Biogeochemistry over the sulfidic​/auriferous zone near Ajjanahalli, Chitradurga Schist belt, Dharwar craton, souther India

The sulfidic​/auriferous ridge near Ajjanahalli in Chitradurga schist belt is chiefly constituted by BIF and flanked on either side by a closely knit assembly of phyllitic schists and carbonaceous shales. The mineralization is confined to the shear zone and the ore minerals are disseminated in the fracture filling quartz​/carbonate veins of epigenetic nature. Pyrite, chalcopyrite, arsenopyrite and pyrrhotite are the chief sulfide minerals within which gold is hosted. The mineralized ridge is covered by fairly thick natural vegetation including herbs, shrubs and tree species. About half meter to meter thick well drained, coarse textured silty to gravelly soil form a crust over this mineralized ridge. The area is a semi-​arid tract with moderate annual rainfall of ∼500mm. In the present study, this auriferous zone has been chosen for geobotanical and biogeochem. study to appraise the morphol. characteristics of plant species and the metal dispersion pattern in them in relation to metal content in the ground. Soil geochem. suggests a poor to moderate accumulation of Cu (139-​627 ppm)​, most in the range of 250-​600 ppm. Many other metals show normal dispersion. Arsenic is in high concn. ranging from 695-​5470 ppm. Similarly, Se also shows anomalous content (1304-​2136 ppm)​. Plants show inhomogeneous accumulation of metals. Cu is in poor concn., probably owing to its poor content in soil. Slightly alk. nature of soil could have also hindered its dissoln. and the consequent mobility. Arsenic is significantly accumulated in a Fern species (Pteris vittata) which is confined only to the mine-​sections in mineralized ridge. Interestingly, a clear higher uptake pattern of Au by this Fern is also noticed, whereas other species do not show such comparable patterns of As and Au. Gold in soil also shows moderate concn. (40-​320 ppb)​. Thus, Fern species hint at the possibility of considering it as a local indicator for gold in sulfidic zones. But, detailed sampling of this species is necessary before ascertaining its value as an index plant. Other metals like Co, Zn and Mn, though present in variable concns. in plants, they do not reflect much significance. Se, though toxic to plants at higher concns., was found to be enriched in plants, aptly reflecting the sulfidic nature of the substratum

Biogeochemistry over the sulfidic/auriferous zone near Ajjanahalli in the central part of Chitradurga schist belt, Dharwar craton.

The sulfidic/auriferous ridge near Ajjanahalli in Chitradurga schist belt is chiefly constituted by BIF and flanked on either side by a closely knit assembly of phyllitic schists and carbonaceous shales. The mineralization is confined to the shear zone and the ore minerals are disseminated in the fracture filling quartz/carbonate veins of epigenetic nature.Pyrite, chalcopyrite, arsenopyrite and pyrrhotite are the chief sulfide minerals within which gold is hosted. The mineralized ridge is covered by fairly thick natural vegetation including herbs, shrubs and tree species. About half meter to meter thick well drained, coarse textured silty to gravelly soil form a crust over this mineralized ridge. The area is a semi-arid tract with moderate annual rainfall of ~500mm. In the present study, this auriferous zone has been chosen for geobotanical and biogeochemical study to appraise the morphological characteristics of plant species and the metal dispersion pattern in them in relation to metal content in the ground. Soil geochemistry suggests a poor to moderate accumulation of Cu (139-627 ppm), most in the range of 250-600 ppm. Many other metals show normal dispersion.Arsenic is in high concentration ranging from 695-5470 ppm. Similarly, Se also shows anomalous content (1304-2136ppm).Plants show inhomogeneous accumulation of metals. Cu is in poor concentration, probably owing to its poor contentin soil. Slightly alkaline nature of soil could have also hindered its dissolution and the consequent mobility. Arsenic is significantly accumulated in a Fern species (Pteris vittata) which is confined only to the mine-sections in mineralized ridge. Interestingly, a clear higher uptake pattern of Au by this Fern is also noticed, whereas other species do not show such comparable patterns of As and Au. Gold in soil also shows moderate concentration (40-320 ppb). Thus, Fern species hint at the possibility of considering it as a local indicator for gold in sulfidic zones. But, detailed sampling of this species is necessary before ascertaining its value as an index plant. Other metals like Co, Zn and Mn, though present in variable concentrations in plants, they do not reflect much significance. Se, though toxic to plants at higher concentrations, was found to be enriched in plants, aptly reflecting the sulfidic nature of the substratu

Biogeochemical study over the copper mineralized areas of Kalyadi, Karnataka.

