A colleague and I visited a university geology department in the vicinity of a project we were working to do a presentation on the geology of the project. We had been invited by a professor who’s research interests were amongst other things, mafic-ultramafic rocks and he had invited us as our project was hosted by such. He relayed an interesting story to us that day: Final-year students in the department were required to complete a research project and a student had approached the professor and told him that he is interested in doing a project about ultramafic rocks. The professor immediately retorted: “No! Ultramafic are only for the masters!” The professor was of course not talking about Masters’ degree students but referring to the dedication and possible the intelligence required to understand these rock types and their associated deposits.
I am not sure I completely agree with the professor but the ore forming processes of magmatic sulphide deposits and the accompanying complexities, chemical variables and their interactions are definitely a challenge for most geologists. I probably would have had a much better understanding of these deposits earlier if such a professor had taken the time to explain the concepts to me in simple, understandable terms. Today, and hopefully going forward I will touch on some aspects of these very interesting deposits in a way that will make them understandable to most.
Magmatic sulphide deposits are of course important from an exploration and mining perspective because they host economic quantities of copper, nickel, and the platinum group elements (PGE). There are many factors which govern their size and grade, and the development of different exploration tools based on the interplay between these factors can be very exciting. For now, I will focus on a minor element in the earth’s upper mantle, sulphur.
Magmas that form mafic-ultrmafic (high in Mg, low in Si) rocks originate from the earth’s upper mantle, the region just below the earth’s crust. Sulphur is a minor component of this region and occurs at around 250 ppm. As rocks in this region melt and form the aforementioned magmas, sulphur is melted with and a certain amount goes into solution into the magma. In the same way that water, due to its immediate chemistry and temperature can only receive a certain amount of salt into solution, so also with a magma when it comes to sulphur. The rest of the sulphur segregates out of the magma at its source as a immiscible (“does not mix”) sulphide melt, possibly remaining in the source region. This principle is known as the solubility of an element in a certain liquid.
One of the most central principles in the formation of a sulphide deposit is the consequent change in solubility of sulphur in a mafic-ultramafic magma as the temperature of the magma changes (cooling after and during emplacement). The amount of sulphur a magma can accommodate decreases rapidly as the temperature decreases. In fact it decreases exponentially, due to the further influence of other chemical changes in the magma as well.
Why is this non-linear, exponentially decreasing nature of sulphur solubility so important? Imagine a basaltic magma busy crystallizing somewhere in the earth’s lower crust. This magma has about 0.1% sulphur, which is currently not being incorporated into the major rock-forming minerals. So, material is being removed from the liquid part of the magma into mineral crystals, leaving behind the sulphur, and so increasing the sulphur content in the liquid. Eventually, the liquid cannot accommodate sulphur anymore as it reaches its sulphur solubility point and starts to exsolve (separate out) as a sulphur-rich melt. Iron (Fe) is sulphur loving (chalcophilic) and thus this suphide rich melt picks up Fe out of the magma and becomes FeS rich, dense and sinks to the bottom of the magma chamber where it then crystallizes well-known ore minerals such as pyrrhotite, chalcopyrite, pentlandite and pyrite.
But, for large amounts of sulphide to segregate out it would seem that the magma must be somehow pushed into “sulphur super-solubility”. This is where things get very tricky and…very theoretical. Some of the proposed mechanisms that may cause this include magma mixing, very rapid drop in magma temperature, rapid differentiation (removal of elements from magma as certain minerals crystallize very fast) and finally magma contamination (by the surrounding rocks.
There remains much to be still said but I trust this gives a basic understanding of the forming process. We can later talk about things like partition coefficients, R-factor, fractionation and then the exploration tools. Do you have experience in magmatic sulphide deposits? Let me know your thoughts in the comments below! For some this post might have been too academic, don’t worry, we will soon be back discussing the gun-slinging, cowbow business of feasibility studies and project development!
The ideas presented in this post are reviewed in a well written paper by Maier, Barnes and de Waal which can be found here.