Andrew Jedemann HBSc thesis abstract

The Coldwell Alkaline Complex was emplaced as a Midcontinent Rift-related intrusion into Archean-aged rocks of the Schreiber-Hemlo Greenstone belt, centered around the town of Marathon, Ontario. Although the complex consists of predominantly alkaline rocks, numerous PGE-(Cu) mineralized gabbroic intrusions also comprise a substantial portion of the complex and are of considerable economic interest. The PGE-(Cu) mineralized sub-ophitic to ophitic gabbros within the Marathon deposit of the Eastern Gabbro Suite have been studied in detail by numerous workers, leading to many interpretations of genetic models for the emplacement of the gabbros and subsequent mineralization.

This study focuses on characterizing two trenches in the Boyer Lake district, located along the northeastern border of the complex that consist of predominantly PGE-(Cu) mineralized coarse-grained to pegmatitic sub-ophitic olivine gabbros as well as many smaller ultramafic intrusions and pyroxenites that are in contact with Archean footwall rocks. Of particular interest, is an area located on Trench 2 that consists of olivine- and plagioclase-rich olivine gabbros, and does not exhibit the typical sub-ophitic texture of olivine gabbros elsewhere on the trenches as well as in the Marathon deposit, rather poikilitic olivines that commonly contain thin partial rims of pyroxene. Additionally, this zone is characterized by enriched PGE mineralization, a chalcopyrite  bornite sulphide assemblage and geochemical signatures that are all characteristic features of the W-Horizon, which is a known lens of high-grade PGE mineralization within the Marathon deposit. Mineral chemistry data in conjunction with Sm-Nd and Sr isotope analyses suggest that the PGE-enriched olivine- and plagioclase-rich zone consists of very primitive intrusions, and likely represents some of the earliest, if not earliest emplaced rocks comprising the study area in contrast to more pyroxene-rich and less mineralized sub-ophitic gabbros. The chaotic nature of the intrusions, variable geochemistry, PGE grades and mineralization styles suggests that the genesis of the Boyer Showing may best fit with a previously hypothesized dynamic magma conduit model that has been postulated for the genesis of the gabbros of the Marathon deposit.