The A-type granites of Jharsuguda at the western margin of the Singhbhum craton in India host several Nb-, Ta-, and Bebearing pegmatites.Detailed SEM and electron-microprobe analysis reveal potential LREE-carbonate mineralization through the occurrence of predominantly bastnäsite-(La) and bastnäsite-(Ce) and rarely bastnäsite-(Nd), parisite, synchysite-(Ce) and thorian synchysite-(Ce) in these granites.Both F-and OHdominant members are present.A few of the bastnäsite-(Ce) show very high content of ThO 2 (up to 14.6 wt.%), and the concentration of ThO 2 in the thorian synchysite-(Ce) varies from 4.8 to 13.1 wt.%.LREE-carbonates replace primary allanite and zircon, forming pseudomorphs varying in size from 40 µm to 500 µm.These pseudomorphs generally have two zones: an inner zone of LREE carbonate, which is surrounded by a zone enriched in Si, Al, and Fe containing very low concentrations of REEs and Ca.Signatures of oscillatory zoning and inclusions of residual zircon are sometimes observed in these pseudomorphs.Rare cerite-(Ce) with 68.7 wt.% Ce 2 O 3 , which is higher than in any so far reported cerite, is also encountered.The elevated Ce 2 O 3 concentration, the positive Ce-anomaly in its REE pattern, and the association with kaolinite suggest that cerite was formed by hydrothermal alteration of cerianite.Minor alteration of zircons is observed in the granites.While the fresh zircons are devoid of inclusions, the altered ones contain numerous mineral inclusions, including that of monazite.Altered zircons are sometimes associated with LREE-bearing carbonates.The compositional data indicate the replacement of primary minerals by a halogen, phosphate, and carbon-bearing fluid.
Abstract The Central Indian Tectonic Zone (CITZ) is a Proterozoic suture along which the Northern and Southern Indian Blocks are inferred to have amalgamated forming the Greater Indian Landmass. In this study, we use the metamorphic and geochronological evolution of the Gangpur Schist Belt (GSB) and neighbouring crustal units to constrain crustal accretion processes associated with the amalgamation of the Northern and Southern Indian Blocks. The GSB sandwiched between the Bonai Granite pluton of the Singhbhum craton and granite gneisses of the Chhotanagpur Gneiss Complex (CGC) links the CITZ and the North Singhbhum Mobile Belt. New zircon age data constrain the emplacement of the Bonai Granite at 3,370 ± 10 Ma, while the magmatic protoliths of the Chhotanagpur gneisses were emplaced at c . 1.65 Ga. The sediments in the southern part of the Gangpur basin were derived from the Singhbhum craton, whereas those in the northern part were derived dominantly from the CGC. Sedimentation is estimated to have taken place between c . 1.65 and c . 1.45 Ga. The Upper Bonai/Darjing Group rocks of the basin underwent major metamorphic episodes at c . 1.56 and c . 1.45 Ga, while the Gangpur Group of rocks were metamorphosed at c . 1.45 and c . 0.97 Ga. Based on thermobarometric studies and zircon–monazite geochronology, we infer that the geological history of the GSB is similar to that of the North Singhbhum Mobile Belt with the Upper Bonai/Darjing and the Gangpur Groups being the westward extensions of the southern and northern domains of the North Singhbhum Mobile Belt respectively. We propose a three‐stage model of crustal accretion across the Singhbhum craton—GSB/North Singhbhum Mobile Belt—CGC contact. The magmatic protoliths of the Chhotanagpur Gneisses were emplaced at c . 1.65 Ga in an arc setting. The earliest accretion event at c . 1.56 Ga involved northward subduction and amalgamation of the Upper Bonai Group with the Singhbhum craton followed by accretion of the Gangpur Group with the Singhbhum craton–Upper Bonai Group composite at c . 1.45 Ga. Finally, continent–continent collision at c . 0.96 Ga led to the accretion of the CGC with the Singhbhum craton–Upper Bonai Group–Gangpur Group crustal units, synchronous with emplacement of pegmatitic granites. The geological events recorded in the GSB and other units of the CITZ only partially overlap with those in the Trans North China Orogen and the Capricorn Orogen of Western Australia, indicating that these suture zones are not correlatable.
