Controls of Asbestos Mineralisation in the Roro Ultramafics, Singhbhum District, Bihar

The ultramafics of Roro are layered bodies comprising two major lithological units-serpentinised pyroxenites (SPX) and serpentinised saxonites (SSX). Ribbon zones of chrysotile mineralisation are found localised in thin granular serpentinite (SR) horizons at the contacts of 'the SPX and SSX layers in cross folded portions of these ultramafic bodies.

The SSX rock is typically a clot-bearing one, comprising poikilitic clots of saxonitic composition embedded in relatively olivine-rich and more serpentinised groundmass. Two controls for chrysotile localisation can be readily identified in the Roro ultramafics. The lithological control is provided by extensively serpentinised interfaces of the already partially serpentinisied pyroxenitic and saxonitic layers. These horizons of secondary serpentinisation (viz., the SR horizons) appear to have resulted from either a fusion of the SPX and SSX rocks at their contacts or, alternately, from the segregation of the SSX clots at the interfaces of these layers and their subsequent serpentinisation. The composition of the SSX rock therefore appears to have been a vital factor in fibre development in the SR horizons. The structural control is considered to have been provided by the superposition of structural deformations due to Singhbhum orogeny over those caused by the older Iron ore orogeny.

Fibre development is indicated to be a dynamothermal process in the structure-restricted zones of secondary serpentinisation in an already serpentinised ultramafic body.