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

• Main Author: Lande, Lauren L.
• Other Authors: Newberry, Rainer, Larsen, Jessica, Twelker, Evan, Keskinen, Mary
• Format: Thesis
• Language: English
• Published: 2016
• Subjects: Fairbanks; Fish Lake; alaska range; Alaska
• Online Access: http://hdl.handle.net/11122/6828

Thesis (M.S.) University of Alaska Fairbanks, 2016 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 ...