The paper
Simmons, S. F., Brown, K. L., and Tutolo, B.M., 2016, Hydrothemal transport of Ag, Au, Cu, Pb, Te, Zn, and other metals and metalloids in New Zealand geothermal systems: spatial patterns, fluid-mineral equilibria, and implications for epithermal mineralization: Economic Geology, v. 111, p. 589-618.What it says
A couple posts back I wrote about one of my favorite papers of all time, the one about how to make a gold deposit. That paper was a prelude to this one. The authors sampled geothermal systems on the North Island of New Zealand at surface and at depths of up to 3 km where the water is between 200 and >300 degrees C. The samples were analyzed for a broad suite of metals and mineral and ligand equilibria were calculated for these waters.There are a lot of interesting bits in the paper and I read it three weeks ago, so I'll just cover a few of the more important ones here.
Some metals show similar concentrations and metal ratios to the local rock units, suggesting derivation from country rocks. However, the economically important metals, Au, Ag, and Cu, have a magmatic source.
The ligands that transport metals in hydrothemal fluids have a strong influence on the depositional mechanism of those transported metals. The authors found little evidence to support that the metal content of the fluids are limiting the equilibria. They also confirm one of those foundational things I learned in my economic geology courses: boiling leads to metal deposition. This is why the deep waters sampled have variable, but often high, metal concentrations, but the surface waters have much, much lower concentrations of Au, Ag, Te, Cu, and Pb, the elements commonly found in epithermal ore deposits.
Why it matters
There are two important conclusions that are relevant to exploration:First, there is a deep (magmatic) source supplying most of the Au, Ag, Cu, and Te.
Second, boiling is the main depositional mechanism for those elements, which are the economic drivers of most epithermal deposits. Multi-element geochemistry can give reliable clues about the depth of exposure.
Why I read it
After reading some interesting papers about pluton emplacement and explosive volcanism (will write that up soon) and other papers that just caught my eye, I decided that if I'm going to read just a couple papers every month (or quarter), they ought to be relevant to my job. As I've mentioned before, I work on a lot of epithermal deposits, so this one, an outgrowth of one of my all time favorite papers, was a natural pick.Odds and Ends
Happy New Year! One of my resolutions is to read a couple papers a month and write them up here. Hold me to it, Internet.
