Looking deeper for gold

Materials World magazine
,
1 Jan 2018

The elusive origin of gold may have been discovered, as Ellis Davies reports.

The origin of gold has never had a convincing explanation. An area of mystery since ancient times, the coveted metal has, however, been traced to a possible origin, the deepest regions of Earth, by a team of researchers at the University of Granada (UGR), Spain. The team found evidence of the Earth’s internal movements favouring the ascent and concentration of gold in Argentinean Patagonia, which represents the first register of gold found under the South American continent, at a depth of 70km.

‘The direct application of this research is to have a new approach for searching ore deposits, by using the study, not only of the mineral deposits themselves, but of the whole lithosphere – the crust and underlying mantle,’ said José María González Jiménez of UGR.

The Earth is divided into three large layers – crust, mantle and core. Mankind has only managed to explore the crust of the Earth, where we find rare earths and mining materials. The mantle, however, is the layer separating the nucleus from the crust, and its upper limit is around 17km under the oceans and 70km under the continents. Although we cannot access the mantle, it can reach us through volcanic eruptions, which can bring small fragments – xenoliths – from the mantle to the surface. It is in these xenoliths that researchers found tiny native gold particles that originated deep in the mantle.

The formation of mineral deposits is generally attributed to an origin within the crust in which they are found, but this explanation does not take into account the role of mantle movements and ascents. The researchers say that this new evidence could contribute to a more advanced exploration of mineral deposits that have previously not considered the mantle as an origin. 

Jiménez told Materials World, ‘We have found micro-sized particles of native gold in a mantle-derived xenolith hosted in the lava of a Patagonian volcano. This xenolith is a fragment of mantle rock from >70km deep brought to the surface by a volcanic eruption. The interesting point is that this volcano has sampled the mantle beneath the ore-productive crust of the Deseado Massif. We found the gold particles when studying thin sections of the mantle rock under conventional and electron scanning microscopy.’

The impact of history

Focusing on the region of Argentinean Patagonia, the researchers found a high concentration of gold in the crust, and from this were able to hypothesis that the mantle in that region is unique in having a tendency to generate gold deposits on the surface because of its history – stemming back 200 million years to when Africa and South America were part of the same continent. Their separation was caused by the ascent of a plume from the deep mantle, which broke the crust, causing the separation. The ascent of the plume enriched the environment with metals, which would later generate the conditions for the creation of gold deposits. This process was caused by the movement of a tectonic plate under another, allowing the circulation of metal-rich fluids through the cracks, which precipitated the metals and concentrated them near the surface. 

The researchers hope that their new approach, when tested in other localities, will be useful for predicting future resources.