S. Farooq
Department of Geology
Aligarh Muslim University

Sulfide mineralization occurs in the Precambrian sericite-chlorite schists of pelitic composition at Barigaon near Askot in the Lesser Himalayan region of Kumaon.  The schists, forming the lowermost horizon of an inverted sequence of metamorphic rocks, known as the Askot Crystallines, show effects of polymetamorphism - progressive regional type superimposed by dynamic and retrograde type.  The Askot Crystallines have been considered by most workers to constitute a southern klippe of the vast Almora-Dudhatoli Nappe that was pushed southwards from the Central Crystallines of the higher Himalaya over the younger sedimentary rocks of the Lesser Himalaya.  The thrusting was accompanied by intrusion of dioritic magmas along the thrust plane.  Petrochemical examinations reveal that the Crystallines have been hybridized by the magmas near the thrust contact.

The rock types constituting the Crystallines include sericite-chlorite and garnetiferous muscovite-biotite schists, augen and permeation gneisses, and migmatites.  Field and laboratory investigations reveal that the transition from the lowest grade schists to the highest grade migmatites, and from pelitic to granitic composition is perfectly gradational and that the granitic gneisses and migmatites are the product of metasomatic granitization that was syngenetic with the regional metamorphism.  The various planar and linear structural elements preserved in the Crystallines, particularly in the low-, and medium grade schists, bear evidence of at least three tectonic episodes to which the rocks were subjected.  The orebody at Barigaon has been affected by the last of these three episodes and is hence considered older than this but younger than the first two.

Petrological and mineralogical investigations support a multistage alteration-mineralization model.  The orebody is surrounded by concentric zones of altered wall-rock.  Two phases of alteration are recognized - one pre dating the sulfide mineralization while the other syngenetic with it. The alteration preceding the sulfide mineralization is of sericitic-argillic type and is marked by the destruction of rock textures, alteration of preexisting rock minerals, and introduction of a few minerals into the country rocks viz., sericite and andalusite.  Sericitization, kaolinization, and silicification accompanying this alteration are conspicuous in concentric zones around the orebody.  The alteration, as revealed by petrochemical studies, was produced by fluids which were late differentiates of a granitic magma.  The initial temperature of the fluids is estimated to have been 375^o C, with pressures below 4.0 Kb.  The pH of the solutions, which were acidic in nature, decreased further as the fluids entered the country rocks.  There was, however, an increase in the pH as the solutions migrated further into the unaltered rocks and the H⁺ ion was lost due to boiling in response to reduced pressure.  The second phase of alteration - the propylitic type, accompanying the sulfide mineralization, is marked by the introduction of epidote, fluorite, apatite, tourmaline and phlogopite.

Field investigations reveal that subsequent to its localization, the orebody was sheared along planes trending NNE-SSW.  Studies of exsolution textures of the sulfides indicate that the temperature of the mineralizing solutions was about 5000^oC.  The mineralizing solutions were alkaline in nature but the pH decreased continuously as the fluids migrated away from the fissures into the country rocks.  The deposition of metallic sulfides was effected by a decrease in the pH.

The occurrence of sulfides in the form of a massive lode, stringers, and disseminations is suggestive of transportation of metals by mechanical transfer and infiltration, and also by diffusion.

Sulfide bodies also occur in the granitic gneisses along the Gurji Gad near Askot.  Preliminary investigations reveal a genetic relationship of the two mineralized zones.