calendar Add meeting dates to your calendar.


Paper No. 8
Presentation Time: 3:55 PM


HART, Craig J.R., Mineral Deposit Research Unit, Department of Earth & Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada, LANG, James R., Hunter Dickinson Services, Inc, 1020-800 West Pender St, Vancouver, BC V6C 2V6, Canada, GOLDFARB, Richard J., United States Geological Survey, Box 25046, MS 973, Denver, CO 80225 and FARMER, G. Lang, Department of Geological Sciences, University of Colorado, Boulder, CO 80309,

The Pebble porphyry Cu-Au-Mo deposits in southwestern Alaska represent one of the world’s largest magmatic-hydrothermal mineral deposits. The deposits are directly associated with three lithologically and geochemically diverse plutonic suites that intruded Jura-Cretaceous Kahiltna flysch strata over a ~10 m.y. period in the mid-Cretaceous from 97 to 88 Ma.

1) The oldest suite consists of intrusions of coarse-grained pyroxene gabbro, hornblende diorite sills, and granodiorite sills that are laterally extensive, up to 300 m thick, and host mineralization. U-Pb dates indicate the units are 96 Ma, or slightly older.

2) Several alkalic units form an intrusive and breccia complex, and several smaller solitary stocks. Biotite pyroxenite forms a large, texturally-variable body with fine-grained diopsidic phases and coarse-grained phases with abundant biotite, magnetite and apatite. Several quartz-deficient monzonitic units with K-feldspar megacrysts occur locally, and cut the mafic rocks. Intrusive breccias occur throughout the complex. The rocks are alkaline, high-K, and silica-saturated. U-Pb dates indicate intrusion at ~96 Ma.

3) The youngest, most volumetrically and economically important suite is dominated by quartz-phyric hornblende granodiorite and monzodiorite, and forms the large Kaskanak batholith, as well as the four intrusions that host the Pebble West Zone, and the larger body in the centre of the East Zone. The Kaskanak has accessory magnetite and titanite, the Pebble plutons are similar, with sparse phenocrysts of equant K-feldspar. Calc-alkaline geochemistry characterize all units. U-Pb zircon dates of 91-89 Ma and Re-Os molybdenite dates of ~89.5 Ma establishes a temporal relationship of these magmas with mineralization.

All three plutonic suites are isotopically primitive indicating melt derivation from a youthfully-enriched lithosphere or mantle. Associated volcanic rocks are absent throughout the district, and coeval intrusions are not recognized, therefore evidence for a mid-Cretaceous volcano-plutonic arc is lacking. The alkali precursor magmas, their relationships with the calc-alkaline suites, as well as an atypical non-arc setting for Pebble, may have genetic implications for its size, enrichments in of Cu, Au and Mo, and large gold endowment.

Meeting Home page GSA Home Page