Eric Potter HBSc thesis abstract
The Lac de Gras Kimberlite field is located approximately 295 kilometers north of the town of Yellowknife, within the Slave structural province. Since the discovery of the first kimberlite pipe in 1991, nearly two hundred kimberlite pipes have been identified in the Lac de Gras region, more than fifty of which are diamond-bearing. The kimberlites studied thus far have been of two broad textural types, hypabyssal and volcaniclastic (pyroclastic). Three kimberlites of each textural type were selected for study, and the spinel grains within each kimberlite studied. Roughly eighty spinel grains were analyzed, producing approximately four hundred analyses.
From these analyses, it was found that the hypabyssal kimberlite spinels followed essentially two evolutionary trends. The majority of the spinel grains analyzed followed the magnesian ulvöspinel magmatic trend, the characteristic magmatic trend observed in archetypal kimberlites. This trend consisted of euhedral titanian magnesium aluminous chromite (TIMAC) cores mantled by magnesian ulvöspinel - ulvöspinel magnetite series (MUM) compositions. When plotted on a reduced spinel prism, the trend begins near the MgCr2O4-FeCr2O4 join and heads towards the rear rectangular face and up towards the Mg2TiO4-Fe2TiO4 apex. The secondary evolutionary trend observed was the pleonaste reaction trend. This consisted of either: euhedral pleonaste cores mantled by TIMAC or MUM compositions, or more commonly, TIMAC cores, mantled by pleonaste compositions followed by an iron-rich rim. In terms of morphology, the hypabyssal kimberlites were dominated by atoll grains rimmed by a thin magnetite layer. In addition, the atoll grains appear to have preferentially nucleated about phlogopite phenocrysts. In the pyroclastic kimberlites, the spinel grains displayed little or no spinel evolution in terms of core and mantle textures. Rather, the magnesian ulvöspinel trend was preserved in two distinct grain populations, unlike the hypabyssal units, which exhibited core and mantle textures. In terms of morphology, these units were dominated by corroded atoll grains of TIMAC compositions, with subeuhedral to euhedral groundmass spinel grains of MUM compositions forming the rest of the population.