Enrichment of PGE through interaction of evolved boninitic magmas with early formed cumulates in a gabbro–breccia zone of the Mesoarchean Nuasahi massif (eastern India)

The Mesoarchean Nuasahi chromite deposits of the Singhbhum Craton in eastern India consist of a lower chromite-bearing ultramafic unit and an upper magnetite-bearing gabbroic unit. The ultramafic unit is a ∼5 km long and ∼400 m wide linear belt trending NNW-SSE with a general north-easterly dip. The chromitite ore bodies are hosted in the dunite that is flanked by the orthopyroxenite. The rocks of the ultramafic unit including the chromitite crystallized from a primitive boninitic magma, whereas the gabbro unit formed from an evolved boninitic magma. A shear zone (10–75 m wide) is present at the upper contact of the ultramafic unit. This shear zone consists of a breccia comprising millimeter- to meter-sized fragments of chromitite and serpentinized rocks of the ultramafic unit enclosed in a pegmatitic and hybridized gabbroic matrix. The shear zone was formed late synkinematically with respect to the main gabbroic intrusion and intruded by a hydrous mafic magma comagmatic with the evolved boninitic magma that formed the gabbro unit. Both sulfide-free and sulfide-bearing zones with platinum group element (PGE) enrichment are present in the breccia zone. The PGE mineralogy in sulfide-rich assemblages is dominated by minerals containing Pd, Pt, Sb, Bi, Te, S, and/or As. Samples from the gabbro unit and the breccia zone have total PGE concentrations ranging from 3 to 116 ppb and 258 to 24,100 ppb, respectively. The sulfide-rich assemblages of the breccia zone are Pd-rich and have Pd/Ir ratios of 13–1,750 and Pd/Pt ratios of 1–73. The PGE-enriched sulfide-bearing assemblages of the breccia zone are characterized by (1) extensive development of secondary hydrous minerals in the altered parts of fragments and in the matrix of the breccia, (2) coarsening of grain size in the altered parts of the chromitite fragments, and (3) extensive alteration of primary chromite to more Fe-rich chromite with inclusions of chlorite, rutile, ilmenite, magnetite, chalcopyrite, and PGE-bearing chalcogenides. Unaltered parts of the massive chromitite fragments from the breccia zone show PGE ratios (Pd/Ir = 2.5) similar to massive chromitite (Pd/Ir = 0.4–6.6) of the ultramafic unit. The Ir-group PGE (IPGE: Ir, Os, Ru) of the sulfide-rich breccia assemblages were contributed from the ultramafic–chromitite breccia. Samples of the gabbro unit have fractionated primitive mantle-normalized patterns, IPGE depletion (Pd/Ir = 24–1,227) and Ni-depletion due to early removal of olivine and chromite from the primitive boninitic magma that formed the ultramafic unit. Samples of the gabbro and the breccia zone have negative Nb, Th, Zr, and Hf anomalies, indicating derivation from a depleted mantle source. The Cu/Pd ratios of the PGE-mineralized samples of the breccia zone (2.0 × 10³–3.2 × 10³) are lower than mantle (6.2 × 10³) suggesting that the parental boninitic magma (Archean high-Mg lava: Cu/Pd ratio ∼1.3 × 10³; komatiite: Cu/Pd ratio ∼8 × 10³) was sulfur-undersaturated. Samples of the ultramafic unit, gabbro and the mineralized breccia zone, have a narrow range of incompatible trace element ratios indicating a cogenetic relationship. The ultramafic rocks and the gabbros have relatively constant subchondritic Nb/Ta ratios (ultramafic rocks: Nb/Ta = 4.1–8.8; gabbro unit: Nb/Ta = 11.5–13.2), whereas samples of the breccia zone are characterized by highly variable Nb/Ta ratios (Nb/Ta = 2.5–16.6) and show evidence of metasomatism. The enrichment of light rare earth element and mobile incompatible elements in the mineralized samples provides supporting evidence for metasomatism. The interaction of the ultramafic fragments with the evolved fluid-rich mafic magma was key to the formation of the PGE mineralization in the Nuasahi massif.     

Rain height in relation to 0°C isotherm height for satellite communication over the Indian Subcontinent

Summary ¶The 0°C isotherm height, a parameter needed for the estimation of attenuation of microwave and millimetre wave for earth-space communication, has been estimated for different stations spread over India. The variations of 0°C isotherm height for different seasons over these stations are presented. Attenuations of radio wave due to rain at frequencies 10GHz and above have also been estimated for few stations using the 0°C isotherm height so derived. The results are useful for radio systems designers.     

