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A NEW GENETIC MODEL FOR THE GIANT Ni-Cu-PGE SULFIDE DEPOSITS ASSOCIATED WITH THE SIBERIAN FLOOD BASALTS

Chusi Li and Edward M. Ripley

Department of Geological Sciences, Indiana University, Bloomington, Indiana 47401

Anthony J. Naldrett

Department of Geology, University of Toronto, Ontario, Canada M5S 3B1

Corresponding author: e-mail, cli{at}indiana.edu

The Kharaelakh intrusion is one of several sill-like, multiphase gabbroic intrusions that host world-class Ni-Cu-PGE deposits in the Noril’sk-Talnakh region. The sulfide ores of the Kharaelakh intrusion are characterized by elevated ³⁴S values (10–12) and high PGE concentrations (e.g., up to10 ppm Pt). The ³⁴S values require addition of crustal S with elevated isotope ratios such as evaporite-bearing country rocks (³⁴S, ~20) and R factors (magma/sulfide mass ratios) of <400 during sulfide segregation, whereas the Pt concentration requires a much higher R factor (>2,000) if Pt content in the magma is similar to the values in the coeval, most undepleted lavas (~10 ppb). Such a discrepancy can be explained by sulfide resorption by a new flux of mantle-derived magma into a deep staging chamber to form PGE-enriched magma. Interaction of the PGE-enriched magma with evaporite-bearing country rocks in the plumbing system at higher crustal levels could have produced sulfide liquids with high PGE contents as well as elevated ³⁴S values. Mass-balance calculations indicate that <0.9 wt percent anhydrite assimilation from evaporite-bearing country rocks is required to explain the elevated ³⁴S values of the sulfide ores. This model, although developed purely based on data from the Kharaelakh deposit, can also be applied to other coeval Ni-Cu-PGE deposits. For the coeval Noril’sk-I and Talnakh deposits different depths of anhydrite assimilation in the respective plumbing systems are required. This model differs from the previously proposed models in detail but reenforces the idea that chalcophile element depletion in continental flood basalts is a useful exploration tool for Ni-Cu-PGE deposits associated with coeval subvolcanic intrusions.