203 Old and the New, Long and the Short: Perspectives on Integration of Timescales of Magmatic Processes: Special Session Related to MGPV Awards to Madison Myers and Anita Grunder

We encourage papers that challenge and give insights into the interplay between timescales, methodologies, and processes to build a cohesive understanding of the lifespan, evolution, storage, and eruption of magma, from plutonic to volcanic systems.

Tuesday, 21 October 2025
1:30 PM - 5:30 PM
HBGCC, 216AB

The Role of Voluminous Long-Lived Arc Magmatism in Porphyry Mineral Deposit Formation

Paper number: 203-13
Program: Technical Sessions
Time: 4:45 PM – 5:00 PM

Authors

Abstract

Long-lived magmatism of several giant porphyry Cu-Mo-Au, lode and epithermal ore deposits suggests that temporal magmatic evolution is a factor in producing ore fluids. At Sulphurets, British Columbia, Au/Cu and Mo/Cu ratios of ores temporally increase over ~11 m.y.  in concert with zircon delta18O increase documenting crustal contribution to magmas. Two examples of ore-forming magmatism are the Boulder batholith, Montana (BB), and the El Abra-Fortuna batholith, Chile (EAF), both emplaced concurrent with tectonic shortening and crustal thickening.

            In Montana, the Cretaceous BB is preceded by eruption of Elkhorn Mountain Volcanics (~10,000 km3, ~86 to 80? Ma) that evolved from basaltic andesite, andesite, and dacite to late rhyolite ignimbrite and are folded and cut by reverse faults.  Minor volumes of diorite to granite (~83 to 78 Ma) were succeeded by homogeneous Butte hornblende-biotite granite (BG, 77-76 Ma) comprising 80-volume% of the BB (5,000 km2 or >25,000 km3).  Hornblende barometry indicates BG exposures range 5-9 km paleodepth. The giant Butte ore deposit (~38 Mt Cu resource) is associated with small volume granite porphyry dikes (66-60? Ma).  Both pre-BG and the youngest porphyries contain abundant Proterozoic zircons, which are rare in BG.   Zircon ages and magma volumes indicate high magmatic flux both in early to intermediate stages (EMV & BG), and decreased flux in late stage, low-temperature ore-forming porphyries derived >10-km-depth. 

            In Chile, the Icanche Fm Eocene andesite-dacite lavas (52-44 Ma, >1000 km2) were folded and reverse-faulted. Alkalic pyroxene-biotite diorite to quartz monzodiorite stocks (43-41 Ma, 100 km2/500 km3) intrude these faults.  Later hornblende-biotite quartz diorite, granodiorite, and minor granite (40-37 Ma, ~320 km2/>1600 km3) of the Fortuna-El Abra complex are emplaced at 4-5 km depth.  Small volumes of Chuquicamata granodiorite porphyries (35-33 Ma) are more deeply emplaced.   Porphyry Cu-Mo ore ages are 37-35 Ma (EAF, ~50 Mt Cu) and 35-31 Ma at the Chuquicamata cluster (120 Mt Cu).  Magmatism migrated temporally southeastward over the last 12 m.y., and youngest magmas and ore fluids are sourced >10 km depth. 

            Protracted evolution of magmas to silicic compositions requires both long-lived translithospheric hydrous magmatism and likely mid-crustal sources of late ore fluids.  Repeated deep magmatic recharge and transcrustal assimilation-fractional crystallization produced late enrichment in key ore components water, sulfur, chlorine and metals (Cu, Mo, ±Au-As).

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414104

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