Sulfide Mineralization at Askot, Kumaun Himalaya
The sulfide mineralization at Barigaon near Askote is controlled by the regional structural patterns of the Crystallines. The orebody is emplaced along the S1 planes and is elongated parallel to the fold axes of F3 generation. Two generations of sulfide minerals are recognized. Since here, as at anywhere else in the Himalaya, the rate of erosion is greater than the rate of weathering, no gossan is associated with the mineralized zone, and as such, no zone of supergene enrichment can be anticipated.
The orebody at Barigaon is 3-17 m thick and is traceable over a strike length of 400 m in boreholes. The drilling operation was part of the exploratory program of the Directorate of Geology and Mining, Uttar Pradesh. The strike of the orebody is 110o-115o. The orebody is a massive replacement type with an outer fringe of disseminated sulfides. The massive part is about 17 m thick near Barigaon, where it has been intersected by the adits driven into it from the foot-wall side by the Mineral Exploration Corporation of India. In the sulfide zone, rock textures are characterised by complete replacement of the country rock resulting in a nearly complete destruction of original textures, and in places of intense replacement,.even structures. The massive ore has resulted from the continued growth and coalescence of innumerable centres of replacement. The massive sulfides gradually and imperceptibly grade into disseminated ore. In places the massive ore terminates abruptly against the altered country rock indicating that the orebody has been sheared. Residual rock fragments are evidence of incomplete replacement in the outer fringe of the massive sulfide zone. Coarse texture of the sulfides in the massive sulfide zone is common, indicating high temperatures of formation. The disseminated ore constitutes only a small portion (upto 8 percent) of the mineralized rock. The sulfides are peppered through the host rocks in the form of specks and blebs, generally accompanied by small veins and stringers.
In the massive zone the amount of introduced gangue is moderate to high whereas in the disseminated zone it is small. Replacement banding is especially well developed in the outer fringe where the ore minerals have replaced extremely fine foliated schists. A banded and schistose texture with stringers parallel to the schistosity is apparent. In the inner sulfide zone gneissoid textures resulting from complete and vigorous replacement of the country rock have resulted. The banding of the ore minerals is due to simple progressive replacement of the schists. Both hard and soft sulfide minerals have a more or less equigranular allotriomorphic texture. The banding arises from the more ready access of the mineralizing solutions along the planes of schistosity of the invaded rock, coupled with the replacement of silicate minerals, viz., the sheet silicates, in bands parallel to the schistosity. The common order of deposition is arsenopyrite, sphalerite, galena, cubanite, and chalcopyrite. The emplacement of the orebody has clearly been influenced by the structure of the rocks, since this orebody is elongated parallel to the F3 fold axes. The S1 schistosity planes provided in themselves channelways to conduct ore fluids to rocks susceptible to replacement and served as loci for ore deposition. For the hydrothermal activity at Askote, a multiple alteration-mineralization model is proposed.
Primary mineralization, related to sericitic-argillic alteration is marked by the introduction of pyrite with subordinate sphalerite, galena, and chalcopyrite. Secondary mineralization is synchronous with propylitic alteration and was accompanied by profuse mineralization of arsenopyrite, sphalerite, galena, chalcopyrite, cubanite, pyrrhotite, and some other sulfide minerals occurring in minor amounts.
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