Magma Mixing as a Trigger for Sulphide Saturation in the UG2 Chromitite (Bushveld): Evidence from the Silicate and Sulphide Melt Inclusions in Chromite

Abstract

AbstractIt is of great importance to understand the origin of UG2 chromitite reefs and reasons why some chromitite reefs contain relatively high contents of platinum group elements (PGEs: Os, Ir, Ru, Rh, Pt, Pd) or highly siderophile elements (HSEs: Au, Re, PGE). This paper documents sulphide‐silicate assemblages enclosed in chromite grains from the UG2 chromitite. These are formed as a result of crystallisation of sulphide and silicate melts that are trapped during chromite crystallisation. The inclusions display negative crystal shapes ranging from several micrometres to 100 μm in size. Interstitial sulphide assemblages lack pyrrhotite and consist of chalcopyrite, pentlandite and some pyrite. The electron microprobe data of these sulphides show that the pentlandite grains present in some of the sulphide inclusions have a significantly higher iron (Fe) and lower nickel (Ni) content than the pentlandite in the rock matrix. Pyrite and chalcopyrite show no difference. The contrast in composition between inter‐cumulus plagioclase (An68) and plagioclase enclosed in chromite (An13), as well as the presence of quartz, is consistent with the existence of a felsic melt at the time of chromite saturation. Detailed studies of HSE distribution in the sulphides and chromite were conducted by LA‐ICP‐MS (laser ablation‐inductively coupled plasma‐mass spectrometry), which showed the following. (I) Chromite contained no detectable HSE in solid solution. (II) HSE distribution in sulphide assemblages interstitial to chromite was variable. In general, Pd, Rh, Ru and Ir occurred dominantly in pentlandite, whereas Os, Pt and Au were detected only in matrix sulphide grains and were clearly associated with Bi and Te. (III) In the sulphide inclusions, (a) pyrrhotite did not contain any significant amount of HSE, (b) chalcopyrite contained only some Rh compared to the other sulphides, (c) pentlandite was the main host for Pd, (d) pyrite contained most of the Ru, Os, Ir and Re, (e) Pt and Rh were closely associated with Bi forming a continuous rim between pyrite and pentlandite and (f) no Au was detected. These results show that the use of ArF excimer laser to produce high‐resolution trace element maps provides information that cannot be obtained by conventional (spot) LA‐ICP‐MS analysis or trace element maps that use relatively large beam diameters.