Ultramafics intercalated with metasedimentaries in Kalyadi area host copper mineralization and the overburden of this mineralized zone has been studied from the point of geobotany and biogeochemistry. In the mineralized zone, the soil cover is very thin (1-1.5 ft), and supports sparse and stunted vegetation. Shrubs dominate the herbs and trees. Dodonaea viscosa, Cassia auriculata,Maytenus emerginata, Pavetta indica, Erythro xylon, Tecoma stans, Aerva lanata, Hyptis suaveolens, Atylosia albicans,Stachyterphyta indica, Chromolaena odarata are the plant species which have been studied and sampled. Morphologically, nosignificant changes could be made out from the species growing in the mineralized area. Depending upon the distribution and homogeneity of the plant species sampling has been carried out following a grid pattern. The elements analyzed for plant species and soil included Cu, Cr, Zn, Ni, Co, Mn and Mg. Trace element study of soil and floral species indicates moderate but inhomogenous dispersion of Cu from the bed rock source. Only a few samples of Leguminosae, Dodonaea viscosas, Erthyron xylon, Leucas ciliata and Pavetta indica showed slightly higher values. Thus, these species are considered as better up-takers of copper. Restricted Cr,Ni and Co in almost all the analyzed plants could be due to their lesser mobility. The overall biogeochemical observation has helped to recognize Leucas ciliata, Cassia auriculata and Erthyron xylon as species slightly favourable for Cu up-take and could serve as local indicators. Statchyterphyta indica is recognized as a good up-taker of Zn

Biogeochemical Study of Copper Mineralized Zone near Kalyadi, Karnataka

Ultramafics intercalated with metasedimentaries in Kalyadi area host copper mineralization and the overburden of this mineralized zone has been studied from the point of geobotany and biogeochemistry. In the mineralized zone, the soil cover is very thin (1–1.5 ft), and supports sparse and stunted vegetation. Shrubs dominate the herbs and trees. Dodonaea viscosa, Cassia auriculata, Maytenus emerginata, Pavetta indica, Erythro xylon, Tecoma stans, Aerva lanata, Hyptis suaveolens, Atylosia albicans, Stachyterphyta indica, Chromolaena odarata are the plant species which have been studied and sampled. Morphologically, no significant changes could be made out from the species growing in the mineralized area. Depending upon the distribution and homogeneity of the plant species sampling has been carried out following a grid pattern. The elements analyzed for plant species and soil included Cu, Cr, Zn, Ni, Co, Mn and Mg. Trace element study of soil and floral species indicates moderate but inhomogenous dispersion of Cu from the bed rock source. Only a few samples of Leguminosae, Dodonaea viscosas, Erthyron xylon, Leucas ciliata and Pavetta indica showed slightly higher values. Thus, these species are considered as better up-takers of copper. Restricted Cr, Ni and Co in almost all the analyzed plants could be due to their lesser mobility. The overall biogeochemical observation has helped to recognize Leucas ciliata, Cassia auriculata and Erthyron xylon as species slightly favourable for Cu up-take and could serve as local indicators. Statchyterphyta indica is recognized as a good up-taker of Zn

Physical volcanology and geochemistry of Paleoarchean komatiite lava flows from the western Dharwar craton, southern India: Implications. for Archean mantle evolution and continental growth.

