April 29. Carlin gold

The Roberts Mountains Formation in Eureka County, Nevada, is mostly carbonate, limestone and dolomite that was laid down in a shallow Silurian sea. Interesting rocks, certainly, but geologists and prospectors alike walked over those rocks for decades without realizing it was the host to one of the largest gold deposits on earth. 

They missed the gold because it’s in the rock as tiny tiny grains – often smaller than a micron. A micron is one-one thousandth of a millimeter. Pretty small. Gold was discovered in 1961, and the first mine began production in 1965, near Carlin, Nevada, which gives its name to these “Carlin-type” gold deposits. 

Goldstrike Mine, Carlin Trend, Nevada

What happened is that the limestones and dolomites, which are soluble in even slightly acidic water, were pretty much turned into a really finely porous sponge – many tiny holes were dissolved in the rock. And the water that dissolved the holes – or maybe later water – was mineral rich, and carried gold in solution that precipitated into those little holes. That happened long after the Silurian rocks were laid down around 417 million years ago. The gold mineralization of these rocks probably happened more like 40 million years ago, when Nevada was beginning to be pulled apart and big normal faults were starting to form. Those faults probably served as conduits for the hot, acidic, mineral-rich waters to percolate through the Silurian strata.

Exactly where the mineral-rich waters came from is controversial – did they pick up gold as they passed through older rocks, leaching out the gold and then redepositing it here? Or did the water come out of magma, molten rock, deeper in the earth? There’s geochemical and geophysical evidence to support the magmatic idea, that the stuff came from molten rock deep down in the earth’s crust, but I don’t think this question is fully resolved.

The gold ore at Carlin is typically only 1 to 10 grams of gold per ton of rock. A gram is about the mass of a paper clip, so you can see how the gold must have been really thinly scattered through the tons of rock – but there was a lot of it. The Carlin Trend in Nevada has produced way more gold than the Mother Lode in California, and today The Silver State – Nevada – produces about 80% of all the gold mined in the United States, and more than 10% of world gold production. The Carlin mines passed the 50-million-ounce mark in 2002 and 70 million ounces by 2008, and they’re still going strong. New mines continue to be opened along the 5-by-40-mile zone, whose production is more than $85 billion at 2010 gold prices.

Much of the gold at Carlin is produced from open-pit mines. It’s the second-largest gold district in the world, second to the Witwatersrand in South Africa, where the gold comes from underground mines almost 2½ miles deep, the deepest on earth. With Carlin approaching 100 million ounces of gold, it’s a distant second to Witwatersrand, which has produced about 1.5 billion ounces of gold since it was discovered in 1886. That’s about half the gold ever mined on the planet. The origin of the gold there is about as different as possible from Carlin. The South African gold is related to a huge meteorite impact more than two billion years ago, back in the Precambrian.

The main use – 38% – of gold in the United States is in electronic components, because gold is an excellent non-reactive conductor. Things like computers are among the main consumers. Jewelry takes another third, coins amount to about 19% of consumption, and all the gold in all the new gold crowns and other dental uses adds up to about 5% of US gold each year.

Thanks to Carlin, Nevada, the US is a net exporter of gold, but China produces twice as much. Australia is #2 in world gold production, and the US and Russia are usually about tied for third place. 

—Richard I. Gibson 

Photo of Goldstrike Mine, Carlin Trend, from USGS 

Links:

http://en.wikipedia.org/wiki/Carlin%E2%80%93type_gold_deposit
http://www.geotimes.org/apr06/feature_GoldOrigins.html
http://minerals.usgs.gov/west/projects/nngd.htm
http://goldinvestingnews.com/6070/the-geology-of-the-carlin-trend.html