Kira Arnold HBSc thesis abstract

The Coldwell Complex of the Superior Province is known to host multiple lamprophyric dykes varying from ultramafic to alkaline affinities. The Ney’s Lookout dyke is located within the Center II portion of the Coldwell Complex, crosscutting assimilated syenites. This area of Center II is highly brecciated and assimilated, which is interpreted to be representative as evidence of the cauldron collapse. The lamprophyre is devoid of diamonds, and has not been thoroughly analyzed other than brief field assessments. The objective of this research was to classify the lamprophyre according to the IUGS classifications of lamprophyre-clan rocks.

The main minerals that comprise the lamprophyre are euhedral zoned pyroxene phenocrysts and flow-aligned anorthoclase laths in a fine-grained groundmass composed of biotite, pyroxene, amphibole and feldspars. The pyroxene phenocrysts are poikilitic with inclusions of biotite, amphibole and relic olivine similar to the groundmass. Relic chloritized olivine phenocrysts are present in the groundmass and as inclusions in pyroxenes. The pyroxenes present in the sample are classified as diopside with minor augite. Mineralogical compositions of the Ney’s Lookout amphibole classify as Ferroan Paragasitic Hornblende. When compared to other lamprophyres within the Coldwell Complex, there were mineralogical and textural similarities between Ney’s Lookout Lamprophyre and local sannaites, such as porphyritic texture and mantling of amphiboles on pyroxene. Aside from minor variances in modal mineralogy, the lamprophyre best resembles a sannaite under the alkaline lamprophyre classification.

The chill margin along the dyke is a fine-grained rim with no contact metamorphism present, interpreted as representing a moderate emplacement temperature. Laths of anorthoclase are directionally aligned in the lamprophyre and are interpreted as flow textures. Zonation in the pyroxene is distinct in the phenocrysts, representing increasing Cr and Ti outward toward the rim with increased Fe in the core and decreasing amounts in the rim. Uralitization is the altertion of pyroxene phenocrysts to amphibole around the outer rim. The zonation and uralitization often occur from magma mixing events.