Use of hydrochemical parameters for the identification of fresh groundwater resources,Potharlanka Island,India

A hydrochemical study has been carried out on the fresh groundwater resources of Potharlanka, Krishna Delta, India. Groundwater samples were collected at 58 sites and analyzed in June and December 2001. The groundwater is mildly alkaline with a pH of 7.2–8.2, electrical conductivity (EC) varies from 645–4,700 µS/cm in June 2001 (pre-monsoon) and from 605–5,770 µS/cm in December 2001 (post-monsoon). More than 75% of the samples have >1000 mg/l TDS which is higher than the maximum permissible limit for potable water. Na and Cl are the dominating cations and these are directly proportional to TDS. Extremely low HCO ₃/Cl and variable high Mg/Ca (molar ratios) indicated the transformation of the fresh groundwater aquifer systems to saline. Groundwater of this island is classified as Na–Cl, Na–Ca–Cl–HCO ₃, Na–Mg–Cl–SO ₄ and mixed types. A high percentage of mixed water types indicates the possibility of simultaneous fresh groundwater dilution activity along with a seawater ingression/intrusion process. Low rainfall and excessive withdrawal of groundwater has caused the increase of saline water intrusion.     

Origin of the UG2 chromitite layer, Bushveld Complex

Chromitite layers are common in large mafic layered intrusions.A widely accepted hypothesis holds that the chromitites formedas a consequence of injection and mixing of a chemically relativelyprimitive magma into a chamber occupied by more evolved magma.This forces supersaturation of the mixture in chromite, whichupon crystallization accumulates on the magma chamber floorto form a nearly monomineralic layer. To evaluate this and othergenetic hypotheses to explain the chromitite layers of the BushveldComplex, we have conducted a detailed study of the silicate-richlayers immediately above and below the UG2 chromitite and anotherchromitite layer lower in the stratigraphic section, at thetop of the Lower Critical Zone. The UG2 chromitite is well knownbecause it is enriched in the platinum-group elements and extendsfor nearly the entire 400 km strike length of the eastern andwestern limbs of the Bushveld Complex. Where we have studiedthe sequence in the central sector of the eastern Bushveld,the UG2 chromitite is embedded in a massive, 25 m thick plagioclasepyroxenite consisting of 60–70 vol. % granular (cumulus)orthopyroxene with interstitial plagioclase, clinopyroxene,and accessory phases. Throughout the entire pyroxenite layerorthopyroxene exhibits no stratigraphic variations in majoror minor elements (Mg-number = 79·3–81·1).However, the 6 m of pyroxenite below the chromitite (footwallpyroxenite) is petrographically distinct from the 17 m of hangingwall pyroxenite. Among the differences are (1) phlogopite, K-feldspar,and quartz are ubiquitous and locally abundant in the footwallpyroxenite but generally absent in the hanging wall pyroxenite,and (2) plagioclase in the footwall pyroxenite is distinctlymore sodic and potassic than that in the hanging wall pyroxenite(An_45–60 vs An_70–75). The Lower Critical Zone chromititeis also hosted by orthopyroxenite, but in this case the rocksabove and below the chromitite are texturally and compositionallyidentical. For the UG2, we interpret the interstitial assemblageof the footwall pyroxenite to represent either interstitialmelt that formed in situ by fractional crystallization or chemicallyevolved melt that infiltrated from below. In either case, themelt was trapped in the footwall pyroxenite because the overlyingUG2 chromitite was less permeable. If this interpretation iscorrect, the footwall and hanging wall pyroxenites were essentiallyidentical when they initially formed. However, all the modelsof chromitite formation that call on mixing of magmas of differentcompositions or on other processes that result in changes inthe chemical or physical conditions attendant on the magma predictthat the rocks immediately above and below the chromitite layersshould be different. This leads us to propose that the Bushveldchromitites formed by injection of new batches of magma witha composition similar to the resident magma but carrying a suspendedload of chromite crystals. The model is supported by the commonobservation of phenocrysts, including those of chromite, inlavas and hypabyssal rocks, and by chromite abundances in lavasand peridotite sills associated with the Bushveld Complex indicatingthat geologically reasonable amounts of magma can account foreven the massive, 70 cm thick UG2 chromitite. The model requiressome crystallization to have occurred in a deeper chamber, forwhich there is ample geochemical evidence. KEY WORDS: Bushveld complex; chromite; crystal-laden magma; crustal contamination; magma mixing; UG2 chromitite     

