Mineral Investigation

The Salumber-Ghatol belt in Rajasthan, India, situated along southern margin of the Aravalli Craton, hosts a cluster of Cu-Au deposits in calcitic and dolomitic marbles that belong to Debari Group of the Paleo-mesoproterozoic Aravalli Supergroup. The Fe-Mn rich dolomitic marble of the Delwara Formation hosts Cu-Au-Fe-oxide mineralization at Ghagri and associated distal K-Fe-Mg rich altered rocks (cryptocrystalline microcline + magnesioriebeckite + magnetite + phlogopite) and proximal feldspathised carbonate rocks (medium grained albite + microcline + dolomite + magnetite). The calcitic marble of Mukandpura Formation hosts Dugocha Cu-Au deposit with development of distal graphitetourmaline-bearing albitites and proximal albite-microcline-magnetite rocks. Calcite and dolomite carbonates of Bhukia region with development of albite-actinolite-bearing alteration assemblages host the largest of the Cu-Au deposits in this belt. The second generation folds and associated ductile-brittle shear zones of the multiply deformed events constitute conduits for the mineralizing fluids at all locations in this belt.Carbon isotope data for the dolomitic marbles of Ghagri area suggest mixed isotopic signatures: (i) normal marine values close to zero per mil (mean= −0.269 ‰) and (ii) enriched positive values (mean= + 4.04 ‰) akin to the 1.9 Ga global event of positive excursion. The calcitic marbles of Dugocha area have significantly depleted δ13C values that are interpreted to be the result of pervasive inter-mixing of isotopically distinct carbon derived from primary depositional carbonates, hydrothermally exchanged carbonates, with significant influence of the organic carbon. The petrographic distinction of calcitic and dolomitic marbles of Bhukia area is corroborated by δ13C values that show a bimodal distribution of near 0 per mil and depleted values of −3 per mil. Fluid inclusion studies suggest predominance of saline brines in the ore fluids. The δ34S values for all the three deposits are narrowly constrained between 10.4 and 15.1 ‰, which suggests a common metasedimentary source for sulfur.Identical chondrite-normalized REE patterns and primitive mantle-normalized multi-element patterns suggest a common Na- and/or K-source for the formation of Ghagri and Dugocha-Bhukia feldspathic rocks. Based on mineralization styles and attendant alteration patterns, it is argued that the mineral deposits of Salumber-Ghatol belt in the Aravalli Supergroup (i) represent sedimentary carbonate-hosted Cu-Au deposits belonging to the wider class of Proterozoic Iron-Ore-copper-Gold type (IOCG-like) mineral deposits; (ii) the fluids responsible for the mineralization are generated from common sub-crustal source and (iii) during mineralization, the influx of Na-K-rich fluids is so intense that it created ‘artificial’ hyper-saline conditions in the normal depositional basin that facilitated crystallization of scapolite, brucite, spadaite, povondraite and phlogopite bearing mineral assemblage.

The present study emphasises occurrence, mineral assemblages and mineral chemistry of the Neoproterozoic Degana Peraluminous Granite (DPG) around Rewat Hill and its fertility. Based on field evidences and nature of occurrences of wolframite, the tungsten mineralization of the Rewat Hill has been classified into five types; Type-I: quartz–wolframite vein, Type-II: quartz–muscovite–wolframite–polymetallic sulphide vein, Type-III: DPG hosted wolframite, Type-IV: stock-works hosted wolframite in phyllite and Type-V: gravel bed hosted wolframite. Tungsten mineralization in DPG was established in the exposed part by various workers but extension of fertile DPG and its potentiality for tungsten and associated mineralization in the soil-covered area around Rewat Hill remained unexplored. In this study, sub-surface exploration through vertical and inclined boreholes studies has been established which suggest a possible extension of fertile DPG, i.e., Type-I and Type-III tungsten mineralizati...

The Palaeoproterozoic Debari Group of Aravalli Supergroup near Dugocha, Rajasthan exposes carbonalbite-tourmaline bearing exhalite rock termed carbonaceous metaexhalite. This unit occurs within dolomites of Jagpura Formation and hosts gold-bismuth bearing sulphide mineralisation either within it or along dolomite-metaexhalite contact zones. Polyphase deformation and mineral recrystallisation under green schist facies of metamorphism characterise these rocks. Geochemically, the carbonaceous metaexhalites have a wide silica range (51 wt% to 63 wt%) and high total alkali (7.85 wt % to 13.20 wt%). EPMA studies of silicates indicate that the main feldspar in the rock is albite and the tourmalines are richer in titanium and chromium. The associated sulphides possess high salinity fluid inclusions (up to 19 wt% NaCl equivalent) and δ 34 S isotopic values of 12-14 permil. The latter values match well with those of the modern hydrotherms along Discovery Deep median trench of Red Sea. The mos...

Geological and geochemical characteristics of the litho units exposed in and around Kalalikhera area of Pur-Banera belt have confirmed the presence of felsic volcanic rock unit named Kalalikhera felsic volcanic, with characteristic features such as (i) abundance of lapilli composed of polygonal quartz and feldspar and (ii) presence of relict bipyramidal quartz. These volcanics are associated with thin bands of chert. Geochemically, the volcanics are found to be of rhyolitic composition. Development of garnet porphyroblasts along with preferred orientation of the silicate minerals of rhyolite indicates a low to medium grade metamorphism and deformation of the units. Monazite geochemistry of the volcanics indicated towards the metamorphic origin of monazites, with REE pattern show steep fractionation trend from La to Lu. The in-situ chemical age of 2192 ± 57 Ma recorded from xenotime is interpreted as indicative of the opening of the Aravalli basin with the onset of rifting leading to the formation of Kalalikhera volcanics. The estimated ages of 1784 ± 92, 1351 ± 45 and 1026 ± 57 Ma from monazite analyses are interpreted as different metamorphic events associated with (i) closure of the basin and the onset of Aravalli orogeny, (ii) further imprints of Delhi orogeny, and (iii) with late Grenvillian age imprints.

