ISOTOPIC INVESTIGATIONS OF THE NI-SULFIDE NOVA-BOLLINGER DEPOSIT, WESTERN AUSTRALIA: S, SR AND SM-ND SYSTEMS

The Nova-Bollinger igneous suite (ca. 1300 Ma) in Western Australia occurs within the Frazer Zone of the Albany-Frazer Orogen and hosts economically significant Ni-sulfide mineralization. The system represents “conduit-style” mineralization broadly divided into the Nova, to the East, and Bollinger, to the West, orebodies with a combined resource estimate of 13.1Mt at about 2 wt.% Ni, 0.8 wt.% Cu and 0.1 wt.% Co. Ni-sulfide mineralization is localized within the host rocks and into the country rocks to a depth of about 50m, with ample evidence of sulfide-country rock interaction. Sulfur, Sr, and Sm-Nd isotopes have been used to evaluate the role of crustal contamination of the Nova-Bollinger parental magmas. All mineralization types in the system are characterized by δ³⁴S values between +1.6 and 8.0‰ with an average value of about 4.3‰ which is not significantly anomalous relative to uncontaminated mantle values near 0‰. No significant ∆³³S was observed in neither of these sulfide ores. The values for both δ ³⁴S and ∆³³S are remarkably like those from the Savannah deposit and Octagonal prospect in Western Australia. The initial ⁸⁷Sr/⁸⁶Sr and ¹⁴⁷Sm/¹⁴³Nd ratios of Nova-Bollinger igneous rocks at the age of formation of 1300Ma are similar to those of CHUR_1300Ma, however Bollinger globular sulfides with ɛSr values of 63.1 and 144.5 and ɛNd values of -1.8 and -5.7 are suggestive of more pronounced crustal contamination in the system.

The country rocks of the Snowys Dam Formation, locally enriched in iron and sulfur, have been used as a proxy to a potential crustal contaminant for a lack of a more sulfidic lithology in the Frazer Zone. These metasedimentary sulfides show similar ranges in values for both δ ³⁴S and ∆³³S, but their Sr, and Sm-Nd isotopic ranges are much greater than observed at Nova-Bollinger.

Bulk assimilation of the country rock sulfides, as modelled in both silicate and sulfide portion of the system, of up to 3% contamination would have been sufficient to produce the degree of crustal contamination recorded in the Nova-Bollinger ores. Alternatively, or if a contaminant with higher S isotopic values was responsible for sulfide saturation in the system, the low degrees of crustal contamination may also be related to isotopic exchange between contaminated and pristine magmas in the conduit. The high Ni grades and low PGE tenors of the Nova-Bollinger ores are in part related to the dynamic conduit environment and sulfide liquid segregation saturation prior to Ni removal by the crystallization of olivine.