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11.
In the Dharwar Craton, southern India, gold deposits are found mostly along the six arcuate shear zones passing through late Archaean greenstone belts (2.7 Ga). One such shear zone complex extends for about 400 km within and along the Ramagiri–Hungund schist belt. The Penakacherla sector of this shear zone is excellently exposed, enabling a detailed investigation of synorogenic gold mineralisation and its relationship to associated hydrothermal alteration.Metamorphism and deformation under NE–SW compression associated with Archaean subduction processes converted mafic volcanic rocks into amphibolites and intermediate to felsic volcanic rocks into quartz mica schists. Continued compression generated a 50–100-m-wide shear zone complex consisting of mafic phyllonites. Advection of hydrothermal fluids through this shear zone and reaction between fluids and the mafic phyllonites resulted in a silicified, K-metasomatic assemblage mainly consisting of chlorite, amphibole, K-mica, plagioclase, ankerite, quartz, Fe-oxides, pyrite, chalcopyrite and arsenopyrite. Networks of quartz and carbonate veinlets, a few millimeters to a few centimeters thick, formed along the foliation planes giving rise to microscopic alteration envelope, in which individual veinlet systems are merged into one another to form a composite alteration system. Gold is found within these quartz veinlets, mafic phyllonites and at their mutual contacts.Hydrothermal fluids have modified the primary major, minor, trace and LREE compositions of host rocks such that their mutual behaviour became non-systematic. Some HFSE and HREE also show minor mobility but the overall REE pattern generally resembles that of the precursor mafic volcanic rocks. Mass and volume loss/gain by Si and Ca has made significant impact on Al, Ti and Zr abundances, which are generally immobile during hydrothermal alteration. However, element pairs such as Zr–Hf, V–Sc and Nb–Ta maintain primary inter-element ratios, although their absolute abundances are drastically diluted. Similarly, ΣREE in highly silicified and carbonatised samples are reduced, but patterns remain similar to those of relatively least altered mafic phyllonites with (LaN/Yb)N between 1 and 3. In some samples, LREE enrichment is observed elevating in (LaN/Yb)N from 3 to 11. Pathfinder elements and base metals such as As, Cd, Cu, Pb, Zn and Sb have been added along with the Au and Ag.δ13C of carbon varies from −16‰ to −21‰ suggesting a biogenic origin, whereas coexisting pyrite δ34S ranges from 1‰ to 3‰, pointing towards the involvement of magmatic or average crustal sulphur. Overall concentrations of K, Rb, Sr, Ba, Nb, Ta, Ti, Cs, Cr, Co, V, Y and Sc and many of the ratios such as K/Rb, La/Sc, La/Yb indicate that metamorphism, devolatilisation and dehydration of an oceanic subducting slab might have partially contributed the mineralising fluids and generated the alteration assemblage observed in the host rocks. Fluid sources were mantle and greenstone belt dehydration and devolatilisation generating observed compositional and alteration diversity. 相似文献
12.
