Steve Flank HBSc thesis abstract

The Hele Sill is an elliptically shaped ultramafic intrusion emplaced within the sedimentary rocks of the Kama Hill Formation. It is recognized as being part of the earliest phase of magmatism associated with the onset of the 1.1 Ga Midcontinent rift. These early melts are picritic in composition and have rare earth element compositions that show enrichment in the light rare earth elements and fractionation through the heavy rare earth elements interpreted to indicate a primitive, mantle source.

The stratigraphy of the Hele Sill can be divided, using petrographic and geochemical characteristics, into five zones. The Upper Marginal Zone comprises vari-textured leucogabbro that contains fragments of partially digested hanging wall rocks, and potassic pegmatitic pods. The Upper Gabbroic Zone is a layered sequence of subophitic gabbro to cumulate melagabbronorite, and displays a trend of Ca and Mg enrichment and Na and Si depletion towards its base. A rapid transition to lherzolites and wehrlites of the Ultramafic Zone is due to the crystallization and accumulation of cumulus olivine and pyroxenes, which show a strong cumulate texture. Megacrystic clinopyroxene, orthopyroxene and plagioclase poikilitically enclose cumulate minerals indicating residual liquids diffused efficiently. The Lower Gabbroic Zone and Lower Marginal Zone form a 10 - 20m marginal reversal, with rock compositions becoming more felsic towards the lower chill margin. Rock types change from melagabbronorite to gabbro, and show the strongest negative Nb anomalies found in the intrusion.

The rocks of the Hele Sill are interpreted to have formed from an ultramafic magma that underwent fractionation and accumulated mafic mineral phases. The effects of contamination from the Kama Hill Formation are strongest in the Lower Marginal Zone, which show increases in alkalis and incompatible elements. The bulk composition of the Hele Sill is not significantly affected by this local contamination. However, an earlier, more pervasive contamination may have occurred as the parent magma staged within the basement rocks of the Quetico Subprovince as indicated by the overall negative Nb anomaly not expected from a primitive mantle source rock. This trend is observed in the middle section of the nearby Osler Group volcanic rocks. It is suggested that the Hele Sill is coeval with this stage of volcanism and represents the intrusive equivalent of the Middle Osler Group Volcanics.

Steve is currently working as a geologist for HTX Minerals, in Sudbury

For more information about this thesis contact Dr Pete Hollings