Informatics Publishing Limited

M. Santosh ¹

Affiliations
1 Centre for Earth Sciences, Trivandrum 695 031, IN

Abstract

No Abstract.

Full Text

   

M. Santosh ¹, N. G. K. Nair ¹

Affiliations
1 Centre for Earth Science Studies, P. B. No. 2235, Trivandrum 695010, IN

Abstract

The granite of Chengannoor, Alleppey district, Kerala, having an intrusive relationship with Precambrian charnockite-cordierite gneiss sequence is inferred to be a post-kinematic granite of magmatic origin. The major mineral constituents are mesoperthitic K-feldspar with quartz and two generations of plagioclase. A late/post intrusive potash metasomatism caused changes in the mineralogy and chemical composition of the granite.

Recent K-Ar dating of hornblende from the Chengannoor granite assigns an age of 550 m.y. for the granite (Soman, Golubyev and Santosh. Ind. Jour, Earth Science) (in press).

Full Text

     

N. G. K Nair ¹, K. Soman ¹, M. Santosh ¹, M. M. Arakelyants ², V. N. Golubyev ²

Affiliations
1 Centre for Earth Science Studies, Trivandrum 695010, IN
2 IGEM, Academy of Sciences, Moscow, USSR, RU

Abstract

Biotite separates from three granite plutons in the Northern part of the Kerala region, namely, Kalpatta. Ambalavayal and Thaluru yield K-Ar ages of 512±20 m.y., 560±30 m.y. and 710±20 m y. respectively. The age data are comparable with those reported for similar granitic intrusives of the south Indian region.

Full Text

   

M. Santosh ¹

Affiliations
1 Centre for Earth Science Studies, P. B. 2235, Sasthamangalam, Trivandrum 695 010, IN

Abstract

Fluid inclusion studies in a number of doubly polished plates of zircons showing interesting features are documented. The equilibrium balanced shapes of inclusions and their pattern of distribution are suggestive of slow growth rate, largely controlIed by stacking mechanism, resulting in completed layerites. Development of tensional fractures which eventually healed by dendritic growth and typical necking-down textures of long tabular inclusions are recorded. Thermometric studies show tbat the early fluids were of high density (0.90-0.85 g/cm³)CO₂ type with probable traces of CH₄ and/or N₂, entrapped at a pressure of 4.5 Kb. The fluids evolved into pure CO₂ (O.83 - 0.68 g/cm³) and to mixed CO₂-H₂O around 650°C and 3.7 Kb due to formation of vapour phase, while a fraction of the melt was sti!l present. The composition of fluids recorded suggests repeated CO₂ activity in south India with a peak during Late Precambrian-Early Palaeozoic times, effecting the generation and emplacement of alkaline plutons.

Full Text

     

M. Santosh ¹, N. G. K. Nair ¹

Affiliations
1 Centre for Earth Science Studies, P. B. No. 2235, Sasthamangalam. Trivandrum 695010, IN

Abstract

A syenite pluton occurring ncar Angadimogar in Cannanore district. northern Kerala and emplaced close to the continental margin and along a NW-SE trending faultlineament is reported. Alkali feldspar constitutes the dominant mineral with subordinate albitic plagioclase and quartz. corresponding to nordmarkitic composition. The syenite shows unique assemblage of accessory minerals. namely hornblende, riebeckite. crocidolite and acmite with diopside, bi.otite. zircon, sphene and calcite. High alkali content, with dominance of Na₂0 over K₂0, moderate transition dement contents and low Rb values are characteristic. Petrochemical features suggest melt equilibration from a K-rich, Rb-depleted mantle source, with subsequent amphibole and alkali feldspar fractionation. Significant increase in fO₂, fH₂O and peralkalinity towards the residual stage is deduced. The syenite belongs to the group of Late Precambrian-Early Palaeozoic alkaline intrusives of the region, with its generation and emplacement correlatable with the taphrogenesis of the western continental margin of lndia.

Full Text

     

M. Santosh ¹

Affiliations
1 Centre for Earth Science Studies, P.B. 2235, Trivandrum 695 010, IN

Abstract

Major and trace element geochemistry of coexisting alkali feldspar and plagioclase pairs from granite and alkali feldspars from the associated pegmatites of the Ambalavayal molybdenite prospect in Wynad district, Kerala, are presented. The element levels in both the feldspars show an overall comparability with the values reported from elsewhere. The nature of distribution of elements between coexisting feldspars is generally consistent with their crystal chemical behaviour. X-ray studies of alkali feldspars show a triclinic symmetry with the triclinicity values ranging from 0.2955 for feldspar from the granite to 0.6742 for feldspar from the pegmatite. Mol per cent feldspar end members range from Or_56.43Ab_40.52An_3.05in alkali feldspar of the granite to Or_74.00Ab_22.51An_3.19 in alkali feldspar of the pegmatite. The crystallization temperature of the alkali feldspar-plagioclase pairs in the granite is estimated to be between 704-740°C and that of alkali feldspar in the pegmatite to be between 525-580°C.

