Geochemistry of late-archean bhavani (mettupalayam) mafic/ultramafic complex, Southern India: implications for platinum group element mineralization Satyanarayanan M.1,*, Balaram V.1, Sylvester P.J.2, Rao D.V. Subba1, Charan S.N.1, Shaffer Michael3, Dar Ali Mohammed4, Anbarasu K.4 1National Geophysical Research Institute (Council of Scientific and Industrial Research), Uppal Road, Hyderabad-500007 2Dept of Earth Sciences, Memorial University, St. John's, NL, Canada, A1B 3X5 3CREAIT Network, Micro-Analysis Facility, Inco-Innovation Centre Memorial University, St. John's, NL, Canada, A1C 5S7 4Dept of Geology, Periyar University, Salem-636011 *m_sat2001@yahoo.com
Online published on 3 December, 2011. Abstract The mafic and ultramafic intrusive rocks near Punjampuliyampatti, Odhimalai, Karappadi and Solavanur in late-Archean Bhavani complex within the districts of Coimbatore and Erode, Tamil Nadu (India), were studied for possible PGE mineralization. The mafic/ultramafic Bhavani complex occurs as structurally concordant lenses and sheets within the supracrustal gneiss formations, and is composed of gabbro, metagabbro, metapyroxenite to hornblendites with disseminations of chromites. The meta-gabbro (± garnet) is a medium grained inequigranular granoblastic polygonal textured melanocratic hornblende gabbro to pyroxene hornblende-gabbro consisting of plagioclase (19–36 wt.%), pyroxene (5–42 wt.%) and hornblende (9–56 wt.%). The plagioclase bearing hornblende pyroxenites and pyroxene-hornblendites display similar texture with garnet as an accessory (0.2 wt.%). Chromites and sulfides (pyrrhotite, pyrite, chalcopyrite and pentlandite) occur as irregular disseminations. Sulfides are associated with hornblende, magnetite, chlorite, ilmenite and sometimes quartz, while chromite is associated mainly with the pyroxenes. Both the pyroxenite and hornblendite show consistent and complementary compositions with SiO2 ranging from 46.6 to 53.5%, Al2O3 from 6 to 14%, MgO from 5.3 to 9.9%, CaO from 2 to 16 and TiO2 from 0.2 to 0.7%, while the average composition of SiO2, Al2O3, MgO, CaO and TiO2 in gabbro varies (in %) from 49.6–53.3, 10–16, 3.6–13.1, 8–14 and 0.4–1.8. Bulk composition indicates that the magma type is tholeiitic but tending towards calc-alkaline nature, and may have formed in an active continental margin setting. The Mg# ranges from 34 to 73 for gabbro (including meta-gabbro) and 29 to 82 for the ultramafic rocks indicating crystallization from both primitive and evolved magmas as well as its later alteration. PGEs in representative gabbro samples and the concentrations of Ru (8–22 ng/g), Rh (3–6 ng/g), Pd (713–816 ng/g), Ir (1–3 ng/g), Pt (28–37 ng/g), Au (42–64 ng/g), Cu (10–128 μg/g), Ni (206–1276 μg/g), normalized to primitive mantle values, show that PPGE (Pd-PGE) are enriched over IPGE (Ir-PGE) with a distinct positive Pd-anomaly, possibly due to partial melting of the upper mantle under ultra-high temperature metasomatic conditions. The Pd-Ir ratio varies from 238–758 and Pt-Pd ratio from 0.03–0.07, which when plotted against Cu indicate that the PGEs were extracted from their mantle source by S-undersaturated magma. Top Keywords Geochemistry, Mafic-ultramafic complex, PGE, Bhavani, Southern India. Top |