In peninsular India, gold mineralisation is confined to the

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1 CHAPTER EIGHT MINERALISATION PATTERNS

2 ~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

3 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.

4 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.

5 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).

6 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

7 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).

8 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

9 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

10 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

11 (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.

12 ~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.

13 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;

14 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

15 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

16 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.,

Practice Test Rocks and Minerals. Name. Page 1

Name Practice Test Rocks and Minerals 1. Which rock would be the best source of the mineral garnet? A) basalt B) limestone C) schist D) slate 2. Which mineral is mined for its iron content? A) hematite