Magmatic sulphides: why Cu, Ni & PGE get in!

There seems to be a lot of interest in magmatic sulphide deposits from readers, or should I rather say an interest to better understand these interesting deposits. My last post focused on the central role of sulphur in these magmatic systems, how the sulphide portion of the magma separates from the silicate portion and then how the sulphide melt deposits itself within the magma chamber. We also spoke about how Fe gets into the sulphide melt. But what about the important stuff, the nickel, copper and PGE?

ARM's nickel sulphide Nkomati mine in South Africa. It's Cu/Ni ratio ma suggest Bushveld lineage...

ARM’s nickel sulphide Nkomati mine in South Africa. It’s Cu/Ni ratio ma suggest Bushveld lineage…

What determines how much Ni, Cu and PGE goes into the sulphide portion with the Fe? This after all determines whether the eventual sulphide deposit will be potentially economic or not. So today we will have a brief look at an important factor, which addresses these questions and takes us one step further in understanding this deposit type, partition coefficients.

Let’s imagine again a magma crystallizing somewhere in the earth’s crust, it has already reached sulphur solubility and started to separate out a sulphide melt component. We therefore now have a two-phase system, the silicate melt and the newly exsolved sulphide melt. The chalcophilic (sulphur-loving) Fe has also now started to move into the sulphide melt. Ni, Cu and PGE are however also strongly chalcophilic and thus will also start to be taken up into the sulphide portion along with the Fe. But what determines the concentration of these elements in the final sulphide melt/ deposit?

It is the intrinsic chemical affinity (attraction for) of Ni, Cu and PGE for either the sulphide or silicate melt that determine how much goes where. This affinity in our magmatic system is expressed as the partition coefficient. In academic literature it is referred to as the “D value”. It simply expresses how strongly an element is preferentially partitioned into one of the two phase by taking the ratio of the concentration of an element (eg. Ni) in the final sulphide melt and the concentration in the silicate melt and so giving the following equation:

Formula for the Partition Coeffient (D Value) from Maier, Barnes and de Waal.

Formula for the Partition Coefficient (D Value) from Maier, Barnes and de Waal.

D values can either be determined by experiment or by empirical observation. Experimental values for Ni and Cu range between 200 – 500 and 200 – 1400 respectively. This means that Ni, for example, will likely be 200 – 500 times more concentrated in the sulphide melt than in the silicate melt. The empirically observed value for PGE is about 100 000!

From an exploration perspective the partition coefficient is useful in making rough predictions about the metal concentrations of sulphides that crystallize from magmas of known composition. It is only a rough estimate but really simple. If you have two of the three variables of the equation, you can calculate the third. For example, relatively “standard” basaltic magmas (Karoo, Deccan, etc) contain approximately 100 ppm Cu and Ni. Magmatic sulphide ores crystallizing from such magmas therefore may contain 2 – 10% Cu and 3 – 4% Ni.

So, to summarise, Ni, Cu and PGE goes into the sulphide melt due to its strong chalcophile nature, but the amount that goes in is determined by, amongst other things, the original parent magma composition and therefore the partition coefficients of these elements in that particular magma.

Interestingly, komatiite magmas have significantly more Ni than basalts (around 1000 ppm), but D values (Ni) for komatiites are around 100. As a result sulphide ores resulting from these magmas may have more than 10% Ni. This examples shows how economic factors can drive regional exploration in that if a grade of 4 – 5% nickel grade is needed to make an economically viable resource, exploration should rather focus on komitiites as opposed to basalts.

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