Metamorphism and Fluid Inclusion Studies of Charnockites and Associated Rocks from the Madurai Granulite Block , Southern India with Special Reference to Graphite Mineralization

The present research is aimed at studying the charnockites and associated rocks of the Madurai Granulite Block (MGB), especially in terms of their field settings, texture, mineralogy, and mineral chemistry analyzing their petrogenesis with the help of thermobarometrical studies and geochronological constraints. The mechanism of charnockitization by the influx of CO2 rich fluids and its relation to the graphite mineralization is actually a matter of discussion and study. The objectives of the present study are, to delineate petrological and structural relationship of charnockites and associated gneissic rocks, to study the field and petrogenetic aspects of graphite mineralization in the MGB, to establish and re-evaluate the P-T conditions of formation of the rocks with the aid of thermbarometric computations and to compare with the earlier studies, characterization of graphite with XRD, Raman spectroscopy and isotope studies together with a search in to its genesis and its relation to the high-grade metamorphism of the terrain, to evaluate the role of CO2 bearing fluids in the processes of charnockitization as well as in the genesis of graphite within the high-grade terrain and to delineate the metamorphic geochronology of selected rocks using ‘monazite dating’ technique with EPMA.

Novel heating/cooling stage designed for fluid inclusion microthermometry of large stalagmite sections

Liquid–vapour homogenisation temperatures of fluid inclusions in stalagmites are used for quantitative temperature reconstructions in paleoclimate research. Specifically for this application, we have developed a novel heating/cooling stage that can be operated with large stalagmite sections of up to 17 × 35 mm2 to simplify and improve the chronological reconstruction of paleotemperature time-series. The stage is designed for use of an oil immersion objective and a high-NA condenser front lens to obtain high-resolution images for bubble radius measurements. The temperature accuracy of the stage is better than ± 0.1 °C with a precision (reproducibility) of ± 0.02 °C.

Quartz recrystallization regimes, c-axis texture transitions and fluid inclusion reequilibration in a prograde greenschist to amphibolite facies mylonite zone (Ribeira Shear Zone, SE Brazil)

The crystal-plastic behavior of quartz mylonites from the Ribeira Shear Zone (SE Brazil), a major strike-slip structure that was active during a prograde metamorphic phase related to the Neoproterozoic Brasiliano-Pan African Orogeny, was investigated using a multi-method approach. Geothermobarometry results indicate deformational conditions ranging from similar to 300 to similar to 630 degrees C and 500-700 MPa. A strong correlation between mapped metamorphic zones and a dominance of different dynamic recrystallization mechanisms of quartz occurs within the mylonite zone. Bulging recrystallization (BLG) dominates within the chlorite zone between 300 and 410 degrees C, subgrain rotation recrystallization (SGR) operates within the biotite zone from 410 to 520 degrees C, and grain boundary migration recrystallization (GBM) dominates in the garnet zone above 520 degrees C. The development of quartz c-axis textures is mainly governed by temperature and dynamic recrystallization mechanisms. Textures from BLG zone mylonites are characterized by maxima around Z; SGR zone mylonites display single girdles or asymmetric type I crossed girdles; and GBM zone mylonites comprise maxima around Y and intermediate between X and Z. The scarcity or absence of water-bearing fluid inclusions in quartz mylonites from the SGR and GBM zones, which are dominated by carbonic inclusions, suggests water-deficient conditions, whereas BLG zone mylonites are dominated by water-bearing inclusions. This evidence indicates that water was available in the protoliths but has been eliminated with increasing deformation and deformation temperature. No effect of the water content variation on the quartz microstructural and recrystallized grain size evolution was detected, and little influence on c-axis texture development was observed. Most of the fluid inclusion densities were reequilibrated during the shear zone exhumation history, recording a decompression in the range of 300-500 MPa, while microstructural reequilibration effects related to the prograde metamorphism are largely preserved. Fluid inclusion microstructures and densities from two SGR zone samples preserved evidence for a near isothermal compression within the interior of the Ribeira Shear Zone during the prograde metamorphism. (C) 2009 Elsevier B.V. All rights reserved.

