Abstract:

The Bokan Mountain complex is a small (6.5 km2) Jurassic (~177. 2 ± 0.2 Ma, U-Pb zircon) peralkaline granitic intrusion located near the southern tip of Prince of Wales Island (Alaska). The complex hosts several uranium and thorium prospects, one of which was mined intermittently between 1957 and 1971. The uranium mineralization is structurally controlled and mainly occurs in a plunging pipe-like ore body up to 24 m in diameter with ~ 1 wt. % U3O8 and 3 wt. %ThO2. In addition, there are several major rare earth element (REE) prospects related to the complex which predominantly occur as clusters of mineralized felsic dikes and alteration (metasomatically enriched) halos associated with NW-trending shear-fault zones, some of which can be traced for > 2 km. A recent mineral resource estimate has reported 3.7 million tonnes grading 0.75 wt. % ΣREE and Y oxides from two main zones.<br/>The complex is a circular intrusive body composed of highly fractionated peralkaline granitic rocks hosted by a Paleozoic basement which includes granitoids dated at 469.2 ± 3.9 Ma (U-Pb zircon). In turn the complex was intruded by two suites of Cretaceous dikes dated at 150 and 105 Ma (40Ar/39Ar). The peralkaline granites are typically leucocratic with 2-10 vol. % of sodic amphibole (arfvedsonite) and sodic clinopyroxene (aegirine). The intrusion is composed of two main units: an outer ring made up predominantly of aegirine granite and a core composed of arfvedsonite granite; locally, a zone of mixed aplite-pegmatite borders the complex. The granitic rocks are high in alkalis (8-10 wt. %) with an agpaitic index [(Na+K)/Al] >1, are ferroan with high Fe2O3 (~3.5-5 wt. %) but low MgO (<0.04 wt. %) and with the (FeO*/(MgO+FeO*) ratio ><br/>0.95. The rocks are characterized by high contents of high-field-strength-elements (HFSE), REE (particularly the heavy REEs), Th and U and low contents of CaO (<0.2 wt. %), Sr, Ba and Eu. The composition of the rocks reflects extensive fractional crystallization, in part due to the role of volatiles (including fluorine) and alkalis. Volatiles and alkalis are known to depress the solidus temperature and suppress crystallization of accessory minerals, which contain the bulk of HFSE and REE, until the latest stage of fractionation. The parent magma was likely derived from low degree partial melting of the lithospheric mantle metasomatically enriched in HFSE, REE, Th and U and possibly halogens during an earlier subduction zone related event, probably during the Ordovician. At an advanced stage of fractional crystallization, the magma became fluid-saturated and over-pressured, which led to the escape of a fluid-rich, highly fractionated melt and deposition of the major part of the U, Th, HFSE and REE mineralization. When these highly fractionated melts and aqueous fluids were exsolved they followed pre-existing zones of weakness (i.e., faults and shears) that penetrated through the complex and into the surrounding country rocks for up to several km from the intrusion. Although mineralization in the complex initially occurred in the zones of peralkaline granitic pegmatites and aplites, they and the surrounding host rocks were extensively overprinted at moderate temperature by hydrothermal fluids which caused a significant metasomatic remobilization of rare metals. Thus, most of the REE and HFSE mineralization reflect both an initial magmatic event, but more importantly a pervasive hydrothermal metasomatism which upgraded the mineralized zones.