Evaluation of sea water ingress into an Indian atoll

Groundwater is the prime source of fresh water in most part of world. The groundwater floats in the form of thin lens which is vulnerable to various stresses such as tide, cyclone, draught, abstraction, etc. The problem of getting this meager resource of fresh groundwater sustained for longer time, becomes more difficult task on tiny atoll with large population depending on it. In order to develop sustainable management scheme and identify vulnerable part of aquifer, systematic assessment of groundwater quality on such island have become imperative. Detailed hydrochemical study has been carried out to identify potential fresh groundwater resources on Andrott Island, UT of Lakshadweep, India. The analysis has given an early signal of deterioration in groundwater quality in some parts of the island during non-monsoon period, whereas the quality becomes slightly better during monsoon period. The study suggests immediate measures for arresting the deterioration in groundwater quality as well as augmentation for restoration of aquifer in some parts of the island.     

Birkhoff’S Theorem In The Einstein–Cartan–Kalb–Ramond Theory Of Gravity

It is shown that an analog of Birkhoff’s theorem of general relativity exists in the Einstein–Cartan–Kalb–Ramond (ECKR) theory of gravity when Kalb–Ramond (KR) field strength, which occurs in the theory is independent of time.     

Petrological and Geochemical Characteristics of Archean Gneisses and Granitoids from Bastar Craton, Central India – Implication for Subduction Related Magmatism

The gneisses and granitoids of Bastar craton (with rock suites up to 3.5 Ga) show calc-alkaline trondhjemitic characteristics. The rocks are enriched in both LILE and HFSE than primordial mantle. They have also relatively higher abundances of LILE and strong depletion at P and Ti in the multielement diagram. The depletion of Ti and P indicates retention of these elements by titanite and/or apatite during partial melting. It is proposed that subduction of an oceanic slab and its consequent melting led to the formation of the protoliths of the gneisses without much interaction with the mantle wedge. The granitoids represent temporally distinct suites formed in response to further melting of slab at greater depth and interaction of magma with the mantle wedge during their transport to the crust.     

Some rain rate and rain attenuation statistics at millimeter wavelengths in the tropics

Summary A study of rain rate and rain induced attenuation characteristics over two tropical stations in India has been made. The observed rain rate distributions differ significantly when compared with the CCIR report. The refractive indices have been calculated for certain frequencies at a tropical rain temperature of 20°C showing the variational pattern at different millimeter wavelengths. The present analysis also reveals that in the range 0.05 cm–0.3 cm drop diameter the variations of scattering amplitudes are systematic while in the range 0.4 cm–0.6 cm they are irregular.With 6 Figures     

A comparison of two techniques for calculating groundwater arsenic-related lung,bladder and liver cancer disease burden using data from Chakdha block,West Bengal

Calculation of excess disease burden for As exposed populations is becoming increasingly important to enable quantitative estimation of the impacts of various As mitigation options. There are several methods by which such calculations may be carried out. In this study, two methods, recently applied to estimating groundwater As-related health risks in southern Asia, to estimate disease burden arising from lung, bladder and liver cancer from As exposure for an As-effected area of West Bengal have been compared. Both utilized calculated distributions of exposure of the studied population to As from groundwater. Method (I) then entailed calculating disease burden by combining published background rates for death and disability adjusted life years (DALYs) and standard mortality ratios (SMRs) for excess health impacts related to As exposure, whilst for Method (II), disease burden from As exposure was estimated using the basic DALY formula, combined with incidence rates based on the NRC multistage Weibull model. Dose–response data for both methods were based on studies in Taiwan.     

Major ion chemistry,weathering processes and water quality assessment in upper catchment of Damodar River basin,India

The chemical characteristics of surface, groundwater and mine water of the upper catchment of the Damodar River basin were studied to evaluate the major ion chemistry, geochemical processes controlling water composition and suitability of water for domestic, industrial and irrigation uses. Water samples from ponds, lakes, rivers, reservoirs and groundwater were collected and analysed for pH, EC, TDS, F, Cl, HCO₃, SO₄, NO_3, Ca, Mg, Na and K. In general, Ca, Na, Mg, HCO₃ and Cl dominate, except in samples from mining areas which have higher concentration of SO₄. Water chemistry of the area reflects continental weathering, aided by mining and other anthropogenic impacts. Limiting groundwater use for domestic purposes are contents of TDS, F, Cl, SO₄, NO₃ and TH that exceed the desirable limits in water collected from mining and urban areas. The calculated values of SAR, RSC and %Na indicate good to permissible use of water for irrigation. High salinity, %Na, Mg-hazard and RSC values at some sites limit use for agricultural purposes.