Geochemistry of Ultramafic Lenses in the Granitoids of the Southeastern Flanks of Shimoga Supracrustal Belt (karnataka) with a Note on the Distribution of Platinum Group Elements and Minerals

Almost Thirty Individual Lenses of Ultramatites Occur Within about 150 Km²of the Basement Granitords of the Peninsular Gneiss forming Southeastern flanks of the Shimoga Supracrustal Belt. They form Small elongated Bodies (<50 M X l Km) which are most Commonly Oriented Parallel to the General Northwesterly Strike of the Basement Granitoids.

The Ultramafites fall Mainly in the Range of Chromitite-dunite-peridotite-pyroxenite. the Original Textures as well as Mineralogy of the Rocks are Considerably Modified by Pervasive Deuteric and Low-grade Metamorphic Alterations. Chlorite (and/or Antigorite) and Fe-cr Oxides are the Ubiquitous Minerals. Amphibole is abundant only in Hornblendite. Carbonate, Epidote, talc, Magnetite and Sphene Occur in Minor Amounts. The Re1ics of the Original Olivine and Pyroxenes are very Rare.

Geochemically, the Ultramafites include High-cr, High-mg, High-a1 and High-fe Types. of the Four, High-cr and -Mg types Account for the Largest Proportion, whereas High-A1 and Fe-types Constitute only Local Variations. the overall High Magnesia (25.6 Wt%) and Alumina (9.5%), Iow Lime (2.8 %) and very Low Alkalis (< 0.1%) are Distinctive Major Element Geochemical Features. Marked Enrichment of Ni (1317 Pprn) in High-Cr and -Mg Types, Ti (3.9%) and V (1176 ppm) in High-fe Type and Zr (291 Ppm) in High-A1 Type are among Distinctive Trace Element Geochemical Features. Low REE (3.2 - 18.5 Ppm) and Nearly Flat Chondrite Normalized Patterns Characterize High-Cr and -Mg Types. the High-A1 Type has Contrastingly High REE (141.6 Ppm) and Markedly Enriched in LREE.

The High-Cr and -Mg Types Bear Geochemical Signatures of Derivation from Higher Degree of Partial Melting of Garnet-lherzolite Layer of the Mantle. Mixing of Primitive and Evolved Magmas is Indicated by the Composite Association of High-Cr, -Mg, and -Fe Units Noted in some of the Ultramafic Bodies. the High-a1 Type with Strong Enrichment in Ree and Zr Bears Evidence of Derivation from Segregation of Late Stage Residual Liquids. Kornatiitic Bulk Composition of the Parental Magma is Indicated by some of the High-Cr and -Mg Ulyarnafites. the Chromite-dunlte-peridotite-pyroxentite Association Strongly Suggests that the Ultramafite Lenses of the Area Constitute Parts of a Tectonically Emplaced Layered Body.

Analysis of as many as 27 Samples of Ultramafites, Covering all the important Petrographic Variations has not Revealed either Anomalies in the Content of Pt and Pd or Possible Potential of PGE minerailzation. Samples from the layered Ultrarnafite body to the East of Rangapura, Containing Variable Proportions of Chromite Bands, However, have Yielded Significantly High Os, Ir, Ru and Rh Values (the Total of these Ranges from 130 to 240 Ppb). these Samples also Contain Minute Platinum Group Mineral (pgm) Inclusions, Mainly in the Range of Ladrite-irarsile and less Frequently those Corresponding to Erlichmanite, Osarsite, Irarsite and Bowieite. the Pge Mineral Inclusions are Early formed Phases of Deep Seated Crystallization which became Entrapped in Later Crystallized Chrornite and Silicates. the Ubiquitous Hydrothermal and Low-grade Metamorphic Alterations have not Apparently Caused Significant Migration of Pge.