Paragenesis of the Askot Sulfides

A generalised paragenetic sequence of the sulfide and gangue minerals is given in fig. 1. The gangue minerals precede the sulfides in the general order of introduction. The first minerals to be introduced were quartz, epidote, andalusite, and sericite. These were followed by the first generation arsenopyrite, sphalerite, bismuth, galena, chalcopyrite, marcasite and pyrite. The first generation sulfides have a generally simultaneous deposition. In some cases, however, replacement of one mineral by another is discernible. The emplacement of first generation gangue and sulfide minerals was closely associated with sericitic-argillic alteration. The deposition of first generation sulfides was followed by a period of very slow hydrothermal activity during which only hydrothermal quartz was deposited.

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 reactivation of the hydrothermal activity is associated with a period of fracturing and is marked by the introduction of first generation actinolite, calcite, fluorite, apatite, phlogopite, tourmaline and second generation epidote. These minerals have, in general, an overlapping deposition but in most cases the sequence can be established and is depicted in fig. 1. The introduction of these silicate minerals was closely followed, in order, by magnetite, sphalerite, galena, arsenopyrite, cubanite and chalcopyrite. Gudmundite, pyrrhotite and second generation bismuth were deposited simultaneously with cubanite. Some cubanite, however, was formed after these minerals.

The second generation sulfides were followed by the second generation gangue minerals viz., andalusite, actinolite, calcite, phlogopite, tourmaline, apatite, chlorite and sericite, and third generation epidote. These minerals were more or less simultaneously deposited. Second generation pyrite and pyrrhotite which post-date the other second generation sulfide minerals appear to have been simultaneous with the second generation gangue minerals.

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