Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Himanshu K. Sachan ¹

Affiliations
1 Wadia Institute of Himalayan Geology, Dehradun - 248 001, IN

Abstract

The Deri-Ambaji massive sulphide deposits are associated with the Precambrian rocks of Delhi Supergroup. Detailed fluid inclusion studies of quartz associated with the host rock of sulphide deposit as well as with ores show high density CO₂ (1.05-1.08 g/cm³) as the dominant fluid phase and C0₂-H₂0 (0.94-0.98 g/cm³) and minor H₂O inclusions are also present. The isochore for the high density fluid inclusions indicate the pressure limit of 7.5 Kbars, at the upper temperature limit 800°C, which is in accordance with the estimation by mineralogic thermobarometery. The lower P-T limit is of 2.5 Kbar at 500°C, which indicates that the transition of upper amphibolite to granulite facies of metamorphism has taken place in the area. The P-T path delineated from combined mineralogical and fluid data is characterised by T-convex nature, which is indicative of isothermal crustal uplift, probably promoted by extensional tectonics.

Keywords

Fluid Inclusions, Thermobarometry, Metamorphism, Deri-Ambaji, Delhi Supergroup, Gujarat, Rajasthan.

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Himanshu K. Sachan ¹, Sumit K. Ghosh ¹

Affiliations
1 Wadia Institute of Himalayan Geology, Dehradun - 248 001, IN

Abstract

The fluid inclusion study in quartz overgrowth in the siliciclastics of Neoproterozoic Nagthat Formation reveals that the quartz cementation took place in the range of 80°C to 130°C with a salinity range of 0.82 to 9.18 wt. % NaCl. The microthermometric data, combined with petrographic and geologic evidences pennitted to constrain the nature of cementing fluids and source of cement. The silica for quartz cementation was derived partly from both the internal source (quartz and feldspar dissolution) and external source (pore fluids). It can be further suggested that the quartz cementation in the lower stratigraphic level (near the Chandpur Formation) is mainly by marine water while in the upper stratigraphic level (near the Blaini Formation), meteoric water has played an important role for the same.

Keywords

Fluid Inclusion, Neoproterozoic, Nagthat Formation, Lesser Himalaya, Uttar Pradesh.

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Himanshu K. Sachan ¹, Barun K. Mukherjee ², Talat Ahmad ³

Affiliations
1 Wadia Institute of Himalayan Geology, DehraDun - 248 001, IN
2 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, JP
3 Department of Geology, Delhi University , Delhi 110 007, IN

Abstract

Microthermometric study of fluid inclusions in gabbro from the Nidar ophiolitic sequence revealed that brine rich fluid (34-45 wt% NaC1) at the temperature of more than 400°C were predominant throughout the gabbro High saline inclusion homogenized by halite dissolution at the temperature of 276-400°C. All the moderate to high temperature(l30-390°C) inclusions are liquid and vapour dominated, commonly secondary in origin, majority having seawater-Like salinity( 17- 8 wt% NaC1).

The highly saline fluid was probably derived by phase separation of hydrothermal seawater or an exsolved magmatic aqueous phase at the temperature of 250-412°C. The migration of the Phase-Separated along fractures resulted in segregation of the vapour and brines and preferential entrapment of High-Saline inclusions at depth. These brines may have originated when seawater approached the top of the magma chamber. The subsequent fracturing of the gabbro at the temperature of 130-390°C facilitated the entry of seawater which pervasively altered the gabbro. This hydration processes resulted into the formation of secondary minerals or by mixing with Phase-Separated fluid, resulting in salinities Two-Times higher than that of seawater.

Keywords

Brine, Fluld Inclusions, Nidar Ophiolite, Ladakh.

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R. Islam ¹, Sumit K. Ghosh ¹, Himanshu K. Sachan ¹

Affiliations
1 Wadia Institute of Himalayan Geology, 33 General Mahadeo Singh Road, Dehra Dun - 248 001, IN

Abstract

The shallow marine Nagthat siliciclastics of the Jaunsar Group show extensive development of medium to thickly bedded arenites and wackes, and represent the terminal Proterozoic siliciclastic episodes of the Lesser Himalaya. Chemical variation between arenites and wackes of Nagthat siliciclastics is related to the elemental distribution within the different mineralogical phases. Inverse correlation of most of the major oxides and trace elements (Ni, Cr, Co and Nb) with SiO₂, indicate quartz dilution effect. The majority of arenites show the common presence of plagioclase, K-feldspar and clay minerals, but lack ferromagnesium minerals and therefore indicate minimal decomposition of felspar and greater decomposition of ferromagnesian minerals. This phenomenon is well documented by the positive correlation of Fe₂O₂^(t) Ni, CO, K₂O, Rb, Sr, Th, Nb and Y with Al₂O₃ hereby suggesting that elemental distributions are mainly controlled by clay minerals. A moderate correlation exists between Al₂O₃ and La and Yb suggesting that REEs are similarly controlled in part by clays or micas. The plots of Nagthat siliciclastics in A-CN-K diagram, along with their enriched K₂O contents suggest variable degree of post-depositional K-metasomatism. Presence of higher amounts of monocrystalline quartz than the polycrystalline quartz, characteristic major and trace element contents, and enriched LREE pattern with negative Eu anomaly indicate a source of granite or granitic gneiss. High Eu/Eu ratio, low Ni concentration and low La/Co, Th/Co and Cr/Th ratios corroborate the acidic source. Granite or granitic gneiss source signature, along with indicative palaeocurrent directions, suggest that the detritus for Nagthat siliciclastics may have been derived from the southerly direction where Banded Gneissic Complex (BGC) of Aravalli-Bundelkhand craton is widely developed.

Keywords

Neoproterozoic, Nagthat Siliciclastics, Source Rock, Geochemistry, Banded Gneissic Complex (BGC), Kumaun Lesser Himalaya.

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