A comparative study of Pleistocene phosphorites from the continental slope off western India 总被引：1，自引：0，他引：1
V. Purnachandra Rao S. W. A. Naqvi M. Dileep Kumar D. Cardinal A. Michard D. V. Borole E. Jacobs & R. Natarajan 《Sedimentology》2000,47(5):945-960
Two types of phosphorite recovered from the continental slope off western India are described. The first type, phosphorite 1, comprises a hard, grey nodule composed of carbonate fluorapatite (CFA) and calcite as major minerals. The phosphorite consists of light‐brown microcrystalline apatite containing a few skeletal fragments and planktonic foraminifera. Scanning electron microscope (SEM) studies show evidence of dissolution of skeletal calcite and filling of the resulting cavities by phosphate composed of ovoid to rod‐shaped apatite microparticles. Apatite also occurs as coatings on these particles. The P2O5 content of the phosphorite is 29%, and the CO2 content of the CFA is about 4·5%. The rare‐earth element (REE) abundance (ΣREE=2·02 μg g–1) is lower than in other modern phosphorites. The 87Sr/86Sr ratio and ?Nd value of this sample are 0·70921 and –9·9 respectively. The 14C age found through accelerator mass spectrometry (AMS) dating (18 720 ± 120 years BP) is much younger than that determined by the U‐series method (100 ka). The second type, phosphorite 2, comprises a friable, light‐brown nodule consisting of CFA as the only major mineral, with a CO2 content of the CFA of 4·5%. In thin section, the phosphate is light brown and homogeneous, and a few bone fragments are present. The P2O5 content is 33%, and REE contents (ΣREE = 0·18 μg g–1) are lower than in phosphorite 1. The age of phosphorite 2 is >300 ka. Phosphorite 1 appears to have formed during the late Pleistocene through replacement of carbonate by phosphate; phosphorite 2 is also of Pleistocene age but is much older than phosphorite 1. The initial substrate for phosphorite 2 was a fish coprolite, which was subsequently phosphatized during slow sedimentation under low‐energy conditions. Microbial mediation is evident in both phosphorites. The colour, density and P2O5 content of the phosphorites are found to be dependent on the nature of the initial substrates and physico‐chemical conditions during phosphatization. The CO2 content of the CFA is not related to the precursor carbonate phase. The nature of sediments, rates of sedimentation and the time spent undergoing phosphogenesis at the sediment–water interface may control REE concentrations in phosphorites. 相似文献
13.
M. Shakir Khan M. Zubair Deepak Verma A. H. Naqvi Ameer Azam M. K. Bhardwaj 《Environmental Earth Sciences》2011,63(2):279-282
Radon and its progeny have been recognized as one of the major contributors to the natural radiation and health hazards in
the human dwellings. Even lung cancer is expected if it is present in enhanced levels beyond maximum permissible limit. This
paper reports the measurements of indoor radon and its progeny in the urban dwellings of the Etah district of Uttar Pradesh
province in Northern India using the cellulose nitrate (LR-115 type-II) plastic track detectors. It is found that the values
of radon concentration vary from 3.52 to 248.64 Bq m−3 with a standard deviation of 69.19. The values of radon progeny concentration vary from 0.38 to 26.88 mWL with a standard
deviation of 7.48. The effective dose has been calculated and found to vary from 0.05 to 3.76 mSv year−1 with a standard deviation of 1.05. The lifetime fatality risk is found to vary from 0.04 × 10−4 to 2.90 × 10−4. The results have been compared with the results reported in the rural areas of the same district. 相似文献
14.
Hasan Raja Naqvi Javed Mallick Laishram Mirana Devi Masood Ahsan Siddiqui 《Arabian Journal of Geosciences》2013,6(10):4045-4056
The present comparative study is multi-temporal in nature. The Revised Universal Soil Loss Equation (RUSLE), remote sensing, and GIS were used to model the soil loss estimation for soil conservation and vegetation rehabilitation in Nun Nadi watershed for the years 2000 and 2009. The estimated mean soil loss for the year 2000 and 2009 is 3,283.11 and 1,419.39 Mg?ha?1 year?1, respectively. The study finds that about 80 % area has low or least risk of erosion and about 7 % is exposed to high or very high risk which indicates the improvement in terms of soil loss if we compare the data of both the time periods. The findings show that the rainfall, LULC change, and elevation are the main responsible factors for the soil loss in Nun Nadi watershed. Conservation measures have been adopted; however, the problem still remains serious and demands urgent attention. 相似文献
15.
