Arizona Geological Society
2026 Speaker Series
Tuesday, 31 March 2026 | 5:30 - 8:00 PM
Location: Hexagon Mining Division Office
40 East Congress Street, Suite 150, Tucson, Arizona 85701
Parking: On the street or parking garage (Old Pueblo Parking)
Social Hour - Sandwiches from Beyond Bread (5:30-6:20 PM MST), Presentation (6:25 PM MST)
NOTE: This event will not be livestreamed.
AGS and SEG Publications will be available
for sale at the meeting.
Will need cash or check (no credit cards)
For those planning to attend the event, please register by 6:00 PM on Sunday, March 29, 2026. Please register early so we have an accurate number of attendees to order the right amount of food.
The Arizona Geological Society thanks Hexagon
for generously providing the venue and drinks
Porphyry Copper Formation Driven by Water-Fluxed Crustal Melting during Flat Subduction: Insights form the Laramide Belt, SW USA
by Thomas Lamont, Associate Professor,
Department of Geoscience, University of Nevada, Las Vegas
Abstract: The prevailing view of porphyry copper formation along convergent plate boundaries involves deep crustal differentiation of metal-bearing juvenile magmas derived from the mantle wedge above a subduction zone. However, many major porphyry districts formed during periods of flat-slab subduction when the mantle wedge would have been reduced or absent, leaving the source of the ore-forming magmas unclear. A key example is the Late Cretaceous-Paleocene Laramide Porphyry Province in Arizona, which formed during flat-slab subduction of the Farallon Plate beneath western North America between ca. 75-50 Ma. By combining geochronology, thermobarometry, and isotope geochemistry to investigate the deep crustal processes taking place during Laramide porphyry mineralization, we show that the Laramide granitic host rocks did not derive from the mantle wedge, but instead from a Proterozoic crustal source that was potentially pre-enriched in copper. This source underwent water-fluxed melting between ~73 and 60 Ma, coincident with peak granitic magmatism (~78–50 Ma), porphyry genesis (~73–56 Ma), and flat-slab subduction (~70–40 Ma). Water-fluxed crustal melting was driven by volatiles derived from the leading edge of the Farallon slab, without extensive magmatic or metallogenic input from the mantle wedge. This petrogenetic model is further supported by low Ti concentrations in zircon (>6 ppm Ti) from syn-mineral and peraluminous granites across Arizona and New Mexico which requires magmas to be 'super wet' (>6 wt% H₂O). Contrastingly pre-mineral intrusions and volcanics have much higher Ti concentrations in zircon due to hotter and drier (<2 wt% H2O) magma associated with pre-cursor ore-barren steep subduction.
We propose that water-fluxed crustal melting occurred in a geodynamic 'sweet spot' located directly above the leading edge of the flat-slab at the point the subducting slab was shallowing from a steep to low angle geometry. At this location, elevated crustal temperatures would have prevailed due to a combination of thickened crust, above and laterally displaced mantle wedge to the sides. Volatiles released from the dehydrating flat slab would have moved upward directly into the Proterozoic lower crust, promoting crustal hydration and melting. Resultant rheological weakening and isostatic adjustments from flat-slab subduction may have caused the locus of contractional deformation to migrate eastward further inland ahead of the leading edge of the flat slab, although on timescales >15 Myr, continual underthrusting of the Farallon slab would have ultimately cooled the North American crust. This may explain the systematic spatial-temporal patterns of mineralization at any given location: (1) pre-mineralization contraction and barren volcanism and granitic magmatism related to steep subduction; (2) syn-mineralization “super wet” granitic magmatism and water-fluxed crustal anatexis during slab flattening; and (3) post-mineralization ore-barren magmatism, tectonic quiescence, and cooling related to continued underthrusting of the Farallon plate.
The Central Andes show similar spatial-temporal magmatic and mineralization trends, invoking a similar flat-slab regime associated with Late Eocene–Miocene porphyry copper formation. We therefore propose that other convergent plate boundaries with flat-slab regimes experienced a similar mechanism of ore formation triggered by volatile-mediated lower-crustal melting.
Bio: Thomas Lamont is a field geologist and petrologist interested in how tectonic processes influence ore-formation. He is originally from Liverpool in NW England, UK, and obtained his undergraduate degree and PhD from the University of Oxford, UK (2019). His PhD thesis focused on investigating accretionary mountain building processes and the pre-extensional evolution of metamorphic core complexes in Cyclades, Greece, supervised by Professor Mike Searle and Professor Dave Waters. He then did a postdoc at the University of St Andrews, UK, where he investigated subduction zone, ultra-high temperature metamorphism and granite petrogenesis with Professor Richard White. He then moved to the University of Bristol to undertake a BHP funded project with Professor Frances Cooper, where he investigated the geodynamic controls on porphyry copper deposits in the SW USA. He is now an Assistant Professor at the University of Nevada Las Vegas where he is focusing research efforts on flat-slab subduction metamorphic processes and the upper plate thermal, mechanical and metallogenic response, using the SW USA as a case study.
Hexagon Mining Division Office - 40 East Congress Street,
Suite 150, Tucson, Arizona 85701
