Post on 30-Apr-2018
~t is obvious fact that the distribution and occurrence of
gold in rocks, unlike other minerals is very patchy and
irregular, not following the general laws of mineralisation
as an metalliferous beds and veins.
In peninsular India, gold mineralisation is confined to the
Archaean and Proterozoic greenstone belts (Dixit, 1980).
Evidences of mineralisation are pbserved on strike length of
2.5 km extended from Chigargunta in the north to Nandymadugu
in the south. Mineralisation occurs both in the mafic and
felsic units.
The surface indication of mineralisation is in the form of a
highly tectonized zones with vein quartz, maroon to reddish
brown heavily iron stained out crops and ancient workings
(Plate 1, B) .
The width of the ore zone varies between 0.5m and 10m. The
gold bearing ore bodies occur in the form of linear narrow
tabular North-South lodes trending parallel to the regional
foliation and have generally narrow widths. The strike
length of the individual lodes range from 250 to 500 metres.
The mineralisation is met with on the flanks of the hill
well within the mafic litho units in the northern parts of
the prospect. The mineralised zones are observed very close
to the contact between mafic and felsic formation as we
proceed in southern direction of the prospect area.
classification types:
The vein gold deposits of India :may broadly be divided into
three categories as - (i) This includes the principal gold
deposits which are being actively worked at present;
(ii) Those fields which have yielded some gold on
considerable development work in the past but remain
abandoned till now as they were considered to be low grade
or marginal grade in the past; (iii) Shallow workings or
minor occurrences whose potentiality remain unassessed or on
preliminary assessment found to be non-commercial or of mere
academic interest. Since Chigargunta area is considered to
be a highly profitable and productive zone considering the
present day bullion rate of the world, despite the fact
these same zone had been abandoned in the past. Hence, it
comes under category 11.
old deposits are broadly classified into two types based on
their mode of origin. One of these is of primary origin to
which belong the massive, veins, or lodes, stock works and
replacement deposits and the other type is of secondary
origin, which includes residual and placer deposits. Out of
the primary origin the vein or the lode deposits are the
most important. In the present area the vein type deposits
are highly suggestive of primary origin (Ziauddin, 1967a).
Niggli (1929) classified the epigenetic ore deposits into
volcanic group or near surface and plutonic group or deep
seated on the lines of the classification of the igneous
rocks. The plutonic deposits are divided into hydrothermal,
pegmatitic-pneumatolytic, and orthomagmatic sub-groups
depending upon whether the ores formed from liquids or gases
or as direct crystallization products in the magma itself.
According to him Gold-Silver association is related to
volcanic group, and the Iron-Copper-Gold association related
to hydrothermal sub-group of plutonic group that has been
formed under deep seated conditions. In the present area of
study, the Iron-Copper-Gold association is assigned the ore
deposits are of plutonic grbup.
� old occurs in native form in association with sulphides
such as arsenopyrite, pyrrhotite, pyrite and galena, which
occur in minor quantities. The sulphides are present to a
certain extent and are not as dominant as they are usually
found in some of the sulphidic western lodes of Kolar gold
fields and also the near by Mallappakonda gold prospect. The
fact being that the recovery of gold in metallurgical
processing of the ore, are expected to be high, and also the
mutual boundary relationship between gold and other
sulphides is highly suggestive of the "free milling type"
gold ore.
Pathfinder element:
Arsenic which is occurring in gaunge and ore mineral is
taken as a path finder element for gold deposits due to the
fact of its observed association as arsenopyrite with pyrite
in the ore mounts. Arsenic is strongly oxyphile and tends to
form oxides or hydrated oxides. It is absorbed as ferric
hydroxide precipitating arsenic as anion complex AsOs. Gold
is noted by a siderophilic element. Hence the association of
arsenic and sulphide of iron with gold is commonly seen
(Pushkar Singh and Jagannadha Rao, 1980) .
Hydrothermal mineralisation:
Magma is the direct source of the most of the materials of
endogenetic mineral deposits. Magmas are hot silicate melts
that contains metals in very small quantities and are
generated inside the earth, which are regarded as the chief
sources of the vein material. During the course of cooling
of the magma, gold is separated in native state or in
combination with the other elements like Cu, Ag, As, Sb, Se,
Te, Ba and S. The separation of gold from the rest of the
silicates is brought out by differentiation process as a
result of physico-chemical condition prevailing upon the
magma. The magmatic differentiation gives rise to an end
product of magmatic fluids in which there may be
concentration of metalliferous mineral deposits, that are
originally present in the magma include gold as hydrothermal
solutions. The hydrothermal solutions may lose their mineral
content by metasomatic replacement or replacement, to form
replacement deposits.
Generally replacement dominates under the conditions of
higher temperatures and pressures near the intrusive, where
hypothermal deposits are formed, which is the
characteristic of mesothermal zone. The association of
pyrite, pyrrhotite, and galena confirms the vein type of
mineralisation (Lindgreen, 1907).
