CHAPTER EIGHT

MINERALISATION PATTERNS

~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.