by N G Nair Manager (Mines) Malanjkhand Copper Project Malanjkhand Copper Project,
Khetri Copper Complex and Malanjkhand Copper Project
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Nair Profile
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A Geologist from Malanjkhand India
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BHRAT GOLD MINES LIMITED, “MY FIRST EXPERIENCE IN INDIAN UNDER GROUND MINES.” MINES AT KOLAR GOLD FIELDS, KARNATAKA STATE, INDIA.
EXPLORATION STRATEGY FOR GOLD IN SOUTHERN INDIAN SHIELD
INTROUDCTION
In southern India gold mineralisation is associated with the Archaean greenstone belts with metavolcanics/ acid volcanic, contract zones of basic and acidic rocks, meta-ultramafites, sulphidic Banded Iron Formation (BIF), quartz pebble basal conglomerates, granodiortes corridors of greenstone belts and in the granulite belt (vidadharan, 2003), Auriferous nature of Late-Archaean-Early Proterozoic greenstone belt (>3000 –2000 m.y) is well established and Gold is being produced from some of these belts since historic times. (Subrahmanyam, 2001). Despite the favourable geological milieu and gold metallogeny of the Dharwar Craton green stone belt being comparable to the world famous gold fertile provicnces of Yilgran Craton of Western Provinces of Canada, Rhodesian and Kaapaval cratons of South Africa, the established gold potential of the Dharwar craton, excluding the Kolar, Ramagiri and Hutti gold fields, is about 2 2kg.sq.km. Vis-à-vis the established potential of over 25 kg/sq.km. for the South African. Candian and Australian provinces. The alarmingly low known potential suggests distinct possibilities of substantial gold resources in the Dharwar Craton, which remains to be explored by well planned exploration strategy. The demand for gold in India is more than 500 tonnes per annum, while the country’s production from the South Indian Shield has come down to less than 2 tonnes annually. Therefore, it is imperative to fully assess the gold potential of the gold fertile greenstone belts of the Southern Indian Shield and exploration strategy is proposed to locate additional indigenous gold potential.
CONGENIAL GELOGICAL MILLEU FOR GOLD
Since gold occurs in diverse geological set-up, all the favorable geological environments in south India have been discussed.
1. Archean Granite-green stone belt:
The Archaean greenstone belts are widely developed in the Dharwar Craton of Karnataka and Andhra Pradesh States and partly in Tamil Nadu State. More than 99% of total primary gold in the country comes from a vast expanse of over 40,000 sq km in the Dharwar Craton (kar and Biswas, 2001). The cratonic area is divided into eastern and western blocks by Closepet Granite (2400-2000 m.y). From the viewpoint of gold mineralisation, the late Archaean greenstone belts of eastern block with dominant basic / acid volcanices with different eqisodic granitoids and granitic intrusions from the potential area. The discovery of gold deposits in granodiorite corridor in Dona Section of Jonnagiri Schist Belt (JSB) withtranscrutal shear zone in the structural continuity between Kadiri-Kolar Schist Belt in the north is path-breaking (Sreerama chandra Rao, 2001). The green stone belts of western block of Dharwar Supergroup of the late Archaean (2600 – 2400 m.y) and Sargur Group of Mid-Archaean age (>3000 m.y) have relatively lower enciidence of gold. Gold mineralisation in both the blocks is predominantly of primary in nature, gold-quartz vein type hosted both in metavolcanics, granodiorite (Dona Sector) and metaasedimentaries, mostely being epigenetic vein / lode type and occasionally sysngenetic strata bound type, as in banded iron formations. The secondary gold mineralisation in the basal conglomerates of the Dharwar Supper-group and in present day placers is only of academic interest and value.
2. Southern Granulite Terrainn (SGT)
The high-grade granulitic rocks of kerala and Tamil Nadu contain gold prospects. In Nilambur and Attapadi areas, the gold deposits occur within or in the proximity of the Wayanad-Moyar Shear Zone and some of the new deposits discovered fall wihin the Bhavani Shear zone. Both the deposits fall within the Souther granulite Terrain (SGT). Occurrences of gold in Wayand and Nilambur Gold belt have been known for several decades and the metal was extracted both from alluvium and reef deposits.
3. Supracrustal Belts:
Occurance of gold has been reported from several high grade supracrustal belts fo limited dimensions south of Mysore and elsewhere. Almost all the supracrustal belts grouped under “Sargur Group” occur as enclaves with the poloyphase granite-gneiss-migmatite association (Peninsular Genissic Complex). Basement for all these belts cannot be distinguished from the penisular Gness. There are reports of a large number of ancient workings for gold in many of the supracrustal enclasves, which include Kempinkote prospect of Nuggihalli Schist Belt.
4. Quartiz-pebble congiomerate and placer gold;
Gold occurrence in the quartiz pebble baseal conglomerates of the Dharwar Supergroup and present day river courses (placer gold) is of academic interest only.
