Geology and uranium-thorium mineral deposits of the Bokan Mountain Granite Complex, southeastern Alaska

Abstract

The Bokan Mountain (Kendrick Bay) uranium-thorium deposits are associated with a Late Jurassic peralkaline granite ring-dike complex. The uranium-thorium deposits form pipe-like bodies along contacts, or occur as pods in en echelon northwest-striking shear zones. The host intrusive is a multiphased body consisting mainly of riebeckite granite porphyry and subordinately of pegmatite-aplite, aegirine granite porphyry, and eight other lesser rock types; the ore is closely associated with an albitized aegirine syenite. The ore consists of thorium and rare earth-rich rock containing uranothorite, uraninite, and generally less than 2% of sulfide species. About 100,000 tons (89,000 t) of ore have been mined at an average grade of about 1% U 3O 8 and nearly 3% ThO 2, mainly from the Ross-Adams pipe. The pipe itself lies along the contact of aegirine syenite and aegirine granite porphyry; it is as much as 24 m across and was mined along strike for over 300 m. The ore in shear zones occurs in lenses as much as 3 m thick and 30 m in strike length. Wallrock alteration within and adjacent to orebodies consists of pervasive hydrothermal albite and lesser amounts of chlorite, fluorite, calcite, quartz, sericite, and tourmaline. Hematite is present in the outer-distal parts of the ore zones. The minerals produced to date occur within the outer annular zone of the Bokan Granite Complex in which aegirine is present as the major alkali ferro-magnesian mineral. Ore emplacement occurred during the first subsidence event in association with devolatilization of the magma chamber. The pod-like ore zones appear to have formed during regional faulting synchronous with magma crystallization and subsequent hydrothermal events. Subsequent magma crystallization yielded riebeckite granites that were depleted in U and Th. Filling temperatures in ore stage fluid inclusions within quartz range from 320 to 331°C with pressure corrections increasing the temperature of formation to 420°C or higher, depending on dissolved gas content. Sulfur isotopic analyses on pyrite, galena, sphalerite and pyrrhotite indicate disequilibrium, but δ 34S H 2S is estimated at 7.6‰. Carbon and oxygen isotopic analyses on ore stage calcite yield δ 18O H 2O of 6.8 to 8.1‰ and δ 13C ΣC of −4.3 to −7.0‰. The oxygen and carbon isotopic data support a magmatic origin for the calcite and associated thorium-uranium deposits.