Palaeoarchaean (3.38–3.35 Ga) komatiites from the Jayachamaraja Pura (J.C. Pura) and Banasandra greenstone belts of the western Dharwar craton, southern India were erupted as submarine lava flows. These high-temperature (1450–1550°C), low-viscosity lavas produced thick, massive, polygonal jointed sheet flows with sporadic flow top breccias. Thick olivine cumulate zones within differentiated komatiites suggest channel/conduit facies. Compound, undifferentiated flow fields developed marginal-lobate thin flows with several spinifex-textured lobes. Individual lobes experienced two distinct vesiculation episodes and grew by inflation. Occasionally komatiite flows form pillows and quench fragmented hyaloclastites. J.C. Pura komatiite lavas represent massive coherent facies with minor channel facies, whilst the Bansandra komatiites correspond to compound flow fields interspersed with pillow facies. The komatiites are metamorphosed to greenschist facies and consist of serpentine-talc ± carbonate, actinolite–tremolite with remnants of primary olivine, chromite, and pyroxene. The majority of the studied samples are komatiites (22.46–42.41 wt.% MgO) whilst a few are komatiitic basalts (12.94–16.18 wt.% MgO) extending into basaltic (7.71 – 10.80 wt.% MgO) composition. The studied komatiites are Al-depleted Barberton type whilst komatiite basalts belong to the Al-undepleted Munro type. Trace element data suggest variable fractionation of garnet, olivine, pyroxene, and chromite. Incompatible element ratios (Nb/Th, Nb/U, Zr/Y Nb/Y) show that the komatiites were derived from heterogeneous sources ranging from depleted to primitive mantle. CaO/Al2O3 and (Gd/Yb)N ratios show that the Al-depleted komatiite magmas were generated at great depth (350–400 km) by 40–50% partial melting of deep mantle with or without garnet (majorite?) in residue whilst komatiite basalts and basalts were generated at shallow depth in an ascending plume. The widespread Palaeoarchaean deep depleted mantle-derived komatiite volcanism and sub-contemporaneous TTG accretion implies a major earlier episode of mantle differentiation and crustal growth during ca. 3.6–3.8 Ga

Physical volcanology and geochemistry of Palaeoarchaean komatiite lava flows from the western Dharwar craton, southern India: implications for Archaean mantle evolution and crustal growth

Palaeoarchaean (3.38–3.35 Ga) komatiites from the Jayachamaraja Pura (J.C. Pura) and Banasandra greenstone belts of the western Dharwar craton, southern India were erupted as submarine lava flows. These high-temperature (1450–1550°C), low-viscosity lavas produced thick, massive, polygonal jointed sheet flows with sporadic flow top breccias. Thick olivine cumulate zones within differentiated komatiites suggest channel/conduit facies. Compound, undifferentiated flow fields developed marginal-lobate thin flows with several spinifex-textured lobes. Individual lobes experienced two distinct vesiculation episodes and grew by inflation. Occasionally komatiite flows form pillows and quench fragmented hyaloclastites. J.C. Pura komatiite lavas represent massive coherent facies with minor channel facies, whilst the Bansandra komatiites correspond to compound flow fields interspersed with pillow facies. The komatiites are metamorphosed to greenschist facies and consist of serpentine-talc ± carbonate, actinolite–tremolite with remnants of primary olivine, chromite, and pyroxene. The majority of the studied samples are komatiites (22.46–42.41 wt.% MgO) whilst a few are komatiitic basalts (12.94–16.18 wt.% MgO) extending into basaltic (7.71 – 10.80 wt.% MgO) composition. The studied komatiites are Al-depleted Barberton type whilst komatiite basalts belong to the Al-undepleted Munro type. Trace element data suggest variable fractionation of garnet, olivine, pyroxene, and chromite. Incompatible element ratios (Nb/Th, Nb/U, Zr/Y Nb/Y) show that the komatiites were derived from heterogeneous sources ranging from depleted to primitive mantle. CaO/Al2O3 and (Gd/Yb)N ratios show that the Al-depleted komatiite magmas were generated at great depth (350–400 km) by 40–50% partial melting of deep mantle with or without garnet (majorite?) in residue whilst komatiite basalts and basalts were generated at shallow depth in an ascending plume. The widespread Palaeoarchaean deep depleted mantle-derived komatiite volcanism and sub-contemporaneous TTG accretion implies a major earlier episode of mantle differentiation and crustal growth during ca. 3.6–3.8 Ga

Nickel anomalies in the ultrmafic profiles of Jayachamarajapura schist belt, Western Dharwar craton

Jayachamarajapura schist belt consists predominantly of ultramafic rocks (mainly komatiite) with minor metasediments of the Sargur group in the Western Dharwar Craton. Sampling of plant species and soil over high magnesia komatiite-bearing areas of this belt has been carried out. Out of the seven plant species analysed, one species, i.e. Vicoa indica has indicated higher Ni values (540-896 ppm), and its potential as a local indicator for Ni has been recognized. The soil samples have yielded distinct signatures of Ni (3126- 12,406 ppm) and Co (382-1071 ppm). The high Mg content of the soil samples indicates that the soil profile is mostly derived from undifferentiated komatiitic bed rocks, and the observed anomalies of Ni and Co warrant a detailed study of this belt for possible Ni-Co mineralization. Copyright of Current Science (00113891) is the property of Indian Academy of Sciences and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract

Anatomy of 2.57-2.52 Ga granitoid plutons in the eastern Dharwar craton, southern India: Implications for magma chamber processes and crustal evolution

