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The bauxites deposits of Kachchh area in Gujarat are investigated to characterize them based on mineralogical and petrographic studies. The major bauxitic mineral in these occurrences is gibbsite, with minor concentration of boehmite and diaspore. Apart from the bauxitic minerals, the other associate minerals are kaolin, calcite, alunite and the iron ore minerals such as hematite and goethite and titanium rich anatase. The iron ore minerals (hematite and goethite) are 10-50microns in size and are disseminated throughout the oolitic and pisolitic bauxitic minerals. At places the goethite exhibits colloform texture. The preservation of basaltic texture in some of the samples indicate that the insitu nature of these bauxites, which are formed by the alteration of calcic plagioclase from the parent basalt. Although, the basalt occurs as the main parent rock for these bauxites, the presence of calcite in some of the samples represent the possibility of having a limestone parent rock at least in some of the bauxite occurrences.

Characterization of fluid inclusions in graphite-bearing charnockites from the southwestern part of the Madurai Granulite Block in southern India reveals a probable relation with the formation and break down of graphite during the high-grade metamorphism. The first-generation monophase pure CO(2) inclusions, the composition of which is confirmed by laser Raman spectroscopy, recorded moderate density (0.77-0.87 g/cc) corresponding to low tapping pressure (around 2 kb) than that of the peak granulite-facies metamorphism. The precipitation of graphite, as inferred from graphite inclusions and delta(13)C values of the graphite from the outcrops, is interpreted as the cause of this lowering of fluid density. An intermediate generation of pseudosecondary inclusions resulted from the re-equilibration or modification of the first-generation fluids and the CO(2). formed is interpreted to be the oxidation product from graphite. The youngest generation of fluids which caused widespread retrogression of the granulites is a low-temperature (similar to 350 degrees C) high-saline (32.4-52.0 wt% NaCl equivalent) brine. Carbon isotope data on the graphite from the charnockites show delta(13)C values ranging from -11.3 to -19.9 parts per thousand, suggesting a possibility of mixing of carbon sources, relating to earlier biogenic and later CO(2) fluid influx. Combining the information gathered from petrologic, fluid inclusion and carbon stable isotope data, we model the fluid evolution in the massive charnockites of the southwestern Madurai Granulite Block. (c) 2009

The discussion on our paper (Pandalai et al. 2003) by Kolb et al. (2003, DOI 10.1007/s00126-002-0345-9) raises important issues related to the genesis of the Hutti gold deposit. The main points raised by Kolb et al. (2003) relate to the estimation of P and T of ore formation and the chronological (and genetic) order of vein formation in the shear zones at Hutti as identified by them. Rogers et al. (2002) and Kolb et al. (2003) recognize five stages of deformation (D1–D5), one prograde (M1) and two retrograde (M2, M3) stages of metamorphism, and four stages of quartz vein formation with attendant alteration. Rogers et al. (2002) and Kolb et al. (2001, 2003) do not refer to previously published literature on structural studies of the Hutti–Maski schist belt. Nevertheless, the deformational stages identified by Kolb et al. (2003) need to be placed in the framework of the regional tectonic history of the Hutti–Maski belt. Structural studies on this belt are reported in Roy (1979, 1991) and Biswas (1990). These studies identify three phases of deformation and metamorphism that are codified here as D1 , D2 , D3 * and M1 , M2 * , M3 * respectively to distinguish them from those of Kolb et al. (2003). A brief summary of the structural framework is given below before discussing the points raised in Kolb et al. (2003). The first deformational stage (D1 ) led to the development of tight isoclinal (F1 ) folds, penetrative schistosity (S1 ), synchronous prograde metamorphism (M1 ) and syntectonic emplacement of tonalite and tronjhemite around the Hutti–Maski belt. Additionally, mylonite and mylonitic foliation (Sm ) developed in narrow shear zones. On account of the parallelism of Sm * with S1 * it is inferred that they developed during the phase of D1 * deformation. The prograde M1 * phase of metamorphism is low grade (greenschist facies), but rises to medium grade (amphibolite facies) with development of garnet and cordierite in the north-eastern peripheral zone of the greenstone belt. The D2 * deformation stage gave rise to small and large-scale (F2 ) folds with subvertical axial planes (S2 ) and axes that plunge 50 to 60 NW. F2 folds bring F1 isoclinal folds into parallelism with F2 * limbs and are responsible for the map pattern of the exposed rocks. S2 *

