Information on the host rocks


Rock Alteration


Post-Depositional Effects in the Sulfide Ores of Askot

Movements within the orebody during deposition are indicated to have caused an extensive fracturing of the brittle sulfide and other minerals viz., arsenopyrite, pyrite, magnetit and some sphalerite. Such movements resulted in marginal granulation and brecciation of the sulfide minerals. It can be visualised that as mineralization progressed, the originally fine foliated rocks became progressively more competent so that they yielded to small intra-mineralization movements by developing fractures across the schistosity.

Of the various fractures that have been recognised the first pre-dates the introduction of first generation sulfides. After the introduction of the first generation gangue minerals, the rocks were fractured prior to the introduction of first generation sulfides. The second phase of fracturing marks the period intervening between the introduction of first and second generation sulfides and hence the period between the sericitic-argillic and propylitic alterations. The third phase was contemporaneous with the deposition of second generation galena and has affected the second generation sphalerite which is profusely replaced along these fractures by second-generation galena and cubanite.

Fig 1: Paragenetic Sequence of Sulfide and Gangue Minerals, alteration types, and estimated temperature of mineralization at Barigaon (vertical lines represent periods of fracturing).

The fourth phase marks the waning part of the mineralization period and has affected all but the latest formed gangue minerals and some chalcopyrite, pyrrhotite and pyrite. The last formed pyrrhotite and second generation gangue minerals replace earlier minerals along these fractures. The fifth phase of fracturing post-dates the hydrothermal activity and has affected even the last formed gangue minerals. Fig. 1 represents the periods of fracturing in relation to the hydrothermal activity and mineralization.

Notes & Handouts

The Himalayas

Kumaon Himalayas

Askot Basemetals



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