Abstract The Cuddapah basin in southern India, consisting of the Palnad, Srisailam, Kurnool and Papaghni sub-basins, contains unmetamorphosed and undeformed sediments deposited during a long span of time in the Proterozoic. In the absence of robust age constraints, there is considerable confusion regarding the relative timing of sedimentation in these sub-basins. In this study, U–Pb isotopic dating of zircon and U–Th–Pb total dating of monazite and uraninite from the gritty quartzite that supposedly belongs to the formation Banganapalle Quartzite have been used to constrain the beginning of sedimentation in the Palnad sub-basin. Magmatic and detrital zircons recording an age of 2.53 Ga indicate that the sediments were derived from the granitic basement or similar sources and were deposited after 2.53 Ga. Hydrothermally altered zircons both in the basement and the cover provide concordant ages of 2.32 and 2.12 Ga and date two major hydrothermal events. Thus, the gritty quartzite must have been deposited sometime between 2.53 and 2.12 Ga and represents the earliest sediments in the Cuddapah basin. Monazite and uraninite give a wide spectrum of ages between 2.5 Ga and 150 Ma, which indicates several pulses of hydrothermal activity over a considerable time span, both in the basement granite and the overlying quartzite. The new age constraints suggest that the gritty quartzite may be stratigraphically equivalent to the Gulcheru Quartzite that is the oldest unit in the Cuddapah basin, and that a sedimentary/erosional hiatus exists above it.
Li-rich micas have often been used to characterize the magmatic and hydrothermal evolution of granites.This study highlights the suitability of Li-micas as tracers of hydrothermal W-mineralization associated with the Neoproterozoic Degana granite in western India.Based on micro-textural relations and mineral chemical zoning, muscovites in the mineralized granites can be grouped into three types: 1) igneous muscovite, formed during the granite crystallization, 2) hydrothermal muscovite formed from alteration of the K-feldspar, 3) hydrothermal muscovite occurring in mineralized quartz veins.Some of the muscovites show further post-mineralization alteration possibly in response to regional tectonothermal events.The major and trace element chemistry of hydrothermal muscovite indicates different substitution mechanisms such as, Si 2 LiAl -3 and SiLiAl -1 R -1 where R = (Fe 2+ + Mg + Mn) that controlled the incorporation of Li.Ti-in-quartz temperatures of 319°C to 362°C were obtained from hydrothermal quartz in the mineralized quartz vein which is slightly lower than the maximum temperature obtained from fluid inclusion study (420°C; [1]).The high Rb, Li, Nb, Ta and F composition of the altered mica along with the presence of fluorite suggest a fluorine-rich late fractionated magmatic hydrothermal fluid source.The major and trace element composition of mica (interme diate compositions between phlogopite-zinnwaldite-muscovite) from granite, greisen, and the different stages represented in the veins are studied in details and their chemistry indicate a potential mixing of endmember fluids fluid from different sources.A reactive transport model is attempted to model the chemical evolution of the hydrothermal system during the interaction of the Degana granite by the ore-bearing fluid.The approximated model is then compared with the petrographic evidences.
Abstract The distribution of Au and associated trace elements in pyrite and arsenopyrite from late Archean Hutti and Hira-Buddini orogenic gold deposits, eastern Dharwar Craton, southern India was investigated by laser ablation-inductively coupled plasma-mass spectrometry. X-ray element maps acquired by electron probe microanalyser reveal oscillatory zoning of Co and As indicating the crystallization of pyrite and arsenopyrite in an episodic fluid flow regime in which fluid salinity fluctuated due to fault-valve actions. The absence of any relationship between Au and As in pyrite obviate the role of As in the incorporation of Au into pyrite, particularly here and may be generally the case in orogenic gold deposits. On the other hand, positive correlations of Au with Cu, Ag and Te suggest possible influence of these chalcophile elements in the enhanced gold concentrationin sulfides. Pb-Bi-Te-Au-Ag bearing micro-particles (<2 μm) are observed exclusively in micro-fractures and pores in arsenopyrite. The absence of replacement features and element gradient suggests direct precipitation of Pb, Bi, Te, Au and Ag from a fluid that was unreactive towards arsenopyrite. An intermittent fall in fluid pressure caused by the fault-valve action would have resulted in the sporadic precipitation of Au, Pb, Ag, Bi and Te.
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