D.V. Subba Rao

Geology, geochemistry and tectonic setting of Archaean greywackes from Karnataka nucleus, India

More than 80% of the area covered by the Chitradurga and Shimoga schist belts (3.0-2.6 Ga) of Karnataka nucleus, India, is made up of greywackes. Feldspathic, lithic and quartzose greywackes are deposited, respectively, along the western, northern and the eastern margin of the Chitradurga schist belt. Greywackes of the Shimoga schist belt show a large compositional scatter between feldspathic and lithic types. Compositional differences indicate variable provenance for the different localities. Tonalite-trondhjemite gneisses (TTG), mafic-ultramafic schists, basic-intermediate-acid volcanic rocks, sedimentary rocks like banded iron-formation, quartzites and cherts, and K-granites in minor quantity, were exposed in the source area. Volcanic debris was also contributed from contemporaneous volcanism in the basin. Greywackes in this region are invariably interbedded with fine-grained, biotite-rich phyllites and schists. Marked compositional differences are observed between the greywackes and the interbedded phyllites. The greywackes have K2O/Na2O < 1, whereas phyllites show large variation in K2O/Na2O ratios ranging from 1.5 to 15.0. High Cr-Ni content of the greywackes and phyllites of the Chitradurga belt are indicative of an important contribution from ultramafic rocks. Light REE enrichment and negative Eu anomalies are particularly pronounced in the mudpacks of the greywackes represented by phyllite. The REE and other chemical data, such as La/Th, Ti/Zr and K2/MgO ratios, indicate that these greywackes were supplied from a juvenile continental nucleus and were laid down on a simatic and converging active continental margin. An Archaean miniplate model with low-angle subduction and sedimentary basins of moderate size is proposed to explain the geochemical and tectonic features of the schist belts.

Geology and geochemistry of arenite–quartzwacke from the Late Archaean Sandur schist belt—implications for provenance and accretion processes

Abstract Detailed geological, petrological and geochemical studies have been carried out on an arenite–quartzwacke suite of rocks constituting a part of the Late Archaean Sandur schist belt in Dharwar craton, southern India for understanding the nature of provenance for these sedimentary rocks. The arenite–quartzwacke consists of rounded to sub-rounded and angular fragments of monocrystalline–polycrystalline quartz, quartzite and chert embedded in a fine-grained matrix of quartz and sericite. While arenites are more siliceous (SiO 2 , 80–92 wt.%), the quartzwacke have relatively lower silica content (ca. 69–78 wt.%). The arenites and quartzwackes have CIA values ranging from 76 to 96 which suggest intense chemical weathering. This is further corroborated by the positive correlation between Al 2 O 3 and TiO 2 in both these rock types. The ACNK modeling of arenites and quartzwackes show evidence for addition of K 2 O during later metasomatic alteration. In the ACNKFM ternary diagram all the samples plot along a mixing line between chlorite and sericite indicating alteration during K-metasomatism and the presence of mafic rocks in the source. The high concentration of HFSE such as Zr, Hf, Nb and Ta and the trace element ratios Th/Sc, La/Sc, Th/U and Ce/Th in the arenite–quartzwacke indicate a mixed provenance. The rare earth element modeling of quartzwackes considering tonalite, granite and amphibolite end members in the provenance suggests equal proportions of mafic and felsic end members. A composition comprising of 25% tonalite+25% granite+50% amphibolite in the provenance appears to match with the observed range of REE patterns of quartzwackes. The presence of higher proportions of granite in the provenance is evidenced by the large negative Eu anomalies in these sediments. Field evidence and structural discordance suggest that the arenite–quartzwacke suite is an allochthonous part of the Sandur schist belt.

Archaean stromatolites from the Chitradurga schist belt, Dharwar Craton, South India

Abstract Stromatolites have been found in the cherty dolomite member of the Vanivilas Formation of the Chitradurga Group belonging to > 2600 Ma old Archaean Dharwar Supergroup, South India. They occur in the Bhimasamudra, Marikanve and Dodguni areas of the Chitradurga schist belt. The stromatolites of the Bhimasamudra and Marikanve areas are described here for the first time. The stromatolites of Bhimasamudra consist of cylindrical and terete columns, as well as pseudocolumns with concave tops developed on a wavy base. Coalescence of columns and γ-type branching have been noticed. The Marikanve stromatolites are stratiform, often grading into pseudocolumnar and dome-shaped types. Naked stromatolite columns showing β-type and markedly divergent branching are found in the Dodguni area. The Bhimasamudra and Dodguni stromatolites probably developed in an intertidal environment and the Marikanve ones in a subtidal environment.

Integrated geophysical and geological studies for mineral prospecting in Betul-Chhindwara belt (BCB), Central India

Integrated study combining high resolution electrical resistivity tomography and time domain induced polarization was carried out in Betul-Chhindwara belt Madhya Pradesh, Central India in order to evaluate and delineate the polymetallic sulphide mineralization, its nature, type of deposit and depth. On interpretation of the models results clear cut anomalies revealed showing chargeability ∼2 to 54 mV/V up to a maximum depth of 131m. This range of chargeability signify signature of metallic conductor. Nevertheless the basement rock is clearly mapped, showing substantial resistivity contrast. In addition detailed analysis of the integrated results from geology, geochemistry and Scanning Electron Microscope–Energy Dispersive X-ray Spectroscopy resulted from in situ rock samples shows good correlation with resistivity and IP results. This integrated study confirms the presence of conducting sulphide mineral ore body and the results and findings need test drilling at the geophysical anomalous site(s) to confirm the depth persistence and evaluate the metallic conductor.

