Scientists have made a groundbreaking discovery about the origins of Earth's gold deposits, shedding light on the intricate processes beneath our feet. A recent study, led by Christian Timm of the GEOMAR Helmholtz Centre for Ocean Research Kiel, has revealed a fascinating mechanism that enriches the Earth's mantle with gold, ultimately leading to the formation of valuable ore deposits.
The research focused on the Kermadec island arc, located northeast of New Zealand. This region is known for its submarine volcanoes, which have been studied over two decades. The team analyzed 66 glass samples collected from these volcanoes, revealing astonishing gold concentrations up to six nanograms per gram. This might seem insignificant, but it's significantly higher than what's found at mid-ocean ridges, another major volcanic source.
What's particularly intriguing is the role of mantle melting. Gold doesn't freely move through mantle rock; it's tightly bonded to sulfide minerals. For gold to be released into magma, these sulfides must break down, and that requires high temperatures. The Kermadec glasses provided crucial data, showing that melting occurred at temperatures above the sulfide liquidus, where liquid sulfide dominates and allows silver and copper to behave similarly.
The study also uncovered a 'second melt' phenomenon. During partial melting, copper is more readily extracted into magma than gold. This results in a higher gold-to-copper ratio in the residual rock. If this rock melts again, the new magma inherits this elevated ratio. The most gold-rich samples came from the northern segment of the arc, particularly the Putoto volcano, indicating a history of repeated melting events.
Interestingly, the study refutes the idea that fluids rising from the subducting Pacific Plate are the primary source of gold. Instead, it suggests that water mainly facilitates mantle melting, and the key factor for high gold concentrations is the high and repeated degree of melting. This process is a slow, repeated 'cooking' of the same rock, transforming trace amounts of gold into enriched magmas.
The implications of this research are significant. While the gold concentrations in these magmas are not yet mineable, understanding the magmatic starting point is crucial. Magmas with more gold in the upper crust provide hydrothermal fluids with a richer source to work with. The Kermadec data indicates that arc magmas in settings with depleted, repeatedly melted mantle have a larger gold endowment compared to magmas elsewhere.
This study represents the most comprehensive regional analysis of gold behavior in primitive arc magmas to date. It highlights the intricate relationship between mantle melting, gold enrichment, and the formation of valuable ore deposits. As Timm notes, the alchemy of gold begins long before it reaches the surface, with the transfer of gold from the mantle into a melt that eventually forms volcanoes.
