of the Altered Zone around the Askot Sulfide Body
terms metamorphic, secondary, and hydrothermal shall be used herein to
indicate respectively original minerals of the country rocks, those that
have been recrystallized, redistributed or altered by the hydrothermal
activity, and those that have been emplaced as a result of metasomatic
chemical additions. It must be emphasized that it may not have
always been possible to distinguish clearly between secondary and
hydrothermal minerals on the basis of petrographic criteria alone.
Geochemical considerations were in some instances very helpful in
differentiating. Petrographic methods have been of little value in
determining the mineralogy of feldspar alteration products. The
identification of clay minerals has not been possible, and pyrophyllite is
very difficult to distinguish from sericite by optical means.
In the fresh country rock, quartz occurs as medium sized, euhedral, and
slightly strained grains. Towards the zone of alteration it is
progressively replaced by secondary quartz along grain boundaries and,
fractures. In the outer zone of alteration secondary quartz is very
abundant and replaces metamorphic quartz. The metamorphic quartz
occurs as isolated porphyroblasts with embayed boundaries, surrounded by
microcrystalline secondary quartz (fig. 1).
Fig 1: Metamorphic
quartz in the zone of hydrothermal alteration being replaced by
microcrystalline secondary quatrz along grain boundaries and
quartz is clear whereas the secondary quartz is diffused containing tiny
inclusions of other minerals. Quartz in the zone of silicification/feldspathization
is generally derived from the breakdown of feldspars in the zone of
argillic alteration. This can be readily seen in Figure 7 wherein
the SiO2 curve indicates a depletion of silica in the zone of
argillic alteration and its enrichment in the zone of silicification/feldspathization.
Though the unaltered country rocks are sericitic, almost all the sericite
in the altered zone is of secondary origin. The mineral occurs as
minute shreds as an alteration product of K-feldspars. Small
quantities have also been introduced by hydrothermal solutions as a result
of potassium metasomatism. Sericitization of the country rocks took
place along S1 and S3 planes (fig. 2) along which
the mineral occurs with no apparent orientation.
Sericitization of the country rocks proceeding along the foliation
planes S1 and S3. Sulfide minerals occur as disseminations
with silica, tourmaline, some epidote, and phlogopite, it constitutes the
entire bulk of the altered country rocks in the immediate vicinity of the
sulfide zone. Within the sulfide zone, the sulfide minerals replace
sericite, whereas in the zone of sericitic alteration, sulfides are seen
as disseminations in sericite. From the textural studies of the
sulfide and altered zone it is interpreted that sericite generally formed
earlier than the sulfides. Minor amounts, however, may have
formed even after the emplacement of
sulfides, since some sericite appears to be replacing sulfide minerals.
The unaltered country rocks are profoundly chloritic but in the zone of
hydrothermal alteration the effects of bleaching suggest that the
ferromagnesian minerals were broken down by the initial hydrothermal
activity. Biotite and chlorite encountered in the zone of alteration
are of late hydrothermal origin. Minor chlorite occurs as scaly
masses in the outer fringe of the zone of sericitic, alteration and also
in the zone of argillization and silicification. It is occasionally
an alteration product of biotite. In the zone of silicification,
garnet is seen altering to chlorite (fig. 3).
Fig 3: Garnet
crystals containing spiral trains of inclusions. This garnet
can be seen altering to chlorite.
penninite variety usually occurs as an alteration product of garnet, while
prochlorite is of hydrothermal origin. Hydrothermal prochlorite is
also noticed associated in appreciable quantities with epidote in veins
that have undergone propylitic alteration.
Biotite has been emplaced along the foliation planes of the country rocks.
Such biotite is of a secondary origin and can be readily recognised by its
cross-cutting relationship with rock schistosity (fig. 4).
Rock fabric disturbed by the growth of garnet crystals -
post-tectonic crystallization. Note the presence of later
introduced biotite flakes that have cross-cutting relationship
with the main schistosity.
the zone of silicification/feldspathization biotite forms 2-10 percent of
the total silicate minerals, whereas in the zone of sericitic alteration
it is rarely more than 2 percent.
Epidote occurs generally as granular aggregates of distinct elongate
crystals. This mineral invades the country rocks along fractures and
foliation planes in the sericitized rocks. Epidotization is most
intense in the inner zone of sericitic alteration and in the outer fringe
of the sulfide orebody. Early formed epidote replaces the earlier
sulfides and in turn is replaced by the later sulfides. Some epidote
is also seen replacing the second generation sulfides (fig. 5).