Monsoon-induced coastal upwelling, land run-off, benthic and atmospheric inputs make the western Indian shelf waters biologically productive that is expected to lead to high rates of mineralisation of organic matter (OM) in the sediments. Dissimilatory sulphate reduction (SR) is a major pathway of OM mineralisation in near-shore marine sediments owing to depletion of other energetically more profitable electron acceptors (O2, NO3 ?, Mn and Fe oxides) within few millimetres of the sediment-water interface. We carried out first ever study to quantify SR rates in the inner shelf sediments off Goa (central west coast of India) using the 35S radiotracer technique. The highest rates were recorded in the upper 10 cm of the sediment cores and decreased gradually thereafter below detection. Despite significant SR activity in the upper ～12 to 21 cm at most of the sites, pore water sulphate concentrations generally did not show much variation with depth. The depth integrated SR rate (0.066–0.46 mol m?2 year?1) decreased with increasing water depth. Free sulphide was present in low concentrations (0–3 μM) in pore waters at shallow stations (depth <30 m). However, high build-up of sulphide (100–600 μM) in pore waters was observed at two deeper stations (depths 39 and 48 m), 7–11 cm below the sediment-water interface. The total iron content of the sediment decreased from ～7 to 5 % from the shallowest to the deepest station. The high pyrite content indicates that the shelf sediments act as a sink for sulphide accounting for the low free sulphide levels in pore water. In the moderately organic rich (2–3.5 %) sediments off Goa, the measured SR rates are much lower than those reported from other upwelling areas, especially off Namibia and Peru. The amount of organic carbon remineralised via sulphate reduction was ～0.52 mol m?2 year?1. With an estimated average organic carbon accumulation rate of ～5.6 (±0.5) mol m?2 year?1, it appears that the bulk of organic matter gets preserved in sediments in the study region. 相似文献
16.
Geochemical and petrological studies on serpentinised ultramafics of Kadakola, Mysore State, India, indicate that the Archaean mantle beneath the Indian Shield was of peridotitic in nature. This protomantle in the Archaean period was undifferentiated with higher concentrations of siderophile and transitional elements. 相似文献
17.
The Banded Iron-Formation (BIF) of the Kushtagi schist belt, Dharwar Craton is interbedded with metavolcanics. The oxide fades cherty (Al2O3 < 2%) and shaley (Al2O3 > 2%) BIFs show large-scale variations in their major and trace elements abundance. Cherty Banded Iron-Formation (CBIF) is depleted in Al2O3, TiO2, Zr, Hf and other trace elements like Cr, Ni, Co, Rb, Sr, V, Y and REE in comparison to Shaley Banded Iron-Formation (SBIF). Depleted REE, positive Eu anomalies and the flat to HREE-enriched pattern of CBIF indicate that Fe and SiO2 for these BIFs were added to ambient ocean water by hydrothermal solutions at the AMOR vent sites. It is inferred that the higher amount of hydrothermal fluid flux with a higher exit temperature provided enormous quantities of iron and silica. Fine-grained sedimentation in the basin gave rise to the observed variability in the composition of BIF. During transgression a wave base was raised up, consequently deposition of CBIF became possible, whereas, during the regressive stage, these chemical sediments were buried by and/or mixed with the terrigenous sediments resulting in deposition of SBIF and interbedded shales. Volcaniclastic activity within the basin appears to have contributed significantly to the composition of some SBIF and shales. The hydrothermal exhalative hypothesis combined with the Archaean miniplate model explains most of the chemical features of the BIFs of greenstone belts. 相似文献
18.
