Eastern Dharwar Craton: Evidence for more diamond deposits

When you travel by road from Bengaluru to Hyderabad, you will come across quite a few outcrops of dark coloured rocks. These rock outcrops have low silica content of about 50 percent and are rich in magnesium and ferric compounds. By combining and shortening the words magnesium and ferric, geologists call them mafic rocks. Generally, these rocks are extremely tough and are quarried for crushed stone, road metal or for building stones.

These outcrops have been investigated extensively in the last few decades and there is now a consensus that they were formed due to magmatic upwelling into pre-existing fractures, which then solidified as intrusive bodies, more resistant to weathering than the surrounding structures. So, in time, these mafic rocks rose above the surroundings, like a natural wall or a ridge which geologists call dykes.

If you are travelling by a low flying aircraft, you notice that these mafic dykes are a few tens to a hundred or more metres in length and width. You may also notice that many of these dykes are parallel and some even appear to be radiating from some source, creating an appearance of groups. Geologists call these formations mafic dyke swarms.

Most of the magmatic upwelling, from about two and a half billion years ago, intruded into the Archean rocks formed earlier, in the Dharwar craton, a discernible block of the Indian continent, covering most of the Telangana, Andhra Pradesh, and Karnataka of today.  There was repeated mafic magmatic upwelling, with some millions of years in between. Mantle plumes strained against the Archean craton from different directions, and generated ripples of mafic dykes trending in different directions - East to West, North to South. North East to South West and North West to South West.

Distribution of mafic dykes across the Dharwar craton. Google earth image displaying the mafic dykes adjacent to the southern fringe of the Cuddapah basin marked with red square. 

Such intrusions of magmatic plumes can often destroy the diamondiferous mantle keel, more than 150 kilometres under the earth, where most diamonds are formed and preserved in the diamond stability field. These diamonds may also be brought closer to the surface in kimberlite rocks, named after rocks where diamond mining started in Kimberley, South Africa. 

More than 150 kimberlite locations were reported from different regions of the Eastern Dharwar craton. Among these, the Raichur and Wajrakarur kimberlite fields are diamondiferous.    

After all, till the 19^th century, before diamond mining started in Kimberley, India was a major source of diamonds – especially along the alluvium in the Krishna river basin. These diamonds, including the legendary Koh-i-noor, were perhaps washed down from the weathered kimberlites in that area.

Scientists at the CSIR-NGRI recently investigated the mantle plume activity that formed the mafic dykes and its impact on the diamondiferous kimberlite roots in the mantle keel – the mechanically resistant, buoyant and diamondiferous mantle, under the eastern parts of the Dharwar craton.

In the vast region between the south of Hyderabad and the north west of Chennai, they sampled seven dykes and three sills, sheet like intrusions. Three were from E–W trending dykes, one from a N–S dyke in the periphery of the Cuddapah basin and three from mafic sills of the Cuddapah basin.

The geologists crushed the samples and separated baddeleyite grains from all the samples. Baddeleyite is a crystal of the zirconium oxide mineral found in low silica rocks and, since it is quite stable, it can be used for determining radiometric age. The scientists thermally extracted lead from baddeleyite grains and, by determining the ratio of different isotopes of the lead, they calculated the age of the rock samples. Lead atoms of atomic weights 206, 207, and 208 are radiogenic, daughters of uranium and thorium. By determining their ratios with lead of atomic weight 246, it is not too difficult to calculate the age of the rocks, since the half-lives of uranium and thorium are known. The technique is simple and provides an estimate of the age of a rock with a 0.1% precision.

The team thus identified at least eight different times when the upwelling magma created the dykes in the eastern Dharwar region.

“The dating of the samples collected showed that the first of these happened about 2367 million years ago”, says E. Nagaraju, CSIR-NGRI.

This was followed by other intrusions ~2253, ~2216, ~2210, ~2082, ~2180, ~1886, and ~1864 million years ago.

“The mafic event at about 1864 million years ago had escaped the notice of earlier researchers who worked on the mafic dykes”, says V. Parashuramulu, CSIR-NGRI.

Among these mafic dyke swarms, two were parallel and six were radiating. The radiating dykes converged to a single point, probably the source, or plume head, for that particular dyke swarm. Based on this concept, the scientists traced four mantle plume centres The plume centre in the western part, active around 2367, 2210 and 2180 million years ago, is adjacent to the non-diamondiferous Narayana pet kimberlite field. But the plume head at about 2082 million years ago was below the Cuddapah basin, of the present day Dharwar craton, situated within a 100 km radius of the diamond-bearing Wajrakarur kimberlite field.

Location of the inferred mantle plume head (~200 km diameter) below the Cuddapah basin. Arrows indicate the mantle plume locations of the 2367 Ma and of the 1886 Ma dyke swarms of the Dharwar craton.

“After the magmatic intrusions about 1886-1864 million years ago, there were no significant mantle plume events. The lithospheric keel must have cooled and restored the conditions for diamond formation. Thus, kimberlites formed 1100 Ma ago escaped destruction, and diamonds were retained by these kimberlites”, says D. Srinivasa Sarma, CSIR-NGRI.

Red squares and blue diamonds in the map above show potential kimberlite areas

“The mantle plume activity does not always destroy the mantle keel of diamondiferous kimberlite roots. At least, that is what we see from the case of the Dharwar craton”, says N. Ramesh Babu, CSIR-NGRI. 

Imagine: if you are walking along this region, you may have diamonds under your feet, far below, perhaps many kilometres under the ground. However, to find locations amenable to mining, many more scientific studies would be required.

Reference:

D. Srinivasa Sarma., V. Parashuramulu., M. Santosh., E. Nagaraju., N. Ramesh Babu., 2020. Pb–Pb baddeleyite ages of mafic dyke swarms from the Dharwar Craton: Implications for Paleoproterozoic LIPs and diamond potential of mantle keel. Geoscience Frontiers 11 2127-2139. DOI: 10.1016/j.gsf.2020.05.014