University of Western Australia: Research find carbon-rich fluid makes high-grade gold deposits

New research from The University of Western Australia and Curtin University has discovered how carbon and metal nanoparticles in the Earth’s crust can boost the concentration of gold deposits.

The study Nanoparticle suspensions from carbon-rich fluid make high-grade gold deposits, published in Nature Communications, examined ultra-high-grade specimens of gold from five mines across the globe, which formed at various depths in the Earth’s crust.

Lead author Dr Laura Petrella, from the Centre for Exploration Targeting at UWA, said researchers found the circulation of fluids rich in carbon and metals in the Earth’s crust produced economic gold deposits with an enrichment of 100 to 10,000-fold.

“In rare occasions gold concentration is so high that large nuggets are formed in quartz veins by processes that still confuse scientists,” Dr Petrella said.

“We discovered the first evidence of nanoparticles of precious metals such as gold, electrum, silver oxide and copper, preserved in carbon and/or silica inclusions in large gold grains.”

The particles discovered are very small (100,000 times smaller than the width of a human hair) but large in comparison to metal-bearing molecules that were originally believed to be the only substance to promote the formation of metals in the Earth’s crust — this increases by 5,000 times the potential concentration of gold in the fluids.

Dr Nicolas Thebaud, co-author and project lead from UWA’s School of Earth Sciences, said the existence of metal nanoparticles had previously been suspected but this study was the first to document the systematic association with carbon.

“The study explores for the first time the role of carbon in concentrating gold in the Earth’s crustal fluid,” Dr Thebaud said. “It delves into how carbon may promote formation of nanoparticles that aggregate to form exceptional gold grades in veins mined today.

“The discovery makes us rethink the way metals can be transported in the Earth’s crust fluids and opens the scientific community to new ideas regarding the compositions of these mysterious crustal fluids.”

Mining ultra-high-grade orebodies is associated with more energy-efficient processes as well as a smaller environmental footprint — both characteristics are essential for a more sustainable mining sector.

“Our work aims at further understanding the formation of high-grade, high-tonnage orebodies so that mineral explorers may develop the tools for a more efficient targeting of these deposits,” Dr Petrella said.