Recent works suggest Proterozoic plate convergence along the southeastern margin of India which led to amalgamation of the high grade Eastern Ghats belt (EGB) and adjoining fold-and-thrust belts to the East Dhrawar craton. Two major thrusts namely the Vellikonda thrust at the western margin of the Nellore Schist belt (NSB) and the Maidukuru thrust at the western margin of the Nallamalai fold belt (NFB) accommodate significant upper crustal shortening, which is indicated by juxtaposition of geological terranes with distinct tectonostratigraphy, varying deformation intensity, structural styles and metamorphic grade. Kinematic analysis of structures and fabric of the fault zone rocks in these intracontinental thrust zones and the hanging wall and footwall rocks suggest spatially heterogeneous partitioning of strain into various combinations of E-W shortening, top-to-west shear on stratum parallel subhorizontal detachments or on easterly dipping thrusts, and a strike slip component. Although relatively less prominent than the other two components of the strain triangle, non-orthogonal slickenfibres associated with flexural slip folds and mylonitic foliation-stretching lineation orientation geometry within the arcuate NSB and NFB indicate left lateral strike slip subparallel to the overall N-S trend. On the whole an inclined transpression is inferred to have controlled the spatially heterogeneous development of thrust related fabric in the terrane between the Eastern Ghats belt south of the Godavari graben and the East Dharwar craton.

Fault-slip analysis along Gani-Kalva and Kona faults were performed.Extensional to transtensive regime dominated the post-lower Cuddapah or Kurnool sub-basin opening.Late compressional to transpressive/pure strike-slip regimes dominated the post-Kurnool coeval to the Pan-African event.Multiple fault-slips suggest Cuddapah basin evolution punctuated through various deformational eventsThe sedimentary basins are often influenced by adjoining mobile belts as is the case of the Proterozoic Cuddapah basin in southern India, lying in proximity of the Eastern Ghats granulite belt (EGB) and the Southern granulite terrain (SGT). Two major faults – Gani-Kalva and Kona faults – in the western part of the Cuddapah basin have been examined in this work to comment on the changes in paleostress conditions affecting the basin evolution punctuated by several stages of extensional and compressional regimes in the time interval 1.9 Ga to 0.5 Ga. The tensor solutions were obtained for the mesoscale faults and fault striae using the improved Right Dihedron method followed by the Rotational Optimization method (TENSOR, Delvaux and Sperner, 2003). Combining the paleostress field reconstruction and other structural/stratigraphic attributes we propose a possible scenario of punctuated evolution of the Cuddapah basin and its inversion. The stress regimes are tentatively correlated with multiple stages of basin opening and deformation of basin in-fill, representing the pre-Grenvillian through the Pan-African orogeny. The paleostress conditions derived from various stratigraphic horizons provide an opportunity to comment on the changes in tectonic stresses including several stages of extensional and compressive regimes in the Cuddapah basin.The tensor solutions were obtained for the mesoscale faults and fault striae in the neighbourhood of above two fault lines using the improved Right Dihedron method followed by the Rotational Optimization method (TENSOR, Delvaux and Sperner, 2003). The most prominent stress states recorded around the two fault lines can be classified into those affecting 1) the Paleoproterozoic lower Cuddapah successions, 2) the younger Kurnool Group and 3) both the groups. Integrating the stress states of the fault-slip analysis obtained from different unconformity bounded rock groups of lower Cuddapah and other geologic data, we suggest extensional to transtensive/pure strike-slip regime to be active during the first phase of basin opening and c. 1.89 Ga mafic igneous activity. Late compressional to transpressive/pure strike-slip regimes are well recorded from the younger Kurnool Group representing late Neoproterozoic deformations. The tensor solutions from different unconformity bound stratigraphic horizons constrain different states of deformation. We relate the extensional to transtensive/pure strike-slip regimes from lower Cuddapah to the ongoing basin opening events and the associated Palaeoproterozoic igneous activities. The late Neoproterozoic to early Paloezoic compressional to transpressive/strike-slip regime present the possible effects of compressional activities happening around the cratonic margin. The present analysis lends further support to the idea that estimation of paleostress states from fault-slip data even in regions of relatively weak deformation in continental interior are important while assessing influence of plate margin stresses on the continental interior.

Two major structural events characterize the tectonic evolution of the northern part of the Nallamalai Fold Belt (NFB): (1) the Late Paleoproterozoic or Early Mesoproterozoic Dn1 compression due to the thrusting of granitic gneiss and Nellore Schist Belt (NSB) over the eastern part of the NFB possibly related to the Columbia supercontinental activities; and (2) the Late Neoproterozoic-Early Cambrian Dn2 deformation in the NFB due to its overthrust movement on the western and northern part of the undeformed Cuddapah basin (i.e., the Kurnool and the Palnad sub-basins). The late structures with NW to N vergent hanging wall lithounits (NFB) are represented by several map-scale imbricate thrust sheets, flat and ramp structures, asymmetrical to overturned folds, and structures associated with nappe. The nature of deformation in the footwall (Kurnool Group in Palnad sub-basin) and the hanging wall (NFB) structures are also elaborated in view of the possible regional tectonics. Progressive oroclinal model is also proposed here to explain the changes in the structural trend of NFB from NE to E. The variations of the NFB structures in conjunction with the regional structures connotes of thin-skinned deformation during Pan-African period, which are examined to constrain the possible plate kinematics related to the convergence of East Gondwana fragments.