Gold is the oldest precious metal known to mankind and most cherished treasure and wealth of individuals and nations. It is known in India since antiquity. The metal is most prized as an object of beauty and for its unique properties like stability, electrical conductivity, malleability, reflectivity and ductility. The name gold is derived from historic English word ‘geolo’ (yellow) and the chemical symbol ‘Au’ is derived from the Latin name ‘aurum’ (glowing dawn). Refined gold and silver in the form of bars and ingots are called bullion. Gold has a characteristic metallic yellow colour, but can be black or ruby when finely divided. When alloyed with other metals the colour changes.
Gold, silver, PGE elements: viz, platinum, palladium, rhodium, iridium, ruthenium, and osmium, are known as pristine noble / precious metals because of their least reactivity to corrosive agents, oxidation and tarnishing. They exhibit siderophilic and chalcophilic characters. However, metals like silver, osmium and ruthenium are far from being noble. Gold, copper and silver are called coinage metals as these metals were used for minting.
Pure gold is tasteless, non-toxic and non-irritating when ingested and is sometimes used as a food decoration in the form of gold leaf / foil. Gold has only one stable and naturally occurring isotope, Au197, 36 radioisotopes and 32 nuclear isomers. Pure gold is too soft for ordinary use and is typically hardened by alloying with copper or other metals. The gold content of gold alloys is measured in carats (k), pure gold being designated as 24k. Nitric acid has long been used to confirm the presence of gold in items, and this is the origin of the colloquial term "acid test". Some plants can absorb and accumulate gold. Certain species of bacteria may release, decompose or precipitate gold.
The unsuccessful attempts of medieval alchemists to produce gold from basemetals with a mythical substance called the “philosopher’s stone” laid the foundations for chemistry and metallurgy. Minerals like pyrite, chalcopyrite, and millerite with strong metallic character and having similar yellowish coloration are referred to as “fools gold”. Due to unique properties, relative scarcity of deposits and complete recyclabilty, gold commanded great value since time immemorial. Gold is repository of wealth and a convenient, reliable, stable and universally accepted medium of exchange
Gold evolves as a siderophile element from the iron-nickel core at crustal spreading centers. During partial melting of the mantle, gold along with metals derived from sulphides rise with basaltic (magmatic) fluids into the crust along mid-oceanic ridges and at subduction zones. It is then associated with complex processes involving convection, subduction, partial melting, hydrothermal processing, weathering, erosion and deposition before being returned to the mantle for recycling again at subduction centers. The deep seated ore bearing solutions containing gold are of both magmatic and metamorphic origin. The temperature, pressure, pH, salinity, redox, and sulphur content hydrothermal fluids influence the metal carrying capacity.
Geochronological data of gold metallogeny revealed major periods of enrichment as Archaean and Proterozoic. Gold occurs in a variety of litho assemblages, and multiple geological environments / settings such as greenstone belts, mantle derived intrusions, diaperic juvenile plutons and granulites. In the Indian subcontinent, prominent granite greenstone belts of Peninsular Sheild are located in Dharwar, Bastar, Singhbhum and Rajasthan cratons. The Dharwar craton, with two blocks viz the eastern and western, hosts the maximum number of gold occurrences. The Eastern block provides an important and favourable lithologic, structural and stratigraphic milieu for gold mineralization and hosts major deposits like Kolar and Hutti.
In the northwestern Indian Shield, gold occurs in association with copper in the Archaean greenstone-like sequence (at Dhani Basri, in Mangalwar Complex) and Proterozoic metavolcanosediments (at Bhukia and Dugocha, in Aravalli Supergroup) with enrichment in the latter. Gold also occurs in Palaeo / recent river alluvium placers, laterite, soil and regolith. Puga geothermal system is a “ hot spring type epithermal gold deposit in the making, in the Ladakh region of Jammu and Kashmir. The distribution of gold occurrences in India is shown in Fig-1
Fig-1 Gold and associated Metal Prospects in India (Click on the map to see a larger view)
Romans were probably the first to organize gold mining and exploration in a systematic fashion. The evidence of large scale fabrication of gold art objects and jewellery were discovered by archaeologists dating back to about 3000 BC in Sumerian tombs at Ur in Mesopotamia. Gold was smelted in Egypt and Sumaria about 3500 BC.
