• Anurag Tripathi

      Articles written in Journal of Earth System Science

    • Lithologic boundaries from gravity and magnetic anomalies over Proterozoic Dalma volcanics

      Pramod Kumar Yadav P K Adhikari Shalivahan Srivastava Ved P Maurya Anurag Tripathi Shailendra Singh Roshan K Singh Ashish K Bage

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      Dalma volcanics (DVs) has intruded the older Singhbhum Group of Metapelites. Despite DVs being rich in mineralisation, its boundaries are not clearly demarcated. Gravity and magnetic surveys have been attempted for mapping the boundaries in DVs. These surveys were made in the northern fringeof the DVs over an area of ∼0.70 km2 along 13 parallel lines at 50 m spacing. The data was acquired at ∼25 m spacing. The surveys were taken for determination of lithological boundaries, depths and nature of causative source using Euler depth solutions and radially averaged power spectrum (RAPS).Residual anomaly maps of gravity and magnetic intensity show the same trend as that of Bouguer gravity anomaly and total magnetic intensity anomaly map indicating towards shallow sources. The magnetic map in general follows the same pattern as that of gravity anomaly maps. The map shows coincident highgravity and magnetic anomalies. These anomalies together with resistivity signatures confirm that the northern fringe of DVs hosts volcanogenic massive sulphide settings. The Euler depth solution delineated the lateral boundaries and nature of the source. It seems that the source is of spherical nature lying withina depth range of 25–40 m. The obtained lithological (vertical) units from RAPS are between Lower DVs, Upper DVs and Singhbhum Group Metapelites at depths of ∼15, ∼25 and ∼40 m, respectively. The metallogeny is associated with the Upper DVs and the corresponding delineated lithological (vertical) unit is indicative of the top of the ore body. Good agreement is observed with the geological successionfrom the drilling data and resistivity data. The findings suggest that the northern fringe of DVs could be a preferred target for drilling.

    • Audio-magnetotelluric investigation of sulfide mineralization in Proterozoic–Archean greenstone belts of Eastern Indian Craton

      Shailendra Singh Ved P Maurya Roshan K Singh Shalivahan Srivastava Anurag Tripathi P K Adhikari

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      Greenstone belts are well known for gold occurrences at different regions of the world. The Dhanjori basin in the eastern Singhbhum region shows major characteristics of a rifted greenstone belt. Initially, we conducted 14 audio-magnetotelluric (AMT) measurements for a profile of ~20 km in the frequencyrange of 1 kHz to 10 Hz over this rather complex geologic environment covering Dhanjori Volcanics (DhV) and Kolhan Group (KG). Subsequently, gravity and magnetic surveys were also conducted over this AMT profile. The purpose of the survey was to identify and map conductive features and to relate them to metallogeny of the area along with the mapping of the basement of Dhanjori basin. The strike analysis showed N30◦W strike for DhV for all the frequencies and for sites over KG domain in the frequency range of 100–10 Hz, but for KG domain, the obtained strike in 1 kHz to 100 Hz is N45◦E. As the combination of transverse electric (TE), transverse magnetic (TM) and tipper (Tzy) can recoverthe electrical signature in complex geological environment, we discuss the conductivity model obtained from TE+TM+Tzy only. The inversion was carried for the regional profile with 14 sites and for 7 sites over KG domain. Conductivity model shows two well resolved conductors, one each in KG and Quartz Pebble Conglomerate Dhanjori (QPCD) domains respectively showing common linked concordant features between these regional and KG profiles. The conductors are interpreted as sulfide mineralization linked with QPCD group of rocks which may host gold. These conductors are also horizontally disposed due to the intrusive younger Mayurbhanj Granite. These intrusives correlate well with the gravitymodeling as well. The thickness of the Dhanjori basin at the central is about 3.0 km, similar to that from gravity modeling. The conductivity model also indicates the presence of shallow conductors, but could not be resolved due to lack of high frequency data. However, the results from the close-by drill site indicate the presence of shallow sulfide mineralization hosting gold. The deep level conductors delineated from AMT studies are associated with gravity high and low magnetic. ICP-AES results of Dhanjori samples show significant concentration of gold ~5.0 g/t, which is of economic consideration. Thus, it can be inferred that the conductors have evidences of sulfide mineralization which host gold.