Full Text

     

M. Santosh ¹, N. G. K. Nair ¹, K. Pande ², K. Gopalan ²

Affiliations
1 Centre for Earth Science Studies, P. B. 2235, Sasthamangalam, Trivandrum 695 010, IN
2 Physical Research Laboratory, Navarangpura, Ahmedabad 380 009, IN

Abstract

Rubidium and strontium determinations are reported for seven whole-rock samples of the Ambalavayal granite of northern Kerala. The data yield a well defined Rb-Sr isochron, corresponding to an age of 595 ± 20 Ma and initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7171 ± 0.0022. The age marks a regime of Late Precambrian-Early Palaeozoic granite magmatism in the southwestern Indian shield. The agreement of the Rb/Sr age with the previously measured K/Ar age of 560± 30 Ma indicates no significant secondary thermal event in the region. The evolved initial Sr ratio suggests either an exclusive origin or significant crustal contamination of magma derived from a deeper source.

Full Text

   

M. Santosh ¹, Marasu Yoshida ²

Affiliations
1 Centre for Earth Science Studies, P. B. 2235, Sasthamangalam, Trivandrum 695 010, IN
2 Department of Geosciences, Osaka City University, Osaka, JP

Abstract

Field and microstructural evidences indicating retrogressive-transformation of banded charnockite to leptynite (garnet gneiss) through a transition zone of khondalite (garnet biotite gneiss) is reported from two localities in south Kerala. In the same localities, formation of acid charnockite patches are also noted. Structural analyses show that the breaking event post-dates the formation and isoclinal folding of banded charnockite, but clearly pre-dates the making of patchy charnockite.

Full Text

     

M. Santosh ¹

Affiliations
1 Centre for Earth Science Studies, P.B. 2235, Sasthamangalam, Trivandrum 695010, IN

Abstract

Two pegmatites, namely, Bandarcbua and Kharonia, show ideal mineralogical zoning with a massive quartz core surrounded by a muscovite·plagioclase-perthite·quartz and muscovite-quartz zone. Aqueous two phase inclusions with 40% vapour and three phase inclusions with C0₂(liquid)+CO₂ (gas)+H₂0(liquid) occur as primary inclusions in quartz from the feldspathic zone. Aqueous three phase inclusions with NaCl daughter mineral coexisting with gaseous inclusions with 60-80% vapour occur as pseudosecondary inclusions in quartz from the feldspathic zone and as primary inclusions in quartz from the muscovite-quartz zone. Quartz from migmatitic gneisses show the common occurrenee of monophase carbonic inclusions. Heating-freezing data assign CO₂ densities of 0.81-0.90 g/cm³ for the peak metamorphic fluids which were entrapped at 4.2 Kb pressure at a temperature of 680°C. The feldspathic zone crystallized from moderate density (0.6 g/cm³) and salinity (10 wt, % NaCI) fluids. Coexisting H₂0 and CO₂-H₂0 inclusions record a temperature of 630°C and a pressure of 2.4 Kb for the equilibration of the feldspathic zone. Subsequent adiabatic decompression resulted in the boiling of fluids with enhanced density (1.O g/cm³) and salinity (32 wt, % NaCl). Boiling, which precipitated bulk of the mineralization, occurred at temperatures of 360°C to 320°C and at pressures of 110-90 bars, corresponding to 1100-850 metres depth. The fluid characters suggest a probable relationship between regional metamorphism and the genesis of the mineralized pegmatites of the Bihar mica belt.

Full Text

     

M. Santosh ¹, P. K. Thampi ¹, S. S. Iyer ², Marina B. A. Vasconcellos ²

Affiliations
1 Centre for Earth Science Studies, P. B. 2235, Sasthamangalam, Trivandrum 695010, IN
2 Departmento de Processos Especiais-MEC, Institute de Resquisas Energ. e Nucleares, Caixa Postal 11049-Butanta, Sao Paulo, BR

Abstract

Rare earth element abundance in carbonatite associated with the 740 Ma old alkaline complex of Munnar is reported. High values of La (up to 232 ppm) and Ce (up to 261 ppm), moderate Sm (upto 11 ppm), with low Tb (up to 1.1 ppm), Yb (upto 3 ppm) and Lu (up to 0.6 ppm) are observed, comparable with similar values for carbonatites from other regions. The chondrite normalised patterns exhibit a steep slope from LREE to HREE, with (La/Lu)_cn ranging upto 191. A genetic model involving CO₂ degassing from the mantIe and ion exchange reactions with deep crustal rocks is envisaged, with the carbonatites probably representing a late immiscible fraction that separated from a polymerized alkali silicate melt. REE geochemistry of the carbonatite substantiates the view of a Late Precambrian alkaline magmatic regime in this part of the Indian Shield.

Full Text

     

M. Santosh ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum - 695 031, IN

Abstract

No Abstract.

