Dr. Tom Lamont will deliver the ESE Departmental Seminar on Thursday the 23rd of May 2024: “Geodynamic controls on high-grade hypogene porphyry copper ore-formation: New insights from the Laramide Belt, SW USA”

Join us in room G41 – RSM Building – on Thursday 23rd of May 2024 at 12h15.


The Laramide Belt, SW USA, represents the world’s 2nd most important region in terms of economic copper and formed during a period of low angle subduction of the Farallon Plate beneath western North America between ca. 80-40 Ma. However, only three porphyry copper deposits (Resolution, Arizona ca. 64 Ma; Bingham Canyon, Utah ca. 38 Ma; and Butte, Montana, ca. 63 Ma) are classified as high-grade-hypogene (HGHP; >200 MT at >1% hypogene Cu). We show all three HGHP deposits share similar features: (1) they formed along the margin of relatively rigid cratonic lithosphere, associated with broad areas of thickened (>40-60 km) crust, (2) they occur in localized regions where the azimuth of the maximum principal stress was orthogonal to the craton margin in the ~10 Myrs prior to ore-formation, (3) they formed within a few Myrs following the local relaxation of contractional stress, and (4) they are associated with pre-existing steep ENE-WSW-trending structures aligned parallel with Proterozoic structural lineaments that were optimally oriented to accommodate transtensional movements following a local change in stress field.

We propose that in all three HGHP deposits, contractional stress focussing against rigid cratonic lithosphere occurred due to increased plate coupling and/or end loading during Farallon low-angle subduction. This facilitated deep entrapment of porphyry magmas which locally increased in Cu and volatile contents due to either enhanced deep crustal differentiation and/or volatile mediated crustal anatexis due to interaction of slab derived volatiles with Proterozoic lower crust. This latter hypothesis is consistent with the unradiogenic Sr-Nd-Pb and Re-Os isotopic signatures, and extensive zircon inheritance associated with many Laramide intrusions, as well as the overlapping timing of amphibolite-granulite facies metamorphism and anatexis with the surrounding porphyry copper deposits. Focussed shortening in the lead up to HGHP mineralisation also increased exhumation rates which created a favourable environment for primary high-temperature (600°C) hypogene mineralization to be overprinted by a lower temperature (400°C) secondary hypogene enrichment allowing for extreme alteration telescoping that characterizes HGHP mineralisation. In contrast, the local relaxation in contractional stress swept eastward across the Western USA and occurred due to gradients in gravitational potential energy associated with lateral variations in crustal thickening, or a change in rheology due to hydration and anatexis of the North American lithosphere. This created pathways for deeply entrapped porphyry magmas to rise to the shallow crust, exsolve their volatiles and ultimately form HGHP deposits, whereas on longer timescales, decreased exhumation rates facilitating the preservation of HGHP deposits.

About the speaker

Dr Tom LamontTom is an Assistant Professor in Structural Geology and Tectonics at the University of Nevada, Las Vegas (UNLV). His core research interests are in the tectonic, metamorphic and mineralization processes which occur in subduction zones and orogenic belts. He has been recently investigating the tectonic controls of porphyry copper formation in the Laramide Belt, SW USA. Prior to focussing on porphyry copper formation, Tom worked extensively on Eastern Mediterranean tectonics, investigating the pre-extensional evolution of the Cycladic Islands, ophiolite obduction, and high-pressure and Barrovian metamorphism within the Tinos and Naxos metamorphic core complexes.

Prior to being appointed at UNLV, Tom completed his DPhil at the University of Oxford on structural, metamorphic and strain history of the Aegean Orogeny before taking up postdoctoral positions at the University of St. Andrews (high-pressure–low-temperature metamorphism) and the University of Bristol (tectonic controls of porphyry copper formation).

Getting here