China's giant gold deposit was formed by different geological forces, says paper. Is there more out there?
An international team of scientists has discovered that a giant gold deposit in northern China was formed by magmatic fluids mixing with rainwater, a process different from that of gold found in other parts of the world.
The researchers said their findings could help find gold resources by identifying areas that show past mantle-derived magmatic activities forming intrusive rocks similar to those in Dongping in the eastern North China craton that hosts one of the largest gold provinces in the world.
Lode gold deposits, the world's major source of the precious metal, were generated mostly from metamorphic fluids when they boiled and/or interacted with wall rocks.
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Heated fluids picked up gold when they passed through gold-bearing rocks and deposited gold in favourable locations in the crust after the source rocks were metamorphosed to release gold-bearing fluids.
In China, "the world-class Dongping lode gold deposit has been formed by multiple pulses of magmatic hydrothermal fluids and their mixing with large volumes of meteoric water," the team wrote in a research article published on Tuesday as they detailed the geological process they had uncovered.
"Magmatic fluid pulses were repeatedly exsolved from the underlying magma chamber. Faults and fractures as conduits facilitated ascending of magmatically derived fluids and subsequently their mixing with voluminous meteoric water, processes leading to gold deposition," they said.
"This study opens an opportunity to tightly constrain the origin of lode gold deposits worldwide and other hydrothermal systems that may have generated giant ore deposits in the Earth's crust."
The researchers - from China University of Geosciences, the Chinese Academy of Sciences, the University of Wisconsin-Madison in the US and the German Research Centre for Geosciences - published their findings in the peer-reviewed journal Proceedings of the National Academy of Sciences.
Much of northern China - including what is now Beijing, Tianjin and surrounding Hebei province - sat on an old and stable part of the continental land mass, or lithosphere, known as a "craton", from the Greek word for strength.
One of the authors, Li Jianwei, the dean and a professor of the school of earth resources at the China University of Geosciences in Wuhan, said although the fluid source and formation process of gold in the Dongping deposit was different to other cratons of the world, the composition of the metal showed no essential difference.
"The gold deposits in China are much younger than those developed in other cratons of the world," he added.
"While gold deposits on other cratons mainly formed between 2.8 and 2.5 billion years ago and 2.1 to 1.8 billion years ago, the ones on the North China craton formed around 140 to 120 million years ago."
The North China craton, covering 1.5 million square kilometres (580,000 square miles), is one of the world's oldest, and a rich source of ancient rocks for geologists.
The team said they used the "secondary ion mass spectrometry" technique to analyse the oxygen isotope compositions of garnet, a mineral that can retain the isotopic signature of the fluids that formed it at magmatic temperatures.
They found that the ore fluids were most likely sourced from the degassing of an underlying magma chamber, while water from rain and snow had been involved in ore precipitation throughout the history of mineralisation.
The team said the analysis of garnet could be used to trace the origins and evolution of other giant hydrothermal ore deposits in the Earth's crust.
This article originally appeared on the South China Morning Post (SCMP).
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