by
Dr. Nitish Priyadarshi
Diamond, a gem amongst gems the crystallized carbon and the hardest mineral known is a rare gift from mother earth to humanity. With admantine luster and twinkling habit it fascinated men and women from time immemorial. To win diamonds temples have been profaned, palaces looted, thrones torn to fragments, princes tortured, women strangled, guests poisoned and slaves disemboweled. No strain of fancy in an Arabian Tale has outstripped the marvels of fact in the diamond’s history. So wrote Garner Williams, the General Manager of the renowned Diamond Company De Beers in 1890.
Pliny (A.D.100) described diamond as the king of the stones known only to the kings and to them superficially only as decorative objects. Now diamonds can be possessed by those who are not necessarily kings. It is estimated that about three hundred million women own at least a piece of diamond as jewellery today. Wannenburgh in the year 1990 in his book diamond people, mentioned that each year, men and women spend almost forty billion dollars on about sixty million pieces containing some fifteen million carats of diamonds.
Diamonds occurs in nature in (1) primary form in igneous rocks like kimberlite, lamproites (2) secondary in conglomerates, grits and similar sedimentary rocks, (3) in Quaternary to Recent placers and (4) extra-terrestrial, as in meteorites. Of these different sources most of the diamonds in the world are recovered from the first and the third type.
Minerals which are more abundant than diamond and act as pathfinders to locate diamondiferous source rocks are considered as indicator minerals in exploration for diamonds. These are formed along with diamond in the upper mantle and brought up to the surface as xenocrysts in the kimberlite and lamproite magma. Indicator minerals useful in diamond exploration are pyrope garnet, picro- ilmenite, chrome-diopside, chromite, spinel and micro diamond.
Diamonds as placers deposit:
In geology, a placer deposit or placer is an accumulation of valuable minerals formed by deposition of dense mineral phases in a trap site Placer mining. (pronounced "plass-er") refers to the mining of alluvial deposits for minerals. This may be done by open-pit (also called open-cast mining) or by various forms of tunneling into ancient riverbeds. Excavation may be accomplished using water pressure (hydraulic mining), surface excavating equipment or tunneling equipment.
The name derives from Spanish, placera, meaning "alluvial sand." It refers to mining the precious metal deposits (particularly diamonds, gold and gemstones) found in alluvial deposits—deposits of sand and gravel in modern or ancient stream beds.
The simplest technique to extract diamond or gold from placer ore is panning. In panning, some mined ore is placed in a large metal or plastic pan, combined with a generous amount of water, and agitated so that the gold particles, being of higher density than the other material, settle to the bottom of the pan. The lighter gangue material such as sand, mud and gravel are then washed over the side of the pan, leaving the precious stones behind.
Fig. Aeolian diamond placers of Namibian desert in Africa
The disintegration of primary rock due to weathering, transportation and deposition along with valuable minerals like diamond, gold, tin ore etc., give rise to placers. If the placers are very close to the source, they are known as “eluvial placers”. If shifted down the hill or slope they get grouped as “deluvial placers”. If the material gets accumulated at the foot of the hill due to gravity action it is called “colluvial placers”. The material transported by water, streams and rivers for longer distance deposited away from the source is called “alluvial placers”. Deposition along the shores of the sea results in “beach placers” and in deserts when deposition takes place by wind it is called “aeolian placers”.
The secondary deposits are formed by the weathering of the kimberlite and the lamproite. The diamonds are released from the rock and then, they are transported hundreds of kilometres away to be found in river beds, beach sands, old river beds (sometime found on top of hills deep jungle forest, deserts, etc... Diamonds may also have been transported by glaciers and if the journey has been hard, rough and long, they are not to be found as they have been broken and grinded into near dust.
There is no single, universally applicable method of target selection for alluvial diamond deposits. Climatic and geomorphic history, stratigraphy, structure and tectonic history are important factors that require study and need to be understood for each alluvial diamond province prior to target selection. What is appropriate in one part of the world may be entirely inappropriate in another, but there are some basic principles which must be followed in planning and executing any alluvial diamond exploration program.