Full Text

   

M. Santosh ¹

Affiliations
1 Centre for Earth Science Studies, P. B. 7250, Akkulam, Trivandrum 695 031, IN

Abstract

The 595 Ma old granite pluton punctures Precambrian low-alumina trondhjemites and occurs spatially related to the Moyar and Calicut fault-lineament. Mineralogically, it is a quartz-alkali-feldspar granite. Molybdenite occurs as disseminated flakes in the granite and associated pegmatites and as flaky aggregates in quartz veins. K: Na : Ca relationship suggests an adamellitic composition, whereas SiO₂ vs. Log₁₀K₂O/MgO proportions confirm its alkaline nature. In Harker variation diagrams, the oxide weight percentages show overall magmatic trends. Among trace elements, the notable features are low large ion lithophile element contents, moderate to high transition element levels and extremely high light rare earth element values. High SiO₂, K₂O and Na₂O values, with low abundance of Ba, Rb and Sr and enrichment of Nb and Zr are typical features comparable to alkali granites elsewhere. The Rb, Nb and Y characteristics denote a within-plate affinity. Petrochemical features indicate derivation of the granite from an alkali-enriched partial melt through quartz and alkali feldspar fractionation. The extremely high K/Rb values imply a K-enriched, Rb-depleted deep crustal source. The enrichment of LREE and HFS elements indicate an active role of CO₂-rich volatiles in the petrogenetic regime.

Full Text

     

M. Santosh ¹

Affiliations
1 Centre for Earth Science Studies, P. B. 7250, Akkulam, Trivandrum 695 031, IN

Abstract

The alkali granite pluton of Amba]avayal in Wynad district, Kerala, carries an 800m wide zone of disseminated molybdenite mineralization. Molybdenite also occurs as flakes and flaky aggregates in pegmatites and quartz veins. XRD and geochemical data indicate the molybdenite to be hexagonal 2H₁ polytype. The general mineralogic and geochemical features of the granite are consonant with those of granite molybdenite systems in general. The sulphur isotope values in five molybdenite separates range from -1.62 to + 1.54 ‰, denoting a narrow spread and a single igneous source. Fluid inclusions commonly occur in quartz associated with the granite, pegmatites and quartz veins. Heating-freezing studies show that the granite quartz entrapped high density (0.90-0.95 g/cm³) CO₂-rich fluids at pressures of 4.5-5.2 Kb. Coexisting CO₂ and CO₂-H₂O inclusions in pegmatites yield a P-T estimate of 2.2 Kb and 500°C. Fluid inclusions in the mineralized quartz veins show that the ore-forming fluids (with a maximum salinity of 15 wt. % NaCl) were heterogeneous and the molybdenite precipitation was triggered by adiabatic decompression and 'boiling' at temperatures of 340-360°C and vapor pressures of 110-150 bars. The cooling curve of the granite constructed from combined P-V-T data shows a T-convex path, implying near-isolhermal upward movement of the granite magma. The data are consistent with a genetic model linking magmatism and metallogeny with extensional tectonics in the late Precambrian.

Full Text

     

M. Santosh ¹, D. H. Jackson ², D. P. Mattey ², N. B. W. Harris ²

Affiliations
1 Centre for Earth Science Studies, P. B. 7250, Akkulam, Trivandrum 695031, IN
2 Department of Earth Sciences, Open University, Milton Keynes, MK7 6AA, Buckinghamshire, GB

Abstract

Carbon dioxide-rich inclusions commonly occur in banded charnockites and khondalites of southern Kerala as well as in incipient charnockites formed along oriented zones by desiccation of gneisses. Combined pressure-temperature data from solid phases and CO₂ equation of state barometry from fluid inclusion isochores depict that high density carbon dioxide was the dominant ambient fluid species in all the three rocks. Carbon stable isotope analysis of the fluids using a stepped heating technique in samples collected from Ponmudi, Kottavattom, Manali and Kadakamon show that the banded charnockites and gneisses contain ca. 50-60ppm carbon with δ¹³C values in the range of -8 to -12.3‰. The incipient charnockites are characterised by abundant CO₂ (upto 200 ppm) with δ¹³C in the range, -7.5 to -10.3‰. The carbon stable isotope value of + 1.2‰ obtained for calc-silicate is in ccntrast to the value for the intercalated banded charnockite from Kadakamon (-10‰) depicting that decarbonation was not the source of CO₂, Whereas the heavier carbon-enriched nature of incipient charnockite from Manali may suggest a juvenile source, the carbon isotope values in Ponmudi and Kottavattom suggest a mixed source, possibly due to the interaction of externally derived CO₂ with graphite in the rocks.

Full Text

     

M. Ramakrishnan ¹, M. Santosh ¹, N. S. Reddy ²

Affiliations
1 Centre for Earth Science Studies, Trivandrum 695 031, IN
2 Geological Survey of India, Hyderabad 500 001, IN

Abstract

Patches and oriented zones of 'pseudo-charnockites', similar in appearance to incipient charnockites from the granulite transition zones, are reported for the first time from the pink granites of Gadwal and Kurnool areas in the low-grade metamorphic terrain of northeast Dharwar craton. With a mineral assemblage of quartz, feldspars, micas and amphiboles showing varying degrees of alteration to chlorite and sericite, these zones preserve the signature of hydration following peak metamorphism. Though aqueous brine inclusions (average 6 wt % NaCl equivalent) constitute the dominant fluid phase in the granite and the retrograde zones, the occurrence of coexisting monophase inclusions with a near-pure carbonic fluid (density 0.70-0.80g/cm3) in the retrograde zones invoke special attention. Though carbonic inclusions generally typify dehydration reactions, it is suggested that retrograde reactions of the type which produced the pseudo-charnockite horizons could also result in the entrapment of nearpure CO₂ of moderate to low densities. The carbonic inclusions have apparently resulted from the immiscible separation of a mixed carbonic-aqueous fluid which exsolved into brine and CO₂ upon decompression following the rapid uplift of the Eastern Block along the Chitradurga boundary fault, concomitant with the copious intrusion of younger granitoids.

