#somepapers No. 4: Apatites and ore deposits

The paper

Mao, M., Rukhlov, A.S., Rowins, S.M., Spence, J., and Coogan, L.A., 2016, Apatite Trace element compositions: a robust new tool for mineral exploration. Economic Geology, v. 111, p. 1187-1222. 

What it says

Apatites were selected and separated from rocks representing a wide range of deposit types, from porphyry Cu(-Mo-Au) to orogenic Au to carbonatites. Apatites from a wide compositional range of igneous rocks were also analyzed to see whether this tool could be used to differentiate mineralized from non-mineralized rocks. 

The minerals were analyzed for a suite of >30 major and trace elements on the microprobe and laser ablation ICP-MS instruments. The analyses were then fed into ioGAS software, a geochemical package that is commonly used in the exploration industry to look for geochemical trends and correlations. The software developed a series of discrimination functions to separate the analytical results into predefined groups. The software was able to find functions that grouped apatites from the different deposit types. These are ugly equations, but they work. Some deposit type groupings overlap more than others, but the groupings do appear to hold.

The paper (and its appendices) presents a good overview of apatite mineral chemistry in the introduction. The results contain a great set of apatite chemistry and good discussion about some features that differentiate, say, porphyry Cu-Mo from an alkalic porphyry Cu-Au deposit.

Why it matters

New ore deposits are getting harder to find, and increasingly exploration is looking for deposits under cover where in place rock is difficult or impossible to put a hammer on. This tool is best used on stream sediment and other samples that aren't in place, and the paper strongly suggests using it in conjunction with other datasets. Put enough of these together and a target or vector comes together.

Why I read it

This is the paper I wanted to write around 2008. A big part of my master's research was probing apatites from a porphyry deposit, mainly to see how much S was substituting in the P sites. Since then, I've been really interested in apatite chemistry. In 2007 I got a small grant to analyze hydrothermal apatites from a variety of ore deposits. I zapped them with the microprobe at BYU and the laser ablation unit at Oregon State. I collected ~300 pretty good apatite analyses.

Then we had a kid and I had to get a real job. All non-dissertation research was set aside.

Using apatite chemistry in this way is interesting, but I don't do this kind of geology for my job. I am a resource geologist, which means most of the time I get to really do geology it's a big scale. But in my heart, I'm a mineralogist. Any paper that presents a novel use for mineral chemistry catches my eye.

Odds and ends

Even though this paper beat me as first to dump a pile of apatite data from all sorts of ore deposits, I still think there's life in my handful of analyses. The big reason is that I am confident that all my apatites are hydrothermal. If I'm reading this paper right, the apatites were a mix of igneous and hydrothermal, in most cases (sample descriptions are in Appendix 1). My hunch (or hypothesis, I suppose) is that there are should be some real and important differences between the chemistry of hydrothermal and igneous apatites. A lot of these differences should survive hydrothermal alteration, thanks to paired substitutions that take the mineral chemistry away from "ideal" compositions. Someday I'll get back to it and see.

In the end, the mix of hydrothermal and igneous apatites in this study don't matter a great deal. The point is to find these apatites in sediment after most of the rest of the rock has been scattered or succumbed to chemical weathering. the groupings that fall out of the ioGAS analysis appear fairly robust. Whether that is because the chemistry is related to the initial conditions of formation or has been changed through hydrothermal processes, it might not be relevant. If it works, it works. Explorationists are often happy to leave it to the academics to figure out why.