#somepapers No. 2: Melt inclusions and the Bingham Canyon Cu-Mo(-Au) deposit

The paper:

Zhang, D. and Audetat, A., 2017, What Caused the formation of the giant Bingham Canyon Porphyry Cu-Mo-Au deposit? Insights from melt inclusions and magmatic sulfides. Economic Geology, v. 112, p. 221-244.

What it says:

A lot of research has been thrown at trying to find out why some intrusions are mineralized, and why some of those deposits are so big. This study sampled a bunch of rocks around the Bingham Canyon porphyry Cu-Mo(-Au) deposit to analyze melt inclusions and magmatic sulfides to determine the initial composition of the magma that was the source of metals in the giant deposit.

Porphyry deposits form when water exsolves from crystallizing magmas in the shallow crust (5-15 km) and carries sulfur and metals up until they are deposited in stockwork quartz veins. One idea of why deposits are big is that they formed from magmas enriched in the elements that are concentrated in the ore deposit. Whole rock data don't always give the initial composition of the magma because offgassing, alteration, and other effects can leave the rock with a different composition than the initial magma. Melt inclusions, small pockets of melt trapped in crystallizing minerals, are less affected by the changes that happen as magmas cool. Melt inclusions are analyzed by laser ablation ICP-MS. Corrections and internal standardization are used to translate the raw data into whole rock numbers we recognize.

The results of this work suggest the source magma for the Bingham deposit was a mixture of about 40% mafic magma with 60 % rhyolitic magma. The composition along this mixing trend is pretty normal. They compared the composition of magmas from several mineralized and non-mineralized intrusions. It isn't particularly high in sulfur or any of the metals in the deposit (~1300 ppm S, 50-90 ppm Cu, 0.8-2.0 ppb Au, 2-3 ppm Mo). It doesn't have significantly more water than most arc magmas. The deciding factor, then, must be a large volume of this (disappointingly) normal magma. In reality, it isn't really that large a volume of magma. The fluid must efficiently extract the constituents from ~150 km^3 of magma, which isn't really that big of a pluton. This is consistent with some other studies that do this sort of mass balance (I need to finish mine!).

Why it matters:

Most of the world's metals come from big deposits like Bingham. Researchers are always trying to find what is different about these giants so that exploration geologists can find more.

Why I read it:

This paper has it all! Melt inclusions, magmatic sulfides, mass balance, magma mixing! All that, and it's about the Bingham Canyon deposit. I worked at Bingham for a bit over 4 years, and have written a couple papers on it. I think of it as my mine, and am always happy to read more about it. 

I am interested in the details of how deposits form. The practical applications of these aren't always apparent, but pile up enough knowledge about the compositions of source magmas, or the distribution of deposits, or the details of fluid pathways, and practical applications to finding or expanding ore deposits drop out. 

I'm especially interested in the formation of magmatic-hydrothermal deposits (porphyries and skarns). I like the igneous rocks and the minerals that form as part of the hydrothermal system. From a hard science point of view, I am especially fascinated the composition/evolution of the magma that feeds these big deposits. Although right now I don't work in a porphyry  deposit, I hope to work on these kinds of intrusions (mineralized or not) again someday. 

Odds and ends

One of the things I thought about a lot during my PhD research is what magma is, how it forms plutons, and what constitutes a "magma chamber". This paper doesn't really define magma chamber, but it does use that term when discussing how much magma is needed to feed this deposit. There are some important implications about how you define magma chamber, when talking about the sources of these deposits. What you really need to form one of these is to efficiently extract the S, Cu, etc. from >150 km^3 of melt. It's hard to sustain that volume of melt as one big pot of molten rock. More likely, you'll end up with a larger volume for your "magma chamber" that has some percentage of crystals that still allows the melt to communicate when it comes time to exsolve the mineralizing fluids. It's a really interesting problem to me. Like I said, I thought a lot about it during my research, but my data didn't really cooperate much in answering any of these questions.