STRUCTURAL CONTROLS ON EMPLACEMENT OF EARLY JURASSIC PERALKALINE GRANITE AND RARE EARTH ELEMENT MINERALIZATION AT BOKAN MOUNTAIN, SOUTHEAST ALASKA

The Bokan Mountain complex (BMC) in southeast Alaska hosts structurally controlled U and rare earth element (REE) deposits, including the high-grade-U Ross Adams mine. The BMC stock is 3 km in diameter, consists of arfvedsonite-quartz porphyritic peralkaline granite, and has a U-Pb zircon age of 177±1 Ma. Andalusite in host argillite suggests epizonal emplacement in an Ordovician oceanic arc complex in the Alexander terrane (AT). Mineralization is located in pegmatites, dikes, and shear zones which are cut by unmineralized ~150 and ~105 Ma dikes.

Steeply dipping mineralized dikes extend up to 6 km NW and SE from the BMC stock. The Dotson dike system consists of interwoven mm- to m-scale dikes in a zone 50 m wide. Dikes have chilled margins and apophyses. Many have quartz-albite cores and rare earth oxide (REO) selvages. Nested dikes with REO selvages indicate multiple intrusive episodes. Quartz-filled tension gashes and vein offsets locally indicate sinistral kinematics.

Analysis of structures shows no pre-existing fabric influenced BMC emplacement. High-T mineral lineations in granite are locally folded. Pervasive protoclastic textures in granite include faulted feldspars. Pegmatites, dikes, and shear zones within and peripheral to the BMC trend NW-SE. Mineralization at the mine is located in en echelon veins striking 303º in a shear zone striking ~350º. Radiometric surveys of the BMC show planar W and NW patterns which suggest structures host radioactive minerals. BMC granite and dikes are cut by steep W- to NW-trending faults. FeREO mineralization coats fault surfaces and permeates granite up to 5m. Sinistral faults in host marble contain fluorite-REO. FeREO on faults are attributed to late hydrothermal fluids. Mineralization was concentrated by remobilization of REE during successive synkinematic fluid injections.

Isotopic signatures and elevated incompatible elements in BMC granite are consistent with emplacement in an extensional setting. High positive ε_Nd values support a mantle source. Early Jurassic rocks in the AT are limited to the BMC, the Dora Bay pluton 25 km to NW, a dike 80 km to NW, and bimodal Moffat volcanic rocks 100 km to SE of the BMC, all dated at ~175-180 Ma. These rocks lack evidence for arc association and may represent a rift event or magmatic activity on sinistral structures during AT migration.