A PETROLOGICAL MODEL FOR EMPLACEMENT OF THE ULTRAMAFIC NI-CU-PGE ALPHA COMPLEX, EASTERN INTERIOR, ALASKA

The Alpha complex is a mineralized Ni-Cu-PGE mafic-ultramafic intrusive complex located within the Wrangellia Terrane in the Eureka Creek area of the Eastern Alaska Range. The complex, a 30-km long, 3-km wide, sill-form body, consists of cyclically alternating dunite, wehrlite, and clinopyroxenite, and locally contains hornblende. The complex has been variably interpreted as a layered mafic intrusion, a differentiated mafic-ultramafic complex, and as a multiphase complex of sills and possibly ultramafic extrusive rocks. Previous industry-led exploration has yet to converge on a geological model for the complex that adequately explains multiple aspects of the observed mineralization and crystallization patterns. Due to poor exposure and a lack of chilled margins, it is not immediately clear whether this is a single body or a multi-sill complex.

Our initial transect shows that the igneous complex has at least eight cyclical transitions between dunite and clinopyroxenite. At the broadest scale, the sample transect can be split spatially into at least two zones with differing olivine compositions (Fo_80-83 and Fo_83-88); smaller cyclical variations occur within these divisions. The forsterite component of olivine correlates with modal mineralogical patterns such that clinopyroxenite has the least primitive olivine composition. Ni contents in olivine become increasingly depleted within each layer, and Ni contents of olivine associated with pentlandite are significantly lower than olivine with similar forsterite content, where sulfides are absent. Chromite in the complex displays a strongly developed Fe-Ti trend and is Ti-enriched (up to 3.8 wt %)—a characteristic of intrusions from flood basalt regions. Chromite is depleted in Cr and enriched in Fe³⁺ in clinopyroxenites relative to dunite within a single layer. Ultramafic rocks contain minor amounts of hornblende and tremolite, but orthopyroxene is absent. Modal composition patterns and variations in olivine forsterite compositions within and between zones supports a hypothesis of repeated sill injections from an evolving or multiple magma source. The absence of chilled margins may reflect a high temperature of emplacement and high heat transfer to enclosing rocks.