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.
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 375o
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.
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 5000oC.
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.
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.
bodies also occur in the granitic gneisses along the Gurji Gad near Askot. Preliminary investigations reveal a genetic relationship of
the two mineralized zones.
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