A petrological model for emplacement of the ultramafic Ni-Cu-PGE alpha complex, eastern Alaska Range

Abstract

The Alpha complex, also known as the Fish Lake complex, is a mineralized Ni-Cu-PGE (platinum group element) mafic-ultramafic intrusive complex located within the Wrangellia terrane in the Eastern Alaska Range. The complex, a 30-km long, 3-km wide, sill-form body, consists of alternating layers of dunite, wehrlite, and clinopyroxenite. 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. I found that the complex consists of numerous individual sills ranging from 30-200 m thick. These can be identified based on the forsterite (Fo) component of olivine. Sills have primitive margins with high Fo contents in olivine that grade inwards to evolved cores with lower Fo olivine. Some of the sills have distinctly wehrlitic centers that were identifiable in the field. High Fo contents in olivine (up to Fo 87) and high Cr/(Cr+Al) in spinel (0.46-0.96) suggest a primitive basaltic parental magma. Individual zones in the complex contain clinopyroxene with a range of TiO₂ values; these TiO₂ values reflect the associated melt and can be used to differentiate between different magma series. Clinopyroxene compositions suggest three different magmatic compositions: a low-TiO₂ magma in the upper unit, an intermediate magma in the basal unit, and a high-TiO₂ magma in the central unit. Ultramafic sills in the upper unit contain the most primitive olivine with a range in MgO contents (Fo 87-83) and clinopyroxene with low-TiO₂ values (<0.4 wt.%). The central ultramafic sill has the most evolved olivine (Fo 84-79) compositions and clinopyroxene with high-TiO₂ values (up to 0.9 wt.%). The theoretical wt.% TiO₂ and Mg number (Mg/(Mg+Fe²+)) of the parental melts were calculated from clinopyroxene compositions and they suggest that the low-TiO₂ sills are similar to the Lower Nikolai basalt, and the high-TiO₂ sills are similar to the high-TiO₂ Upper Nikolai basalt. Intermediate TiO₂ compositions in clinopyroxene and disequilibrium compositions in olivine in the basal sills indicate an origin from magmatic mixing. One sample has a bimodal olivine phenocryst assemblage, with one group averaging Fo 80.5 and the other group averaging Fo 82.5. This sample is nearby another sample with high-MgO olivine (Fo 86). These compositions may record multiple magmatic inputs from mixing of the Lower Nikolai magma series with the Upper Nikolai magma series. PGE ratios record two distinct Pt versus Pd trends: a low Pd:Pt trend and a high Pd:Pt trend. Elevated Pd in the Upper Nikolai basalt and low Pd in the Lower Nikolai basalt suggest high Pd in the central sill is related to crystallization from the Upper Nikolai basalt. When exploring for sills with high PGE content, high-TiO₂ contents in clinopyroxene may be a good indicator of whether or not a sill has the potential to host elevated PGE mineralization.