Alteration geochemistry and fluid inclusion characteristics of the greenstone-hosted gold deposit of Hutti, Eastern Dharwar Craton, India

Gold mineralization of the Hutti mine, southern India, is situated in closely spaced laminated quartz veins and associated alteration haloes along steeply dipping shear zones within a sequence of rather uniform amphibolites. Intense shearing has resulted in large-scale mylonitization of the wall rocks. Anastomosing shear zones, with intervening lensoid bodies of unsheared amphibolites, are characteristic features of the deposit. The general pattern of symmetrical alteration comprises a distal zone of chlorite-rich rock, with a proximal biotite-rich zone adjacent to laminated quartz veins. Arsenopyrite thermometry yielded a temperature range of 350-477 °C for the biotite alteration zone, which preceded the formation of the laminated quartz veins. Mass balance calculations on the alteration zones indicate a gradual mass and volume loss during alteration. The alteration is accompanied by intense potash metasomatism and addition of sulfur, which resulted in the formation of arsenopyrite, pyrrhotite, and pyrite. Results of fluid inclusion studies suggest that low salinity (3.9-13.5 wt% NaCl equivalent) H₂O-CO₂ rich fluids were responsible for gold-rich laminated quartz vein formation in the Hutti deposit. These fluids constituted a later counterpart of the protracted fluid activity that first formed the biotite alteration zone. The estimated P-T values range from 1.0 to 1.7 kbar at 280-320 °C. These data, along with the alteration assemblages and the characteristic gold-sulfide association, both in the altered wall rock and laminated quartz veins, suggest that gold, transported as reduced bisulfide complexes, was deposited in response to sulfidation reactions in the wall rocks. Comparison of P-T conditions of formation of gold-quartz veins at Hutti with two other large gold deposits in the eastern Dharwar Craton, namely Kolar (1.8 kbar/280 °C) and western Ramagiri (1.45-1.7 kbar/240-270 °C), indicates broadly similar lode-gold forming conditions in the Dharwar Craton.     

Proper depiction of monsoon depression through IRS-P4 MSMR

In this paper, daily variations of satellite-derived geophysical parameters such as integrated water vapour (IWV), cloud liquid water content (CLW), sea surface temperature (SST) and sea surface wind speed (SSW) have been studied for a case of monsoon depression that formed over the Bay of Bengal during 19th-24th August 2000. For this purpose, IRS P4 MSMR satellite data have been utilized over the domain equator — 25‡N and 40‡-100‡E. An integrated approach of satellite data obtained from IRS-P4, METEOSAT-5 and INSAT was made for getting a signal for the development of monsoon depression over the Indian region. Variations in deep convective activity obtained through visible, infrared and OLR data at 06 UTC was thoroughly analyzed for the complete life cycle of monsoon depression. Geophysical parameters obtained through IRS-P4 satellite data were compared with vorticity, convergence and divergence at 850 and 200 hPa levels generated through cloud motion vectors (CMVs) and water vapour wind vectors (WVWVs) obtained from METEOSAT-5 satellite. This comparison was made for finding proper consistency of geophysical parameters with dynamical aspects of major convective activity of the depression. From the results of this study it is revealed that there was strengthening of sea surface winds to the south of low-pressure area prior to the formation of depression. This indicated the possibility of increase in cyclonic vorticity in the lower troposphere. Hence, wind field at 850 hPa with satellite input of CMVs in objective analysis of wind field using optimum interpolation (OI) scheme was computed. Maximum cyclonic vorticity field at 850 hPa was obtained in the region of depression just one day before its formation. Similarly, with the same procedure maximum anticyclonic vorticity was observed at 200 hPa with WVWVs input. Consistent convergence and divergence at 850 and 200 hPa was noticed with respect to these vorticities. In association with these developments, we could get lowest values of OLR (120 W/m² ) associated with major convective activity that was consistent with the maximum values of integrated water vapour (6-8gm/cm²) and cloud liquid water content (50-60 mg/cm² ) persisting particularly in the southwest sector of the monsoon depression.     

Impact of moisture variations on the circulation of the south-west monsoon

The impact of moisture anomalies on the circulation of the south-west Indian monsoon has been studied with a general circulation model. Newtonian relaxation is adopted to subject the model atmosphere under sustained moisture anomalies. The impact of negative anomalies of moisture was seen as a divergent circulation anomaly, while the positive anomaly was a stronger convergent anomaly. Although the humidity fields display a resilient behaviour, and relax back to normal patterns 1–2 days after the forcing terms in humidity are withdrawn, the circulation anomalies created by the moisture variation keeps growing. A feedback between positive moisture anomalies and low level convergence exists, which is terminated in the absence of external forcings.     