The repeated occurrence of the certain characteristic
minerals are also known as the 'geologic thermometers', like
pyrite, arsenopyrite, pyrrhotite, tourmaline, garnet which
were highly suggestive of hypothermal condition of the ore
formation (i. e. , temperature ranges between 3000 C and 5000 C
and the presssure is very high) in hydrothermal deposits.
The same may be drawn from the presence of pegmatites, as
the pegmatites are developed in deep seated high pressure
environments (Park and Macdiarmid, 1975). The deposits of
Chigargunta area must be endogenetic type (Safonov et al.,
1980) as evidenced by the presence of so-called high
temperatures and pressures at depths. Ultimately the
resultant ore deposits of hypothermal zone, have been
brought to the surface through orogenic process and erosion.
Consequently, these deposits are more abundant in
metamorphic rocks and also the rocks of older geological
periods. In the present area, the mineralisation is confined
to metamorphic rocks of Dharwarian age, which supports the
hypothermal nature of hydrothermal process (Park and
Macdiarmid, 1975) .
Hypothermal deposits commonly occupy attenuated crests of
folds or shear zones. They also have a tendency to follow
drag folds and to replace country rocks selectively. Close
pitching folds and drag folds are important for localizers
for replacement deposits (Park and' Macdiarmid, 1975).
Generally mineralisation is met within the crestal portions
of the folds (Narayanaswami et al., 1960; Narayanaswamy,
1963) .
In the present area of investigation, gold mineralisation is
confined to thin bands of sulphide metachert, tuff sulphide
quartz veins within the meta gabbro horizon in an isoclinal
synform in the northern portion of the area; and tp an
auriferous su1phi.de lode to the southern end of the fold.
Each limb of the fold is about 300m in length. Out of the
two limbs of the folded structure the strike persistence of
mineralisation appears better on the over turned eastern
limb. This view may support the mineralisation in
Chigargunta area is hypothermal type under hydrothermal
process. Gold bearing quartz lodes are considered typical
examples of hydrothermal injections (Radhakrishna, 1976).
The presence of pitching ore shoots and the gradual decrease
of gold content at deeper levels, are the characteristic
features of gold mineralisation in hydrothermal process.
(~arayanaswami et al., 1960; Ziauddin and Narayanaswami,
1974) .
Gold is weakly disseminated in the mafic and ultra mafic
host rocks and has got mobilised and concentrated in payable
quartz veins and lodes in a secondary setting through
granite intrusion, metamorphism and structural modification
(~adhakrishna, 1983) .
The development of large extensive lodes is favoured in
greater depths as there are no abrupt changes there.
Accordingly the large persistent veins and replacement
deposits like the prospect area are highly indicative of
hypothermal nature of the ore deposits in hydrothermal
process (Park and Macdiarmid, 1975) .
The action of hydrothermal solutions on rocks is understood
by wall rock alteration. Induced cavities like shear zones
and rock alteration openings play more important role. The
compressive and tensional forces may be due to the effect of
crustal disturbances from time to time in the earth in the
ancient past. These forces operating on rocks, accompanied
by faulting, constitute long and continuous channel, ways
for solutions. Subsequently these are occupied,by metals and
are formed as fissure veins (Bateman, 1942).
Shear zones result where fractures, instead of being
concentrated in one or two single breaks are exposed in
innumerable closely spaced and more or less parallel
discontinuous surfaces of deep seated rupture and crushing
which are due to high temperatures and pressures. These
shear zones make excellent channel ways for mineralising
solutions which is evidenced by the copious water flows
where and when cut by tunnels in mines (Bateman, 1942). Wall
rocks that have been altered by solutions are found to be
more convenient for mineralisation and these maintain
equilibrium with that of mineralising solutions.
It has long been observed that hydrothermal deposits are
generally accompanied by a band of alteration of the wall
rocks readily visible to the eye. The presence of the
products of wall rock alteration like tourmaline, muscovite,
biotite, sphene, hornblende, magnetite, feldspar and garnet
which are of high temperature origin highly conclusive of
wall rock &l,teration in hydrothermal process and also
further confirms the schistose natu,re to the belt
(Bateman, 1942). The width of the alteration zone is highly
.variable. The wall rock alteration is observed to be altered
to varying degrees. It is noticed that the intensity
decreases outward from the vein.
~ineralisation in mafic unit:
Geochemical studies reveal back ground gold values are
higher in older mafic rocks. It generally leads to the
conclusion that the source of gold in the gold quartz veins
is the mafic rocks themselves and that this disseminated
gold has got concentrated in veins and shear zones, as a
later tectonic and thermal activity (Anantha Iyer and
Vasudeva Murthy, 1967). In the mafic unit, mineralisation is
confined to thin bands (0.5m to 4m) of metamorphosed
siliceous rocks with stratiform pyritic sulphide ore, highly
tectonized (mylonite zones) with pods. However one can also
view the minor amount of gold in the amphibolites is due to
the impregnation of hydrothermal solutions carrying gold
into the country rocks. The fact that such minor amounts of
gold is present only in rocks that are adjacent to quartz
veins carrying gold leads to the above surmise.