Keywords

Andhra Pradesh, Dharwar Craton, Fluid Inclusion, Greenstone Terrain, Pseudo-Charnockite.

Full Text

     

K. R. Hari ¹, M. Santosh ², A. C. Chatterjee ³

Affiliations
1 Department of Geology, Govt. Arts & Science College, Durg, M. P., IN
2 Centre for Earth Science Studies, P.B.7250, Akkulam, Trivandrum 695 031, IN
3 School of Studies in Geology, Vikram University, Ujjain, M.P., IN

Abstract

Fluid inclusion studies in olivine phenocrysts from alkali olivine basalt, iddingsite basalt and three-phenocryst basalt of the Pavagad igneous suite, an outlier of the Deccan Trap, indicate the presence of highly evolved primary silicate-melt inclusions with various daughter crystals of olivine, pyroxene, plagioclase and opaques. High temperature experiments using a heating stage indicate homogenization in the range of 1040 to 1240 °C. The gas bubble within these inclusions show melting close to the triple point forpure carbon dioxide and homogenize into liquid phase between 25 and 31 °C, denoting densities of 0,70-0.50 g/cm³. Trapping pressure estimated from CO₂ equation of state barometry is about 3.8 kbar, which translates to a depth of formation of ca. 10 km. This correlates well with an independent depth estimate of 10-14 km derived from combined melt and fluid homogenization data. The melt inclusion chardcteristics in the Pavagad suite suggesllhatlhe olivines equilibrated from a slow cooling system at high pressure conditions, the parental melt having a probable alkali olivine basalt composition.

Keywords

Silicate-Melt Inclusions, Olivine, Daughter Crystals, Homogenization, Temperature, Pressure, Pavagad Igneous Suite, Deccan Trap, Fluid Inclusions.

Full Text

     

P. K. Omana ¹, M. Santosh ¹

Affiliations
1 Centre for Earth Science Studies, P. B. 7250, Akkulam, Thuruvikkal Post, Trivandrum 695031, IN

Abstract

Laterite weathering profiles in the Nilambur gold prospect of northern Kerala contain dust, grains and nuggets of gold in the various zones. A close examination of the morphological parameters of gold particles separated from various horizons reveal that those occurring towards the upper part of the weathering profiles have nearspherical or ovoidal shapes with regular grain contours and dull lustre. A porous network of numerous etched pits characterise the grain surface. Towards the lower horizons, the degree of etching becomes less and the grains are xenomorphic with plane crystal faces and irregular to jagged contours. Grain-size in the laterite gold is almost four times larger than that of primary gold in vein quartz, while in individual weathering profiles grain-size shows a relative iDcrease from top to bottom. suggesting dissolution. Various textural patterns including arborescent dendrites, tufts. filaments and petalites of secondary gold have been identified. Spectacular incipient growth of painted gold with increased fineness and bright lustre characterise many of the grains, typifying low temperature chemical reprecipitation. An evaluation of the mechanisms of supergene gold formation suggests that ferrolysis reaction involving oxidation of pyrite was fundamental in mobilizing gold in the Nilambur weathering profiles, testified to by the intimate association between iron oxides and supergene gold grains. Migration was effected by capillarity and fluctuations in water table level. Precipitation was triggered by the reduction of anionic complexes in acidic profiles. The nature of occurrence, morphology and texture parameters of gold grains in the Nilambur laterites indicate that supergene gold genesis is dictated by distinct chemical reactions related to natural weathering processes.

Keywords

Laterite Gold, Gold-Morphology, Nilambur, Kerala.

Full Text

     

M. Santosh ¹, M. Jayananda ², B. Mahabaleswar ²

Affiliations
1 Department of Geosciences, Osaka City University, Osaka 558, JP
2 Department of Geology, Bangalore University, Bangalore 560 056, IN

Abstract

Fluid inclusion studies in the Closepet granite reveal the common occurrence of trapped fluids which show melting temperatures and laser excited Raman spectral-characteristics close to those for pure CO₂. The quartz-bound inclusions in this polyphase intrusive define two major genetic categories: an earlier CO₂-rich fluid with only minor traces of water and a late mixed carbonic-aqueous fluid. Compelling evidence for fluid immiscibility is preserved by coexisting inclusions with widely varying filling ratios and similar filling temperatures. Salinities upto 10 wt per cent NaCl are estimated from microthermometry, which ascribe relatively higher temperatures for fluid immiscihility in the CO₂-H₂O system. Our results indicate that this late Archaean granite body which truncates regional metamorphic isograds has probably been a major carrier of CO₂-rich volatiles, subsolidus exsolution and channelisation of which along structural locales caused incipient dehydration and the formation of Kabbaldurga-type arrested granulites in many adjacent localities. Our model seeks to associate advective transfer of both heat and volatiles required for dehydration with the Closepet magma.

