In nature, gold most often occurs in its native state (that is, as a metal), though usually alloyed with silver. Native gold contains usually eight to ten percent silver, but often much more – alloys with a silver content over 20% are called electrum. As the amount of silver increases, the color becomes whiter and the specific gravity becomes lower.
Ores bearing native gold consist of grains or microscopic particles of metallic gold embedded in rock, often in association with veins of quartz or sulfide minerals like pyrite. These are called “lode” deposits. Native gold is also found in the form of free flakes, grains or larger nuggets that have been eroded from rocks and end up in alluvial deposits (called placer deposits). Such free gold is always richer at the surface of gold-bearing veins owing to the oxidation of accompanying minerals followed by weathering, and washing of the dust into streams and rivers, where it collects and can be welded by water action to form nuggets.
Gold sometimes occurs in minerals in chemical composition with other elements, especially in association with tellurium. Examples are calaverite, sylvanite, nagyagite, petzite and krennerite. Gold also occurs rarely as a mercury-gold amalgam, and in very low concentrations in seawater.
The formation of Gold
The most common natural method of concentration of gold is through the ancient action of hot fluid inside the Earth’s crust. (Fluids deep in the crust are heated by the Earth’s internal heat. As they move towards the surface they cool down.)
The fluids moved through the rocks over a large area and “dissolved” the gold. When these fluids cooled or reacted with other rocks the dissolved gold precipitated (came out of the fluid) in cracks or fractures forming veins.
If the fluids move over a large enough area, and dissolve the gold for a long enough period of time, gold can be concentrated in amounts in the parts per thousand or even greater.
As well as gold, the fluids carried other dissolved minerals, such as quartz. This is why gold is often found with quartz. These are known as primary gold deposits and to extract the gold the rock containing the veins of gold has to be dug up (mined), crushed and processed.
The rocks containing the gold veins have now been exposed on the surface and are eroding away. The gold that these rocks contained has been washed down into creeks to form alluvial gold deposits. Here, the gold is further concentrated by the action of water.
Because gold is heavier than most of the material moved by a creek or river, it can become concentrated in hollows and trapped in the bed of the river. These are known as secondary gold deposits and they can be worked using a gold pan, cradle.
Properties of Gold
Gold is a very rare substance making up only five ten-millionths of the Earth’s outer layer.
(Imagine 10 million Smarties in one place and only 5 of them were made of gold!).
Its rarity and its physical properties have made it one of the most prized of Earth’s natural resources.
Gold, like iron, copper, lead, tin etc. is a metal. Metals are good conductors of heat and electricity and are almost all solid at room temperature (with the exception of mercury). They are malleable and ductile.
|Properties of Gold|
|Mineral:||usually found as a native metal|
|Hardness:||2.5-3 on Mohs Scale|
Gold is heavy – it weighs over nineteen times more than water, and is almost twice as heavy as lead.
If you had enough Gold to fill a one litre milk carton, it would weigh 19.3 kilograms, the same volume of milk weighs only one kilogram.
Gold is quite soft. It is slightly harder than a fingernail but not as hard as a coin or glass.
Gold, like most metals, can be hammered into thin sheets (malleable) or drawn out into thin wires (ductile).
This has made gold sought after for a wide range of applications, like jewellery and in electronics.
“Gold leaf” for example, is gold that has been beaten into a sheet less than one tenth of a millimetre thick.
It is then used for lettering on honour rolls in schools, or for putting gold onto picture frames and ornaments.
Gold in India
India is one of the world’s largest consumers of gold. Several agencies have license to import gold and silver in response to demand. Besides being a major consumer, India has massive refined gold reserves estimated at over 10 000t, with the bulk of gold being held as jewellery. Most gold is sold to Indian nationals.
Gold mineralization in India has been established to occur in diverse geological environment. These are similar to known gold hosting geological environments all over the world. The potential geological environments for primary and secondary gold prospects recognized so far as follows:
a) Archaean granite-greenstone belts hosting lode type deposits. Such setting is common in the states of Karnataka and Andhra Pradesh and some minor ones in Tamil Nadu and Orissa.
b) Early Proterozoic volcano-sedimentary sequences of the supracrustal/fold belts hosting lode gold and volcanogenic massive sulphide type deposits. It includes occurrences in Sonakhan, Sakoli, Mahakoshal, Kotri and Raigarh belts of Central India; continuation of Mahakoshal belt in Northern India; Aravalli belt of Western India and Singhbhum belt of Eastern India.
c) The high grade granulitic rocks of Kerala and Tamil Nadu.
d) Secondary gold in laterite and in placers. These include reported occurrences in laterite caps over supra-crystal rocks of southern, central and eastern India and also occurrences of placer gold in fluviatile sand in the Peninsular and sub-Himalayan areas.
The Archaean granite-greenstone assemblage in the Dharwar Craton of southern India has so far been the most promising gold producer generating almost 99% primary gold of the country.
Hutti Gold Mine Company is India’s only producer of gold, producing approximately 3 t of gold a year from reserves estimated at 600t. The only other gold producer in India, Bahrat Gold Mines has ceased operations due to exhaustion of high grade reserves, depth of operations and high overheads. Bharat was active in the Karnatake and Andhra Pradesh regions.
Hutti Gold Mines Co , the sole primary producer of gold, is owned by the Government of Karnataka. The company has taken new initiatives to tap the Hutti South Block at the Hirabudini mine and to adopt bio-leaching technology. It is also modernising its ball mill, and is diversifying into gold refining, the production of gold coins and hallmarking. Gold deposits in the vicinity of Hutti are estimated to contain very substantial resources. Elsewhere, new deposits, with average grades ranging from 0.1 to 2.6 g/t Au, were discovered some years ago in the Chitradurga and Sandur districts of Karnataka.
Gold production has recently come mainly from the copper smelters as a by-product. Birla Copper and Sterlite have produced more gold than the only working gold mine at Hutti in Karnataka. The former produced about 5.2 t in 2003-04, and Sterlite produced around 5.1 t
Although in last few years there was significant ore resource augmentation, but the country has not witnessed any radical change in gold production from primary gold ore. The major producer will be the Hindustan Gold Mines Limited [HGML] from its three existing mines. Historically India has been a leading producer of gold in the world with Kolar, Hutti, Ramagiri and Gadag belts of the Dharwar craton as the major contributors. Outside the Dharwar craton, Kunderkocha and Lawa mines of Singhbhum craton and Wayanad Gold Field of Tamil Nadu and Kerala have contributed to the gold production in a small way. In the present time the production of gold is around 3-4 tonnes/annum and that is produced in two forms namely the primary gold and the secondary one as a by-product from base metal mining. Hutti Gold Mines Company Limited (HGML) is the primary gold producer from its three mines located in Karnataka.