The pressure, temperature and composition of ore fluids that resulted in gold deposition in the Archean, greenstone-hosted Hutti deposit have been studied using fluid inclusions and the compositions of arsenopyrite and chlorite. Five types of fluids have been identified in fluid inclusions in quartz veins associated with mineralization. They are (1) monophase CO2-rich fluid; (2) low-salinity (0 to 14 wt% NaCl equivalent) and high-salinity (16 to 23 wt% NaCl equiv.) aqueous fluids; (3) high-salinity (28 to 40 wt% NaCl equiv.), polyphase aqueous fluids; (4) CO2-H2O-NaCl fluids of low salinity (0-8 wt% NaCl equiv.); and (5) a few carbonic inclusions with halite +/-nahcolite. The diversity of entrapped fluid composition is explained in terms of changes in fluid pressure and temperature which affect a more or less uniform supply of primary low-salinity CO2-H2O-NaCl fluid to the shear zone. Geothermobarometric studies indicate that during mineralization temperature ranged between 360 and 240degreesC, and fluid pressure between 3,600 and 1,600 bar. The data are interpreted in terms of the cyclic fault-valve mechanism for active shear zones. Deposition of gold and sulfides has been studied on the basis of constraints from the composition of wall-rock chlorite, ore-mineral assemblages, and textural features. Tubular channels, 20 to 100 pm wide and up to 500 mum long that arise from fractures and C-planes in sheared quartz veins are reported for the first time. The channels have pyrrhotite, arsenopyrite, pyrite and gold at their distal ends, with calcite filling up the remaining part. These channels form in response to increases in T and P, by dissolution of quartz grains, guided by dislocations in them. At the P-T conditions of interest, gold and sulfide deposition takes place in the shears and fractures of quartz veins from CO2-H2O-NaCl ore fluid of low salinity and pH due to changes in phase compositions that occur during the process of shear failure of the enclosing rocks. In the wall rock where pH is buffered, gold deposition takes place from the predominant Au(HS)(2)(-) species with progressive sulfide deposition and decrease in YES, from 0.01 to 0.001 mol/kg as T falls from 360 to 240degreesC.

The Proterozoic, carbonate-hosted, Zawarmala Pb-Zn deposit has stratiform, banded pyrite-sphalerite ore, as well as discordant, sphalerite-galena veins and rich massive galena ore shoots. The mechanisms of deposition of discordant vein and massive ores have been studied using speciation and solubility calculations based on ore fluid characteristics and physical parameters inferred from petrography, fluid inclusion studies, and leachate analysis. Massive galena ore, which is the economic ore type, cuts across the earlier, F2-related, vein-type ore and is distinct from the vein-type ore in terms of mineralogy, texture, and fluid inclusion characteristics. Fluid inclusions in quartz associated with the ores show that fluids forming vein-type ore are low-salinity (4.3–14.7 wt % NaCl equiv) H2O-NaCl fluids. The trapping temperature of vein-type ore fluid was estimated to be in the range of 395° to 290°C at a pressure of about 1,450 bars. Fluids associated with massive galena ore are H2O-CO2-NaCl fluids of lower salinity (about 3–4 wt % NaCl equiv). The wide variation in XCO2 of fluid inclusions that occur in close proximity to each other within the same sample of massive ore suggests heterogeneous entrapment of the low-salinity H2O-CO2-NaCl fluids. From the intersection of isochores of local, homogeneously entrapped CO2 and aqueous biphase fluids of unmixed CO2-H2O-NaCl fluid, trapping temperature fluid was estimated to be between 250° and 150°C at pressures of 2,000 to 750 bars. Despite the low salinity of the ore fluids, speciation studies show that chloride complexes are dominant over most of the temperature range of interest in both ore fluids. The only bisulfide complex of importance is \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Pb(HS)_{2}^{0}\) \end{document}, which is dominant below 175°C in fluids associated with massive galena ore. Data from fluid inclusions and solubility calculations reveal that cooling is the important mechanism of vein-type ore deposition. In contrast, physicochemical data indicate that an increase in reduced sulfur, by mixing a regional, high metal content, low sulfur fluid with a local, high sulfur fluid at the site of deposition, was the mechanism for massive galena ore deposition. The difference in oxidation state (and therefore also in metal content) of the two fluids is inferred to be a significant factor that caused differences in the modes of deposition of the two ore types and the relatively high value of XCO2 (0.2033) in fluids associated with massive galena ore.

Fluid inclusions and melt inclusions have been considered as possible guides for mineral exploration by many researchers. However, the minuteness of the fluid inclusions and the multiplicity of generations within the same deposit have always posed serious problems in the proper utilization of fluid-inclusion data in prospecting.