In India, references to gold are found in the Rig Veda, the Puranas, Mahabharata and Ramayana and other epics but there is no record of ancient mining places and periods. It finds mention by different names in our scriptures since ages. Gold was mined in India 1800 years before the present or 200 A.D. as proved by carbon-14 dating of timber recovered from Hutti Gold Mines. Evidence of gold mining activity is spread over length and breadth of India, but obscured due to lack of preservation of data, leading to discovery and rediscovery at many places. Presence of old workings, mine dumps, panning sites, muck heaps, earthen retorts, slag and ore grinding - pounding implements along auriferous tracts in India are commonly observed. Ancient gold mining was carried in two distinct periods, of which earliest one commenced at an unknown date and continued up to about 500-600 A.D. while the second episode of intensive and extensive exploration and mining resumed after a gap of over 1000 years, which led to discovery of Kolar and Hutti gold filelds (1870 A.D).
Kolar, the second deepest (3200 m) gold mine in the world, survived for 110 years and the Hutti gold field witnessed four periods of widely separated exploration and mining Viz, Pre Asokan +2000 years old, Nizam period 1886-1920; 1937 to 1947; and the present and most successful from 1947 onwards. The deepest known old working in the world of about 250m in length and over 195m in depth is located on the Main reef at Hutti gold mines
The advent of worldwide gold rushes in 19th century laid the foundation for present day mining activity. The gold boom was experienced in India with discovery and mining in Kolar, Hutti, Gadag, Ramagiri, Honalli, Wynad, in the South and a few in North viz, Lawa, Mysara, Pahardia, Kundrekocha, etc. Most of the old workings were closed due to dwindling production and prevailing cost-benefits.
The extraction of gold involves ore crushing, concentration, slurrying, scrubbing, desliming, gravity processing, leaching (chemical, bacterial, carbon-in pulp and carbon in leach), floatation, ion-exchange, fluxing, amalgamation, cynadation, roasting, and smelting. Selection of metal extraction method or combination depends on nature occurrence of gold, fineness, grain size, rock type, time taken, accuracy, precision, economics, efficiency, hazards and ecological / environmental factors. The Carbon-in Pulp (CIP) leaching process is used to treat low grade oxidized ores at Ajjanahalli
The unique combination of chemical and physical properties makes it invaluable in numerous of everyday appliances and applications and jewellery. The metal forms a vital component of many medical, industrial and electrical applications. Gold alloys are used for electroplating, granulation, pressing, and lamination. High reflectivity (99 %) of gold makes it ideal for Infrared heaters, cookers, as shielding for spacecrafts and satellites and in life saving face shields for astronauts and fire fighters
Gold is used in spot plating, strip plating, and reduction in thickness of plated surface, ayurvedic preparations, treatment of prostate cancer, arthritis, medical diagnosis, radio therapy and medical research. Gold has been made into thread and used in embroidery of clothes. Gold plated palladium, tin and nickel are used in electronics. Titanium and chrome alloys are used in dentistry. High prices of gold promote the use of basemetal-clad gold in electronics, electrical and jewelry products. Gold performs critical functions in computers, communication equipment, spacecraft, jet aircraft engines, and a host of other products.
Naturally, gold is never pure. Native gold, in itself, is an alloy, normally containing an isomorphic mixture of gold and silver in the proportion of 4 to 15% (rarely <1%). Electrum, amalgam tellurides and bismuthites are the common alloys. Gold forms thio (S2O32-), halide, organic and cyanide complexes besides colloids or sols.
India ranked 6th in the world with a Gold production of 19.5 tonnes during the year 1905, whereas the production of gold from primary source during the year 2007 is only 2.490 tonnes. Kolar mine has produced more than 800 tonnes of gold before its closure in 2001. Presently gold is produced from three mines viz Hutti, Uti, Hirabuddni (HGML) in Karnataka and as by product from basemetal sulphide deposits of Khetri (Rajasthan), Mosabani, Singhbhum(Jharkhand), in public sector and Kundrekocha in private sector in the decreasing order.
In India, the total gold production in the year (2006-07) was 12.82 tonnes, ( 0.5% of world production), of which 2.36 tonnes is from primary source, 127 kg from basemetal mines as by product, and the remaining 10.34 tonnes recovered from secondary source by smelting of imported copper concentrates by HINDALCO at Dahej in Bharuch dist, Gujarat (IBM year book 2007, and mineral commodity summaries-2006)
India is the world’s largest consumer of gold ie, about 700 to 800 tonnes per annum (30% of world production) thus indicating a huge gap between demand and indigenous supply. India is a minor producer and major consumer of gold. To meet the domestic demand, India imported monetary and non-monetary gold to tune of 715 tonnes during 2006-07. The world production of gold was estimated at 2,430 tonnes in 2005. Production of gold mainly comes from South Africa -the world leader, Australia, USA, China, Peru, Russia, Indonesia, and Canada.
Contributed by M. Karunaker Reddy and R.L Jat, Western Region, Jaipur
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