Ni-Au-PGE Mineralization in the Ultramafic Body at Shankaraghatta, Shimoga Schist Belt, Karnataka: A Mineralogical and Geochemical Study

Association of Ni and PGE with the already known gold mineralization is recognized in about 5 m wide sulphide zone in the ultramafic rock exposed between Bhadra Reservoir and northern border of Kuvempu University Campus. The mineralization is picked up over a strike length of almost 3 km. The mineralized ultramafite, which is a metadunite, contains about 94% serpentine, 3% sulphides, 1% magnesite and 0.5% Fe-Cr oxide. Millerite and pentlandite are the main carriers of Ni. Gold varies from native gold to electrum. Melonite greatly dominates michenerite, merenskyite and Pd-Ti-Bi-Sb alloy among Pd minerals identified. Pt is present both as sperrylite and as irarsite. Au as well as Pd-Pt minerals occur as minute inclusions of 1 to 10 microns in size. About 50% of these inclusions are in sulphides, 10% in silicates and the remaining are located along the contacts between the sulphides and silicates.

The ultramafite contains isolated pods and patches of chlorite-Rock which are contrastingly enriched in Al, Fe, Ti, Mn, REE, Y, Th, U, Zn and Zr and depleted in Si, Mg, Ni, Cr, As and Bi. These chlorite pods are interpreted as products of late stage residual liquid of ultramafic magma.

The Shankaraghatta ultramafite is not a layered intrusion. The primary economic target in this case is Au and Ni and not PGE as in stratabound deposits. Here the distribution of Ni, Au and PGE is controlled by sulphides. Concentration of the metals in sulphide melt by liquid immiscibility is suggested. Very high MgO and contrastingly low Al₂O₃ and TiO₂ of the mineralized metadunite of Shankaraghatta suggest its formation from ultramafic magma derived from high degree (20-25%) of partial melting of fertile mantle associated with plume activity.