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31.
The present study is confined to the northern part of the Khetri copper belt that extends for about 100 km in northern Rajasthan. Mineralization is more or less strata-bound and is confined to the garnetiferous chlorite schist and banded amphibolite quartzite, occurring towards the middle of the Proterozoic Delhi Supergroup. Preserved sedimentary features and re-estimation of the composition of the pre-metamorphic rocks suggest that the latter were deposited in shallow marine environment characterized by tidal activity. Cordierite-orthoamphibole-cummingtonite rock occurring in the neighbourhood of the ores is discussed, and is suggested to be isochemically metamorphosed sediment. The rocks together with the ores were deformed in two phases and metamorphosed in two progressive and one retrogressive events of metamorphism. Study of the host rocks suggests that the maximum temperature and pressure attained during metamorphism are respectively 550–600°C and < 5.5 kb. Principal ore minerals in Madan Kudan are chalcopyrite, pyrrhotite, pyrite and locally magnetite. In Kolihan these are chalcophyrite, pyrrhotite and cubanite. Subordinate phases are sphalerite, ilmenite, arsenopyrite, mackinawite, molybdenite, cobaltite and pentlandite. The last two are very rare. Gangue minerals comprise quartz, chlorite, garnet, amphiboles, biotite, scapolite, plagioclase and graphite. The ores are metamorphosed at temperatures > 491°C. Sulfide assemblages are explained in terms of fS
2 during metamorphism. Co-folding of the ore zone with the host rocks, confinement of the ores to the carbonaceous pelites or semi-pelitic rocks, strata-bound and locally even stratiform nature of the orebodies, lack of finite wall rock alteration, metamorphism of the ores in the thermal range similar to that for the host rocks, absence of spatial and temporal relationship with the granitic rocks of the region led the authors to conclude that the entire mineralization was originally sedimentary-diagenetic. Any loss of primitive features and development of incongruency are due to subsequent deformation and metamorphism to which the ores and their hosts were together subjected. 相似文献
32.
Soumen Sarkar 《Sedimentary Geology》1981,29(4):241-282
The late Proterozoic, intertidal Lower Bhander Sandstone (Bhander Group, Vindhyan Supergroup) developed around Maihar, central India, is characterized by alternations of sandstone and shale in different scales and shows profuse ripple marks of widely varying morphology. Visual examination of their external morphology led to the identification of wave ripple, current ripple and others of intermediate character.Standard deviation and average of ripple spacing and height of symmetrical and assymmetrical ripples show genetically significant differences analogous to those obtained by Harms (1969) for wave- and current-generated ripples. Different dimensionless parameters, e.g., R.I., R.S.I., S.I., etc., processed separately for the two types of ripples, show a wide variation in their range which encompasses the total spectrum of values stipulated for wave and current ripples. However, the frequency of any particular genetic type of ripple differs widely when analysed in terms of different dimensionless parameters. Several scatter plots, prepared after Tanner (1967) also indicate the presence of various genetic types of ripples, but there are ripples for which results remain inconclusive. Furthermore, scatter plots involving the vertical form index (ripple length/ripple height) and median grain size of a few asymmetrical ripples, following Reineck and Wunderlich (1968a), led to the discrimination between current ripple and wave ripple and the distinction is grossly consistent with the results obtained by other means.Ripple spacing, ripple index and grain-size data of a few representative samples of ripples of possible wave origin, analysed after Tanner (1971) and Allen (1979) indicate that they were generated in a shallow basin with restricted fetch.Internally, the ripples, irrespective of their symmetry, are often characterized by unidirectional bundles of foresets consisting of rhythmically alternating sand and mud laminae. The sets of cross-laminae may be complexly organized with planar or curved erosional boundaries separating them. In many instances internal structures typical of wave ripples are also noted.Inconsistencies, however, exist between the results obtained by application of different criteria in interpretation of these ripple marks. The limitations in applicability of 相似文献
33.
A. P. Mitra Y. V. Somayajulu S. P. Singal S. C. Majumdar T. R. Tyagi B. M. Reddy S. K. Agarwal B. S. Gera A. B. Ghosh S. K. Sarkar 《Boundary-Layer Meteorology》1977,11(1):103-116
A number of troposphere-monitoring experiments were pressed into service during a large tropospheric disturbance which lasted for several days in December 1974. The experiments operated intensively included a monostatic Sodar system, a satellite radio beacon receiver at very low elevation angles, Meteorological Radiosondes, a 250-kW weather radar, a microwave LOS link and a microbarograph. This multi-pronged campaign yielded very useful results and the correspondences between various observations were striking. The Sodar and microwave link observations were visibly affected only when N reached a value of about –70 units; however, the satellite beacon observations showed anomalous behaviour even for lower N values. Rather spectacular coincidences occurred on one night: the large fading (up to 10 db) in the LOS link; highly disturbed conditions displayed in the entire Sodar range; the largest tropospheric effect on the satellite beacon system; and the most active superrefraction of the radar echoes. It was also noticed that these effects are obvious only during nights, while they remain masked or dampened in the sun-lit hours. 相似文献
34.
