In the western part of Bundelkhand massif, a caldera with intra-caldera sediments, known as Dhala Formation, occurs as an outlier in and around Mohar village of Shivpuri district, Madhya Pradesh. For the first time, occurrence of peperite is being reported from the basal part of the Dhala sediment. Two types of peperites have been recognized: blocky and fluidal or globular with variable morphology. In peperitic zones, features like soft sediment deformations, presence of sediment into the rhyolite along cracks, vesiculation of the sediments and other evidences suggestive of sediment fluidization are some definite characteristics of interaction of hot magma with wet sediments forming peperite. The occurrence of peperites reflects the contemporaniety of deposition of the Dhala sediments and volcanism, which is well in accordance to the volcanic origin of Dhala structure. Further, the nature of unconformity between the Dhala and overlying Kaimur which is characterized by merely a few centimeter thick pebbly/conglomeratic bed does not appear to represent a large hiatus as expected between the Semri and Kaimur of Vindhyan Supergroup. So, the contemporaniety of the Dhala Formation (at least the lower part) as reflected by occurrence of peperites, coupled with the available age of the rhyolite and the nature of the unconformity between the Dhala and overlying Kaimur provide convincing evidence to correlate the Dhala Formation with the Lower part of the Kaimur and unlikely with the Semri Group or Bijawar as proposed earlier.
Low grade uranium mineralisation associated with phosphorites/phosphatic sediments and carbonaceous shale has been well established at and/or close to Krol-Tal contact in some of sub-basins of inner Krol belt such as Nigalidhar, Mussoorie and Garhwal synclines in the realm of lesser Himalaya. Unlike the other sub-basins of the belt, though the Nainital syncline is sparsely phosphatic and carbonaceous, carbonate hosted uranium occurrence is reported for the first time from the syncline. The carbonate-hosted uranium occurrences in Krol D of Nainital syncline are of limited extent with values ranging from 200 ppm to 840 ppm eU3O8. Radioactivity is associated with fractured dolostone/dolomitic limestone and pelloidal limestone. Uranium phases mostly occur in association with carbonaceous, ferruginous matters and clays. However, ultrafine pitchblende grains do occur associated with idiotopic pyrite aggregates suggesting initial trapping of uranium by organic matters present in sediments which later remobilised and precipitated as fine pitchblende. Subsequently, second phase of remobilisation of uranium under oxidising condition gave rise uraniferous goethite, limonite and other iron oxyhydoxides. In contrast to other sub-basins, the Krol-Tal contact in Nainital syncline does not appear to hold any promising phosphate and uranium mineralisation due to less suitable basinal morphology and low Ca/Mg ratio.
The diamonds sourced from the Mesoproterozoic Atri pipe are white to brown in color and often plastically deformed, with two morphological populations present, octahedra dominant in the finer sizes, and resorbed dodecahedra in the coarser sizes. The color and resorbed shapes have some resemblance to properties of diamonds from the Majhgawan lamproite 80 km to the east-northeast and of the brown, plastically deformed diamonds from the Mesoproterozoic Argyle lamproite pipe in Australia. The diamond infrared spectra indicate low to moderate nitrogen content and IaA to IaAB aggregation and show occasional spikes related to hydrogen and carbonate presence, which suggests diamond formation through cooling of hydrous fluids that contain both CH4 and CO2. Platelet peaks are commonly prominent and show regular correlation with the IaB component, unlike the irregular diamonds from Argyle. The diamond inclusions are dominantly peridotitic olivine and Cr-rich magnesiochromite formed from depleted lithospheric mantle at ~150-km depth at an estimated 1,140° to 1,200°C, corresponding to a typical cratonic geotherm equivalent to 40-mW/m2 model surface heat flow. The predominance of harzburgitic-type inclusions and the lack of eclogitic-type indicators in the heavy mineral concentrates suggest that both the diamonds and mantle section sampled by the Atri intrusion are mainly or entirely peridotitic in nature, unlike those of the Argyle lamproite, despite their similar craton-marginal tectonic positions.
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