5. Gold in laterite.
Gold has been reported from some of the laterite cappings over the supracrustal rocks of Southern India
The greenstone supracrustal belts in Andhra Pradesh are “kolar style” belts in the “Eastern Tectonic Belts” (ETB) of the Dharware Craton. The granite-greenstone belts in Andhra Pradesh constitute part of the most important Eastern Gold Province in the south Indian shield. The gold fertile Ramagiri and Southern extention of Kolar Schist belets and Jonnagiri, Kadiri, Veligallu, gadwall, Tsundupalle, Peddarvuru, Nellore and other lesser known and other minor schist belts in Andhra Pradesh bear striking similarities to the prolific gold producing greenstone belts of the world in terms of their favourable lithoteconic milieu. Exploration carried out in recent years in these schist belts led to the discovery of low to medium grade resources in several trget zones of lode-gold type and auriferous BIF (sysgenetic) type deposits. The discovery of important gold-quartz lode type deposit within sheared diapiric domal tonalite-granodiorite and granite plutons along the sheared margins of Jonnagiri Schist Belt in Dona Sector, has opened new vistas for exploration of gold in strikingly similar set-up elsewhere (Sreeramachandra Rao, 2001)
Exploration of gold was being carried out mostly in the south extention of Kolar, Ramagiri and Jonnagiri Schist belts of A.P sicne early 1990s, while regional surveys for gold were carried out in other belts of A.P.
Exploration for gold in four blocks namely Chennabhavi, Kothapalle, Ompratima-Gantalappa and Jibitil blocks (from north to south) falling in the central part of the Ramagiri – Penakacherla Schist Belt was carried out by IBM and GSI. GSI (196 
assessed 0.7 million tones of ore averaging 7 g/t with 1.98m in thickness upto 300 m depth in Yeppamana mine section. Bhrat Gold Mines Ltd (BGML) produced a total of 1371.16 kg of gold by treating 51615 tonnes of ore from this mine till it was closed in May, 1999. GSI continued surveys and exploration in the Ramagiri-Penakacherla belt since 1981. Based on the present status, ore reserves of all categories in Ramagiri Gold Field (RGF) and Ramampuram – Bhadrampalle Gold Fields total 0.91 million tones with gold values reanging from 1.42 to 5.60 g/t of which reserves of ore grades over 3.0 g/t total 0.59 million tonees in Ompratima – Gantlappa, Chennabhavi, Kothapalle, Jubitil blocks of RGF sectors and Ramapuram and Bhadrampalle blocks of Ramapuram-Bhadrampalle Gold Fields sector.
Integrated ground surveys involving geological, gechemical and geophysical surveys carried out in the souther extension of Kolar Schiest Belt led to the discovery of Kudithanapalli prospect located 8 km south of Kolar Gold Fields near the eastern margin of the belt. Two zones
1) quartz reefs and strings in sheared and hydrothermally altered schistose amphibolite
2) the contact zone of amphibolite with quartzo –fels pathic gnesses are the loci for gold and molybdenum mineralisations. The two zones extend each for a length of 200 m (cumulative length 400 m). A total reserve of 0.42 million tones has been estimated with an average grade of 1.2 g/t and 0.1% Mo and width of 2.2 m upto explored depth of 180 m.
In dona sector of Jonnagiri Schist Belt (JSB) the gold is hosted mostly in tonalite-granodiorite and also localized within the mafic part of the southeastern part of the belt, comprises Dona Temple, Dona East, Dona North and Dona South blocks. The dona North and Dona South blocks. The Dona Sector of JSB contains mesothermal quartzvein gold mineralisation, which are hosted in sheared and altered tonalite-granodiorite rock. The discovery of gold deposits in the Dona Sector of JSB associated with ductile and beittle-ductile shears along the tectonic margins in the lithouits of the tonalite-granodiorite suite has opened new vistas for exploration of Gold in similar settings elsewhere. The Dona Sector has gold prospects with probable total resources of 7.77 million tones with an average grade of 1.77 million tones with an average grad of 1.7 g/t in Dona East Block, 0.098 million tones of 3.92 g/t with true thickness of of 1.81 m in Dona North Block, 0.104 million tones in eight auriferous zones of Dona South Block with grade ranging from 1.16 ti 6.28 g/t Au and thickness from 1.22 to 5.38m.
Regional gechemical surveys carried out in Kadiri Schist Belt by GSI (1981-93) over an area of 250 sq km resulted in collection of 4500 samples from various lithounits and adjoining granite pahses including bedrock, soil, stream sediment, termite and trench samples. The chemical analytical result did not indicate significant gold values in respect of the samples collected Kadiri Schist Belt.
Regional geochemical surveys carried out by GSI in Gadwal Schist Belt led to delineation of five prospective blocks for gold and basemetal mineralisation, the prominent one being chelamalapuram, which on test drilling, indicated traces of gold (manimum 2 g/t Au) in two sulpide bearing pegmatite bodies within granite and volcanic congomeate.
Regional geochemical surveys and large scale mapping were carried out by target areas of gold and multimtal sulphide mineralisation associated with several shear zones in BIF. Analytical results of 1008 lithogeochemical samples collected on 1 km x 1 km grid, 1560 random samples, 20 steam sediment samples and 205 trench samples led to delineation of 9 blocks namely 1) Errakonda Block.
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A Mining Engineer from Copper Mines in India
NARAYANAN NAIR