Tourmaline from the Archean G.R.Halli gold deposit,Chitradurga greenstone belt,Dharwar craton （India）： Implications for the gold metallogeny

Tourmaline occurs as a minor but important mineral in the alteration zc,ne of the Archean orogenic gold deposit of Guddadarangavanahalli （G.R.Halli） in the Chitradurga greenst~ne belt of the western Dharwar craton, southern India. It occurs in the distal alteration halo of the G.R.Halli golcl deposit as （a） clusters of very fine grained aggregates which form a minor constituent in the natrix of the altered metabasalt （AMB tourmaline） and （b） in quartz-carbonate veins （vein tourmaline）. ~[＇he vein tourmaline, based upon the association of specific carbonate minerals, is further grouped as （i） albite-tourmaline-ankerite-quartz veins （vein-1 tourmaline） and （ii） albite-tourmaline-calcite-quartz veins （vein-2 tourmaline）. Both the AMB tourmaline and the vein tourmalines （vein-I and vein-2） belong to the alkali group and are clas- sified under schorl-dravite series. Tourmalines occurring in the veins are zoned while the AMB tour- malines are unzoned. Mineral chemistry and discrimination diagrams 1eveal that cores and rims of the vein tourmalines are distinctly different. Core composition of the ve：n tourmalines is similar to the composition of the AMB tourmaline. The formation of the AMB tourmaline and cores of the vein tour- malines are proposed to be related to the regional D1 deformational event associated with the emplacement of the adjoining ca. 2.61 Ga Chitradurga granite whilst rims of the vein tourmalines vis-a- vis gold mineralization is spatially linked to the juvenile magmatic accretion （2.56-2.50 Ga） east of the studied area in the western part of the eastern Dharwar craton.     

Characterization of Sediments for Sustainable Development: State of the Art

Sediments are the particulate matter that are transported by the flow of water and eventually get deposited at the bottom of water bodies, such as oceans, rivers, lagoons, and so forth. The deposition of these materials due to continued intervention of natural as well as anthropogenic activities is a serious concern as the storage capacity (or health) of the system is reduced or otherwise affected. Keeping in view the emerging challenges associated with proper disposal and subsequent utilization of sediments, as a manmade resource, their extraction from such locations becomes necessary. Hence, it is essential to characterize the sediments in order to devise strategies to exploit them as a manmade resource in the best possible manner. With this in view, this article highlights the importance of different attributes and aspects of the sediments (viz., physical, chemical, morphological, geotechnical, toxicological, and microbial characteristics) deposited in the water bodies that will help in assessing their potential as a substitute material to natural resources.     

Occurrence of paragonite in the hydrothermalwall rock alteration zone of G. R. Halli gold deposit, Chitradurga Schist Belt, Western Dharwar Craton, Southern India

Paragonite (NaAl₂Si₃AlO₁₀(OH)₂), a dioctahedral sodium mica, occurs as a minor phase in the wall rock alteration assemblage of the Guddadarangavanahalli (G. R. Halli) gold deposit of Western Dharwar Craton, Karnataka. It occurs as colourless to pale yellowish grains and scaly aggregates in chlorite, ankerite and quartz rich matrix of the altered metabasalt. Its presence is noticed in the distal to near proximal wall rock alteration zone, mainly in the southern part of the G. R. Halli gold deposit. Presence of paragonite is identified by X-Ray Diffraction (XRD) and Electron Probe Micro-Analyzer (EPMA) studies. The Na₂O content of the paragonite varies between 5.96 wt% and 7.8 wt% whereas K₂O ranges from 0.44–2.68 wt%. Al₂O₃ is measured between 37.73 wt% and 39.72 wt% whereas SiO₂ varies from 45.06–47.19 wt%. The relative proportion of Na and K in paragonite varies between Na₉₆K₄ and Na₇₇K₂₃. The occurrence of paragonite in the wall rock alteration halo of the G. R. Halli gold deposit is proposed as a result of hydrolysis of metamorphosed plagioclase feldspars during the interaction of the hydrothermal fluids and the wall rock.     

Can an annual flood induce changes in channel geomorphology?

