New Paper: Porphyry copper deposits can form much faster than previously thought.
Guest post authored by Lawrence Carter
We are happy to share our new paper:
“A rapid switch in magmatic plumbing taps porphyry copper deposit-forming magmas”.
Published in the October 2022 edition of Nature Scientific Reports, this article addresses the previously fragmented understanding of the magmatic timescales associated with porphyry-deposit formation through study of the the world’s best exposed porphyry system, integrating field observations and geochemistry with state-of-the-art high precision zircon U-Pb geochronology.
We based our study on the Yerington Batholith of Nevada, where tilting of the upper crust has provided one of the world’s best-exposed sections through a magmatic-hydrothermal system. The Yerington represents a section of crust around 8 km thick and ranges from the volcanic to plutonic environments and includes four porphyry deposits. This provides the rare opportunity to study the exposed “root-zone” of a porphyry deposit-forming magmatic system. Because of this unique exposure, previous studies of the Yerington Batholith have underpinned much of our current understanding of how porphyry-type deposits form.
The study involved geochemical analysis and high-precision CA-ID-TIMS geochronology of zircons from carefully selected samples, some of which containing specific magmatic-hydrothermal textures. Success here required high quality zircon recovery from geologically significant samples and was achieved using a Selfrag Lab High Voltage Pulse Fragmenter in our sample preparation circuit to liberate coarse intact zircons from the host rock
Cathodoluminescence (CL) images of zircon grains recovered from Yerington rocks. Spots are from Laser-ablation ICP-MS analysis.
The article presents our new 4-D model for the formation of porphyry-type copper deposits. The current paradigm is of a progressive, multi-million year, arc-scale ramp up in the mineralising potential (‘fertility’) of magmatic systems, however we show that there may instead be a more rapid switch from the intrusion of non-mineralising to porphyry deposit-forming magmas over a period of less than 200 kyrs - an order of magnitude less.
We attribute this to a shift in the magmatic plumbing, from extraction of magmas from mid-crustal reservoirs with relatively poor ore-forming potential to extraction of volatile-rich “fertile” magmas from greater (~30 km) depths. We also show that the distinct “fertile” geochemical signatures associated with ore-forming magmas develop almost instantaneously (<200 kyrs), and are expressed throughout the magmatic system, including in overlying volcanic rocks.
A rapid change in magmatic plumbing to tap porphyry-mineralising magmas: Simplified system paragenesis and conceptual cross-section through the porphyry deposit forming magmatic system.
The rapid nature of the shift in ore-forming potential narrows the temporal-geochemical footprint of magmas associated with porphyry mineralisation and will help exploration geologists discover the next generation of porphyry copper deposits.
Moving forward, we are going to incorporate geochemical and geochronological analysis of other magmatic phases, such as titanite, to provide further constraints on the thermal and time evolution of the porphyry deposit-forming magmatic-hydrothermal system.
Lawrence Carter collecting samples at the Yerington Batholith.
The Importance of Porphyries:
Porphyry-type deposits provide most of the world’s copper and molybdenum, as well as large amounts of gold and other metals, which are of increasing demand for green technologies such as electric vehicles, wind turbines and solar panels, and for power transmission. As such, they are the principle target of many mining companies worldwide. The problem is that most large near-surface deposits have already been found and therefore geologists are having explore deeper and under thicker layers of younger rocks to find them. There is therefore a growing need to better understand where and how porphyry copper deposits form and for new methods to find them.
Our research was supported by the NERC GW4+ DTP programme, a NERC Isotope Facilities grant, Selfrag AG, the Society of Economic Geologists Foundation and the NERC highlight topic ‘FAMOS’. The paper, entitled ‘A rapid change in magma plumbing taps porphyry copper deposit-forming magmas, is published in the journal Scientific Reports and is authored by scientists from Camborne School of Mines, The British Geological Survey, The Natural History Museum, Durham University, Selfrag AG and Lightning Machines
Lawrence is currently a Post-Doctoral Research Associate at The Camborne School of Mines, University of Exeter.
Citation:
Carter, L.C. et al. A rapid change in magma plumbing taps porphyry copper deposit-forming magmas. Sci Rep 12, 17272 (2022). https://doi.org/10.1038/s41598-022-20158-y
Update 30/03/2023
This article was Nature Communication’s 30th most downloaded of 2022, which is amazing - check out the new blog post below: