REDUCED GRANITES AND GOLD VEINS IN THE POGO AREA, EAST-CENTRAL ALASKA

Gold deposits associated with Cretaceous reduced granitic rocks are common in Yukon and Alaska. In the Pogo area of eastern Alaska, shallowly-dipping and structurally controlled >5 M oz gold veins of the Liese Zone and spatially related prospects are associated with an arcuate easterly trending belt of reduced granite to tonalite that compose the older of two Early Cretaceous igneous suites. The younger suite is dioritic in composition. Both calc-alkalic suites have low magnetic susceptibility and oxidation states. The older suite, which includes the Goodpaster Batholith, is weakly peraluminous and contains variable amounts of monazite and zircon. Biotite is common and hornblende is subordinate or absent. The suite is late- to post-kinematic. The late kinematic granitic rocks were emplaced at 109-107 Ma (U-Pb; zircon or monazite) and post-kinematic granitic rocks are as young as 104 Ma (U-Pb zircons). Molybdenite deposition at ~104 Ma in the Liese Zone suggests a temporal relationship to the post-kinematic intrusions . The coexistence of complex saline fluids with CO2-rich inclusions in the veins, as well as a high temperature sulphides assemblage (loellingite and arsenic-rich arsenopyrite) further implies a contribution of magmatic fluid. Based on the data, we interpret gold deposition to be linked to the emplacenent of the older late- to post-kinematic granite suite. A younger intrusive suite, dated at 94.5 Ma (U-Pb; zircon), consists of weakly metaluminous to weakly peraluminous stocks of diorite to quartz monzodiorite. The stocks intrude the older granitic suite and the gold-quartz veins. These rocks contain <5% primary magnetite and are slightly more oxidized than the older granitic suite. Hornblende ± pyroxene dominate and biotite is subordinate. Zircon is abundant, whereas monazite is notably absent. No gold veins are known to be associated with the younger suite. Both the reduced granites and any associated gold veins are older than the Early Cretaceous reduced granites of the Tombstone Suite (92±1Ma) and their associated gold veins, which are widespread in Alaska and Yukon. The two ages of gold deposition associated with reduced granitic intrusions expands the exploration potential for these types of deposits in the region.