Ore Textures and the Late Exsolution of Troilite from Pyrrhotite, Iken Nickel Deposit, Kun-Manie Complex, Amur Oblast, Russian Far East

The magmatic Ni-Co-Cu mineralization in the Iken deposit in the central part of the Kun-Manie complex, Amur Oblast, Russia, hosted by an olivine-bearing websterite, is of a low-sulfide type. The fine-grained disseminations of base metal sulfides (BMS), dominantly pyrrhotite, pentlandite (a major source of Ni of industrial importance), and chalcopyrite, are followed by a scarce Pd-Pt-Ag mineralization. Elevated contents of Al in orthopyroxene (mean 2.78 wt.% Al2O3) along with Al–Na enrichment in clinopyroxene (diopside; mean 5.10 wt.% Al2O3) are associated with highly aluminous compositions of low-chromium members of the spinel–hercynite series. High levels of TiO2 in kaersutite and titanian phlogopite also reflect a pronounced degree of fractionation of the ore-forming melt. Minor portions of sulfide melt are distributed evenly as a result of immiscibility at advanced stages of orthopyroxene crystallization, after the formation of olivine. Differentiated grains of droplet-like BMS largely settled in situ close to grain boundaries of orthopyroxene or occupied interstitial spaces of pyroxenes and olivine in association with spinel–hercynite and fluorapatite. A combination of late saturation in S with relatively quick cooling rates of the hypabyssal body prevented the effective settlement and accumulation of sulfide droplets in the ore zone. The well-developed lamellae of troilite (Fe50S50) exsolved from the host pyrrhotite Fe48S52 during subsolidus cooling, as a consequence of a low-temperature reaction triggered by a sudden drop in fO2. An influx of mantle-derived fluid bearing CO2, CO, and CH4 with the rising magma could be the primary cause of the fO2 reduction. Also, graphite-bearing metasedimentary rocks could have been assimilated. Tiny grains of minerals of noble metals (moncheite and merenskyite with essential amounts of melonite component, sperrylite, hessite, alloy Au63.2Ag36.8, and argentopentlandite) deposited late in a fluid-enriched medium under submagmatic conditions.