Information on the host rocks

 

Petrography
Petrochemistry
Structure

Sulfide Mineralization

Wallrock Alteration Associated with Sulfide Mineralization at Askot

 

Concentric zones of altered rock are associated with the sulfide orebodies in the sericite-chlorite, and muscovite-biotite schists.  The host rocks are melanocratic, fine to medium grained, and subequigranular.  The alteration zones bordering the sulfide bodies in the adjoining granitic geneisses are narrow and inconspicuous, and hence the present study is based entirely on the alteration zones associated with the sulfide bodies in schistose rocks.  The study of wallrock alteration is based on field observations, study of about 200 thin sections and chemical analyses of 30 samples collected during field trips.  A study of changes in modal composition of the host rocks caused by hydrothermal alteration has also been made.

Field observations of the altered zone were, in general, meager, thus precluding uninterrupted observation of defined successions of alteration assemblages.  The most complete coverage of an alteration profile, as also of the sulfide zone, resulted from the investigation of a series of samples collected from the adits driven into the orebody from the footwall side by the Mineral Exploration Corporation of India.

While preparing thin sections and selecting samples for chemical analysis, care was taken to avoid veins and weathered surfaces.  However, this may not have always been possible.  The investigations involved (1) selection of samples to characterize the zoning sequence; (2) petrography; (3) modal calculations, and (4) wet chemical analysis.

The study of altered rocks shows that alteration occurs as concentric zones surrounding the sulfide orebody.  Sericitic-argillic alteration is the dominant type and is characterized by an envelope of secondary minerals around the orebody.  The zones can be easily differentiated according to the intensity of preexisting silicate replacement and mineral associations.  Propylitic alteration is superimposed on the sericitic-argillic alteration and is more conspicuous in the hanging wall than in the foot-wall.

Variations in the volume percent of important secondary/hydrothermal minerals in the zone of alteration, and chemical effects of hydrothermal alteration are presented in fig 1. The volume percent of each mineral was determined by point counting.  Superimposed on the diagram are the variations in the major element compositions of the bulk rock.  There is a striking relationship between the modal composition of the altered zone and major element variation.  Types and extents of alteration zones are evident from these plots and are indicated in fig 1.

 

Notes & Handouts

The Himalayas

Kumaon Himalayas

Askot Basemetals

University

   


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