By Chops

Last month, a piece of geoscience made it (albeit briefly) into the newspapers: namely, the idea that gold forms within earthquakes. This was widely reported, for example ABC Science picked it up and reported it.

Gold from the St Ives Gold Mine, Western Australia
A small sample of ancient gold from Western Australia. New research indicates this gold probably formed in an earthquake 2.5 billion years ago.

Although this news seems quirky more than anything, it is of importance for finding more resources. Although gold is commonly associated with jewellery, it has an equally important role in electronics: modern computers, to name one thing, owe their abilities to gold. Mobile phones have about 50c to $1 worth of gold contained inside them, too; and hey, there’s a heap of mobile phones around these days!

So why is this research important? Gold, while being important, is also terribly difficult to find. Very little gold is contained within the earth. In the area of the earth we can look for it, gold is the seventh rarest chemical element. None of the rarer elements are used or desired as widely as gold, either (though they are more expensive).

Finding gold isn’t helped by the way it tends to form nuggets. If you were digging a hole for gold, you could be right next to a large nugget and not even know it. To understand how you find gold without directly stumbling over it, we need to stand back and look at how mineral deposits form in general.

Mineral deposits of any kind form when minerals are concentrated in a particular area. In most cases, this means that the chemicals that make up that mineral have been transported in a fluid – in some cases, transported large distances from deep within the earth. Think of this fluid as a bus on its daily journey: at each stop, it picks up a few more passengers. A couple of passengers might get off during the journey, but the bus is most likely nearly full at a peak time with people.

When the bus, or our fluid, reaches its depot, it deposits all of its passengers. In the case of chemicals in fluids, this is due to some change in either the physical or chemical state of the environment. The points behind the ‘earthquakes form gold deposits’ story are twofold:
1. The pressure drop caused by earthquakes means the gold falls rapidly out of solution; and,
2. The pressure drop works better to deposit gold in smaller earthquakes (to an extent).

This work ties in nicely with some previous work on ancient (more than 2 and a half billion years old!) gold deposits in Western Australia. By looking at the types of faults and cracks the gold sat in, the workers found that the gold deposits are probably formed in aftershocks of large earthquakes (see the abstract at least in Geology here). This matches nicely with the smaller earthquakes being better for depositing gold: aftershocks are smaller and also much more numerous than main earthquakes.

This research is important because it gives us an idea of what environments gold deposits like to form in. This makes finding them that little bit easier. In our bus analogy, if you want to find a passenger from a bus, you’re better off starting by finding the bus depot.

So although this news seemed quirky and irreverent (“Look! gold forms in earthquakes, so let’s dig the San Andreas fault in California!”), it provides some important clues to help us find something that’s been prized since the ancients yet is of a huge importance to our modern way of life, too.