~ineralisation in felsic unit:
~t is observed that gold mineralisation in the Chigargunta
prospect is well associated with quartz feldspathic mica
schist with massive pyritic sulphides of high temperature
ore minerals, contact of the mafic and felsic rock units and
thin bands of sillimanite-quartzite with pyrite occurring
within the Champion reef. The quartz Xoldspathic mica
schist having quartz, plagioclase, biotite, muscovite, shows
heterogeneous mineralogy. Conformable, pod like greyish blue
quartz veins are associated with these rocks. This rock unit
has been found to be auriferous for a strike length of about
3 km and constitutes the "Main lode" system.
The sillimanite-quartzite occurs as several thin bands
within the Champion reef in the eastern direction of it,
which consists of mainly quartz, sillimanite, muscovite and
pyrite. These bands are auriferous and range in width from
1 m to 20 m. This unit is traceable over 2 krn strike length
in the southern part of the prospect. This is referred to as
the "East lode" system.
The observed gold mineralisation in champion gneiss shoud
be taken as an initiative for the sear:ch of mineralisation
in similar felsic units of other green stone belts.
Modes of mineralisation:
Problems related to mineralisation of gold.at Kolar gold
fields can be divided into several parts; some of them are
related to larger problems related to gold-greenstone
granite association and more specific problems related to
the mode of its formation. In investigating such problems
Satellite imagery forms a useful tool.
Hydrothermal origin is a more consensus concept in which the
intrusive gold bearing siliceous solutions have impregnated
the fissures and weak planes of the country rock and thus
the mineralisation has at least the following components:
A ) A pre-existing host rock which is subjected to
deformation;
B) A hydrothermal source related to deeper crustal
units released as an intrusive fluid and
C) The gold mineralisation is followed by other
geologic episodes.
A similar origin has been proposed for several other gold
deposits and later studies revealed that South African gold
deposits owe their origin to formation of placers and are of
sedimentary origin and in other :places, biota played a vital
role in gold mineralisation. But in case of Kolar gold
field deposit, the satellite imagery supports that the
hydrothermal origin with associated granites playing a vital
role in gold formation and enrichment.
Even if one were to accept that the mineralisation is
hydrothermal, it is necessary to identify the source rock,
whether it is a greenstone or a granite or a pegmatite.
Narayanaswamy et a1 (1960) have laid a greater emphasis on
the role of structures in mineralisation. While they
described the surface geology in a great detail, they could
not establish any correlation between surface geology and
the underground reef workings. It is quite apparent that
they assigned a greater role to structures and tectonics
than that is warranted. Structure and tectonics controlled
the ore localisation but not the ore formation.
The other model for mineralisation is a sedimentation model,
wherein the gold deposition is either purely associated with
clastics/colloids or as a chemical/biogenic precipitations.
The evidences that support this model are the occurrence of
banded haematite quartzites near Oorgaum and the linear
nature of this belt. Apart from the association of the
banded haematite quartzites, there are no other evidences to
support an origin based on sedimentation model, such as
ripple marks, current bedding and other sedimentary
features.
The third model to account for the genesis of the gold in
Kolar gold fields is based on chemical/biochemical
precipitation of gold bearing cherts in a sub-marine
volcanic environment. The basic effusives have been active
in the Archaean basin at deeper zones in which the spilitic
lavas have formed the basement over which the chert/gold has
formed in the channels of the pillow structures. The
composition of the lavas appears to be tholeiitic with
vesicular fabric and these lava types are apparently related
to the gold bearing residual fluids. The deposition is not a
fissure filling type but is a fluid which has
~recipitated, reworked and filled in the channels. The lack
of mineralisation of amphibolites, where the granitic rock
types are absent does not support such a view.
From the limited study now attempted, it is not possible to
reconstruct the genetic model fully. The origin appears to
be due to multiple set of causative phenomenon wherein
sub-marine volcanism, Pillow lava structures, the effusive
basic rocks and the diagenetic processes together with
hydrothermal fluids from granitic sources have had a great
role. There is a definite evidence for tectonism in this
area, and this aided gold localisation. While the
felsic/mafic minerals are affected by deformation. There is
no large scale impact of tectonism on the gold
mineralisation itself.
The valid evidences for the occurrence of telluride phases
together with the occurrence of complex chloro telluride
assemblages together with pegmatitic activity indicate the
possible derivation of gold due to hypogene fluid phase
mineralisation (Safonov et al., 1984; Alexander et al.,
19851.