Types of Wallrock Alteration Associated with the Askot Sulfide Orebody
The most prominent features indicative of rock alteration in the vicinity of the sulfide orebody at Barigaon near Askote are bleaching, softening, reduction in grain size, and a yellow or brown discoloration of the country rocks. The yellow or brown color is due to the hydrous iron oxides originating as a result of decomposition of pyrite and biotite. The effect of bleaching is a result of sericitization. Two major alteration types that can be recognised at Barigaon are:
· I Sericitic-argillic, and
· II Propylitic
Sericitic-argillic alteration is the general term applied here to assemblages containing sericite, quartz, sulfides, variable amount of kaolinite, montmorillonite (?), and pyrophyllite (?). The term propylitic alteration denotes assemblages including epidote, chlorite, tourmaline, phlogopite, apatite, and fluorite. On the basis of the interrelationships of the mineral assemblages a multistage model of alteration is favored, whereby the propylitic alteration was superimposed on an earlier sericitic-argillic alteration. Sericitic-argillic assemblages are products of moderate to strong hydrolytic alteration while the propylitic assemblages denote a weak hydrogen metasomatism to strong calcium, sodium, fluorine, potassium and boron metasomatism.
I. Sericitic-argillic alteration:
All the alteration assemblages herein described show a considerable amount of overlapping. The coexistence of sericite, kaolinite, pyrophyllite (?), and secondary quartz indicates simulatneous sericitic and argillic alterations and silicification. It can be visualised that as long as active circulation of hydrothermal fluids continued along the vein, each zone migrated away from the fissures, that is, it grew at its outer edge and simultaneously receeded at its veinward edge because of encroachment by the next inner zone. This alteration type pre-dates the main stage of sulfide mineralization since most of the minerals of this assemblage are replaced by the sulfide minerals. A 130 mt wide zone surrounding the orebody has been affected by this alteration which appears to be diffused and generally uninfluenced by the structural features of the host rocks. Three distinct alteration zones may be defined from the vein to the unaltered schistose rocks. The boundaries between these zones are gradational and are defined on the basis of differing proportions of hydrothermal/secondary minerals.
Zone of Sericitization
Zone of Argillization
Zone of Silicification/Feldspathization
II. Propylitic Alteration
Propylitization in a narrow zone surrounding the sulfide orebody at Barigaon follows along structural planes e.g. foliation and fractures. Alteration is intense in the vicinity of the orebody. The outer boundaries of the zone of propylitic alteration grade over a short distance into, and slightly beyond the zone of sericitization. Propylitization is more important in the hanging wall side of the orebody. The width of the zone does not exceed 40 mt around the orebody.
Epidote, sericite, tourmaline, phlogopite, apatite and fluorite are the essential minerals constituting this assemblage. Truly speaking, this alteration type is a result of chemical additions rather than alterations of the preexisting minerals. Chemically, the alteration ranges from moderate to strong magnesium, calcium, sodium, fluorine, potassium and boron metasomatism.
It must be mentioned here that since propylitization proceeds along fractures in the sericitized rocks, it can be inferred that the rocks were subjected to a mild fracturing after the sericitic alteration and prior to propylitization. The minerals constituting this assemblage are intimately associated and intergrown with the sulfide minerals indicating that this alteration type and associated processes are syngenetic with the main stage of sulfide mineralization.
Chlorite is found associated in appreciable quantities with this assemblage, but chloritization is not recognised as a separate alteration type. The division into various subtypes, even though the mineral assemblages are essentially ihtergradational, is fortified by the wide range of chemical additions that have accompanied this process of alteration. The following alteration types are recognized:
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