Granular segregation and the Gold Rush

Anyone walking on the beach or through the dunes will often notice areas of dark gray smeared across the surface, often outlining ripples, as if an artist had highlighted the sand's topography with charcoal. I encountered many examples of such designs recently at the dunes of Kelso (above) and Oceano, and on California's beaches. Look closely, and you'll see that these patterns are characterised by concentrations of dark grains, gathered together in a much higher proportion than elsewhere in the sand. Look even more closely, down a microscope, and there they are, little black nuggets nestling among the glittering quartz grains (the example below is from Kelso).

 

As my messing around with kitchen physics showed, sand, like all granular materials, dislikes being mixed and will find endless ways of sorting itself out into its components. Pouring and shaking cause it to segregate by size and shape, transporting it by wind and water differentiates grains by weight (as well as by size and shape), and that's what's going on in the dunes and the beaches. The black nuggets are grains of a mineral much denser than the quartz grains of the same size, and they are therefore slightly less easily moved. With time, the lighter grains are winnowed out and the heavier ones left, concentrated. In the case of the Kelso sand, the heavier mineral is magnetite, an iron oxide; I suspected this, and repaired, again, to the kitchen, to get a fridge magnet which I applied to the sand with the result that the fine black grains flew onto the magnet, piling themselves up into minute towers (see the photo at left; warning - removing the grains from the magnet is far more difficult and can result in scratched white goods).

The natural concentration of minerals by wind, waves and currents is economically important - large deposits of iron were formed this way, today and in the geological past. Such a natural concentration was the cause of one of Thomas Edison’s many business failures. On a fishing trip with friends off the coast of Long Island, Edison put into shore for lunch and found the beach covered with a layer of black sand. He took some home (perhaps to his kitchen) and discovered that the black grains were magnetite. Edison’s enthusiasm ran, as it often did, ahead of his business sense, and he immediately arranged for the purchase of the beach and the manufacture of separating machinery. Unfortunately, by the time he and his colleagues returned to Long Island, a winter storm had reworked the beach and completely removed the black sand.

These kinds of mineral deposits are called placers - in addition to iron, platinum, tungsten, titanium, tin, niobium, zirconium, and other vital elements are all sourced from placers - plus other treasures. The composition of sands betrays their origins, and if they came from the crumbling of precious mineral deposits, then they will contain precious minerals, often more easily accessible than by mining the hard rocks of their origins. Diamonds, rubies, sapphires, garnets, and gold are mined from placer sands in many parts of the world; in places like Namibia, beaches have been stripped to bedrock in the search for precious gemstones. Gold, being heavy and un-reactive, makes for an ideal placer mineral, and such deposits have been exploited for as long as we have been obsessed with gold. An ingenious early method (probably used in ancient Egypt) employed a fleece bag, the woollen side facing inward: water and sediment were passed through the bag, and the heavy gold flakes became embedded in the wool, remaining behind when the bag was emptied of lighter sand and gravel. A similar method was still being used in the mountains of the Caucasus in the 1930s; it also explains Jason and his Golden Fleece.

California was founded on placer sands: forty-niners during the Gold Rush sought the metal not only in subsurface mines, but in the streams and rivers that drained the gold-bearing ores. Panning the stream sediments was backbreaking work, and so a technological breakthrough was called for. It happened in the form of hydraulic mining: miners used high-powered water hoses to erode the valley sides (photo below, USGS). The gold, being heavier than the rest of the dirt, collected in sluices, and everything else drained away; mercury was commonly used to further concentrate the gold. It has been estimated that over a twenty-year period 750 million dump truck loads of sand, mud, and gravel, together with mercury, were flushed into the Central Valley—with dire environmental consequences. The Yuba Valley is just one of the watersheds that remains physically and chemically scarred today, and it can be argued that the environmental movement in the US has its roots there. A few years ago, the New Scientist had a fascinating piece on this - it's only available to subscribers, so I have reproduced it in full at the end of this post. And for further details, go, as usual, to the wonderful USGS search box and put in "hydraulic mining." And where did all this sedimentary debris from hydraulic mining end up? Well, a lot of it is in San Francisco Bay, moving out into the Pacific and forming some remarkable sedimentary features on the way - but more of that in a later post.

And, while I'm at it, I was given by a friend a sand sample from a Yosemite lake, a sample that was reported as containing flakes of gold. I've had a quick look; it's full of the hardly surprising debris of the weathering of the Sierran granites, including many flakes of yellowish mica, red herrings in the search for gold. But, in the middle-right of the photo below is a single grain that looks different - it's still a flake, but more chunky than the typical micas and with a different surface lustre. I'm no mineralogist, and certainly not a precious mineralogist, so if anyone has a view as to whether or not this might be gold, then I'd like to hear from you - but can't yet afford to offer a reward!