Keywords

Closepet Granite, Fluid Inclusions, Immiscibility, Charnockite, Dehydration.

Full Text

     

M. Santosh ¹

Affiliations
1 Centre for Earth Science Studies, P.B. 7250, Akkulam, Thuruvikkal Post, Trivandrum 695 031, IN

Abstract

Ore fluids in the gold-quartz veins in Kolar (Champion Reef) and Wynad (Nilambur) are characterized by CO₂-bearing systems of low salinity and moderate density. Step-wise heating of auriferous quartz, and the isotopic analyses of CO₂ extracted from fluid inclusions yield a tightly constrained δ¹³C range of -5.9 to -9.4%0, providing the first direct evidence for the involvement of mantle carbon in the genesis ofthese Archaean gold deposits. The similarity in the range of carbon isotopic compositions of CO₂ trapped within the Archaean granulites and gold deposits suggests a common source for the fluids. The isotope data are consistent with the derivation of CO₂ from an outgassing sub-lithospheric mantle.

Keywords

Archaean Gold Genesis, Fluid Inclusions, CO₂ , P-T Regimes, Carbon Stable Isotopic Composition, Mantle Carbon.

Full Text

     

M. Santosh ¹

Affiliations
1 Department of Geosciences, Faculty of Science, Osaka City University, Osaka 558, JP

Abstract

From a generally fluid-free deep crust, carbon dioxide influx may occur related to the highest thermal anomaly attending granulite formation. This would manifest itself as high density fluid inclusions captured within different minerals. The transition from H₂O to CO₂ dominated fluids and the concomitant increase in CO₂ densities with metamorphic grade ohserved in regional amphibolite-granulite transitions, and the spectacular increase in the volume of CO₂ recorded from gneiss-incipient charnockite reaction fronts suggest that carbonic fluids cause granulite formation by the dilution of pore fluids and resultant lowering of water activity. Conversely, carbonic inclusions can also result as a consequence of granulite formation. This involves H₂O loss from pre-existing mixed CO₂-H₂O inclusions, by ductile strain-induced leakage during deformation, removal of H₂O to form retrograde hydrous assemblages, or by the absorption of H₂O in local anatectic melt fractions; in all these cases, pure CO₂ "residue" will result.

CO₂ influx need not necessarily manifest itself in charnockite formation alone; the major imprints of this process in aluminous metapelites could be stable isotope alteration. Highest density CO₂ inclusions in a rock need not necessarily represent the peak metamorphic fluid; entrapment and evolution of fluids are dictated by the nature of post-metamorphic P-T-fluid history. Desiccation along structural pathways of enhanced fluid flow is a viable mechanism to destabilize hydrous minerals, but the potential of such a process in generating local melt pockets is a topic which merits further consideration.

Models for the generation of CO₂ are diverse, and can be broadly grouped into internal and external sources. Stable isotope systematics of fluid inclusions and graphites from the southern Indian granulites attest to distinct pulses of external CO₂ often copious and at timescales short of attaining isotopic equilibrium between the influxing fluid and the precipitating graphite. This study proposes a unified model which links Pan-African syenite-chamockite associations and incipient charnockitic alteration of gneisses. Fluid-rich igneous activity of subcrustal origin is envisaged to have contributed the heat and CO₂ necessary to drive structurally-controlled dehydration reactions.

Keywords

Granulite, Fluid Inclusions, CO₂ Fluids.

Full Text

     

M. Santosh ¹

Affiliations
1 University of Paris, FR

Abstract

No Abstract.

Full Text

   

M. Satish Kumar ¹, M. Santosh ¹

Affiliations
1 Centre for Earth Science Studies. P.B.7250, Akkulam, Thuruvikkal Post, Trivandrum 695 031, IN

Abstract

The granite pluton at Kalpatta in northern Kerala is a member of the Late Precambrian magmatic suite emplaced proximal to transcrustal faults in southern India In this study. we report the find of fluorite mineralization within late pegmatite phase associated with the granite. Both purple and colourless fluorites are associated, with X-ray diffraction pattern indicating perfect crystalline nature. Wet chemical analyses show CaF₂ contents of 92 and 96 % for purple and transparent fluorites respectively. Primary fluid inclusions in both fluorite categories belong to aqueous bi-phase type, with T_h between 150-180° C, and salinity in the range of 3 to 9 wt. per cent NaCl equivalent. Qurtz associated with fluorite contains CO₂-H₂O inclusions with T_h(C02) between 8 to 15° C (density 0.85-0.90 g/cm³). The nature of mineralizing fluids suggests that fluorite crystallization occured from low temperature hydrothermal solutions derived as end-products of the frctionation of volatile- and alkli-enriched felsic magma. The mineralization is regarded as a finger-print of the alkaline affinity of Pan-African magmatism in Kerala.

