Information on the host rocks

Petrography

Petrochemistry

Structure

Sulfide Mineralization

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.

1. Zone of Sericitization
The nearest wall-rock alteration zone (20-25 mts wide) is characterised by the predominance of white micas - sericite and pyrophyllite (?), secondary quartz and pyrite crystals plus some chalcopyrite and sphalerite as disseminations.  Andalusite is also present in this zone.  The secondary assemblages replace all the metamorphic minerals, except perhaps some quartz.  Feldspars are completely obliterated.  Sericite makes up as much as 40 percent of silicates whereas secondary quartz accounts for 15 percent.  Alteration is intense in places adjoining the sulfide orebody, where rock textures have been totally obliterated.  In the fringe area sericitic alteration is discontinuous and is guided by the rock fabric.

2. Zone of Argillization
The intermediate wall-rock alteration zone (30-50 mt wide) is easily distinguished because the sericitization is less intense than in the nearest wall-rock alteration.  Towards the sericite zone feldspars are totally obliterated, whereas towards the outer fringe they have generally survived complete alteration, being attacked only along grain boundaries and cleavage planes.  Kaolinite, generally a minor constituent of the feldspar alteration product, is concentrated locally in this zone.  Kaolinite forms 5 percent of the rock volume, sericite 5-15 percent, and chlorite 5 percent.

3. Zone of Silicification/Feldspathization
The outermost wall rock alteration zone (60-75 mt wide) shows an important decrease in sericitization.  Here plagioclase and biotite appear much less altered than in the zone of argillization.  Secondary silica is very abundant in this zone, making up as much as 20-25 percent of the rock.  Feldspars make up 10-15 percent, chlorite 5-10 percent, biotite 5-10 percent, and sericite rarely more than 10 percent.  Secondary quartz replaces metamorphic quartz and feldspars along grain boundaries.  Secondary and hydrothermal plagioclases form a significant part of the assemblage.  The composition ranges from calcic oligoclase to labradorite.  Hydrothermal plagioclases occur as irregular poikiloblasts containing inclusions of quartz and other micaceous minerals.  This manner of occurrence suggests that they are a product of sodium metasomatism during the hydrothermal processes rather than a residual concentration of sodium in preexisting plagioclases as a consequence of the removal of calcium.  Secondary plagioclases are sericitized along fractures and cleavages.  The development of orthoclase is not of much significance.  It is a subordinate mineral in this zone.

 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:

1. Epidotization
Small, elongate, and distinct crystals of epidote are distributed along the foliation planes and fractures.  Epidotization is a result of calcium metasomatism active along fractures in sericitized rocks.  Fluorite and apatite are intimately associated with epidote and their presence is suggestive of accompanying fluorine metasomatism.  Phlogopite occurs in appreciable quantities as short prismatic crystals introduced along foliation and fractures.

2. Tourmalinization
Appreciable quantities of tourmaline and minor amounts of phlogopite occur in the zone of propylitic alteration.  The introduction of phlogopite generally follows the structural trends, whereas tourmaline occurs as disseminated, small, prismatic crystals and fibrous aggregates.  The presence of tourmaline and phlogopite is interpreted to be a result of sodium, magnesium, potassium and boron metasomatism.

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