We present results of field studies for magmatic processes of 2.57-2.52 Ga calc-alkaline plutonic bodies from three corridors in the eastern Dharwar craton (EDC) corresponding to different crustal levels. At deeper levels plutons are bounded by thick zone of migmatites with numerous melt filled shear bands which often overprinted by incipient charnockite. On the other hand in the mid-to-upper crustal levels plutons show relatively sharp contacts and truncates the adjoining basement. The plutons are composite which comprises voluminous intrusive monzodiorite, quartz-monzonite and porphyritic monzogranite in the central part and minor anatectic granites or diatexite at periphery. Numerous xenoliths, Mafic Magmatic Enclaves (MME), disrupted trains of synplutonic mafic dykes are found in both intrusive and anatectic fades. The plutons show magmatic as well as solid-state plastic fabrics defined by magmatic flow banding and C-S fabrics respectively. Crustal scale shear zone network comprising early melt filled NE trending hot ductile dextral shear bands and slightly later colder NW trending sinistral shear bands defined by rotation of mafic boudins, phenocrysts and C-S fabrics. The internal architecture of plutons is attributed to the crustal scale magma chamber processes where voluminous intrusive magmas emplaced into the crust caused reworking of surrounding basement resulting in production of anatectic magmas. Crystallization of voluminous intrusive magmas in the deep crust probably caused development of fractures to mantle depth causing decompression melting of mantle and resultant mafic magmas penetrated the crystallizing host in magma chambers. Field evidences together with published ages and Nd isotope data reveal a spatial link between late Archaean magmatic accretion, reworking and cratonization

Synplutonic mafic dykes from late archaean granitoids in the Eastern Dharwar Craton, southern India

We present a first overview of the synplutonic mafic dykes (mafic injections) from the 2.56 - 2.52 Ga calc-alkaline to potassic plutons in the Eastern Dharwar Craton (EDC). The host plutons comprise voluminous intrusive facies (dark grey clinopyroxene-amphibole rich monzodiorite and quartz monzonite, pinkish grey porphyritic monzogranite and grey granodiorite) located in the central part of individual pluton, whilst subordinate anatectic facies (light grey and pink granite) confined to the periphery. The enclaves found in the plutons include highly angular screens of xenoliths of the basement, rounded to pillowed mafic magmatic enclaves (MME) and most spectacular synplutonic mafic dykes. The similar textures of MME and adjoining synplutonic mafic dykes together with their spatial association and occasional transition of MME to dismembered synplutonic mafic dykes imply a genetic link between them. The synplutonic dykes occur in varying dimension ranging from a few centimeter width upto 200 meters width and are generally dismembered or disrupted and rarely continuous. Necking of dyke along its length and back veining of more leucocratic variant of the host is common feature. They show lobate as well as sharp contacts with chilled margins suggesting their injection during different stages of crystallization of host plutons in magma chamber. Local interaction, mixing and mingling processes are documented in all the studied crustal corridors in the EDC. The observed mixing, mingling, partial hybridization, MME and emplacement of synplutonic mafic dykes can be explained by four stage processes: (1) Mafic magma injected during very early stage of crystallization of host felsic magma, mixing of mafic and felsic host magma results in hybridization with occasional MME; (2) Mafic magma introduced slightly later, the viscosities of two magmas may be different and permit only mingling where by each component retain their identity; (3) When mafic magma injected into crystallizing granitic host magma with significant crystal content, the mafic magma is channeled into early fractures and form dismembered synplutonic mafic dykes and (4) Mafic injections enter into largely crystallized (>80 crystals) granitic host results in continuous dykes with sharp contacts. The origin of mafic magmas may be related to development of fractures to mantle depth during crystallization of host magmas which results in the decompression melting of mantle source. The resultant hot mafic melts with low viscosity rise rapidly into the crystallizing host magma chamber where they interact depending upon the crystallinity and viscosity of the host. These hot mafic injections locally cause reversal of crystallization of the felsic host and induce melting and resultant melts in turn penetrate the crystallizing mafic body as back veining. Field chronology indicates injection of mafic magmas is synchronous with emplacement of anatectic melts and slightly predates the 2.5 Ga metamorphic event which affected the whole Archaean crust. The injection of mafic magmas into the crystallizing host plutons forms the terminal Archaean magmatic event and spatially associated with reworking and cratonization of Archaean crust in the EDC. © Geol. Soc. India