The present study has been a pioneering effort examining the role of an annual flood as a potent stimulus inducing changes in channel geomorphology of the Mayurakshi River, India. Twenty cross sections have been considered for the measurement of various hydro-geomorphic attributes of the river in both the pre- and post-flood conditions in 2018. The study sensed an escalating trend for channel width, width/depth ratio, and wetted perimeter while the reverse was also detected for average depth, maximum depth, cross-sectional area, and hydraulic radius. For example, the width/depth ratio recorded an increase of?~?11%, and the hydraulic radius depicted a decrease of?~?8%. Furthermore, channel asymmetry, bed asymmetry and bed relief index experienced a decrease after the flood. The sudden hydraulic impulse during monsoon flood as manifested in velocity, discharge, specific stream power, Reynolds number, Froude number increases the erosivity of the fluid. Besides the hydraulic factors, bank material (massive sandbank susceptible to hydraulic action and mixed bank constituted by alternate bands of sand and silt, and vulnerable to failure by piping action) brings substantial changes in channel morphology. Moreover, anthropogenic interventions such as sand mining are found to play a significant role in channel behaviour. The role of the multiple factors driving the morphological changes of the cross sections has been unpacked using canonical component analysis.

     

Discovery of Early Tonian Calc-alkaline and Shoshonitic Metamafic Rocks from the North Purulia Shear Zone, Chhotanagpur Gneissic Complex, Eastern India: Implications of Proterozoic Sub-continental Lithospheric Mantle

Reports of shoshonitic rocks in Precambrian terrains are relatively rare. Pl-Grt amphibolites and Hbl-Bt mafic granulites occurring in the migmatitic gneisses of the Chhotanagpur Gneissic Complex(CGC) show calc-alkaline and shoshonitic characteristics. Relict porphyritic, sub-ophitic and poikilitic textures are noted in these rocks. Their parent magma was emplaced during the waning phase of the regional metamorphism. Geochemically, these metamafics are similar to the Group Ⅲ potassic and ultrapo...     

Exposure of developing countries to sea-level rise and storm surges

An increase in sea surface temperature is strongly evident at all latitudes and in all oceans. The scientific evidence to date suggests that increased sea surface temperature will intensify cyclone activity and heighten storm surges. The paper assesses the exposure of (coastal) developing countries to sea-level rise and the intensification of storm surges. Geographic Information System (GIS) software is used to overlay the best available, spatially-disaggregated global data on critical exposed elements (land, population, GDP, agricultural extent and wetlands) with the inundation zones projected with heightened storm surges and a 1 m sea-level rise. Country-level results indicate a significant increase in exposure of developing countries to these climate-induced changes.     

Properties of warm absorbers in active galaxies: a systematic stability curve analysis

Signatures of warm absorbers are seen in soft X-ray spectra of about half of all type 1 Seyfert galaxies observed and in some quasars and blazars. We use the thermal equilibrium curve to study the influence of the shape of the ionizing continuum, density and the chemical composition of the absorbing gas on the existence and nature of the warm absorbers. We describe circumstances in which a stable warm absorber can exist as a multiphase medium or one with continuous variation in pressure. In particular, we find the following results: (i) the warm absorber exists only if the spectral index of the X-ray power-law ionizing continuum  α > 0.2  and has a multiphase nature if  α∼ 0.8  , which interestingly is the spectral index for most of the observed type 1 Seyfert galaxies; (ii) thermal and ionization states of highly dense warm absorbers are sensitive to their density if the ionizing continuum is sufficiently soft, i.e. dominated by the ultraviolet; (iii) absorbing gas with super-solar metallicity is more likely to have a multiphase nature and (iv) the nature of the warm absorber is significantly influenced by the absence of iron and associated elements which are produced in the later stages of star formation history in Type Ia supernovae.     

Tourmaline as metallogenic indicator: Examples from paleo-proterozoic Pb-Zn and Cu-Au deposits of Rajasthan

Petrology and mineral chemistry of ten types of tourmalines from Pb-Zn bearing strata-bound exhalative deposits and Cu-Au-Fe deposits, presumably of IOCG-type, from Rajasthan are presented. The Pb-Zn deposits studied include Agucha, Kayar and Dariba areas in central Rajasthan, each of which is widely separated in space and occurs in isolated but coeval basins within the Archaean basement. The Cu-Au deposits include Ghagri, Bada Talav, Dugocha and Bhukia areas in southeast Rajasthan. The syngenetic Pb-Zn type and the epigenetic Cu-Au-Fe type of deposits are formed during the initial phase of Paleoproterozoic Aravalli Basin evolution with dominant deposition of shelf facies pelite-carbonate sequence of Debari Group. The post-depositional tectono-metamorphic history of the host rocks of Pb-Zn type and Cu-Au type deposits differ significantly with the former deposits showing higher metamorphic impress (amphibolite to lower granulite facies) than the latter (green schist facies).