Wonders of the Universe

THE RAREST OF ALL

Once the centre of the great American gold rush, the 16-1 mine is one of the few gold mines still operating in the state of California today. Digging for gold there with the miners was an enlightening experience. As I peered at seemingly ordinary rocks, I could see glints and glimmers of a familiar yellow colouring, revealing the stones’ precious hidden cargo of gold.

The first twenty-six of the elements are forged in the cores of stars and are distributed through the Universe in their inevitable collapse. But what of the other seventy-two – some of which are vital for life, and many of which we hold most precious? If they are not formed within stellar furnaces, what could their origin possibly be?

In the remote forests of northwestern California, the mountains still hide a secret that made the quiet pine woods the ultimate destination for fortune seekers only a century ago. Although they’re empty today, in the late nineteenth century this was the centre of the California gold rush. Hundreds of thousands of people arrived here, trying anything and everything to get rich, from simple panning to the most advanced mining techniques available. Gold worth billions of dollars was extracted, fuelling the rise of one of the world’s great cities, San Francisco. The insatiable appetite for gold has waned today, but in the forests around Lake Tahoe, the 16–1 mine remains one of the few gold mines still operating in the state of California.

For almost 100 years, miners have been digging for gold in the 16–1, and it is still one of the richest gold deposits in the world, due to a quirk in the local geology. The unique thing about California is that it sits on the divide between the North American tectonic plate and the Pacific tectonic plate. The whole region is one enormous fault line, with thousands of smaller faults running through the rocks of the mountains. When you travel into the mine, which is nothing more than a series of horizontal tunnels at gentle gradients hollowed out of the mountainside, you can see these fault lines everywhere; they reveal their presence as visible boundaries between rock and quartz – a maze of mini-faults. One hundred and forty million years ago, in the Jurassic period when the dinosaurs were running around above the mine, hot water bubbled up and flowed through this rock, carrying a precious cargo. Its water was laden with gold brought up from deep within the Earth, deposited through the seams of quartz. For the last 100 years all the miners have had to do is to follow quartz seams laced with shimmering gold.

The gold that runs all the way through the quartz in the 16–1 mine is unusually pure, at anything up to 85 per cent, and the thick tendrils snaking through the rock glint and glimmer that familiar yellow in the sunlight. The rest is about 14.5 per cent silver, with traces of heavier metals. The area is so rich in gold that it can even be found as simple pure nuggets that can be picked up off river beds, and at the 2010 price of around £900 per troy ounce, it’s obvious why mines like this are still in operation.

If you stop to think about it though, there’s something a bit odd about the value we attach to gold. Throughout history people have gone to extraordinary lengths to get their hands on it, which is odd because it isn’t particularly useful for anything. Copper and iron will help you survive, but gold is next to useless. Most of the gold that we’ve struggled to extract has ended up as jewellery. The only thing that gold has going for it, other than being shiny, is that it is incredibly rare, and this is what drives up its price. All the gold dug out of the ground throughout all of human history – with all the associated tragedy and elation, hardship and riches – would just about fill three Olympic-sized swimming pools.

All the gold dug out of the ground throughout all of human history would just about fill three Olympic-sized swimming pools. It is this almost vanishing scarcity that makes gold so valuable.

It is this almost vanishing scarcity (three swimming pools relative to the size of a planet) that makes gold so valuable; it is just one of many rare elements that are to be found in the most minute of traces within the Earth.

There are over sixty elements heavier than iron in the Universe, some are valuable, such as gold, silver and platinum; some are vital for life, such as copper and zinc; and some are just useful, such as uranium, tin and lead. Very massive stars can produce very tiny amounts of the heavier elements up to bismuth-209 (element number 89) in their cores by a process called neutron capture, but it is known that this makes nowhere near enough to account for the abundances we observe today. There simply haven’t been enough massive stars in the Universe.

The conditions necessary to produce large amounts of the elements beyond iron are only found in the most rare of all celestial events. Blink and you’ll miss them, because in a galaxy of 100 billion stars the conditions violent enough to form substantial amounts of these elements will exist on average for less than two minutes in every century