Prime Meridians – Is This Discovery A Revolution In Copper Exploration?

This week in Pierce Points:

China’s LNG use fell for the first time ever. 2015 shipments were down 1.1% — the first drop since imports started in 2006.

Mining private equity deals jumped 238%. Big investors especially like gold, but deals are becoming more complex.

The world’s #1 copper miner was downgraded. Chile’s Codelco is no longer high grade; but it’s better off than some other firms.

Nigeria warned lazy explorationists. The country wants to take back inactive licenses to jumpstart the mining sector.  

Middle East E&Ps went on a buying spree. Qatar Petroleum and Oman’s Petrogas are using soft markets to buy offshore assets.

Is This Discovery A Revolution In Copper Exploration?

A lot of press this week about new research published recently on copper geology. Which several reporters breathlessly held up as a game-changer in exploration for some of the biggest deposits on the planet. 

The work came from staff at the Camborne School of Mines, Exeter University in Cornwall, U.K. And was published in the journal Nature Geoscience — which is pretty much the holy grail for geological researchers globally. (Personally, I find the regional journals like Journal Of Asian Earth Sciences more useful for exploration — but I digress.)

Popular articles on the paper were quick to point out the scale of this discovery. But were short on details about what researchers actually found — and how it could be used to locate big deposits. 

So let’s take a look. 

One good thing about Nature is that its articles are short. With the Camborne paper running just four pages (as opposed to more specialist journals like Geochimica et Cosmochimica Acta, where dispatches can be 20 pages or more).

In fact, the discovery here can be summed up in one chart:

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The guts of a research discovery, showing that “fertile” porphyries (associated with big copper deposits) have different chemistry than barren ones (source: Nature Geoscience)

Here’s what the researchers did. Looking around the world, they selected porphyry samples from areas that contain big copper deposits, such as Chile. And also picked out porphyries that don’t come with associated mineralization — the “barren” group.

The workers then analyzed certain minerals — plagioclase feldspars — from all of these samples. To determine if there are any chemical differences between the mineralized and unmineralized groups. 

And indeed, there are. As the chart above shows, plagioclase from well-mineralized porphyries shows notably higher amounts of aluminum (Al — which is compared here to the amount of calcium, sodium and potassium, on the vertical axis). 

The data set is indeed convincing. Something is certainly happening with regards to aluminum in places where we find big copper deposits. 

But does that mean we’ve found a silver bullet when it comes to exploring for massive copper ores? 

No. Like all geochemical tools, this one has potential uses in exploration — but it has to be applied in association with the basic observation and analysis that is always key to finding big deposits.

Here’s the main challenge with these findings: they apply to rocks that usually aren’t found at surface.

Just look at the diagram below from the research paper. Where the area containing the plagioclase mineral of interest (shown in yellow) is located a few kilometers below the nearer-surface mineralization that it gives rise to.  

Screen Shot 2016-02-12 at 6.17.50 AM High-aluminum plagioclase could be a sign of big copper deposits. The problem is, plagioclase occurs well below surface.

Of course, this could be useful in a place where drill core can be obtained, revealing the chemistry at depth (in fact, some of the data from the research paper comes from such subsurface samples). But if we’re already drilling, the observation of high or low aluminum is coming somewhat after the fact.

Can the observation about aluminum in plagioclase help us during the more basic stages of fieldwork? Yes. But it takes some careful thought and observation. We’re simply not going to walk onto a hilltop, find aluminum, and drill a discovery hole. 

In many districts, deeper levels in the magmatic system are exposed at surface — through deep erosion. But the challenge is, such outcrops of lower-down rocks can occur kilometers away from the higher-up mineralized zone. Evens tens of kilometers in some cases. 

We thus need to be careful in applying regional mapping and sampling, to create a comprehensive geologic framework for an area. Which could show the presence of high-aluminum plagioclase in one area — indicating an increased likelihood of mineralization somewhere in the district

As with all exploration, we then need to apply two basic principals. 1) Walk around, and 2) Look at the rocks. 

There is one other part of this research that’s worth noting for field hands. The relationship of aluminum to other elements like calcium, sodium and potassium. 

The researchers surmise that aluminum increases near big copper deposits because these systems have a lot of water fluxing through. Which increases the absorption of aluminum inside crystals of plagioclase. 

But as more aluminum is taken up into plagioclase, there’s less room for other components. Meaning that calcium, sodium and potassium seem to be swept away — into fluids, that then can circulate higher, perhaps toward surface. 

It’s thus possible that if we see surface signs of enrichment in calcium (calcite), sodium (salt or albite feldspar), and/or potassium (potassic feldspar or potash) it could mean high-aluminum plagioclase is lurking at depth. Perhaps signalling that copper is nearby. 

Interestingly, that suite of minerals is common as alteration halos in sedimentary copper districts like the African Copperbelt. With albite alteration and evaporites often lying near major deposits here. 

Bottom line, this is great work from a very thoughtful team of researchers — and applied with a solid regional understanding created by rigorous fieldwork, could give us one more powerful indicator in deciding to pursue or reject a prospect. 

Here’s to reading the tea leaves,

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Dave Forest

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