    • Audio-magnetotelluric investigation of Bakreswar Geothermal Province, Eastern India

      Anurag Tripathi Shalivahan S S Ashish K Bage Shailendra Singh Pramod Kumar Yadav

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      Bakreswar Geothermal Province (BGP) lies over the Archaean gneisses and schists formations. The heat flow and geothermal gradient of BGP are $\sim$230 mW/m$^{2}$ and 95$^{\circ}$C/km, respectively. The Bakreswar hot spring (BHS) (23$^{\circ}$52$^\prime$30$^{\prime\prime}$N; 87$^{\circ}$22$^{\prime}$30$^{\prime\prime}$E) is located over NNW–SSE trending fault. It lies in the eastern flank of BGP and is connected by Sone–Narmada–Tapti (SONATA) and deep-seated ONGC fault systems. Earlier studies indicated that the geothermal fluid flowed towards the same path as that of deep ONGC fault. In view of this, 24 audio-magnetotelluric soundings in the frequency range 10 kHz–10 Hz were conducted at one end of the deep ONGC fault, which is about 20 km northwest of BHS. The data analysis indicated that the data are internally consistent. Analysis of dimensionality and strike are indicative of complex conductivity structures. The 3D inversion of full impedance was performed to obtain a conductivity model of the study area and resulted in root mean square (RMS) for sites close to unity. The results have been discussed by extracting cross-section along 3 to 7 profiles (four of the profiles are perpendicular to strike, whereas three profiles were oblique to the strike direction and are perpendicular to east–west trending ONGC faults). Several low conducting zones are observed in the profiles drawn parallel and perpendicular to the strike. These common conductors show consistent model features but modified resistivity and dimension. Depth slices from 200 to 1400 m suggest that the conductors in the northern part of the study area are deeper than the conductors in the south. The study area is dominated by several shallow conductors which coincide with several fractures present in the study area. The conductors are surrounded by a resistive feature that can act as an impermeable bed and probably restricts the flow of water within the zone. The four major conductors along the oblique profiles (profiles perpendicular to deep ONGC fault) coincide with deep ONGC fault, through which probably the geothermal fluid flows.

    • Steel plant slag dumps: A potential source of groundwater contamination

      UJJAL MAL KALYAN ADHIKARI ANURAG TRIPATHI

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      The metallurgical processes in production of iron and steel from ores produce slags (blast furnace slag and basic oxygen furnace slag), which is usually dumped as solid waste by various steel plants around the world. The present study assessed and estimated the impact of slag dump piling up for the last 60 years on the hydrogeological environment in the industrial city of Durgapur in eastern India. Groundwater survey resembles the unconfined nature of shallow aquifer with elevated concentrations of Ca and Mn in the study area. A factor analysis of groundwater physicochemical parameters suggests natural rockwater interaction dominantly controls groundwater composition in the study area. Saturation index of groundwater samples defines supersaturated conditions of aragonite, calcite, dolomite and rhodochrosite near the slag dump. A leaching study of slag samples demonstrates high concentration of Mn in the leachate of the slag dump. Laboratory-scale soil column experiment supplemented with ${HYDRUS 1D }$model was used to evaluate Mn transport through the vadose zone. ${HYDRUS 1D}$model predicts that Mn attain complete breakthrough for 2m thick soil column in the 40th year. Elevated concentrations of Mn and Ca in groundwater samples near the slag dump and the results of leaching study vouch for progressive contamination of groundwater in the area with time.

      $\bf{Highlights}$

      $\bullet$ Slags, the obvious by-product of iron and steel making, have been piled up in different steel plants. Blast furnace (BF) slag finds almost 100% use but there are issues with complete utilisation of steel slag. As a result of which, slags are still getting dumped as solid waste in different plants. The hillocks of slags are vulnerable to weathering and may generate leachate of adverse chemical quality containing metals, non-metals, heavy metals, which may eventually pollute nearby soil regime and groundwater. A case study has been carried out to assess and quantify the groundwater pollution through slag dump derived leachate.

      $\bullet$ Elevated concentrations of Mn and Ca are present in groundwater at close proximities of slag dump. Weathering reactions of primary and/or secondary carbonate minerals present in slag dump result in Mn and Ca contamination in groundwater.

      $\bullet$ Column experiment and numerical simulation reveal that the concentration of Mn in groundwater near slag dump will be increasing with time as the resilience of vadose zone will be nullified with time due to regular leaching of Mn-laden contaminants from overlying slag dump. The Mn plume forming below the slag dump has been spreading following groundwater biogeochemical dynamics of the local groundwater system. If leaching continues, the expanding plume may affect larger areas involving new localities with greater implications in future.

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