IGNEOUS GEOCHEMISTRY, GEOCHRONOLOGY, AND VEIN TEXTURES IN THE GOODPASTER DISTRICT, ALASKA

Au-bearing quartz veins at the Pogo deposit and nearby mineral occurrences in the Tibbs Creek area are broadly coeval with mid-Cretaceous magmatism in the region. New and existing geochemistry and geochronology highlight a series of igneous suites within and adjacent to mineralization. The oldest suite consists of weakly foliated and recrystallized two-mica and garnet-bearing granites to granodiorite. Both granite types are strongly peraluminous and were emplaced ca. 113-108 Ma. An intermediate age suite, characterized by tonalite to granodiorite, was emplaced at ca. 107-105 Ma. A Re-Os age of ca. 104 Ma was determined for molybdenite in a quartz vein in the main Liese zone. Our new Re-Os dating of arsenopyrite coeval with Au appears to corroborate a ca. 106-105 Ma age for Au mineralization. A younger post-mineral suite consists of peraluminous granodiorite to quartz diorite. All igneous suites exhibit steep light and flat middle to heavy REE patterns, indicative of shallow non-garnet residual sources. All suites exhibit strong negative Eu anomalies with weak to absent Ce anomalies. Pearce plots of Y+Nb and Yb+Ta versus Rb indicate a volcanic arc origin for all suites.

Veins in the district exhibit diverse textures ranging from massive quartz with thicknesses locally 1 to >10m, to breccias with angular to milled clasts and bladed calcite. Mineralization is characterized by abundant pyrite, pyrrhotite, arsenopyrite, Au and less abundant As and Bi mineral phases. Brecciation is common in many veins outside the Liese zone. Early Au-bearing quartz clasts are cemented by younger quartz-Au, or arsenopyrite-Au matrix. Milled and rounded arsenopyrite-rock flour matrix exhibits evidence of brittle syn-tectonic deformation. Au locally post-dates the milling event, occurring as rims or as patchy cement between milled arsenopyrite grains. Au-bearing calcite and bladed calcite-quartz veins cut the breccias.

Geochronology suggests a temporal overlap between Au mineralization and emplacement of the tonalite suite. Vein textures suggest at least four generations of Au mineralization in highly brecciated, largely brittle, cyclical deposition events. Observation of Au-bearing calcite and associated bladed calcite suggest boiling occurred locally in the mineralized zones at relatively shallow depths, ca. <1,500 meters.