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Want to strike it rich? Then listen up: there might be new gold in them there hills. Not just undiscovered gold, you understand, but brand new deposits. It seems the glistening metal that remains the ultimate symbol of wealth may be able to collect far faster than anyone thought possible. Giant deposits of the stuff could be laid down within a human lifetime, or even in a matter of days.

For as long as anyone has understood the basics of geology, it has been assumed that it takes millions of years for gold ore to be deposited. Now investigations of a mysterious mine at the centre of a volcano on Lihir Island in Papua New Guinea are changing all that. 鈥淛ust when we thought we had a handle on how gold forms, the rapid formation at Lihir is turning everything we鈥檝e known on its head,鈥� says Mark Hannington, an expert in economic geology at the University of Ottawa, Canada.

The discoveries are not only changing our understanding of the processes that lead to the formation of gold deposits, they could also transform the way prospectors search for their precious prize.

Gold is hard to find, of course; if it wasn鈥檛, it wouldn鈥檛 be so valuable. Normally scattered through the Earth鈥檚 crust at trace levels, it sometimes becomes concentrated in deposits containing myriad metallic specks, usually too small to be seen with the naked eye. A variety of mechanisms are involved in the formation of these deposits, everything from highly pressurised rocks oozing gold-rich fluid deep underground, to rivers leaving traces of gold residue on their banks.

The precious stuff can also be deposited by hot springs and other hydrothermal systems in which water from deep underground, heated by molten rock or magma, rises to the surface carrying dissolved gold with it. It is now estimated that one-fifth of all gold deposits in the world, including the one on Lihir Island, are formed this way.

Whatever the mechanism, everything seemed to point to the process being an extremely slow one. In the early 1990s Edwin McKee of the US Geological Survey used radiometric dating 鈥� a technique based on the rate of decay of isotopes into stable elements 鈥� to assess gold deposits in the Andes, and concluded that it took a million years or more for the deposits to form. Studies of other precious metals have since suggested they might form on shorter timescales, but overall the slow and steady theory of gold ore formation continued to hold sway.

To create a gold deposit, three things must exist: a gold source, a means of transport and a trap. For hydrothermal systems, researchers figured out that magma was the likely source. As plumes of the molten rock start to cool and crystallise deep within the Earth鈥檚 crust, they exude a layer of water-based fluid that rises towards the surface. It has long been known that this fluid is rich with dissolved minerals, including gold and sulphur volatiles, and since the 1970s this was thought to be the means of transport. However, the mechanism of the trap, which causes gold to precipitate out of this fluid as pure metal, remained a mystery.

The means of transport didn鈥檛 quite fit either. Measurements of active hot springs that seemed likely sources of future gold deposits were not turning up enough dissolved gold. Concentrations were so low that it would take millions of years to form a substantial deposit 鈥� far longer than the molten rock underneath was thought to last.

Then in 1985, Kevin Brown, a geochemist at the Wairakei Geothermal Research Centre in New Zealand, made a startling discovery. The steam that drove the plant鈥檚 turbines was generated by allowing superheated, pressurised water from deep underground to expand through small holes in metal plates held at the surface. As some of the water evaporated it left behind a hard, greenish build-up around the hole in one such plate, and when Brown tested this he found it contained a surprisingly large amount of gold. 鈥淚t blew us away when we actually found out what it was,鈥� says Brown, who is now an independent consultant.

He realised that the water everybody had been testing at hot springs had already lost the vast majority of its gold. As water and sulphur volatiles changed from liquid to steam near the surface, he reasoned, they left behind a residue of minerals, including gold, that had been dissolved in the fluid. If he was correct, he had found the missing link in how hydrothermal systems form gold deposits: the fluid was the means of transport, and the trap was a decrease in pressure that boiled off the sulphur, changing the properties of the solution and causing gold to precipitate out of the remaining water. Crucially, this also meant that deposits might form much faster than anyone had thought.

To find out for sure, he needed to obtain water samples from beneath a gold deposit where the hydrothermal system that had created it was still active. There was only one such place, the colossal Ladolam deposit on Lihir Island, one of the youngest and largest gold ore deposits in the world, containing more than 1000 tonnes of gold (see Diagram). There was just one problem: the fluid he needed to sample would be a good kilometre below the surface.

For 20 years, Brown worked on and off to collect the sample. His chance came after miners drilled a series of deep wells under the Ladolam deposit to reduce the temperature and pressure of steam trapped beneath the mine, for safety reasons. Their orientation posed a problem, however. Drilling for some wells began with a vertical shaft sunk at the perimeter of the deposit, which then changed direction to pass underneath it. To capture a sample, Brown needed to devise a container that could be lowered straight down, then crawl at an angle. Once in position, it had to fill with fluid, seal itself shut while deep underground, and return to the surface without leaking a drop of its precious cargo.