The MHD wave instabilities due to non-uniform magnetic field and non-homogeneity of density have been studied. The reference
(coordinate) system considered here is cylindrical type. The General Dispersion Relation (GDR) for the wave propagation in
a gravitating but non-relativistic region has been derived. Similar to common knowledge, the said non-uniformities have been
found to be responsible for the instability of the system. But interestingly many instability factors are produced due to
presence of two types of non-uniformities simultaneously. This theory may add more clues for the event of instabilities, formation
of hot plasma-bed in Galactic Central Region, and mass out-flow from there. Many conditions for instabilities could be obtained
from GDR deduced here. However, a few conditions for critical wavelength of the MHD wave have been obtained in terms of system
parameters (like gradient of magnetic field and rotation).
This theory, in turn, may be helpful for the better understanding of the Explosion Theory of formation of outer structure
of Galaxies like ours. 相似文献
35.
We discuss the prospects of using the redshifted 21-cm emission from neutral hydrogen in the post-reionization epoch to study our Universe. The main aim of the article is to highlight the efforts of Indian scientists in this area with the SKA in mind. It turns out that the intensity mapping surveys from SKA can be instrumental in obtaining tighter constraints on the dark energy models. Cross-correlation of the HI intensity maps with the Ly α forest data can also be useful in measuring the BAO scale. 相似文献
36.
The Kabanga Ni sulfide deposit, Tanzania: I. Geology, petrography, silicate rock geochemistry, and sulfur and oxygen isotopes 总被引:1,自引:0,他引:1
Wolfgang D. Maier Sarah-Jane Barnes Arindam Sarkar Ed Ripley Chusi Li Tim Livesey 《Mineralium Deposita》2010,45(5):419-441
The Kabanga Ni sulfide deposit represents one of the most significant Ni sulfide discoveries of the last two decades, with
current indicated mineral resources of 23.23 Mt at 2.64% Ni and inferred mineral resources of 28.5 Mt at 2.7% Ni (Nov. 2008).
The sulfides are hosted by a suite of ∼1.4 Ga ultramafic–mafic, sill-like, and chonolithic intrusions that form part of the
approximately 500 km long Kabanga–Musongati–Kapalagulu igneous belt in Tanzania and Burundi. The igneous bodies are up to
about 1 km thick and 4 km long. They crystallized from several compositionally distinct magma pulses emplaced into sulfide-bearing
pelitic schists. The first magma was a siliceous high-magnesium basalt (approximately 13.3% MgO) that formed a network of
fine-grained acicular-textured gabbronoritic and orthopyroxenitic sills (Mg# opx 78–88, An plag 45–88). The magma was highly
enriched in incompatible trace elements (LILE, LREE) and had pronounced negative Nb and Ta anomalies and heavy O isotopic
signatures (δ18O +6 to +8). These compositional features are consistent with about 20% contamination of primitive picrite with the sulfidic
pelitic schists. Subsequent magma pulses were more magnesian (approximately 14–15% MgO) and less contaminated (e.g., δ18O +5.1 to +6.6). They injected into the earlier sills, resulting in the formation of medium-grained harzburgites, olivine
orthopyroxenites and orthopyroxenites (Fo 83–89, Mg# opx 86–89), and magmatic breccias consisting of gabbronorite–orthopyroxenite fragments within an olivine-rich matrix. All intrusions
in the Kabanga area contain abundant sulfides (pyrrhotite, pentlandite, and minor chalcopyrite and pyrite). In the lower portions
and the immediate footwall of two of the intrusions, namely Kabanga North and Kabanga Main, there occur numerous layers, lenses,
and veins of massive Ni sulfides reaching a thickness of several meters. The largest amount of high grade, massive sulfide
occurs in the smallest intrusion (Kabanga North). The sulfides have heavy S isotopic signatures (δ34S wr = +10 to +24) that broadly overlap with those of the country rock sulfides, consistent with significant assimilation
of external sulfur from the Karagwe–Ankolean sedimentary sequence. However, based partly on the relatively homogenous distribution
of disseminated sulfides in many of the intrusive rocks, we propose that the Kabanga magmas reached sulfide saturation prior
to final emplacement, in staging chambers or feeder conduits, followed by entrainment of the sulfides during continued magma
ascent. Oxygen isotope data indicate that the mode of sulfide assimilation changed with time. The heavy δ18O ratios of the early magmas are consistent with ingestion of the sedimentary country rocks in bulk. The relatively light
δ18O ratios of the later magmas indicate less bulk assimilation of the country rocks, but in addition the magmas selectively
assimilated additional S, possibly through devolatization of the country rocks or through cannibalization of magmatic sulfides
deposited in the conduits by preceding magma surges. The intrusions were tilted at ca. 1.37 Ga, during the Kibaran orogeny
and associated synkinematic granite plutonism. This caused solid-state mobilization of ductile sulfides into shear zones,
notably along the base of the intrusions where sulfide-hornfels breccias and lenses and layers of massive sulfides may reach
a thickness of >10 m and can extend for several 10 s to >100 m away from the intrusions. These horizons represent an important
exploration target for additional nickel sulfide deposits. 相似文献
37.
