A profound geological mystery has been solved deep beneath the South Pacific, where researchers have identified the specific mechanisms that create Earth's richest gold deposits. A team led by Dr Christian Timm, a marine geologist at the GEOMAR Helmholtz Centre for Ocean Research Kiel, has uncovered the key mechanisms behind the formation of the Earth's largest gold deposits in the deep water-covered regions of the South Pacific as part of his recent study, published in Communications Earth & Environment. The study demonstrated that volcanic island arcs like the Kermadec Islands, New Zealand, act as ‘gold factories’.
The research that the growing concentration of gold in deposits is not coincidental, but results from a series of processes involving repeated melting of the Earth's mantle. The research team proved that the gold was distilled from the mantle and concentrated in the magma through a series of stages prior to arriving at the sea floor by studying volcanic glass. In addition, the research showed that the water released from the subducting tectonic plates acts as a flux that facilitates the enrichment and concentration of gold in the magma before reaching the ocean floor. This geological cycle explains why specific oceanic zones become disproportionately enriched, offering a strategic blueprint for locating future underwater resources.
The ‘gold factory’ beneath the Kermadec Islands
Research published in Nature has revealed that gold can only be enriched under ‘hydrous’ (water-rich) conditions via numerous melting events. When an ocean plate like the Pacific Ocean Plate is being subducted beneath the oceanic portion of the Australian Plate, water in the form of fluids can be forced out of the subducting plate as it descends and escape into the mantle. As more fluid enters the mantle and ‘lowers’ the solidus (or melting point) of the mantle material, there is a progressive increase in melting of the mantle material; however, it is only after multiple cycles of melting that the mantle material can chemically concentrate the ‘chalcophile’ elements (elements that like to associate with sulphur), like gold and copper, in the final product.
66 samples of deep-sea volcanic glass were collected
In order to characterise the processes of melting and enrichment of gold within the mantle, 66 samples of volcanic glass collected from the ocean floor were examined. Unlike crystalline rocks that form from crystal growth following the solidification of lava once cooled, volcanic glass is formed from an instantaneous freeze of the lava following rapid cooling from contact with cold seawater.
The ‘primitive’ glass samples contained higher amounts of gold than are typically found in mid-ocean ridges, thus indicating that some cycles of melting in the deep mantle ‘plumbing’ system resulted in gold enrichment before reaching the volcanoes.
A new blueprint for seafloor resources
The Kermadec arc, previously considered an isolated geological entity, has now been re-evaluated in terms of its geochemical properties, revealing it's also the source region for major previously unrecognised geochemical/mineral resources, including but not limited to gold! This new understanding of the Kermadec arc will help geologists locate important massive seafloor sulfide deposits (associated with precious metals).
This is critically important for providing a global view of how and where noble metals exist on Earth; for understanding the overall 'life cycle' of the gold that you see as solid ore at hydrothermal vent sites to the deep mantle, and to slab-derived fluids.
