27 June 2019
Making metal deposits on the sea floor – and solving a conundrum
Researchers from GNS Science have made a breakthrough in a decades-old debate about how submarine volcanoes form mineral
deposits on the seafloor that are rich in copper and gold.
Since the 1960s, it was believed that metals were leached out of rocks within the top 100m or more of submarine
volcanoes by circulating seawater, which then rises back up inside the volcano to deposit the dissolved metals on the
However, GNS Science-led research points strongly to a two-stage process with the metallic minerals coming from deeper
inside the volcano, having initially been deposited from fluids released by magma. They are then remobilized and
transported further upwards to the seafloor when seawater is allowed to penetrate the volcano and access its interior.
The finding, published in the journal Geology, outlines a mechanism whereby very salty, super-hot metal-rich fluids are expelled from magma but remain trapped within
the volcano because they are too dense to make it all the way up to the seafloor.
Lead author, Cornel de Ronde of GNS Science, says the only way to address the issue was to get direct evidence by
drilling into a hydrothermally active submarine volcano.
The findings of his team’s study come from analysis of drill cores extracted from Brothers volcano in the Kermadec Arc,
400km northeast of the Bay of Plenty coast.
“This is an important advance in the understanding of the way deposits rich in copper and gold are formed on the
seafloor at tectonic plate boundaries around the world,” Dr de Ronde said.
“Copper and gold and other critical metals are being formed continuously at Brothers volcano. It is a modern-day example
of ‘ancient’ deposits of copper now uplifted and exposed on land, many of which are being extracted for use in the green
Dr de Ronde said to successfully transition to a low-carbon economy, the world may need more copper than is currently
being produced. It is currently a vital component in many new technologies such as electric vehicles, wind turbines, and
in hi-tech industrial processes.
Brothers volcano started out like Mount Taranaki, a stratovolcano, and later collapsed into a smaller version of Lake
Taupo, a caldera volcano with faulted walls.
“This collapse provided pathways for heated seawater to penetrate the so-called ring faults inside the caldera and
effectively ‘mine’ the dense metal-rich brines stored deep inside the volcano.
“The fluids interact with the brines and bring them closer to the seafloor where they form the black smoker chimneys
that we see at Brothers volcano.”
The drilling, an extraordinary engineering feat in itself, was done last winter from the International Ocean Discovery
Program ship JOIDES Resolution.
“We now have a much better understanding of the way submarine hydrothermal systems associated with volcanoes work and
the mechanisms that deliver metal-rich minerals to the seafloor,” Dr de Ronde said.
This work is part of a wider research theme at GNS Science aimed at improving the understanding of the origin, makeup,
and potential geohazards of the mostly submerged continent of Te Riu-a-Māui / Zealandia.
The paper, Critical role of caldera in the formation of seafloor mineralization: The case of Brothers volcano, can be found here: https://pubs.geoscienceworld.org/gsa/geology/article/571459/?searchresult=1