Gold rush

As they say, ‘Gold is where you find it’ – gold mineralisation has a great diversity of settings. One of the oddest gold mines is the Ladolam deposit on the island of Lihir off Papua New Guinea — it is also one of the largest, with reserves of around 1300 tons (~41 million troy ounces). There, gold is being extracted from an open pit, cooled by water injection, in the crater of a geothermally active volcano. Aside from that it is one of many different kinds of hydrothermal deposit in which metals are transported and deposited by a plumbing system that delivers hot watery fluids. The hydrothermal system on Lihir is obviously still active, and it is possible to sample the fluid itself by drilling to depths up to a kilometre. Deep sampling is needed to obtain pristine fluids, uncontaminated by mixing with groundwater. Their chemical composition trns out to be surprising (Simmons, S.F. & Brown, K.L. 2006. Gold in magmatic hydrothermal solutions and the rapid formation of a giant ore deposit. Science, v. 314, p. 288-291).

The ground in which the deposit occurs is a breccia produced by explosive decompression when the volcano collapsed in its last magmatic throes, at about 400 ka. It is this brecciation that provided the intricate pathways in which gold was able to precipitate from the hydrothermal fluids. The samples have deuterium and oxygen isotopes that show that it is derived directly from magma. The fluid is extremely saline with very high chloride and sulfate ion concentrations. Around 50 kg of the fluid reaches the surface every second. Because it contains about 15 parts per billion of gold, it is possible to estimate how long it might have taken to produce the gold ore body: a surprisingly rapid 55 thousand years at the current rate of 24 kg of gold per year. Even more surprising is that the Lihir hydrothermal fluid is not particularly rich in gold compared with the fluids emerging from some active volcanoes. For instance Mount Etna is estimated to be delivering up to a tone of gold every year. However, before setting off on a gold rush to extinct volcanoes in the last hydrothermal phase, it is worth bearing in mind that forming a super-rich giant gold deposit requires that both gold transport and deposition are closely synchronised in a small volume of rock, otherwise the gold merely ends up in such a vast volume of rock that its extraction is not economic.

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