  

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From the New Scientist, 12 November, 2005, by Richard Lovett

Histories: The day the gold rush stopped

Although American environmentalism did not begin to flourish until the 1960s, the movement's roots date back to 1878 and a town meeting in Yuba City, California. Not that the assembled townsfolk saw themselves as doing anything so altruistic as protecting nature. Their concern was to save their land and livelihoods from torrents of mud unleashed on them by gold mines upstream. Angry and desperate, they formed the Anti-Debris Association, hired lawyers and went on the offensive. The fight that followed was so bitter it's amazing no blood was spilled. At one point, the association armed an anti-debris militia of 70 men - but law prevailed, and in a decision nearly a century ahead of its time a federal judge decreed that an estimated 45 tonnes of gold must remain in the ground.

IN FEBRUARY 1852, a Connecticut Yankee called Edward Matteson was working his gold claim at a place called American Hill in the Sierra Nevada foothills. There was plenty of gold in the compacted gravel of his claim, but it was widely dispersed and would take months to dig out with a pick and shovel. Perhaps, Matteson thought, there was a better way.

Matteson, like other miners in the early years of the California gold rush, had been using techniques that had hardly changed in millennia. The concept was simple: if you shovelled gold-bearing soil into a sluice box and scoured it with fast-moving water, the lighter sand, gravel and mud would be washed away, leaving the precious metal behind.

Unfortunately, shovelling dirt into a sluice box is hard, time-consuming work - and dangerous if you are digging into the base of a steep cliff. Matteson's brainwave was to attach a canvas hose to a tank of water high above him. Then he could stand safely back and direct the hose so that the water did the excavating and then channelled the debris along a system of ditches into his sluice box.

The Romans had much the same idea: they built dams and ditches to direct water where they wanted it, breaking the dams to create artificial flash floods along the channels. Matteson's innovation was to send the water through a nozzle, magnifying the force of the flow and allowing him to direct it precisely where he wanted.

Thanks to his high-pressure hoses, Matteson's operation became so efficient he could do several weeks' worth of work in a single day. But the new technique, called hydraulic mining, also sent weeks' worth of sediment downriver each day.

Word spread and soon hydraulic mining was booming. Most of the early mines were small and their environmental effects predominantly local. And in the early 1860s severe drought forced many to close. But when the rains returned, so did the miners, this time backed by wealthy investors from as far away as England.

"Hydraulicking" worked best along an 80-kilometre belt of the Sierra Nevada at elevations between 1200 and 1500 metres, where huge deposits of gold-bearing sediments had accumulated in ancient streambeds. There were mines everywhere, but by far the largest was Malakoff Diggins, which was so efficient that it could profitably mine sediments containing only a few pennies' worth of gold per cubic metre.

The mine was run by the well-financed North Bloomfield Gravel Mining Company, which ploughed $3.5 million into the operation. The Diggins eventually produced more than 6 tonnes of gold.

Today all that's left at the site is a 250-hectare pit 200 metres deep. But at the peak of operations, crews laboured day and night in the red-tinted gravels with hoses whose "Little Giant" nozzles could shoot 20-centimetre-thick jets at speeds of up to 50 metres per second - enough force to kill any miner who got in the way. In 1879 a reporter from the San Francisco Bulletin spoke of huge rocks being washed away "like chaff", with "a cloud of red foam" hanging above the points where the water hit the cliff. When water wasn't enough to do the job, entire hillsides were loosened with gunpowder - sometimes 15 to 20 tonnes of rock in a single blast.

When a US federal judge called Lorenzo Sawyer visited the site in the 1880s, he too was impressed. "The excavating power of such a body of water, discharged with such velocity, is enormous," he wrote. He was particularly amazed by the mine's night-time operations, conducted under "brilliant" lights that ran on hydroelectricity. "A night scene of the kind, at the North Bloomfield mine, is in the highest degree weird and startling, and it cannot fail to strike strangers with wonder and admiration."

By 1880, 'hydraulicking' had buried 6000 square kilometres of farmland

But Sawyer wasn't there to admire the scene. He was there because of where all that water, mud and silt was going. The first part of its journey was along a 3-kilometre tunnel through a mountain. The tunnel was effectively a vast sluice box that caught as much as 90 per cent of the gold. The debris, however, carried on into the Yuba river and towards the farmlands below.

