NI ISOTOPE FRACTIONATION BETWEEN KOMATIITE AND SULFIDE MINERALIZATION AT THE NEOARCHEAN HART DEPOSIT, ABITIBI GREENSTONE BELT, CANADA

Initial studies have shown nickel isotope variations in komatiites and associated sulfide minerals, related to large, up to 1‰, abiological fractionation in high-temperature environments. The 2.7 Ga Hart deposit within the Abitibi Greenstone Belt is composed of the main zone and the eastern extension. The main zone of the Hart deposit is located at the base of the basal flow of the komatiitic sequence, and is characterized by massive sulfide grading into semi-massive to net-textured to disseminated sulfide overlain by barren komatiite. The eastern extension occurs 500m east of the main zone, and is 12 to 25m above the base of the second flow in the komatiitic sequence. It consists of semi-massive sulfide grading upsection to disseminated sulfide within barren komatiite.

Bulk rock samples were processed and analyzed following the procedure of Gueguen et al. (2013), at IFREMER in Brest, France. Ni isotopic ratios are reported relative to NIST SRM 986 standard according to the conventional delta notation, with 2σ error generally 0.03 to 0.06‰.

Our data from the Hart deposit shows variation in δ^60/58Ni values from -1.06 to +0.15‰. The least fractionated values are found in barren komatiite (<2% sulfide; -0.10 to +0.15‰), and δ⁶⁰Ni generally decreases as the content of sulfide content increases. Komatiite with disseminated sulfide (2-35% sulfide) exhibit δ⁶⁰Ni values between –0.57 and +0.12‰, and semi-massive (40-75% sulfide) to massive (>75% sulfide) sulfide exhibit overlapping ranges of δ⁶⁰Ni between -1.06 and -0.08‰. This trend is also observed along individual drill holes traversing these two zones with the most negative δ⁶⁰Ni values directly associated with the highest sulfide content with δ⁶⁰Ni values trending towards 0‰ away from the sulfide mineralization.

Ni and S concentrations are negatively correlated with δ⁶⁰Ni values, suggesting that Ni was fractionated as it was incorporated into sulfide liquid from komatiitic magma, preferentially removing the lighter isotopes of Ni, while the komatiitic magma does not exhibit strong fractionation due to a much higher total Ni content.

Gueguen, B., Rouxel, O., Ponzevera, E., Bekker, A., Fouquet, Y. 2013. Ni isotope variations in terrestrial silicate rocks and geological reference materials measured by MC-ICP-MS. Geostandards and Geoanalytical Research, 3; 297-317.