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Gangue Minerals Asociated
with Askot Sulfides
The
term gangue shall be used herein to describe silicate minerals associated
with the metallic sulfides. Mineralogical and textural studies of the
gangue minerals and their relationship with the sulfides are based on a
study of 120 polished thin sections. The gangue minerals are derived from
two sources, viz.:
1.
The hydrothermal activity (hydrothermal), and
2.
Relicts of original country rock (metamorphic).
A
study of the gangue minerals throws light on the manner in which the
orebody was formed, and furnishes information as to the physico-chemical
conditions during deposition. Most gangue minerals occur in two
generations.
1.
Quartz
Quartz is the most abundant gangue mineral. It occurs as anhedral
porphyritic masses containing inclusions of earlier formed sulfides. The
later minerals along grain boundaries and fractures replace it. All quartz
in the zone of massive sulfide mineralization is of hydrothermal origin
whereas in the zone of disseminated sulfides it is mostly of metamorphic
origin, occurring as relicts of unreplaced country rock. Substantial
quantities of quartz in this zone, however, are also of hydrothermal
origin. Quartz appears to have been added ubiquitously and occurs in
complex intergrowths with all other sulfide and gangue minerals. The
process of silicification appears to have become profuse following every
period of fracturing.
2.
Actinolite and Calcite
Actinolite and calcite occur in intimate intergrowths with each other.
Actinolite occurs as long prismatic crystals and columnar aggregates,
whereas calcite occurs as aggregates of fine to medium grained euhedral
crystals. The two minerals make up as much as 75 percent of the gangue
minerals in the central part of the orebody. Actinolite and calcite occur
in two generations. Intergrowths of first generation calcite and
actinolite replace the first generation sulfides and are replaced by the
second generation sulfides, while the second generation of these minerals
replace the second generation sulfides.
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| Fig 1:
Pleochroic haloes in phlogopite formed around tiny inclusions of
unidentified radioactive minerals. Phlogopite is seen to be
replacing epidote and some sulfide minerals. (250X) |
3.
Phlogopite
Phlogopite occurs as medium grained, thick tabular and short prismatic
crystals. The first generation phlogopite replaces first generation
calcite and actinolite and is replaced by the second generation sulfides.
The second generation phlogopite occurs contemporaneously with the second
generation gangue minerals (calcite, actinolite, epidote, etc.) and
replaces the second generation sulfides. Pleochroic halos, resulting from
the inclusion of optically unidentifiable tiny radioactive minerals are
very common in phlogopite (fig. 1).
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Fig 2:
Early formed epidote (high relief) eing replaced by the sulfide
minerals. Second generation epidote is seen replacing the
sulfides (50X), |
4.
Epidote
Epidote occurs as granular to columnar aggregates of fine euhedral
crystals. The first generation epidote replaces the first generation
sulfides and is replaced by the second generation sulfides. The second
generation epidote replaces the second generation sulfides and also some
gangue minerals (fig. 2).
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| Fig 3:
Tourmaline (schorlite) being introduced into the country rocks
along foliation planes. It occurrs as short prismatic
crystals having angular relationships with the schistosity. |
5.
Tourmaline
Tourmaline is a hydrothermal mineral introduced along the foliation. It
occurs as short prismatic crystals (fig. 3) and columnar aggregates. The
first generation tourmaline contains inclusions of first generation
sulfides and is replaced by the second generation sulfides. The second
generation tourmaline occurs in minor amounts and replaces the second
generation sulfides.
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| Fig 4:
Early formed andalusite being replaced by later sulfide
minerals. Note that the cleavage planes of andalusite are
flexed, indicating a deformation of the rocks subsequent to the
emplacement of the mineral. |
6.
Andalusite
Andalusite occurs as aggregates of coarse columnar crystaxs. The first
generation andalusite occurs in association with first generation
actinolite and calcite and is seen as inclusions in first generation
sulfides (fig. 4). The second generation andalusite is seen replacing
second generation sulfides.
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| Fig 5:
Hydrothermaly introduced apatite (light gray, upper left corner)
replacing earlier sulfides and other metamorphic minerals. |
7.
Apatite and Fluorite
Apatite is a hydrothermal mineral and is found as medium to coarse
prismatic crystals (fig. 5) while fluorite occurs as tiny anhedral
crystals (fig. 6). Fluorite and first generation apatite are intimately
associated with each other, replace the first generation sulfides, and are
replaced by the second generation sulfides. The second generation apatite
replaces the second generation sulfides.
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| Fig 6:
Fluorite (anhedral central grain) replacing apatite (inclusions
with moderate relief) and epidote (inclusions with high relief). |
8.
Chlorite and Sericite
Chlorite occurs as small scaly masses, whereas sericite occurs as
aggregates of very fine, elongated crystals. Chlorite is intimately
intergrown, and appears to be contemporaneous with the second generation
gangue minerals. The first generation sericite occurs abundantly as
replacement relicts in the first generation sulfides while the second
generation sericite occurs as a minor phase replacing the second
generation sulfide and gangue minerals. The relative abundance of
various.gangue minerals through the sulfide zone is given in Fig. 7.
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