and also how increased R-factor is required to generate ore grade mineralization. Also evident is that for PGE-rich deposits the R-factor requires is an order of magnitude higher (or more) than it is for the base metals Ni and Cu, and explains why many PGE-rich deposits are associated with large igneous provinces with high volumes of magmatism.
While the R-factor explained how we could achieve grade in mineralization, it also had (and has) broader implications for the exploration for Ni-Cu-PGE sulfide deposits. While obvious, it illustrates the importance of identifying areas that contain potential Ni-Cu-PGE-rich parental magmas (e.g,. picritic to komatiitic magmatic belts; Keays, 1995). The high R-factor also implies that large igneous provinces (LIP), especially those in continental environments, are obvious targets for Ni-Cu-PGE mineralization (e.g., Lightfoot and Hawkesworth, 1997; Lightfoot 2007, and references therein; Begg et al., 2010 and references therein). For example, the largest Ni-Cu-PGE resources outside of Sudbury, are hosted in the Noril’sk-Talnakh area within flood basalts of the Siberian Traps, Russia (e.g., Lightfoot and Hawkesworth, 1997), and the largest PGE resources are hosted within the layered intrusive complexes of the Bushveld Igneous Complex, South Africa (e.g., Cawthorn, 1999; Maier et al., 2013). It also led to recognition of geological settings within LIP that exhibited evidence for repeated magma flow through the Ni-Cu-PGE sulfide forming environment, such as channelized flows in komatiite fields (e.g., Kambalda – Lesher and Arndt, 1995) and magma conduits in intrusions (e.g., Voisey’s Bay; Naldrett, 2010 and references therein).
My next blog post will build this one and cover recent research on metal-rich magmas from the subcontinental lithosphere and their importance for magmatic Ni-Cu-PGE sulfide genesis.