COMPOSITION OF SPINELS IN THE C-14 KIMBER LITE, KIRKLAND LAKE, ONTARIO

Spinels of the C14 kimberlite have been divided into three distinct po pulations: 1) spinels associated with garnet, 2) spinel macrocrysts, a nd 3) groundmass microphenocrysts. Spinels associated with garnet may occur within garnet or as a part of the kelyphitic rim surrounding gar net. They are characterized by a high MG# (Mg/((Mg + Fe2+) = 0.7-0.9) and moderate to low CR# (Cr/(Cr + Al) = 0.7). Spinel macrocrysts are C r-rich (>50 wt. % Cr2O3) with moderate MG# (0.5-0.65). The composition of these spinels and relatively large size (>80 mu m) are consistent with a xenocrystic origin. The microphenocrysts are small ((40 mu m) g rains displaying two distinct compositional trends. The first occurs i n early crystallized chromites that are zoned to higher CR# (0.7-0.9) and lower MG# (0.5) (the Cr-enrichment, CRE trend). The second trend o ccurs in spinels zoned from high CR# (0.9) and moderate MG# (0.5) to l ower values (0.05 and 0.25, respectively) with increasing Fe3+ and Ti. This Fe-Ti-enrichment (FETIE) trend is very similar to the compositio nal spinel trends identified in other group 1 kimberlites and is thoug ht to be due to a late- stage oxidation event. Atoll spinels are a com mon type of groundmass microphenocryst. A model is presented which int erprets these grains as primary growth features from the melt.