Keywords

Fluorite, Fluid Inclusions, Kalpatta Granite, Kerala.

Full Text

     

M. Santosh ¹

Affiliations
1 Centre for Earth Science Studies, P.B. 7250, Akkulam, Thuruvikkal Post, Trivandrum - 695 031, IN

Abstract

The differential solubilities of gold and silver in low temperature fluids lead to Au-Ag decoupling in the weathering front. Selective extraction of silver from the primary ores, or differential precipitation of gold at the fluid front can generate supergene gold deposits with enhanced grade. Gold grains formed by each process will be characterized by distinct evolutionary trends in grain morphology and chemistry. In the laterite-hosted gold mineralization of Nilambur, gold fineness (1000 Au/Au+Ag) increases from 921 in the primary veins to an absolute 1000 in laterites, with a concomitant four-fold increase in grain size. The kinetics and growth of gold particles are dictated by the degree of supersaturation, size of the critical nucleus and the free energy of nucleation, together with time-integrated fluid-rock ratio.

Chemical propagation and physical dislocation of Au during weathering place limitations on the application of conventional exploration techniques, and interpretation of anomalies defined from geochemical surveys on bulk samples. Two cases arc identified in this study: "shadow gold" - where overlying prominentanomaly is unrelated to the barren reef bcneath; and "masked gold"- where the potential of the underlying protore is masked by a weak signal in the overlying anomaly. It is suggested that the gradients in Au conlent within individual grains. and the trends in morphological evolution of gold particles are potential pathfinders for gold exploration in lateritic terranes.

Keywords

Gold, Laterite, Silver, Economic Geology, Nilambur, Kerala.

Full Text

     

Radhi D. Menon ¹, M. Santosh ¹, M. Yoshida ²

Affiliations
1 Centre for Earth Science Studies, P.B. 7250, Akkulam, Trivandrum 695031, IN
2 Department of Geosciences, Osaka City University, Osaka 558, JP

Abstract

Within the gem field in southern Kerala and adjacent Tamil Nadu region, extending over an area of 70 X 35 sq km, a variety of precious and semi-precious stones occur such as chrysoberyl (cymophane or "eat's eye", and alexandrite varieties), ruby, sapphire, emerald, topaz, zircon, and amethyst. The primary mineralization occurs within zoned, complex pegmatites of Pan-African age, emplaced within granulite facies metapelites (khonda1ites), and variably weathered and lateritised. Gemstone deposits of secondary type are associated with stream gravels and placers.

Preliminary fluid inclusion results provide evidence for the involvement of CO₂-enriched felsic melts which interacted with the aluminous supracrustals in the formation of the pegmatite-hosted gem mineralization. Thennal decrepitation offluid inclusions under high vacuum, extraction and quantification of the fluids from inclusions indicate the presence of variable amounts of CO₂ in quartz (72.7 ppm), garnet (141.2 ppm) and chrysoberyl (51.6 ppm). The carbon isotopic composition of CO₂ in quartz indicates a 'Juvenile" magmatic nature, with δ¹³C va1ue of -8 per mil. The gem-bearing pegmatites may therefore represent cryptic pathways through which large volumes of felsic melts migrated, a process which can be correlated to deep-seated extension in the Gondwana crustal fragments during the PanAfrican.

Gem recovery in Kerala, despite vast untapped potential, is largely done through illegal means. We emphasize the need for a gem industry in the State, and the requirement to bring the local prospectors under the fold of artisans so as to make use of their experience and expertise, in gem prospecting.

Keywords

Gemstones, Pegmatites, Fluid Inclusions, Economic Geology, Kerala.

Full Text

     

M. Santosh ¹, Katsuhiko Suzuki ², Akimasa Masuda ³

Affiliations
1 Centre for Earth Science Studies, P.B. 7250, Akkulam, Thuruvikkal Post, Trivandrum 695 031, IN
2 Department of Chemistry, College of Arts & Science, University of Tokyo, Komaba, Meguro, Tokyo 153, JP
3 Department of Chemistry, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182, JP

Abstract

Rhenium-Osmium (Re-Os) dating of two molybdenite samples from the alkali granite and pegmatite of Ambalavayal in northern Kerala (S. India) yielded ages of 567±28 Ma and 566±77 Ma, respectively. These ages closely compare with the previously determined Rb-Sr whole rock age of 595±20 Ma Rb-Sr for granite, and K-Ar biotite age of 560±30 Ma for the pegmatite.

Our study provides the first direct determination of the timing of ore mineralization associated with felsic magmatism in southern India, and reveals the fingerprints of a prominent Pan-African metallogenic event. This timing coincides with the formation of rare metal and gemstone-bearing pegmatites in different parts of southern India, Sri Lanka, Madagascar and East Antarctica. In most cases, the mineralizations are genetically related to felsic magmas emplaced along structural conduits, suggesting that the magmatism and metallogeny are related to deep-seated extension in the cratonized crustal segments of the Gondwana assembly.

