• Vukmanovic, Zoja
• Godel, Belinda
• Fiorentini, Marco
• Reddy, Steven

Abstract

The Merensky Reef is a thin (< 1 m), highly continuous platinum-rich layer in the Bushveld Complex (South Africa). The samples studies here are characteristic of typical Merensky Reef, from bottom to top: lower chromitite layer (0.7-1 cm), coarse grained melanonorite (~10 cm), upper chromitite layer (~1 cm) and overlying melanonorite.Chromites from the lower and upper chromitite layers show morphological differences. Chromites from the lower chromitite layer have complex embayed and branching “amoeboidal” morphologies and often contain small sulphide and silicate inclusions. In contrast, the chromite grains in the upper chromitite are idiomorphic octahedral and are free of silicate inclusions. Both lower and upper chromitites contain interstitial pyroxenes, plagioclase, sulphides (pentlandite, pyrrhotite and chalcopyrite) and platinum group minerals.Electron backscatter diffraction analysis (EBSD) was used to reveal internal microstructures within chromite and sulphide in samples from both chromitite layers. EBSD shows plastic deformation in amoeboidal chromites in the lower chromitite (up to 10° of misorientation), whereas idiomorphic chromites from the upper chromitite show little or no sign of plasticity (≤ 3° of misorientation) and no crystallographic preferred orientation. In both chromitites, all sulphides record crystalo-plastic deformation.The deformation observed in chromites and sulphides is consistent with deformation recorded by the silicates and with the vertical 3D-distribution of the sulphides. All of which are attributed to compaction during cooling of the overlying magma column. According to Godel et al (2007), crystallisation of the lower chromitite layer and the melanorite took place from an injection of new magma on the top of the almost completely solidified anorthosite. The upper chromitite layer formed by a second injection of magma while the melanonorite was still a partially crystalline mush (Cawthorn and Boerst 2006). The more rigid interface between solid anorthosite and overlying magma focused the deformation in the early crystallized phases, chromites and in the late magmatic sulphides. The chromites from the upper chromitite layer crystallised above the “mushy” melanonorite and do not record any signs of deformation, as deformation was accommodated by compaction of the weaker semi-solid melanonorite.The origin of the amoeboidal chromites remains unclear. The larger amoeboidal chromites in the lower layer were more prone to record plastic deformation than adjacent finer idiomorphic chromites, which were more capable of accommodating compaction by sliding past one another rather than deforming internally. Internal microstructures indicate that the amoeboidal grains were not developed by late stage sintering of multiple idiomorphic equant grains as has been suggested previously (Hulbert and Von Gruenewaldt 1985).The amoeboidal grains evidently developed as single crystals, possibly with originally dendritic morphologies, prior to most of the compaction. However, the presence of deformation microstructures in adjacent chalcopyrite grains indicates that compaction-related deformation continued until very close to the solidus temperature of the rock.

Topics

• impedance spectroscopy
• mineral
• polymer
• single crystal
• grain
• inclusion
• phase
• Platinum
• plasticity
• electron backscatter diffraction
• interstitial
• sintering