Radiogenic Osmium in Ganga river sediments of India.

Ganga river Sediments are the major source of radiogenic Osmium.
by
Dr. Nitish Priyadarshi

The Ganga sediments are characterized by high Re/Os (rhenium-osmium) ratios and are extremely radiogenic as evident from their 187 Os/ 188 Os isotopic ratios. Samples from the tributaries Alaknanda, Bhagirathi, Gandak and Ghaghra show pronounced 187 Os/ 188 Os. High Os concentrations combined with sediments flux makes the Ganga an important source for soluble Os isotopic evolution in oceans.

Interest in rhenium-osmium systematics in rivers has risen sharply in recent years due to the revelation of changes associated with sea water Os isotopic compositions during the past 70 Ma. Radiogenic 187 Os is produced from the β- decay of 187 Re. Osmium isotopic composition in sea water is derived from the weathering of basaltic and peridotitic oceanic crust, hydrothermal solutions, additions from cosmic dust and continental weathering products.

Os isotopes in the oceans convey the then prevalent continental weathering processes. The Osmium isotopic composition of the present day sea water is markedly higher since the past 70 Ma. This enhancement in radiogenic Os in sea water is largely attributed to the Himalayan tectonics with its accompanying increased silicate weathering and in particular, chemical weathering of the extremely radiogenic black shales in the lesser Himalayan region.

Marine black shales have been proposed as a source of very radiogenic Os because of their large enrichments in Re and because they can even at trace amounts weather very fast due to oxidative processes. If true, this then has the possibility of tracking exposure and weathering of organic carbon in the form of black shales in the past, e.g. uplift of the Himalayas or the Andes.

According to the article published in Geochim. Cosmochim. Acta in year 1999, Indus and Brahmaputra are less radiogenic than the Ganges, presumably due to the erosion of ophiolitic assemblages exposed along the Indus-Tsangpo suture zone.

The tributaries in the upstream, Alaknanada and Bhagirathi flow through predominantly silicates (shales, phyllites, quartzites, crystalines) and show high Re/ Os ratios of 24.9 and 19.8 respectively. Re is preferentially incorporated into the silicate minerals relative to Os.

Rivers draining the Canadian Shield have radiogenic values at low concentrations (24 to 35 fmol/kg). They are more radiogenic than the Zaire draining the Congo shield (1.4) or the Tapajos draining the Amazon shield (1.33) in the tropics.

Recently osmium concentrations and isotopic compositions in groundwater samples from the Bengal plain have been reported. Groundwaters have Os concentrations (16.9–191.5 pg/kg), about 5–10 times higher than those published for most rivers or seawater. 187Os/188Os varies widely (from 0.96 to 2.79) and is related to the isotopic signatures of the sediments constituting local aquifers.

Table-1 Osmium in Ganga River and its tributaries (in ppt).

Bhagirathi- 32.8
Alaknanda- 21.1
Yamuna- 64.3
Chambal-33.8
Gandak-55.7
Ganga- 37.7 to 51.7

Reference:

Chakrapani, G.J. et.al 2002. Osmium isotopic compositions in Ganga river sediments. Current science, Vol.83, no. 10.

Platinum metal can glitter Jharkhand State of India.

Presence of platinum cannot be ruled out in Jharkhand State of India.

by

Dr. Nitish Priyadarshi

Platinum may be considered one of the precious metals since it is more costly than gold. About 60 percent of that consumed in the United States is for jewelry purposes. It was once used for coinage in Russia until its value exceeded that of the coins. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River. The metal has an excellent resistance to corrosion and high temperatures and has stable electrical properties.

Platinum is only one of a group of related metals consisting of osmium, iridium, palladium, rhodium, and ruthenium. They are not only associated together but also are generally alloyed, and are called, therefore the “platinum metals.” They are very heavy, insoluble in most acids, melt at temperatures of 1,549 degree to 2,700 degree C, and range in hardness from 4.8 to over 7. Iridium is the heaviest metal and osmium the hardest.

Platinum is invariably associated with basic igneous rocks and with the ore minerals characteristic of these rocks. Most of the platinum of the world is intimately associated either with chromite or nickel. Even platinum placers are derived from basic rocks rich in chromite. The platiniferous nickel ores also contain copper and appreciable quantities of gold and silver.

In India, reported values of platinum group of metals worthy of attention are from the pre-cambrian mafic/ultramafic complexes in Sukinda and Nausahi sectors of Orissa and Sitampudi in Tamil Nadu. Geological Survey of India carried out sampling of the chromite ore bodies and their associated rocks. It was observed that the incidence of Platinum group of metals is much less in chromite bodies but it is somewhat more, of 20 to 100 ppb (parts per billion) in the chromite horizons.

Seeing the association of platinum with chromite and its deposits in ultramafic rocks in Jharkhand state of India, presence of platinum cannot be ruled out. Till today no detail research work has been done on the possibilities of platinum in the chromite deposit areas in Jharkhand State. Jojohatu, Hatgamariya, Keshargariya, Roroburu, Chitungburu, Kimsiburu, Kittaburu, Kusmita, Gurgaon, Tonto and Janoa-Ranjrakocha areas must be targeted for platinum deposits.

Chromite deposits of Jharkhand had a pioneering role in the early history of chromite exploitation in India. Small deposits of chromite ore are confined to the southern part of Singhbhum district in Jharkhand. Such deposits are exposed around Jojohatu, Hatgamariya, Keshargariya, Roroburu, Chitungburu, Kimsiburu, Kittaburu. Small occurrences of chromite are also found at Kusmita, Gurgaon, Tonto and Janoa-Ranjrakocha areas. Many of the deposits have been prospected by private parties but abandoned afterwards. The deposits are rather scattered and small and the grade is generally inferior (30-40% Cr2O3).

Jojohatu lies about 25 km to the west of Chaibasa, the district headquarters of Singhbhum. The Jojohatu ultramafic body is spread in three blocks with a cumulative length of 8 km in N-S direction and over a width of 3 km. These blocks are named successively from North to South as Kimsiburu, Kittaburu and Roroburu-Chitungburu. The ultrabasic rocks with which chromite is associated is intrusive into the rocks of Iron Ore Super Group represented in the area.


In one report of B.D. Sharma and others, Chalcopyrite concentrates from the Singhbhum district, India, contained 25-70 ppb Pt, which is greater than amounts found in rocks and chromite. Platinum group of metal are also reported in Kinkel and Kurdeg area of Simdega District.

Reference:
Sharma, B.D. Economic Geology; May 1966; v. 61; no. 3; p. 592-597; DOI: 10.2113/gsecongeo.61.3.592