Sn-polymetallic greisen-type deposits associated with late-stage rapakivi granites, Brazil: fluid inclusion and stable isotope characteristics

Tin-polymetallic greisen-type deposits in the Itu Rapakivi Province and Rondonia Tin Province, Brazil are associated with late-stage rapakivi fluorine-rich peraluminous alkali-feldspar granites. These granites contain topaz and/or muscovite or zinnwaldite and have geochemical characteristics comparable to the low-P sub-type topaz-bearing granites. Stockworks and veins are common in Oriente Novo (Rondonia Tin Province) and Correas (Itu Rapakivi Province) deposits, but in the Santa Barbara deposit (Rondonia Tin Province) a preserved cupola with associated bed-like greisen is predominant. The contrasting mineralization styles reflect different depths of formation, spatial relationship to tin granites, and different wall rock/fluid proportions. The deposits contain a similar rare-metal suite that includes Sri (+/-W, +/-Ta, +/-Nb), and base-metal suite (Zn-Cu-Pb) is present only in Correas deposit. The early fluid inclusions of the Correas and Oriente Novo deposits are (1) low to moderate-salinity (0-19 wt.% NaCl eq.) CO2-bearing aqueous fluids homogenizing at 245-450 degreesC, and (2) aqueous solutions with low CO2, low to moderate salinity (0-14 wt.% NaCl eq.), which homogenize between 100 and 340 T. In the Santa Barbara deposit, the early inclusions are represented by (1) low-salinity (5-12 wt.% NaCl eq.) aqueous fluids with variable CO2 contents, homogenizing at 340 to 390 T, and (2) low-salinity (0-3 wt.% NaCl eq.) aqueous fluid inclusions, which homogenize at 320380 degreesC. Cassiterite, wolframite, columbite-tantalite, scheelite, and sulfide assemblages accompany these fluids. The late fluid in the Oriente Novo and Correas deposit was a low-salinity (0-6 wt.% NaCl eq.) CO2-free aqueous solution, which homogenizes at (100-260 degreesC) and characterizes the sulfide fluorite-sericite association in the Correas deposit. The late fluid in the Santa Barbara deposit has lower salinity (0-3 wt.% NaCl eq.) and characterizes the late-barren-quartz, muscovite and kaolinite veins. Oxygen isotope thermometry coupled with fluid inclusion data suggest hydrothermal activity at 240-450 degreesC, and 1,0-2.6 kbar fluid pressure at Correas and Oriente Novo. The hydrogen isotope composition of breccia-greisen, stockwork, and vein fluids (delta(18)O quartz from 9.9parts per thousand to 10.9parts per thousand, deltaDH(2)O from 4.13parts per thousand to 6.95parts per thousand) is consistent with a fluid that was in equilibrium with granite at temperatures from 450 to 240 degreesC. In the Santa Barbara deposit, the inferred temperatures for quartz-pods and bed-like greisens are much higher (570 and 500 degreesC, respectively), and that for the cassiterite-quartz-veins is 415 degreesC. The oxygen and hydrogen isotope composition of greisen and quartz-pods fluids (delta(18)O(qtz-H2O)=5.5-6.1parts per thousand) indicate that the fluid equilibrated with the albite granite, consistent with a magmatic origin. The values for mica (delta(18)O(mica-H2O)=33-9.8parts per thousand) suggest mixing with meteoric water. Late muscovite veins (delta(18)O(qtz-H2O)=-6.4parts per thousand) and late quartz (delta(18)O(mica-H2O)=-3.8parts per thousand) indicate involvement of a meteoric fluid. Overall, the stable isotope and fluid inclusion data imply three fluid types: (1) an early orthomagmatic fluid, which equilibrated with granite; (2) a mixed orthomagmatic-meteoric fluid; and (3) a late hydrothermal meteoric fluid. The first two were responsible for cassiterite, wolframite, and minor coluChange in the redox conditions related to mixing-of magmatic and meteoric fluids favored important sulfide mineralization in the Correas deposit. (C) 2004 Elsevier B.V. All rights reserved.

The mineralizing fluid in the Piaba gold deposit, São Luís cratonic fragment (NW-Maranhão, Brazil) based on fluid inclusion studies in quartz veins

