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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.
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