Keywords

Geochronology, Molybdenite, Pan-African, Metallogeny, Southern India.

Full Text

     

U. P. Radhika ¹, M. Santosh ¹, H. Wada ²

Affiliations
1 Centre for Earth Science Studies, Thuruvikkal Post, Trivandrum 695 031, IN
2 Institute of Geosciences, Shizuoka University, Shizuoka 422, JP

Abstract

Graphite is commonly associated with the metapelites of upper amphibolite to granulite grade in the Kerala Khondalite Belt (KKB). A number of distinct associations of graphite are identified in this study such as: (I) strataboundand stratiform disseminations in metapelites, (2) veins, coarse flakes, pods and lenses associated with pegmatites, (3) enrichment along shear zones, (4) flakes and books in calc-silicate-charnockite associations and (5) residual concentrations in weathered rocks and laterites. While graphite disseminations in the metapelites (khondalites) occur as small flakes oriented parallel to the metamorphic fabric, the vein and pegmatite hosted graphites are found as coarse Rakes and books. Graphite which fills mesoscopic shears and faults also form coarse aggregates. X-ray diffraction studies indicate a high degree of crystallographic ordering for all the graphite types. Scanning Electron Microscopy reveals perfect crystal forms for graphites from the vein and pegmatite assemblages. Carbon stable isotope analysis of twenty three graphite samples are presented. The disseminated graphite yielded δ¹³C values in the range of -17.5 to -32.1 per mil, characteristic of biogenic origin. Graphite associated with pegmatites shows δ¹³C in the range of -10 to -15.1%, while those from shear planes have values ranging from -8.2 to -12.4%. Heavier carbon enrichment in these cases is suggestive of precipitation from CO₂-rich fluids. The mechanisms which lead to the precipitation of graphite from CO₂-rich fluids are evaluated. Thermodynamic considerations indicate that introduction of CO₂-rich fluids into country rocks under low fO₂, will result in the precipitationof graphite on a quantitative basis. The vein-, pegmatite-, and shear-hosted graphite in Kerala may therefore represent large scale infiltration of CO₂,-rich fluids along structurally controlled pathways.

Full Text

     

M. Santosh ¹, U. P. Radhika ¹

Affiliations
1 Centre for Earth Science Studies, P.B. 7250, Akkulam, Thuruvikkal Post, Trivandrum 695 031, IN

Abstract

Field relations in an orthogneiss quarry within the Palghat Gap region illustrate the development of charnockite alteration halos surrounding alkaline pegmatite dykes. Altered orthopyroxenes in the charnockitic zones are artifacts from concordant melt pockets within the gneisses, where orthopyroxene has developed prior to the charnockitie alteration during regional metamorphic, in a process equivalent to dehydration melting. The charnockitic halos represent metasomatic alteration zones developed through the infiltration of CO₂-H₂O fluids expelled from the dykes at shallow crustal levels. Such examples of greasy green charnockitie alterations could be a widespread phenomenon in different crustal blocks of varying metamorphic grades, and are unrelated to regional prograde granulite processes.

Keywords

Charnockitcs, Fluid Inclusions, Petrology, Palghat Gap, Kerala.

Full Text

     

P. K. Omana ¹, M. Santosh ¹

Affiliations
1 Centre for Earth Science Studies, PB 7250, Akkulam, Thuruvikkal Post, Trivandrum - 695 031, IN

Abstract

A natural example for exceedingly pure gold was recently identified from the laterite weathering profiles of Nilambur, northern Kerala. The laterites are highly ferruginous and nodular, containing saprolite and clayey horizons. Within the various horizons in the laterites, gold is found as small grains or nuggets. In many cases, the oxidised zone contains high concentrations of gold, closely associated with iron oxide/hydroxide matrix and was observed to be crystalline. This occurrence supports dissolution and reprecipitation of gold during weathering.

Laterite samples from different horizons of two representative vertical sections of the weathering profiles in the Maruda area of Nilambur have been analysed for major. minor and trace elements, including gold in this study. The distribution of Fe, Al, Si, Ca, Mg, Na, K, P, Ti and Mn in these profiles can be correlated with the element mobility patterns associated with tropical weathering environment. The results indicate that a predominantly acidic environment is generated during lateritization, which is conducive to gold mobility. Our study confirms that the concentration of naturally purified gold in the laterites was achieved by acidic oxygenated fluids in the weathering front.

Keywords

Economic Geology, Geochemistry, Lateritic Gold, Kerala.