First finding of native gold from the chromitites of the Mesoarchaean Tagadur mines, Nuggihalli Schist Belt, Dharwar Craton, south India

Occurrence of native gold in ultramafic rocks is rather rare. Such occurrences serve as snapshots of the underlying processes leading to their formation and modifications thereafter. Presence of native gold as flakes were earlier reported from the chromites of rock massifs of Kraka, southern Urals, Russia (Kovalev et al. 2007). Gold spherules also occur as tiny inclusions (5–30 μm) in olivine, pyrrohotite and magnetite grains from the ultrabasic rocks of Loveld deposit, South Africa (Rozhkov, 1967). Here we report, probably for the first time, occurrence of native gold from the chromitites of the Mesoarchaean Tagadur Mines of the Nuggihalli Schist Belt (NSB), Dharwar Craton, south India. Geological Survey of India (2006) has already reported significant gold content from the bulk ultramafic rocks of Kempinakote area far south of the current study area. A short discussion on the preliminary petrographic features of the gold grains and its significance is presented.

Gold - sulphide mineralization in ultramafic-mafic-granite complex of Jashpur, Bastar craton, central India: Evidences from geophysical studies

Time Domain Induced Polarization (TDIP) investigation in conjunction with high resolution electrical resistivity tomography surveys carried out at seven sites along a 5.2 km line in Jashpur and Raigarh districts of Chhattisgarh State yielded promising results leading to the demarcation of mineralized zones. These sites are confined to ultramafic-mafic-felsic intrusive complexes representing gabbro-pyroxenite-granite sequences characterized by gold - sulphide mineralization. Two dimensional (2D) geophysical datasets yielded interesting results and a good correlation is found between 2D inverted resistivity tomography and time domain induced polarization (IP) models. Both high resistivity-high chargeability as well as low resistivity-high chargeability conditions have been inferred from these datasets. This observation is consistent with conductive metallic mineralization. Interpretation of the Pharsabahar-Pandripani site, reveals a chargeability magnitude of 9-14 mV/V, which represents a strong metallic conductor associated with gold-sulphide ore body. The results at Samarkachar site are equally significant, showing a good correlation with the presence of a low resistivity and high chargeability zone. Hence, the present study suggests a new scope for application of resistivity and IP tomography surveys for mineral prospecting in favourable geological settings.

Major, trace and REE characteristics of the high grade gold geochemical reference material from Middle Reef, Hutti gold mines, Karnataka

The geological, petrological, mineralogical and geochemical characteristics of the Hutti high-grade gold reference material representing the composite sample of sheared gold-sulphide bearing smoky-grey quartz vein hosted in felsic volcanics (Middle Reef) and hydrothermal altered wall rocks are investigated in the present work. Petrological studies on these rocks reveal the presence of deformed quartz ribbons, plagioclase, chlorite, biotite, stipnomelane, actinolite and abundant native gold, arsenopyrite, pyrite and minor scheelite, monazite and xenotime. The major, trace and REE geochemistry of the host barren rhyolites are interpreted as low Yb adakitic variety which show high Ba, Cr, V and Ni concentrations. REE patterns of the gold-sulphide bearing quartz reef is characterized by a pronounced positive Eu anomaly, LREE depletion and nearly flat HREE with a low total REE which indicate possible contribution from deep-seated fluids to the ore-forming process. These patterns also suggest a source which is enriched in Eu and depleted in LREE either due to crystal fractionation or the formation of scheelite. The associated hydrothermally altered wall rocks from proximal zone, exhibit flat REE patterns coupled with high concentrations of Cu, Zn, Cr, V, Sc and Ni indicating their derivation from secondary epigenetic fluids responsible for metasomatic alteration. The abundant barren schistose amphibolites from distal zone represent enriched type of island arc basalts (IAB). The trace, REE-HFSE data on the rhyolite (adakite) and IAB of Middle Reef in the Hutti schist belt substantiates the inference that Archaean subduction processes in an island arc/convergent margin setting are responsible for adakitic magmatism.

Geochemistry of the archaean greywackes from the Northwestern part of Chitradurga schist belt, dharwar craton, South India — Evidence for granitoid upper crut in the Archaean

Turbidite greywackes of the northwestern part of the Chitradurga Schist Belt constitute a part of >2.6 Ga Cbitradurga Group of Dharwar Supergroup. They consist of a detrital assemblage of mono- and poly-crystalline quartz, microcline and plagioclase feldspar. and volcanic rock fragments. Quartz content and Na 2 O/K 2 O ratios show their quartz intermediate character. In this respect and also in their high FeO(t)+ MgO content they are similar to other Archaean greywackes. The TiO 2 content. K 2 O/Na 2 O, Al 2 O 3 /CaO+Na 2 O and Al 2 O 3 /SiO 2 ratios plotted against Fe 2 O 3 (t)+MgO, although assign an island arc environment for these greywackes, do not unequivocally discriminate between continental and oceanic island arc setting. The Chitradurga greywackes are highly enriched in Zr, Cr and Ni indicating a mixed felsic and mafic source. The contribution from contemporary volcanism in the basin could be significant. The QFL proportions indicate a dissected arc and/or recycled orogenic nature of the provenance. The chemical index of alteration varying from 58 to 63 and Al 2 O 3 /Na 2 O ratio less than 6, indicate relatively unweathered nature of source rock and chemical immaturity of the sediment respectively. The REE patterns show wen-defined negative europium anomalies which reflect granitoid upper crust in the provenance during the Archaean.