Economic geology from first principles

I have to say that I kinda fell into the resources industry by accident. I was offered a job unexpectedly, it sounded like and interesting opportunity, so I said yes. Most of my formal geologic training is in big picture academic geology and associated analytical science, in which concepts and techniques are derived from first principles.

Industrial geology isn’t really like that, but that doesn’t mean that I can’t try. So I will. The law of supply and demand suggests that the price of a mineral should be related to the demand for it, and its abundance. Demand is beyond the prevue of geology, so we will ignore this half of the law for now. Doing this suggests that the price of a mineral should be related to the abundance of that element.


Above is a figure that plots the market price for various elements (US$ per mole) vs their crustal abundance (atomic ppm). If demand was irrelevant and the Earth’s crust was homogeneous, then these elements ought to fall on some sort of negatively sloped trend. As seen from the graph, maybe a third or so of the elements do, but the bulk fall somewhere to the left of the trend.

There are two ways to interpret this. The first is that there is reduced demand for these elements. The second is that the Earth’s crust is not homogeneous. Geology allows us to address this second option.

If the crust was homogeneous, we exploration geologists would be out of work. Mines aren’t just stuck in any old place, they are put where it is most economical to extract a particular resource. In general, this corresponds to am area where geologic processes have concentrated one or more elements of interest. So it is possible that the elements to the left of the trend are elements that are concentrated by geologic processes more easily than other elements.

Have another look at the figure. The elements labeled in yellow are chalcophiles. These are elements that will preferentially form sulphides instead of silicates in the presence of sulphur. This provides an enrichment mechanism not available to elements that only occur as oxides, so that the local concentration of these elements can be orders of magnitude higher than their average crustal abundance.

So if you were curious about why it is that economic geologists are always nattering on about sulphur, this is the reason. Even if the average crustal abundance of Pb is a few ppm, sulphide precipitation can concentrate it to 10% or more, which would place it to the right of Al on this figure.