STABLE ISOTOPE GEOCHEMISTRY OF THE KILMAR MAGNESITE DEPOSITS, GRENVILLE PROVINCE, QUEBEC

The Kilmar magnesite deposits are a series of lenticular, km-scale ore deposits within granulite-facies metasediments of the Morin terrane, Quebec. The terrane is dominated by the 1.15 Ga Morin Anorthosite-Mangerite-Charnockite-Granite suite which intrudes the metasedimentary sequence, and has experienced a polymetamorphic history. The study site, the Dobbie mine, contains magnesite ore surrounded by dolomitic marble. Now removed, this ore body reached a thickness of approximately 20 m and extended ca. 350 m along strike. The ore contains magnesite + dolomite + serpentine while host marble contains dolomite + serpentine ± magnesite ± strontianite ± celestine. Stable isotope ratios of magnesite and dolomite from ore samples and host dolomite suggest minimal influence of metamorphic fluids and preservation of sedimentary values. Carbon isotope ratios of the magnesite (δ¹³C=1.70±0.22‰ PDB, n=7) are identical to other Morin terrane marbles and typical Mesoproterozoic carbonates. Oxygen isotope ratios of magnesite (δ¹⁸O=25.15±0.40‰ SMOW) are high compared to other Grenville marbles, and suggest an evaporitic protolith. Host dolomite is more variable in isotope composition than magnesite ore (δ¹³C= -1.2 to 1.7‰, δ¹⁸O= 22.6 to 26.0‰, n=19). Stable isotope data, mineralogy, and field relations are all consistent with a sedimentary magnesite deposit metamorphosed under granulite-facies conditions.