Lattice dynamics and Raman spectroscopy of protoenstatite Mg2Si2O6

Enstatites (Mg₂Si₂O₆) are important rock forming silicates of the pyroxene group whose structures are characterised by double MgO₆ octahedral bands and single silicate chains. Orthoenstatite transforms to protoenstatite above 1273 K with a doubling of the a axis and a rearrangement of the silicate chains with respect to the Mg₂₊ ions. Lattice dynamical calculations based on a rigid-ion model in the quasi-harmonic approximation provide theoretical estimates of elastic constants, long wavelength phonon modes, phonon dispersion relations, total and partial density of states and inelastic neutron scattering cross-sections of protoenstatite. The computed elastic constants are in good agreement with experimental data. The computed density of states of a chain silicate such as protoenstatite is distinct from that of olivines (forsterite, Mg₂SiO₄ and fayalite, Fe₂-SiO₄) with isolated silicate tetrahedra. The band gaps in the density of states in forsterite are largely due to the separation in the frequency ranges of the external and internal vibrations of the isolated silicate group, whereas in protoenstatite these gaps are filled by the vibrations of the bridging oxygens of the silicate chain. The computed density of states is used to calculate the specific heat, the mean square atomic displacements and temperature factors. Validity of these calculations are supported by Raman scattering measurements. Polarised and unpolarised Raman spectra are obtained from small single crystals of protoenstatite (Li,Sc)_0.6Mg_1.4Si₂O₆ stable at room temperature using the 488 nm or 514.5 nm lines of an Ar⁺ ion laser and a micro-Raman spectrometer with backscattering geometry. The Raman spectra were analysed and interpreted based on the lattice dynamical model. The experimental Raman frequencies and mode assignments (based on polarised single crystal spectra) are in good agreement with those obtained from lattice dynamical calculations.     

Alpha particle haloes in chlorite and cordierite

Summary Effects of the impact of natural long-term irradiation with alpha particles in one chamosite and one cordierite sample were characterised in detail using electron microprobe, Raman microprobe, optical absorption spectroscopy (cordierite only), and transmission electron microscopy (TEM; cordierite only) analysis. In both cases, the impact of ⁴He cores (alpha particles) that were emitted from actinide-bearing mineral inclusions has caused the formation of radiation damage haloes in the host mineral. These haloes have maximum radii of about 33 μm (chamosite) and 47 μm (cordierite). They show notably changed optical properties, i.e., intensified absorption of light as recognised by brown (chamosite) and yellow (cordierite) pleochroism and enhanced or even anomalous interference colours. In spite of the significant disturbance of their short range order, alpha particle haloes are characterised by generally low degrees of structural radiation damage. This is indicated by rather moderate broadening of vibrational bands and, in the case of cordierite, apparently undisturbed electron diffraction patterns in the TEM. Intensive damage, virtually close to an amorphous state, was only found in cordierite up to a few tens of nanometres away from actinide-bearing inclusions. This damage is mainly assigned to recoils of heavy nuclei upon emission of an alpha particle, which have particle trajectory lengths that are three orders of magnitude shorter than those of the alpha particles. Similar to observations on biotite, alpha particle haloes in chamosite and cordierite as observed in the optical microscope may be considered as representative of a very early stage of the metamictisation process.     

Forecasting shield pressures at a longwall face using artificial neural networks

Leg pressure data from all shields of a longwall face are monitored and recorded in the surface computer. An algorithm is developed to detect peak pressures or periodic roof weightings from these pressure data. The intensities and locations of periodic roof weighting are further analyzed using artificial neural network for forecasting of forthcoming shield pressures. The network was trained using data 153 m (500 ft) of face advance. Shield pressures are forecasted for the successive nine mining cycles or approximately 9 m of face advancement. The results obtained validate the efficacy of the developed model.     

Geochemical mobility of Au and Ag during hydrothermal transfer and precipitation: Thermodynamic simulation

The thermodynamic simulation of the geochemical mobility of Au and Ag during their hydrothermal transfer and precipitation was conducted with regard for the formation of continuous nonideal Au-Ag solid solutions (or Au_XAg_1?X) alloys) and with the analysis of the effects of principal physicochemical parameters (temperature, pH, redox potential, and the concentrations of sulfide sulfur and chloride ions) on the solubility of Au-Ag alloys of various composition. Predominant Au and Ag complexes and the types of solutions were identified, which are characterized by different extractive ability with respect to these metals. A tendency in the variations in the Au/Ag ratio in the solid phase during the evolution of the hydrothermal process was revealed with regard for various conditions under which Ag sulfides can occur. The possible effect of the Au_XAg_1?X/solution ratio on the composition of the Au-Ag mineralization was demonstrated, and tendencies in the variations in the composition (fineness) of the alloys during their redeposition were identified.     

Exploration of geothermal activity using time series analysis of subsurface gases data from Bakreswar hot springs area,eastern India

Geothermal power seems to be a potential source of green energy in India. But these renewable energy resources are still ignored in India even after having a lot of potential sources as seen in more than 300 hot springs scattered throughout different geothermal areas of the country. Many of them could be utilized for power generation using the earth’s internal heat. More hours are needed to explore these geothermal areas using geochemical, geophysical techniques, and statistical analysis to qualitatively estimate power harnessing capabilities and sustainability of the areas for generation of geothermal power. In the present paper, attention has been focused to investigate Bakreswar geothermal field of India by continuous (24*7) and online monitoring of terrestrial gases such as He and radioactive gases (²²²Rn) in hot spring emanations of the geothermal area for more than 5 years along with discrete measurement of some other geochemical and geophysical parameters. The discrete measures imply that the seven hot spring vents at the study area are linked to the aquifer through different ways covering non-uniform rock assemblies and ascertain the presence of high amount of radioactive minerals at the underneath terrains. Stable activities with a high-out flux of ²²²Rn and He for a prolonged time period is expected within the reservoir present at the study area as consequence of analysis of the temporal variations and statistical measures of the continuous data sets. Seasonal variation of time series data also recommends that the high amount of radioactive sources present at the crust of the reservoir is able to produce enough quantity of heat irrespective of meteorological effects. The investigation on the power spectra interferes that the geothermal system is still in quite active phase on the influence of tectonic activities. Therefore, the geothermal reservoir present at the Bakreswar geothermal area may be utilized as a constant and continuous heat source for a long time period to run a geothermal power plant.