#somepapers No. 6: How to make a gold deposit

The paper

Simmons, S.F. and Brown, K.L., 2008, Precious metals in modern hydrothermal solutions and implications for the formation of epithermal ore deposits: SEG Newsletter, n. 72, p. 1, 9-12.

Not sure if the link works if you're not an SEG member, but I could find a way to get it to you, if you'd like a copy.

What it says

The water in modern hydrothermal systems has pretty high metal content, and in some geothermal plants, some of these metals precipitates out of solution and is probably forming ore deposits right now. Geologists figured out a long time ago that epithermal (low-T) Au-Ag deposits were associated with hot springs. This paper summarizes some Au and Ag data from geothermal fluids from the Taupo Volcanic Zone in New Zealand and couples this with flow data from the geothermal power plants to get a metal flux through these systems. Assuming some efficient mechanism for getting the metals out of solution, this can deposit enough Au and/or Ag to make a large ore deposit in something like 20,000 years.

Some of the data in the paper comes from the Ladolam Au deposit, which sits above/within an active geothermal system, in case you needed more proof that these systems produce ore deposits.

Why it matters

Understanding the timing and duration of metal deposition in ore-forming systems gives important clues to finding more deposits.

This paper also shows that it doesn't take an unusual setting, or chemistry, or flux to make an ore deposit. What it does take is the plumbing and mechanism to get that gold out of the fluids and keep it in the ground. Exploration geologists should be on the lookout for those features.

Why I read it

I recently took on a new job. Gone are my days worrying about and modeling a giant, well-behaved Carlin type Au deposit in Nevada. Now I'll be working with smaller epithermal deposits, mostly in Mexico. I was looking for the big 100th anniversary summary paper, also by Simmons, when I came across this one.

It is one of my all time favorite scientific papers.

Odds and Ends

I love this paper. I have spend a lot of time thinking about how ore deposits form, why they form, and how long the process takes. This paper addresses all of that. It is still a wonder to me that I can go to a geothermal field and, with some certainty, know what the rocks look like a few hundred meters beneath my feet. I also marvel about the fact that pretty normal fluids can dump a LOT of gold or silver in a short enough time that I can *almost* wrap my head around it. This isn't millions of years, it's thousands. And not too many thousands of years, at that!

It is hard to explain, but this paper pushes all of the emotional buttons that make me so passionate about science.