Colour anomalies and copper exploration in Chile

A kind of scary, but pretty cool, bit of work I got to be a part of recently was exploration in the high Andes of Chile. You can also check out the guest post I wrote for travelinggeologist.com on it. This part of the world is where the giants are… in terms of porphyry-style copper (Cu) ore deposits (e.g., Chuquicamata, El Teniente, Rio Blanco-Los Bronces). These porphyry Cu deposits are extensive magmatic-hydrothermal systems that make large hydrothermal alteration (colour) anomalies on the mountains. And combined with the arid climate of the Atacama desert of Chile, it makes a perfect recipe for spotting, exploring and discovering porphyry Cu deposits.

However, most of these are red herrings, and not related to a fertile Cu deposit… thus is what our job was. In this short blog I will share of bit about the heli-based work I was doing recently, exploring these prominent colour anomalies in the high and dry Andes of Chile.

What are the colour anomalies?

The types of deposits we explore for in Chile are called porphyry deposits (check out my previous post on Chile or for more information on them). Porphyry Cu deposits are related to hydrous porphyritic intrusive rocks, with aqueous high-temperature fluids (i.e., liquid, vapour or supercritical fluid). Copper, sulfur, and other metals and elements precipitate from these fluids (via many different factors, like decompression). The result is large quantities of metals associated with veins, breccias, and disseminated styles of mineralization, but also significant hydrothermal alteration of the surrounding rocks in the area.

Significant alteration from these deposits can make large, prominent colour anomalies that can be seem in the area visually, and/or with the help of multispectral and hyperspectral techniques. Typically they are related to upper level alteration like clays and white micas (e.g., advanced argillic and phyllic alteration). Multispectral and hyperspectral technique use light collected by satellites or air-borne devices that see slightly outside of our visual light spectrum (i.e., 380 – 740 nm), usually at short-wave infrared light wavelengths up to 2500 nm. This means that certain minerals that have characteristic spectral signatures in the short-wave infrared light (like white mica and clays) can be seen and mapped better with images made from these techniques.

The Atacama Desert in Chile is the driest place on earth. There isn’t a lot of vegetation, and there is lots of outcropping rocks. But, that of course means that if there was an exposed or slightly exposure Cu porphyry deposit, then it probably would have been spotted by now. However, these large hydrothermal alteration cells are upper-level expressions of Cu porphyry deposits and more volumetrically extensive, but less diagnostic. So they could led to a mineral deposit at depth, and particularly if they are located at high and hard-to-get-to places, there is a better chance that they have been explored less.

Working with helicopters in the Andes

We used helicopter in the high Andes of Chile during this exploration campaign to get to these colour and spectral anomalies in hard-to-reach places. While I have worked around helicopters before, this was the first time in the high Andes… so things are a bit different there. You are very high (> 4500 m) at the landing sites and thus the air is very thin (so hard to get much lift for helicopters). And this is even trickier in afternoons when the air heats up and becomes less dense, and winds pick up. Thus an early morning start and early afternoon return was the safest option.

Weight is also another important consideration when flying in the Andes, as it is hard to land and take off as high altitudes. Therefore splits of team members were made at lower elevation, and always the helicopter landings and take-offs needed to be “hot” (i.e., helicopter was still running). For example, we would drop a team off below in a dried-up river bed (quebrada), while the other team was dropped off at the landing destination.

As well as basic safety training ahead of time, there are a few things you always need to have, like a survival kit. This would be dropped off at a particular location with items like extra food, water, sleeping bags, tents, etc. And if the helicopter couldn’t come get us at the end of the day we would use it (but luckily during this campaign that never happened).

After helicopter drop offs the rest of the day was left to us, the geologists… walking in high elevations and collecting samples, making observations, etc. Walking so high literally takes your breath away, so it is important to go slow (for examples, experiences in the Peruvian Andes). And up high weather can turn rapidly, from very cold to very hot very fast! So you need to be prepared for anything.

Final thoughts

Working in the high Andes of Chile has its challenges, but also rewards (hopefully more than just experience…). Arid places, like the Atacama in Chile, are perfect for using a combination of colour anomalies and remote sensing techniques to aid in exploration for these porphyry copper deposit (well, as well as many many other underlying dataset to guide us to the right location).

As in most cases in exploration geology, one of the main challenges is just getting to the place of interest. In this case, using a helicopter has its extreme benefits in time saved and reaching inaccessible areas (although at a price tag as you can imagine). But sometimes you need to get up in the air to discover what is below in the earth.

– Stephanie

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