In 1880, California's state engineer had estimated that 6000 square kilometres of farmland had been buried by mining debris. Sand bars had sprung up in rivers all the way to the coast. Hardest hit were the towns of Marysville and Yuba City, directly below the mines. As debris built up in the riverbed, the water rose so fast that the levee builders could hardly keep pace. But by now the people of Yuba City were fighting back. In 1878 they had banded together to form the Anti-Debris Association and sued the owners of the mine. Six years later, a panel of judges headed by Sawyer decided the case.

Judge Sawyer's 56-page ruling is a litany of environmental horrors, but the worst were his projections for the future. So far, the North Bloomfield mine had dumped 90 million cubic metres of debris into the Yuba river. About a quarter of that had reached the lowlands. The rest was still perched upstream, clogging some canyons to depths of up to 50 metres and creeping downward with each spring flood. If there was a really big flood, there was a chance the whole lot would come sliding downriver.

Worse, Sawyer estimated that there were 500 million cubic metres or more of gravel still to excavate. There was no choice but to shut down the mine under the ancient law of nuisance, which prohibits using your property in ways that damage someone else's.

Sawyer didn't ban hydraulic mining outright, though. He ruled that if the miners were to continue, they must find a way to keep the debris on their own property - and prove in advance that it worked. The miners appealed for leave to experiment with remediation techniques while continuing their operations. But Matthew Deady, one of Sawyer's fellow judges, gave that idea short shrift. Asking the people in the valley below to put up with that, he said, "may be likened, at least, to living in the direct pathway of an impending avalanche".

Sawyer's judgement was remarkable because it pre-dated America's best-known environmental rulings by more than 80 years. The stakes were enormous: the judgement would determine California's future as either an agricultural state or a mining state. But Sawyer didn't seem to see it as a trailblazing decision. He was simply applying established law, albeit on an unprecedentedly large scale.

It took years to force out miners who flouted the ruling, but the court's decision effectively killed off hydraulic mining in the California goldfields. Hydraulicking itself has never died: it has been used elsewhere to mine everything from coal to rubies and even aluminium. The difference is that today's mines exploit higher-grade ores, which make it economical to trap the tailings and recirculate the water in closed loops.

The hydraulic miners' legacy still surfaces from time to time in California's courts. In 1986 a flood breached levees built nearly 100 years earlier out of debris washed down from the North Bloomfield mine, resulting in lawsuits over whether such materials were adequate for flood protection. And in 1995 the California supreme court took on the tricky question of who owns those sandbars created by the shifting sediments from the old mines, some of which are now prime sites for urban development.

But nobody ever found an environmentally sound way to reopen the Malakoff Diggins. Today it's a little visited state historical park, even though, if Sawyer's back-of-the-envelope estimate is correct, gold worth $650 million remains in the ground.

Comments

[SteveN]

excellent post. I recommend visiting the Malakoff Diggins to anyone who hasn't yet been there. It's very impressive to see just how big a hole hydraulic mining can dig.

[Lockwood]

The tabular nature and luster of that grain look more like weathered biotite than gold. Biotite often takes on an almost metallic luster (particularly at certain angles) and a golden color as it weathers. If you can find it again under a binocular microscope, try pinning it down and pressing on it with a pin or needle; gold will deform and dent in response, biotite will fragment and shear along the cleavage.

Cool post.

[Lost Geologist]

I'm always uncertain in foto-identifying so don't take this too seriously. The light and colors never quite reach the true values. Though the flake you mention is a little too angular, not the right kind of brightness and a little too rough for my taste of gold.

If you could seperate this flake and test its hardness with a tiny steel needle...perhaps we could have a more positive identity.

[Sandglass]

Thanks for the comments!

I'll make a point of visiting the big hole at Malakoff Diggins when I get back to California (as I intend to) and I'll see if I can relocate that grain (or one like it) and give it a prod with a needle - I must admit I was somewhat suspicious. If I come up with anything, I'll report it.

[Kevin P. Rice]

Ah! And I've learned something, yet again! The meaning behind the name: Placer County, California -- Gold Country. Thanks!

[Water Damage Restoration]

Biotite often takes on an almost metallic luster (particularly at certain angles) and a golden color as it weathers. If you can find it again under a binocular microscope, try pinning it down and pressing on it with a pin or needle; gold will deform and dent in response, biotite will fragment and shear along the cleavage.

Cool post.

[Sandglass]

Thanks for the tip - biotite it is! I will have to seek my fortune by other means....