An experimental investigation to characterise soil macroporosity under different land use and land covers of northeast India 总被引:2,自引:0,他引:2
Sangeeta Shougrakpam Rupak Sarkar Subashisa Dutta 《Journal of Earth System Science》2010,119(5):655-674
Saturated macropore flow is the dominant hydrological process in tropical and subtropical hilly watersheds of northeast India.
The process of infiltration into saturated macroporous soils is primarily controlled by size, network, density, connectivity,
saturation of surrounding soil matrix, and depthwise distribution of macropores. To understand the effects of local land use,
land cover and management practices on soil macroporosity, colour dye infiltration experiments were conducted with ten soil
columns (25 × 25 × 50 cm) collected from different watersheds of the region under similar soil and agro-climatic zones. The
sampling sites included two undisturbed forested hillslopes, two conventionally cultivated paddy fields, two forest lands
abandoned after Jhum cultivation, and two paddy fields, one pineapple plot and one banana plot presently under active cultivation
stage of the Jhum cycle. Digital image analyses of the obtained dye patterns showed that the infiltration patterns differed
significantly for different sites with varying land use, land cover, and cultivation practices. Undisturbed forest soils showed
high degree of soil macroporosity throughout the soil profile, paddy fields revealed sealing of macropores at the topsoil
due to hard pan formation, and Jhum cultivated plots showed disconnected subsoil macropores. The important parameters related
to soil macropores such as maximum and average size of macropores, number of active macropores, and depthwise distribution
of macropores were estimated to characterise the soil macroporosity for the sites. These experimentally derived quantitative
data of soil macroporosity can have wide range of applications in the region such as water quality monitoring and groundwater
pollution assessment due to preferential leaching of solutes and pesticides, study of soil structural properties and infiltration
behaviour of soils, investigation of flash floods in rivers, and hydrological modelling of the watersheds. 相似文献
38.
The present paper deals with the first record of a palynofloral assemblage recovered from the Fulra Limestone Formation exposed at the confluence of Fulra and Panandhro Nalas in the west of Babia hill, Kachchh basin, Gujarat. The recovered palynoflora consists of dinoflagellate cysts, fungal spores and ascostromata, pteridophyte spores, gymnosperm and angiosperm pollen. Some of the important constituents of the palynofloral assemblage are: Lygodiumsporites, Polypodiaceasporites, Polypodiisporites, Margocolporites, Tricolporopilites, Tricolporopollis, Graminidites, Aplanosporites, Phragmothyrites, Spiniferites, Operculodinium and Achomosphaera. Palynological data suggest that the Fulra Limestone Formation was mostly laid down in a shallow marine environment under a warm and humid tropical climate. Abundance of terrestrial palynofossils in some of the samples clearly points towards the relative proximity of the shore. A late middle Eocene age has been assigned for the Fulra Limestone Formation on the basis of recorded palynofossils. 相似文献
39.
Sanjay Sarkar 《Astrophysics and Space Science》2014,349(2):985-993
The paper deals with a spatially homogeneous and anisotropic Bianchi type-I universe filled with two minimally interacting fluids; matter and holographic dark energy components. The nature of the holographic dark energy for Bianchi type-I space time is discussed. An exact solution to Einstein’s field equations in Bianchi type-I line element is obtained using the assumption of linearly varying deceleration parameter. Under the suitable condition, it is observed that the anisotropy parameter of the universe approaches to zero for large cosmic time and the coincidence parameter increases with increasing time. We established a correspondence between the holographic dark energy models with the generalised Chaplygin gas dark energy model. We also reconstructed the potential and dynamics of the scalar field which describes the Chaplygin cosmology. Solution of the field equations shows that a big rip type future singularity will occur for this model. It has been observed that the solutions are compatible with the results of recent observations. 相似文献
40.
S. S. De B. K. Sarkar Bithika Ghosh Manasi Mal B. Ghosh S. Bandyopadhyay S. K. Adhikari A. C. Sen 《Astrophysics and Space Science》1991,184(2):281-285
Coupled equations are derived for different ionospheric parameters through susceptibility dyadic appropriate to the medium. The nature of dispersion within the medium is studied. 相似文献