Geochemical, spatial, and temporal relationships of the intrusives and meta-intrusives of the Pogo deposit, eastern Interior, AK

Abstract

The Pogo deposit is an intrusion-related gold deposit (IRGD) located approximately 90 km southeast of Fairbanks, Alaska. It consists mainly of shallowly NW dipping quartz veins hosted in amphibolite facies paragneiss and predominately granite orthogneiss bodies. To date the deposit has produced over 4 million ounces of gold. U-Pb zircon dating of the orthogneisses shows they have Devonian-Mississippian protolith ages. Dates from the metamorphic zircons, established by microprobe Th data and cathodoluminescence studies, constrain a mid-Cretaceous metamorphic event to ~116 Ma. Recrystallization of kyanite to sillimanite and zircon recrystallization indicates this was a large fluid flux event that predated mineralization by 10 million years. Due to the fine-grained nature of the intrusive rocks at Pogo, identifying rocks in hand sample proved problematic. By combining XRF analysis of the rocks' major and trace elements and age data from this and previous studies, I identified and differentiated several suites of igneous rocks. The oldest is peraluminous granite, emplaced at ~2.5 ± 0.5 kb at ~109-107 Ma, predating mineralization at 104 Ma at a pressure of 2.0 kb. Non-peraluminous granite is less common and of uncertain relationship to the peraluminous granite. Next, temporally, is a body named the Football pluton (and associated dike) of granodioritic to tonalitic composition, emplaced at 2.0 ± 0.5 kb at 103 ± 2 Ma. Not only indistinguishable from age and depth of mineralization, a dike of this body is present downdip underneath the Liese veins (main zone of mineralization), making it the most likely candidate for being the causative pluton for mineralization. The final mid-Cretaceous body is the Liese pluton (and associated dikes), of quartz diorite to tonalite. This forms a large body with E-W dikes cutting the Pogo mineralization and post-dating it at 95.4 ± 0.2 Ma, emplaced at a pressure of 1.0 kb. Thermobarometry and radiometric dating indicate a consistent uplift rate of about 0.6 mm/year during the mid-Cretaceous, 116 to 95 Ma. Initially an extensional event, subduction-related magmatism began at about 105 Ma. At the same time, the thrust faults were re-activated as low angle normal faults that apparently acted as pathways for the Liese mineralization.