The sampler Brown eventually produced was a 2-metre-long titanium cylinder fitted with wheels that enabled the the device to roll downwards under the influence of gravity. Mounted on the side of the sampler near the top was a pair of hinged poles initially folded back against the side like an insect鈥檚 wings. Once the device was lowered into position, a tug on a cable would raise the sampler to wedge the wings into the sides of the well. This triggered a mechanism that drove a sharp needle into a thin sheet of metal on top of the cylinder, making a hole 2 millimetres in diameter. The high-pressure well water would force its way in through a one-way valve, and remain trapped inside the sampler.

鈥淎ll the gold mined in history, 193,000 tonnes, would fit in a cube with sides 22 metres long鈥�

When Brown and geologist Stuart Simmons of the University of Auckland in New Zealand analysed the water, their idea was confirmed. The water contained 15 parts per billion of dissolved gold 鈥� a thousand times the highest concentration ever recorded in the surface waters of hydrothermal systems. The researchers calculated that, given the rate of flow of water in the Lihir system, 24 kilograms of gold are being added to the Ladolam deposit each year, and that the entire ore deposit could have formed in as little as 55,000 years (Science, vol 314, p 288).

Even this rapid rate of formation may be too conservative, according to calculations by Christoph Heinrich of the Swiss Federal Institute of Technology in Zurich. Heinrich has spent more than a decade analysing microscopic samples of hydrothermal fluids trapped inside quartz crystals that formed several kilometres beneath the surface. Smashing the crystals liberates trace amounts of the fluid, and when Heinrich analysed this he found gold concentrations much greater than those at Lihir.

So he thinks the hydrothermal system investigated by Brown and Simmons may have passed its prime and could once have contained greater concentrations of gold. 鈥淲e have found fluids that had a thousand times higher concentrations,鈥� Heinrich says of sites he has studied in Argentina, Indonesia, south-east Europe and the US. 鈥淚f you spin the same argument that they are using with a thousand times higher concentrations, then the time it takes might have been a thousand times shorter 鈥� 50 or 60 years.鈥�

Perfect storm

How so? The Lihir deposit could have formed during a single cataclysmic event, Heinrich suggests. It is known that an eruption on Lihir about 400,000 years ago sent a quarter of the volcanic island tumbling into the sea. This collapse could have allowed the gold-rich fluid below what remained of the volcano to shoot to the surface and precipitate quickly into a concentrated deposit (see Diagram). 鈥淢y best guess is it happened within 50 years, shortly after this collapse,鈥� Heinrich says.

Now we鈥檙e talking 鈥� but could it have happened even faster? Greg Hall, former chief geologist and exploration manager for the Canadian gold-mining company Placer Dome, says it could. 鈥淢y gut feeling looking at Lihir is that it formed in the same time it took Mount St Helens to blow up 鈥� a month, a day, maybe as short as 5 hours,鈥� Hall says. He believes there was a 鈥減erfect storm鈥� of circumstances that came together to form the massive deposit. 鈥淵ou need a big reservoir of magma that is pregnant with gold-rich solution, but you also need a trigger that is fast, like the top of a volcano blowing off, so that you can release the solution instantaneously.鈥�

It wouldn鈥檛 always be that fast. 鈥淚 think you are going to find there is huge variation,鈥� McKee says. 鈥淕old deposits can probably form anywhere from on the order of a million years to a matter of hours.鈥�

So where will the next gold rush occur? Rather than wait for nature to take its course, Simmons and Brown have been trying to use their results to engineer an artificial gold trap. In the 1990s, after Brown discovered gold precipitating in the geothermal plant, he built a couple of vessels that collected gold from water passing from high to low pressure in wells near another New Zealand plant. Each vessel, about the size and shape of an oil drum, contained a series of about 50 steel plates riddled with small holes. The holes allowed water to flow through the vessel while the plates provided a large surface area onto which gold could precipitate.

Disappointingly, the yield wasn鈥檛 worth the effort. 鈥淚t would cost NZ$3 million [US$2 million] to get half a million of gold,鈥� Brown says of the estimated annual cost and revenue, assuming it ran for 10 years. Much of the cost comes from building a vessel to withstand such high pressures. Other costs come from losses at the plant when the well has to be shut down to scrape the gold off the metal plates. 鈥淓very hour that the power station is offline you are just haemorrhaging money,鈥� he says.

Prospects might be improving, however. The price of gold has nearly doubled since Brown鈥檚 first collector was built. And in September last year, the US Department of Energy flew Brown to Tucson, Arizona, to speak about the possibilities of artificial traps at a conference on mineral extraction from hydrothermal waters. Along with Simmons, Brown is now trying to secure funding to test new vessel designs. One possibility is a two-vessel system in which the vessels could be alternately switched in and out to avoid interrupting the plant鈥檚 steam supply while the gold is harvested. Increasing the surface area of the plates and finding materials that speed up the deposition could also help.

鈥淭his is never going to be the next Lihir,鈥� Brown admits, 鈥渂ut it might be a profitable sideline for a power station. The hydrothermal system is already doing the mining for you. All you have to do is extract the gold.鈥