Mineral chemistry of chromite from the Permian Jinbaoshan Pt–Pd–sulphide-bearing ultramafic intrusion in SW China with petrogenetic implications

Abstract

The Jinbaoshan ultramafic intrusion is a member of the late–middle Permian (260 Ma) Emeishan Large Igneous Province in Yunnan Province, SW China and hosts an economical Pt–Pd–sulphide deposit. The intrusion consists mainly of wehrlite (92 vol.%), with minor olivine clinopyroxenite and clinopyroxenite (8 vol.%). The ores, defined as rocks having >0.5 ppm Pt and Pd, contain abundant chromite (up to 20 vol.%) and minor sulphide (<3 vol.%). Sulphide aggregates occur along the silicate grain boundaries and thus clearly crystallized after olivine and chromite. Olivine occurs as euhedral, subhedral or rounded crystals, and varies from 0.3 to 4 mm in size. The olivine has almost constant Fo contents (83 wt.%), and variable Ni contents (0.13 to 0.34 wt.%). There are two types of chromite: (1) relatively homogeneous chromite, as euhedral inclusion in olivine; and (2) zoned chromite as interstitial phase between silicate minerals, and occasionally, as inclusion in olivine pseudomorphs. The zoned chromite can be further divided into type 2a with magnetite rim and type 2b without magnetite rim. Primary chromites, including type 1 chromites and the core of type 2a chromites, have relatively high Cr#'s (51 to 70), a relatively wide range of Mg#'s (13 to 57), variable TiO 2 (1.55 to 9.39 wt.%) and low Ni (NiO≤0.35 wt.%) and Zn (ZnO≤0.72 wt.%). Type 2 chromites formed during serpentinization and were chemically modified toward magnetite rim. The modified chromites have generally lower Cr 2O 3 and Al 2O 3 and higher FeO t (total Fe) than primary chromites. The high Cr contents of the primary chromites and the high MgO/FeO molar ratio (1.47) of the calculated melt composition in equilibrium with olivine in the Jinbaoshan intrusion demonstrated that the Jinbaoshan chromites most likely crystallized from a low-Ti and high-Mg tholeiitic magma derived from a mantle plume in an intra-continental setting. The relatively large amount of chromite in the ores may indicate a direct connection between the chromite crystallization and the ore formation. We infer that the parental magma was S-undersaturated, and remained rich in highly siderophile elements such as Pt and Pd. The parental magma may have experienced early olivine crystallization such that Ir and Os were depleted. This magma intruded the Devonian limestone, sandstone, and slate. Assimilation of the country rocks induced temporary supersaturation of chromites and also triggered S-saturation in the magma. The Jinbaoshan intrusion represents a magma conduit in which magmas deposited excess ultramafic components with Pt–Pd–sulphide enriched horizons due to delayed S-saturation during crystallization.