Before starting field exploration, it is important to have a comprehensive knowledge of (i) the regional geology of the drainage basin under revue; (ii) the constituent lithologies of the gravel being prospected for; (iii) the geomorphic and climatic factors which have affected the exploration area; (iv) the type of deposit to be sought; and (v) any post-depositional processes which may have affected the alluvial deposits being considered for exploration.
World diamond occurrences were found in first in placer deposits which later led to the discovery of many primary kimberlite source rocks. Understanding the mechanism of formation of placers will help in locating areas of concentration of diamond in placers deposits.
Historically diamonds were known to be found only in alluvial deposits in southern India. India led the world in diamond production from the time of their discovery in approximately the 9th century BCE to the mid-18th century AD, but the commercial potential of these sources had been exhausted by the late 18th century and at that time India was eclipsed by Brazil where the first non-Indian diamonds were found in 1725.
Diamondiferous residual rubble and clay found on the mainland of Tanzania developed over weathered kimberlite pipe is a typical example of eluvial placer deposit. Diamondiferous placer of Namibian desert in Africa is a classic example of aeolian placer.
Precambrian diamond bearing conglomerate of Diamantina in Brazil, diamondiferous moraines of Michigan and Wisconsin in United States of America are example of glacial placers.
The diamantiferous gravels of the Belgian Congo, which yield about two-thirds of the world production, are former stream gravels, and the diamonds probably came originally from weathered kimberlite pipes, several of which are known within the Congo. Some diamonds also occur in the Lulua, Zambesi, and other basins.
The Orange River forms a geographic dividing-line between the nations of South Africa and Namibia. For the last hundred million years, the Orange has been carrying eroded diamondiferous kimberlite material from its source on the Kaapvaal Craton, in central South Africa and Botswana.
Diamond-bearing material was deposited in river bank gravels and alluvium as it traveled westward towards the Atlantic Ocean. Other diamondiferous material was re-distributed by wind action, settling to form eluvial deposits in the desert. Material that completed the journey was deposited in beach terrace sediments, or redistributed by northerly ocean currents, to off-shore marine deposits on the sea floor.
These secondary alluvial land and marine deposits are Namibia's only source for diamonds, but the Orange River has left an enormous amount of diamondiferous material in the vast drainage basin stretching 150 miles from Oranjemund to Elizabeth Bay. As the sea-level receded, diamond-bearing sediments were exposed, and eventually covered by blowing sand. Land-based secondary deposits must be excavated from this sedimentary layer that lies beneath a deep layer of surface sand.
In Jharkhand state of India diamonds were reported in Chotanagpur area (ancient name Kokrah) in the Brahmani, Sankh and Koel river basins as mentioned in old records. Records show that near Simah in Palamau district in the Sankh River, near Rajadera (Rajadera is a saucer shaped village, is situated 19 km from Chainpur and about 26 km south of Netarhat) and in Sadni falls originating point of Sankh River, active diamond mines existed. These mines are stated to have yielded in the sixteenth and the seventeenth centuries many large and fine stones especially from Sankh River. Diamonds were washed from the sands and gravels of river Gouel. The river is probably North koel, a tributary of Son. On the banks of this river an ancient township Semah/Semelpur existed. According to the old reports about 8000 people are stated to have worked in these mines.
Raigarh in the upper reaches of Mahanadi river, west of Hirakud, Orissa in India, is also known to have been active in diamond mining since ancient times. Local tribals carry out small scale panning and recover diamonds from alluvial and colluvial placers.
Mr. T.K. Rau of Geological survey of India recovered micro-and macro diamonds from the heavy mineral concentrates of beach samples from the southern tip of India. The diamonds exhibit varied tints of which white, pink and yellow are prominent.
Reference:
· Babu, T.M. Diamonds in India. 1998. Geological Society of India, Bangalore, India.
· Bateman, A.M. 1955. Economic Mineral Deposits, John Wiley and Sons, New York.
· Rau, T.K., 2006. Incidence of diamonds in the beach sands of the Kanyakumari Coast, Tamil Nadu, Journal of the Geological Society of India, vol.67, no1, pp.11-16.
· http://en.wikipedia.org/wiki/Diamonds#Mining.2C_sources_and_production
· http://en.wikipedia.org/wiki/Placer_deposit
· http://en.wikipedia.org/wiki/Placer_mining
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