Fig 5: Early
formed epidote (high relief) being replaced by the sulfide
minerals. Second generation epidote is seen replacing the
is concluded that epidote was being introduced into the country rocks
before the later sulfides were deposited, and continued to be added till
the close of the hydrothermal activity.
The unaltered schistose rocks do not contain tourmaline. The entire
tourmaline in the zone of alteration is hydrothermally produced.
Tourmalinization can be recognised in a 35-40 mts. wide band adjoining the
sulfide orebody. The mineral is intimately associated with epidote.
Two varieties of tourmaline can be recognised -- schorlite, and dravite.
Schorlite occurs in short prismatic crystals showing strongest absorption
normal to the plane of the polarizer. It is introduced into the
country rocks along fractures and foliation planes (fig. 6).
Tourmaline (schorlite) being introduced into the country rocks
along foliation planes. It occurrs as short prismatic
crystals having angular relationships with the schistosity.
occurs as columnar and fibrous radiating aggregates. It replaces and
contains inclusions of several minerals. In the sulfide and inner
zone of sericitic alteration dravite is more abundant, while in the outer
zones, towards the fresh rock side, schorlite is the more important
Hydrothermaly introduced apatite (light gray, upper left corner)
replacing earlier sulfides and other metamorphic minerals.
Apatite is quite abundant in the inner zone of sericitic alteration and
the outer sulfide zone. It occurs as medium sized hydrothermally
formed crystals replacing earlier sulfides and other metamorphic minerals
is intimately associated with apatite and occurs as medium sized anhedral
grains containing inclusions of apatite and epidote (fig. 8).
Fluorite (anhedral central grain) replacing apatite (inclusions
with moderate relief) and epidote (inclusions with high relief).
distribution of fluorite is restricted only to a few meters in the
innermost zone of wallrock alteration and in the sulfide zone. It
occurs as a typical hydrothermal mineral.
occurs as scattered, yellow-brown to pale brown, highly pleochroic and
short prismatic crystals of hydrothermal origin. It is generally
fresh, containing inclusions of sericite, epidote, and other minerals.
It occurs as a minor constituent in the innermost zone of sericitic
Fig 9: Pleochroic
haloes in phlogopite formed around tiny inclusions of unidentified
radiocactive minerals. Phlogopite is seen to be replacing
epidote and some sulfide minerals.
The mineral contains tiny inclusions of unidentified
radioactive minerals that have produced pleochroic halos in phlogopite
are present only in the outer zone of wall rock alteration. Both
plagioclases and K-feldspars are introduced in the silicified country
rocks along the foliation planes. These feldspars occur as
poikiloblasts containing inclusions of quartz and micaceous minerals.
Plagioclases are more abundant than K-feldspars and both increase in
amount towards the fresh rock side. Plagioclases range from calcic
oligoclase to labradorite, while K-feldspars are invariably orthoclase.
Feldspars in the altered zone showing alteration to sericite and
clay minerals along cleavages, fractures and grain boundaries.
The feldspars are untwinned and could only be recognised after staining
following the method of Bailey and Stevens (1960). Feldspars of
metamorphic origin are quite abundant and show considerable signs of
alteration (fig. 10).
are residual products of the hydrolytic breakdown of feldspars releasing
silica and potash. In the outer zones of wall rock alteration relict
feldspars can be seen surrounded by rims of sericite and amorphous clays.
They can also be seen hydrolysed along cleavage planes and fractures (fig.
10). The peak development of clays appears between the outer
sericitic and inner zone of silicification.
occurs in the zone of alteration immediately adjoining the sulfide zone in
the form of clear euhedral crystals and columnar aggregates. It
seems to have been emplaced prior to the sulfides, since it generally
occurs as inclusions in the second-generation sulfides (fig. 11).
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.
Flexed cleavage planes of andalusite are evidence of a mild deformation of
the rocks subsequent to the emplacement of the mineral.
occurs as a typical metamorphic mineral only in the outer zone of the
altered wall-rock. It occurs as aggregates of porphyroblastic masses
being corroded and replaced by secondary quartz and biotite along the
grain boundaries and fractures (fig. 12).
is also a metamorphic mineral and occurs only in the outer zone of wall
Fig 12: Porphyroblasts
of cordierite being corroded and replaced by quartz and biotite
along grain boundaries and fractures. (+Nic)
Three genetic types are recognised - pre-, syn-,
and post-tectonic (Fig. 3 & 4) and all these types are seen altering
occurs as cubes, pyritohedra, and irregular grains. It occurs as a
typical hydrothermal mineral replacing the silicate minerals, and can be
seen altering to limonite along grain boundaries. Pyrite is quite
abundant in the inner zone of sericitic alteration and decreases steadily
in quantity towards the outer zones of wall-rock alteration.