Gold Can Form in a Geological Instant
You can’t say something is old just because it looks old, like gold.
Gold may be a symbol of timelessness and purity – and as an element, it makes an apt symbol – but precious metal deposits have a history in the crust of the Earth. A surprise announcement in Science1 Oct. 13 by New Zealand geologists showed that large gold deposits can form from volcanoes in just thousands of years – not millions. Christoph Heinrich [U of Zurich] commented on this paper in the same issue of Science2 and asked, “How fast does gold trickle out of volcanoes?” The answer depends dramatically on the conditions.
Although the authors of the paper came up with an estimate of 55,000 years for a large gold deposit in Papua New Guinea, a closer reading of the two papers shows that there are many variables and uncertainties. There is no reason to rule out the idea that under proper conditions, large gold deposits could form very rapidly – in centuries or less. Consider these qualifiers and admissions in the two papers. First, the original paper by Simmons and Brown:
- The Ladolam hydrothermal system, on Lihir Island, Papua New Guinea, hosts one of the youngest and largest gold deposits in the world. Several deep (more than 1 kilometer) geothermal wells were drilled beneath the ore bodies to extract water at >275°C and to facilitate open-pit mining. Using a titanium down-hole sampler, we determined that the deep geothermal brine of magmatic origin contains 15 parts per billion gold. At the current gold flux of 24 kilograms per year, this deposit could have formed within 55,000 years. The combination of sustained metal flux and efficient metal precipitation led to the formation of a giant hydrothermal gold deposit in a short period.
- The origins of giant hydrothermal gold deposits are enigmatic. This is because the concentrations of precious metals and flow rates of ore-forming fluids are poorly quantified, and the origins of the metals are unclear.
- The deep geothermal brine contains high gold concentrations [13 to 16 parts per billion (ppb) Au], and these values greatly exceed those measured (0.05 to 0.2 ppb Au) in high-temperature submarine hydrothermal fluids. Except for Sb and Pb, the proportions of Au, Ag, Cu, Mo, Zn, and As in the deep geothermal brine match those in the ore (Fig. 3), suggesting that these elements were not fractionated during deposition.
- The Ladolam heat flow (50 to 70 MW) is modest compared with that of well-known geothermal systems such as Wairakei (420 MW) and Waiotapu (540 MW) in New Zealand. The overall Ladolam gold flux is 24 kg/year, and only 55,000 years would be required to account for all the known gold in the Ladolam ores if we assume constant aqueous gold concentration and fluid flow (50 kg/s), and 100% deposition.
- Although the Ladolam gold concentrations are lower than a theoretical upper limit of 10,000 ppb, as determined by inclusion fluid analyses and calculations of magmatic hydrothermal solutions, the deep Ladolam brine is capable of forming a giant gold deposit within tens of thousands of years.
- That gold transport and deposition operated effectively and in concert on a time scale of several tens to possibly several hundreds of thousands of years emphasizes the importance of synchronizing these processes to generate a giant deposit.
But while the 55,000-years figure makes a nice sound bite, Christoph Heinrich speculates that the actual process could have occurred drastically faster and taken orders of magnitude less time. Notice carefully bullet 5:
- Direct analyses of the gold concentration in these recent fluids demonstrate an intimate link of ore formation to magmatic processes and indicate that metal enrichment occurred in a geologically short period of time. The authors imply that gold mineralization may even go on today, while ore is mined from steaming hot ground in a giant open cut…
- How much fluid is required to form a major ore deposit, and how long does the process take–centuries or millions of years?
- Simmons and Brown calculate that it took 55,000 years to accumulate the 1600 metric tons of gold now contained in the Ladolam deposit. These results open up new questions and will stimulate the ongoing debate about the connection between magmatism, geothermal activity, and ore formation.
- In light of the geological history of Lihir, these observations are consistent with a story for the formation of the giant Ladolam deposit that is even more spectacular than the one envisaged by Simmons and Brown. Most of its gold ore is contained in minerals cementing a highly fragmented rock, which was produced by a dramatic event about half-a-million years ago, when the peak of a former volcano built high above the present area of the deposit collapsed and formed the present semicircle of mountains around the deposit (see the first figure). This sector collapse would have led to sudden decompression of magmatic-hydrothermal fluids beneath the volcano, which originally could have been orders of magnitudes more gold- and sulfur-rich.
- Could a rush of rapidly expanding fluids have formed the deposit in an even shorter period of time than calculated by Simmons and Brown, just after this dramatic event and maybe even on the time scale of a human life? And could the extraordinary geothermal waters sampled today be a mere trickle representing the “spent” ore fluid…?
- Precise radiometric dating of mineral deposition may help to answer these questions, but the interpretation of such data could be challenging. The rocks have been kept at high temperature to the present day, potentially allowing all isotopic clocks to reset themselves continually.
See also the summary of this paper reported on LiveScience: “A giant gold deposit could form in an eyeblink of geologic time, scientists announced today.”
1Simmons and Brown, “Gold in Magmatic Hydrothermal Solutions and the Rapid Formation of a Giant Ore Deposit,” Science, 13 October 2006: Vol. 314. no. 5797, pp. 288-291, DOI: 10.1126/science.1132866.
2Christoph A. Heinrich, “GEOCHEMISTRY: How Fast Does Gold Trickle Out of Volcanoes?”, Science, 13 October 2006: Vol. 314. no. 5797, pp. 263-264, DOI: 10.1126/science.1134456.
Centuries—or within the span of one human lifetime—not millions of years could have been all the time needed to form massive gold deposits. This is rapid geology with a vengeance. All that’s necessary are the right conditions, and it’s gold, gold, GOLD! Imagine James Marshall watching an event like that near Sutter’s Mill.
Remember, Simmons and Brown measured 15 ppb of gold in the brine, but there is a theoretical upper limit of 10,000 ppb, and Heinrich thinks that what we are seeing today could be the tail-end trickle of a “spectacular” event. He claims this volcanic dome collapse was half a million years ago, but what does he know? Just a little while ago, these guys thought gold deposits required millions of years. No human observer was there, but one thing is clear from this report: big things can happen much faster than uniformitarian scientists thought possible.
Don’t miss that last bullet by Heinrich. Some conditions can also cause “all isotopic clocks to reset themselves continually.” Remember that when you hear scientists proclaim confident-sounding dates in the millions and billions of years. They don’t know, and the rocks aren’t telling.
Conditions before, during and after a worldwide flood would have been extremely non-uniform. With the fountains of the great deep bursting from many parts of the world, and volcanic activity occurring on unprecedented scales, it’s possible to envision huge upwellings of hot metal-saturated brine rising up at rates never seen before or since. If that’s how some of these deposits formed, then perhaps God brought some extra blessing out of the devastation of the flood. For the weary descendents of Noah trying to eke out an existence on a cursed planet, he provided a novel form of recreation: the treasure hunt.