Morphology and particle size distribution of olivines and sulphides in the jinchuan Ni-Cu sulphide deposit: Evidence for sulphide percolation in a crystal mush

Abstract

The Jinchuan intrusion is the largest single magmatic nickel sulphide deposit in the world. Sulphide mineralization mainly occurs in two segments (I and II). The crystal size distributions (CSDs) of olivines in the two segments are distinctly different. For all olivines in the two segments, their grain sizes and values of the clustering index R increase with increasing olivine abundance in the cumulates, illustrating overgrowth trends for Segment II olivines and a combination of overgrowth and compaction trends for Segment I olivines. Kinked olivine CSDs and compaction trends in the olivine cumulates of Segment I suggest the presence of two olivine populations with different nucleation rates, one of which most probably grew in a deeper chamber and then became entrained in the ascending magma. In contrast, the simple log-linear CSDs of the olivine cumulates of Segment II could be the result of a single nucleation event in the current magma chamber. In both segments, olivine chadacrysts in orthopyroxene and in clinopyroxene oikocrysts show a progression in grain size from small grains in Ti-poor pyroxene cores to larger grains in Ti-rich rims. This strongly suggests simultaneous in situ nucleation and growth of olivine and both pyroxenes at the top of the crystal mush, with the poikilitic textures developing as the result of faster growth of the pyroxenes relative to olivines. Sulphides in both segments, distributed in the pore spaces between cumulus olivines, form interconnected networks on a scale of centimetres, separated by domains poor in sulphide and occupied by oikocrysts and late-crystallizing interstitial silicates. The small sulphide droplets, having equivalent sphere diameters (ESD) between 80 and 400 lm, are relatively round and show log-linear PSDs, suggesting that they may have nucleated and grown in the mush zones. The irregular morphology and the concave PSDs of the larger sulphide droplets (ESD > 400 lm) suggest that these sulphides are the result of coalescence. CSDs of olivines in sulphide-poor (~4 vol. % disseminated sulphide) cumulates are indistinguishable from those in sulphide-rich samples (~20 vol. % sulphide, net-textured) for both segments. This, along with other textural observations, suggests that the sulphides in sulphide-rich samples percolated through the pore space of the olivine framework and displaced the interstitial silicate liquid, after significant (40-70%) olivine crystallization. Both wetting and non-wetting textures are observed between the sulphides and surrounding olivine, which suggests the presence of two liquids in the pore space and indicates that the extent of displacement of silicate by sulphide melt is rather more extensive than predicted by theory. These textural results for olivine and sulphides suggest that downward and/ or sideward sulphide percolation through the inter-olivine pore space in the mush zone plays a key role in generating the disseminated and net-textured ores in the Jinchuan magmatic Ni-Cu deposit.