Full Text

     

M. Satish Kumar ¹, M. Santosh ¹, S. Yoshikura ², M. Yoshida ¹

Affiliations
1 Department of Geosciences, Osaka City University, Osaka 558, JP
2 Department of Geology, Kochi University, Kochi 780, JP

Abstract

Colour-coded compositional mapping under an electron microprobe provides useful information on chemical gradients within the domain of individual minerals. Here we apply this technique to granulite facies reaction textures from a calc-silicate-charnockite interface at Nuliyam, southern Kerala, to understand the element mobility patterns and mass balance related to fluid-induced metasomatism during deep crustal metamorphism. The contact zone assemblage at Nuliyam is characterized by plagioclase + quartz symplectites after K-feldspar. From element mapping of this reaction interface on micron - scale, and from quantitative analyses of the individal mineral phases under an electron microprobe, we trace the diffusion of K, and addition of Na and Ca, through an open system migration dUling the influx of externally-derived CO₂-rich fluids. Our study provides the first direct evidence for microscale chemical diffusion associated with CO₂-induced charnockite formation in southern Kerala, and illustrates the potential of colour-coded compositional mapping in tracing element mobility patterns.

Keywords

Metamorphic Petrology, EPMA - Compositional Mapping, Calc-Silicate, Charnockite, Nuliyam, Southern Kerala.

Full Text

     

H. M. Rajesh ¹, M. Santosh ¹

Affiliations
1 Centre for Earth Science Studies. PB 7250, Akkulam, Thuruvikkal Post, Trivandrum - 695 031, IN

Abstract

We report the occurrence of primary apatite crystals in association with graphite within alkaline pegmatite dykes in the Kerala Khondalite Belt (KKB). Petrologic and X-ray diffraction studies identify the mineral to be fluorapatite. Fluid inclusion studies show that the fluorapatites contain abundant CO₂-rich inclusions with densities in the range of 0.85-0.72 gcm^-3. A subsequent generation of trail-bound CO₂-HP inclusions with low salinity were entrapped along hydraulic cracks. The abundance of CO₂-rich inclusions, and the intimate association of fluorapatite with fluid-precipitated graphite suggest that the pegmatite dykes hosting fluorapatite were rich in CO₂-dominated fluids. We identify that among the magmatic conduits proposed for the transfer of CO₂-rich fluids in granulite processes in the KKB. the fluorapatite and graphite bearing pegmatites reported in this study form important examples.

Keywords

Petrology, Fluorapatite, Pegmatites, Fluid inclusion, Kerala Khondalite Belt.

Full Text

     

N. G. K. Nair ¹, M. Santosh ¹

Affiliations
1 Centre for Earth Science Studies, P. B. No. 2235, Trivandrum 695010, IN

Abstract

The E-W elongated pluton of Peralimala, Cannanore district, Kerala, which is classified here as an alkali granite, occurs along the Bavali lineament, spatially associated with the late Proterozoic anorthosite-gabbro-granite-granophyre suite of the Ezhimala Igneous Complex. Perthitic K-feldspar, plagioclase of albite-oligoclase composition, quartz alkali amphibole, aegirine and sphene constitute the main mineralogy of the alkali granite. Chemically, the rock shows high content of alkalies with K₂O values ranging up to 10.96 wt. percent. The extremely high levels of K, AI, Ba and Ba/Sr suggest the existence of a phlogopite-rich pocket in the mantle beneath the Bavali lineament from where the melt equilibrated. As peralkaline plutonism is a keynote of pre-rift tectonics, the occurrence of the alkali granite along the Bavali lineament is a manifestation of rift-related alkaline magmatism in this part of the western continental margin of India.

Full Text

     

K. R. Hari ¹, M. Satish Kumar ², M. Santosh ³, Suneet Kumar Rai ¹

Affiliations
1 Government Arts and Science College, Durg, Madhya Pradesh 491 001, IN
2 Department of Geoscience, Osaka City University-559, JP
3 Centre for Earth Science Studies, P.B.No. 7250, Akkulam, Trivandrum - 695 018, IN

Abstract

Carbonate-rich rocks occur as E-W trending dyke-like bodies in the Deccan 'basalts at Chhaktalao area in the lower Narmada valley and show a structurally controlled mode of emplacement. Modal composition shows calcite as the dominent mineral (∼90%). Ankerite, apatite, altered olivine, augite and opaques account for the remaining percentage. Geochemically, these rocks show high CaO (>50%) with low SiO₂, TiO₂, FeO* and K₂O. Trace element concentrations are low compared to average carbonatites. Stable isotope studies show that δ¹³C(-1.74‰ to -2.8‰) and δ¹⁸O(+12.75‰ to +13.35‰) values do not compare with those of "primary igneous carbonatites" eventhough they fall within the wide range of "igneous carbonatites". These rocks can be considered only as carbonate-rich rocks and not as true carbonatites.

Keywords

Petrology, Carbonate-Rich Rocks, Stable Isotope, Nannada Valley, Madhya Pradesh.

Full Text

     

M. Satish Kumar ¹, M. Santosh ¹, H. Wada ²

Affiliations
1 Department of Geosciences, Osaka City University, Sugimoto cho, Sumiyoshi ku 3-3-138, Osaka-558, JP
2 Institute of Geoscience, Shizuoka University, Ohya 836, Shizuoka 422, JP

Abstract

Calcite-graphite isotope exchange thermometer is a useful tool in elucidating the peak metamorphic temperatures in regionally metamorphosed marbles, for which normal cation exchange thermometers are not available.

Keywords

Isotope Thermometry, Granulites, Marbles, Geochemistry, Kerala.

Full Text