O depósito aurífero de Piaba tornou-se a primeira mina em operação no fragmento cratônico São Luís, noroeste do Maranhão. Seu ambiente geológico compreende rochas metavulcanossedimentares do Grupo Aurizona e granitoides da Suíte Tromaí, entre outras unidades menores, formadas em ambiente de arcos de ilhas entre 2240 e 2150 Ma, juntamente com outras unidades menores. A mineralização é caracterizada por uma trama stockwork de veios e vênulas de quartzo com seus halos de alteração (clorita + muscovita + carbonato + pirita + calcopirita e ouro) hospedada em um granodiorito granofírico fino (Granófiro Piaba) e em rocha subvulcânica andesítica do Grupo Aurizona. O corpo mineralizado é espacialmente limitado à zona de cisalhamento ENE-WSW rúptil-dúctil (Falha Piaba). Estudos petrográficos, microtermométricos e por espectroscopia microRaman no quartzo definiram inclusões aquo-carbônicas bifásicas e trifásicas, produzidas por aprisionamento heterogêneo durante separação de fases, e fluidos aquosos tardios. A solução mineralizadora corresponde a um fluido aquo-carbônico composto por CO₂ (5 - 24 mol%, densidade de 0,96 - 0,99 g/cm³), H₂O (74 - 93 mol%), N₂ (< 1 mol%), CH₄ (<1mol%) e 5,5 % em peso NaCl equivalente. O minério depositou a 267 - 302ºC e 1,25 - 2,08 kbar, correspondendo a profundidades de 4 a 7 km, em consonância com o regime estrutural. A composição e o intervalo de P-T do fluido mineralizador, combinados com o caráter redutor (log ƒ_O2 -31,3 a -34,3) e a sulfetação da rocha hospedeira, sugerem que o ouro foi transportado como um complexo sulfetado. O minério foi depositado em consequência da separação de fase, redução da atividade de enxofre e da ƒO₂ pela interação fluido-rocha.

Novel heating/cooling stage designed for fluid inclusion microthermometry of large stalagmite sections

Liquid–vapour homogenisation temperatures of fluid inclusions in stalagmites are used for quantitative temperature reconstructions in paleoclimate research. Specifically for this application, we have developed a novel heating/cooling stage that can be operated with large stalagmite sections of up to 17 × 35 mm2 to simplify and improve the chronological reconstruction of paleotemperature time-series. The stage is designed for use of an oil immersion objective and a high-NA condenser front lens to obtain high-resolution images for bubble radius measurements. The temperature accuracy of the stage is better than ± 0.1 °C with a precision (reproducibility) of ± 0.02 °C.

Morphological ripening of fluid inclusions and coupled zone-refining in quartz crystals revealed by cathodoluminescence imaging: Implications for CL-petrography, fluid inclusion analysis and trace-element geothermometry

Cathodoluminescence (CL) studies have previously shown that some secondary fluid inclusions in luminescent quartz are surrounded by dark, non-luminescent patches, resulting from fracture-sealing by late, trace-element-poor quartz. This finding has led to the tacit generalization that all dark CL patches indicate influx of low temperature, late-stage fluids. In this study we have examined natural and synthetic hydrothermal quartz crystals using CL imaging supplemented by in-situ elemental analysis. The results lead us to propose that all natural, liquid-water-bearing inclusions in quartz, whether trapped on former crystal growth surfaces (i.e., of primary origin) or in healed fractures (i.e., of pseudosecondary or secondary origin), are surrounded by three-dimensional, non-luminescent patches. Cross-cutting relations show that the patches form after entrapment of the fluid inclusions and therefore they are not diagnostic of the timing of fluid entrapment. Instead, the dark patches reveal the mechanism by which fluid inclusions spontaneously approach morphological equilibrium and purify their host quartz over geological time. Fluid inclusions that contain solvent water perpetually dissolve and reprecipitate their walls, gradually adopting low-energy euhedral and equant shapes. Defects in the host quartz constitute solubility gradients that drive physical migration of the inclusions over distances of tens of μm (commonly) up to several mm (rarely). Inclusions thus sequester from their walls any trace elements (e.g., Li, Al, Na, Ti) present in excess of equilibrium concentrations, thereby chemically purifying their host crystals in a process analogous to industrial zone refining. Non-luminescent patches of quartz are left in their wake. Fluid inclusions that contain no liquid water as solvent (e.g., inclusions of low-density H2O vapor or other non-aqueous volatiles) do not undergo this process and therefore do not migrate, do not modify their shapes with time, and are not associated with dark-CL zone-refined patches. This new understanding has implications for the interpretation of solids within fluid inclusions (e.g., Ti- and Al-minerals) and for the elemental analysis of hydrothermal and metamorphic quartz and its fluid inclusions by microbeam methods such as LA-ICPMS and SIMS. As Ti is a common trace element in quartz, its sequestration by fluid inclusions and its depletion in zone-refined patches impacts on applications of the Ti-in-quartz geothermometer.