S. W. A. Naqvi T. Yoshinari J. A. Brandes A. H. Devol D. A. Jayakumar P. V. Narvekar M. A. Altabet L. A. Codispoti 《Journal of Earth System Science》1998,107(4):367-378
Measurements of15N/14N in dissolved molecular nitrogen (N2), nitrate (NO
3
−
) and nitrous oxide (N2O) and18O/16O in N2O [expressed as δ15N and δ18O, relative to atmospheric N2 and oxygen (O2), respectively] have been made in water column at several locations in the Arabian Sea, a region with one of the thickest
and most intense O2 minima observed in the open ocean. Microbially-mediated reduction of NO
3
−
to N2 (denitrification) in the oxygen minimum zone (OMZ) appears to greatly affect the natural isotopic abundances. The δ15N of NO
3
−
increases from 6‰ in deep waters (2500 m) to 15‰ within the core of the denitrifying layer (250–350 m); the δ15N of N2 concurrently decreases from 0.6‰ to 0.20‰ Values of the isotopic fractionation factor (ε) during denitrification estimated
using simple advection-reaction and diffusion-reaction models are 22‰ and 25‰, respectively. A strong decrease in δ15N of NO
3
−
is observed from ∼ 200m (> 11‰) to 80m (∼ 6‰); this is attributed to the input of isotopically light nitrogen through nitrogen
fixation. Isotopic analysis of N2O reveals extremely large enrichments of both15N and18O within the OMZ, presumably due to the preferential reduction of lighter N2O to N2. However, isotopically light N2O is observed to accumulate in high concentrations above the OMZ indicating that the N2O emitted to the atmosphere from this region cannot be very heavy. The isotope data from the intense upwelling zone off the
southwest coast of India, where some of the highest concentrations of N2O ever found at the sea surface are observed, show moderate depletion of15N, but slight enrichment of18O relative to air. These results suggest that the ocean-atmosphere exchange cannot counter inputs of heavier isotopes (particularly18O) associated with the stratospheric back flux, as proposed by previous workers. This calls for additional sources and/or
sinks of N2O in the atmosphere. Also, the N2O isotope data cannot be explained by production through either nitrification or denitrification, suggesting a possible coupling
between the two processes as an important mechanism of N2O production. 相似文献
19.
Biogeochemical ocean-atmosphere transfers in the Arabian Sea 总被引：2，自引：2，他引：2
S. Wajih A. Naqvi Hermann W. Bange Stuart W. Gibb Catherine Goyet Angela D. Hatton Robert C. Upstill-Goddard 《Progress in Oceanography》2005,65(2-4):116
Transfers of some important biogenic atmospheric constituents, carbon dioxide (CO2), methane (CH4), molecular nitrogen (N2), nitrous oxide (N2O), nitrate , ammonia (NH3), methylamines (MAs) and dimethylsulphide (DMS), across the air–sea interface are investigated using published data generated mostly during the Arabian Sea Process Study (1992–1997) of the Joint Global Ocean Flux Study (JGOFS). The most important contribution of the region to biogeochemical fluxes is through the production of N2 and N2O facilitated by an acute, mid-water deficiency of dissolved oxygen (O2); emissions of these gases to the atmosphere from the Arabian Sea are globally significant. For the other constituents, especially CO2, even though the surface concentrations and atmospheric fluxes exhibit extremely large variations both in space and time, arising from the unique physical forcing and associated biogeochemical environment, the overall significance in terms of their global fluxes is not much because of the relatively small area of the Arabian Sea. Distribution and air–sea exchanges of some of these constituents are likely to be greatly influenced by alterations of the subsurface O2 field forced by human-induced eutrophication and/or modifications to the regional hydrography. 相似文献
20.
Siby Kurian Rajesh Agnihotri D. V. Borole S. Wajih A. Naqvi Ana M. Ferreira Carlos Vale 《第四纪科学杂志》2009,24(2):109-116
Using multiple geochemical proxies including specific biomarkers (dinosterol, phytol, stigmasterol and β‐sitosterol) measured in a high‐sedimentation rate core collected from the inner shelf (depth ～45 m) off Goa (India), we reconstruct surface productivity, which is mainly controlled by the monsoon upwelling in this region, during the last ca. 700 a. Surface productivity appears to have varied in tandem with the Konkan–Goa rainfall and sunspot activity during the instrumental period (last 250 a). The productivity proxies also covary with the total solar irradiance reconstructed for the period beyond the instrumental era, within the considerable uncertainty of the age model. This suggests that solar forcing may control coastal upwelling intensity and biological productivity in the eastern Arabian Sea on decadal to centennial timescales. During the late Anthropocene (last ca. 50 a), steep increases in all four biomarkers indicate greatly enhanced productivity in response to high solar irradiance as well as anthropogenic inputs of nutrients. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献