The paper
Johnson, C.L., Dilles, J.H., Kent, A.J.R., Farmer, L. P., Henry, C.D., and Ressel, M.W., 2015. Petrology and Geochemistry of the Emigrant Pass Volcanics, nevada: Implications for a magmatic-hydrothermal origin of the Carlin gold deposits. In Pennell, W.M and Garside, L.J., GSN Symposium, v. 1, p. 391-408.No link this time. I don't know if these papers are accessible online anywhere.
What it says
Rocks form the Emigrant Pass volcanics erupted around 38-36 Ma, right around the same time that the gold deposits on the Carlin Trend were formed. This paper presents new U-Pb zircon dates and geochemistry from many of the units. The dates confirm that the rocks are syn-mineralization. The geochemistry points to crystallization from a water-rich, hydrous magma, the kind that are generally associated with magmatic-hydrothermal deposits. The geochemistry (and some petrographic observations) point to magma mixing and/or crustal contamination of the magma. Compositionally, these rocks overlap more with rocks that are found in productive porphyry copper systems than with rocks from unmineralized intrusions.Why it matters
The dominant genetic model for Carlin type deposits is that they are, more or less, distal magmatic-hydrothermal deposits. This paper adds a bit of evidence to this side.As with most papers written about the geology around ore deposits, the details of the geology matter because we want to be able to find more of whatever is being mined nearby. A robust genetic model helps geologists do that.
Why I read it
I have been meaning to read more papers form the GSN Symposium proceedings since they came out two years ago. So I picked a paper at random from that volume. It was a good random pick because it is: a) about some rocks just over the hill from where I work and b) from a group of people whose work I really respect.Apart from the randomness, it touches on topics that interest me. I'm a big fan of igneous petrology and am always interested to see people try to make ties between igneous geochemistry and (possibly) associated ore deposits. There is often a clear link between the igneous rocks and many types of ore deposits. It has been a lot harder to find links between Carlin type deposits and igneous rocks. The fact that the geochemistry of these rocks is so similar to that of rocks associated with porphyry deposits is important in my mind because it puts things in a framework I know pretty well. I'd like to think it doesn't matter, but the more links I have to a deposit type I understand really, really well makes me more comfortable/confident with the new ideas.
Odds and Ends
I was firmly on the amagmatic side of the Carlin debate for a while. This was largely because I did my MS at the University of Arizona and was told for two years that there wasn't any real evidence for a magmatic origin of Carlin deposits. Since then the magmatic side of the debate has seen a lot more progress than the deeply circulating fluids side. For a while, it seemed the best argument from the magmatic side seemed to be some really, really processed geophysics showing a blob that is supposed to be a big source intrusion, sitting deep, down at the Moho. OK, maybe not that deep. It all seemed pretty arm-wavy.
Since then I've seen some talks and read some papers. And I worked at Bingham, where they have what the U of A crowd called "Carlin like" deposits back in the day. I think about Barney's Canyon and Melco deposits whenever I start to think that there's no way a magmatic fluid could make its way from a porphyry-ish environment to a big gold deposit several km away. Barney's is 5 or 6 km from Bingham. I need to remember that. I also really need to read more about how these things are supposed to work!
I work with a geo who is convinced, against most other evidence and consensus, that these things came from Jurassic intrusions on the trend. "I know what I've seen in the pit," he says. Not sure what to make of that, but there it is, a bit odd and here at the end.
