Estd.: 1981
Department of Geology, RTM Nagpur University, RBD Laxminarayan Campus, Law College Square, Nagpur- 440 001, India
*E-mail: mahakal46@gmail.com
The present paper evaluates the mineral chemistry of olivine, chromite, montmorillonite, orthopyroxene and illmenite within the ultramafic rocks of Sukinda mafic-ultramafic complex of southern Sinhgbhum craton. The minerals chrysotile, antogorite, montmorillonite, olivine, chromite, magnetite, hematite, goethite and quartz were separated from serpentinised peridotite. Orthopyroxene separated from orthopyroxenite. This rock also contains minor olivine, chromite, illmenite, tremolite, chlorite, antigorite, chrysotile and pleonaste etc. Chromitite contains 99% chromite with minor silicate and sulphide minerals. To understand the mineral chemistry of ultramafic rocks, electron probe micro analyses (EPMA) of olivine, chromite, orthopyroxenite and illmenite was carried out. Geochemical composition of olivine and chromite from serpentinised peridotite, orthopyroxene from orthopyroxenite and chromite from chromitite show modest variation.
In serpentinised peridotite, chromite is characterised by low Mg# (5.23 – 42.93) (Mg# = 100 x Mg/(Mg + Fe2+) and high Cr# (83.82 – 89.59) (Cr# = 100 x Cr/(Cr + Al), along with significant enrichment of Al (4.86 – 7.15) and Fe (9.56 – 18.29). Majority of olivine grains from serpentinised peridotide are characterised by high Mg# (83.66 – 98.37), low CaO (0.01 – 0.15) and high MgO (31.44 – 44.04) contents, while FeO (0.49 – 7.38) content show wide range. These olivines are enriched in Cr2O3 (0.02 – 1.16) as compared to the Cr2O3 (0.01 – 1.00) content of orthopyroxene from orthopyroxenite.
Orthopyroxenite from Sukinda massif are Mg-rich enstatite. Its En content varies from 89.1 to 91.89%, which makes up more than 95% of the volume. These results are also consistence with the refractory character. The CaO content of enstatite ranges from 0.02 to 0.77% and it is characterised by high Mg# (83.30 – 98.37). The orthopyroxene from serpentinised peridotite have high content of MgO (30.90 – 39.13), low contents of CaO (0.02 – 0.77) and Al2O3 (0.19 – 0.91), while FeO (0.49 – 5.43) content show wide range. The low content of Ca and Al and the assemblage is typical of boninitic melt.
Keywords: Mineral chemistry, Olivine, Chromite, Sukinda mafic-ultramafic complex, Singhbhum craton, India