Genesis of Panthal Magnesite Deposit, Udhampur District, J&K
Pankaj K Srivastava Department of Geology, University of Jammu, Jammu-180 006
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Introduction The genesis of carbonate hosted sparry magnesite deposit continues to be a subject of controversy and has been discussed between the extremes of mainly synsedimentary origin from sea water in hypersaline environment to an epigenetic replacement model with a wide range of parameters. The magnesite deposits of Lesser Himalaya are confined to the western sector from Kali Valley in Nepal to Jammu and Kashmir in India in the form of detached lensoid to tabular bodies. Geologically these deposits occupy a narrow stratigraphic level being associated with the late Middle Proterozoic carbonate sequences (Joshi, 1995). These magnesite deposits represent veistch type of mineralization of Pohl (1990). Panthal magnesite deposit is one of the important deposits occurring about 7.5 km north of Panthal village which is about 5 km east of Katra town in Udhampur district (J&K). The present paper deals with the genesis of the Panthal magnesite deposit based on fluid inclusion studies and geochemistry. Geological Setup The magnesite deposit in the area under study is associated with Proterozoic Sirban Limestone. It forms a part of Reasi Inlier, which is flanked by rocks belonging to Murree Group with a thin fringe of Subathu in between towards north and northwest, while the southern contact is marked by a thrust with Sirban Limestone overriding the Siwalik. The main lithounits observed in the area are dolostone with stromatolitic bands, belonging to Lower Member of Trikuta Formation. The magnesite deposit occurs as pockets and lensoid bodies within this dolostone. The largest lensoid body present in the area is with outcrop dimension of about (270 x 80) m2. Petrography Magnesite ore body contains mainly magnesite mineral with dolomite, calcite, talc, pyrite, sphalerite and rare quartz as accessory minerals. Replacement of dolomite by magnesite, both at mega and micro level, is present throughout the area. Very minor microcrystalline magnesite is also observed. These normally preserve the original stromatolytic structures of the host dolomite. There is a sharp contrast in the crystallinity pattern between host dolostone (fine grained) and magnesite (medium to coarse grained). On the basis of crystallinity, magnesite is subdivided as medium grained (main body) and coarse grained types, forming a radial or loaf shaped pattern of rhombs (vein type). At places the sparry magnesite is characterized by the presence of stylolites. Fluid Inclusion Study Fluid inclusion study in the host dolomite and magnesite provide necessary clue to the characteristics of fluids that gave rise to formation of these minerals. Three types of fluid inclusions have been observed. Characterization of fluid inclusions suggests that the carbonic (H2O-CO2) and multiphase inclusions are present only in the magnesite while the aqueous biphase inclusions are present in both the magnesite and dolomite. The micro-thermometric results of the fluid inclusions of different varieties of magnesite and dolomite from Panthal magnesite deposit indicate that they do not represent a high temperature hydrothermal activity. Homogenisation temperature as low as 90°C in dolomite and 119°C in magnesite could be representative of a basinal brine or a diagenetic fluid. The
Tm ice for aqueous inclusions ranges between -20.2°C to -1.7°C corresponding to salinity ranges between 22.5 to 2.8 weight percent NaCl equivalent. Density of these inclusions varies from 0.81 to 1.09 g/cm3. The carbonic inclusions show salinity ranging from 1.6 to 14.0 equivalent weight percent NaCl with density of these inclusions ranging from 0.93 to 0.78 gm/cm3. Th is higher than the aqueous inclusions. The multiphase inclusions consist of one daughter crystal. They are more common in the vein type magnesite where they show higher Th than the medium grained magnesite. These inclusions show salinity (30 to 41 equivalent weight percent NaCl) and density. The salinity and density of these inclusions in medium grained magnesite is comparatively low. Geochemistry The host dolomite and magnesite show similar Mn, Si, Al and K contents. The host dolostone has a lower FeO content than magnesite. On the basis of trace element variations with MgO, it is suggested that the Cr and Ni were not present in the Mg bearing solution to incorporate in the magnesite lattice. A traceable amount of Sr is observed in the magnesite which is much lower than the host dolostone. The concentration of Zn in magnesite is higher than Cu and Pb. Enrichment of Zn is also indicated from the presence of sphalerite in magnesite. Total REEs concentration of the magnesite is very low. The (LaN/YbN) ratio ranges from 1.84 to 2.06 while (LaN/LuN) ratio ranges from 1.66 to 2.06 in the magnesite. The chondrite normalized REEs pattern for magnesite is characterized by a moderate negative Eu anomaly with Eu/Eu* varying from 0.40 to 3.25. Genesis The petrographic and geochemical evidence, like presence of dolomite relics within the magnesite at mega- and micro-level and increase of Mn, Fe, Zn in the sparry and vein magnesite over the host dolomite, negative correlation of CaO with MgO, lower LREE/HREE ratios of the magnesite, clearly indicate that the magnesite is formed by the replacement of precursor dolomite by a Mg rich solution (Lugli et al. 2000). The common occurrence of the aqueous fluid, with comparable homogenization temperature and salinity, in magnesite and host dolomite suggests that the aqueous brine was present over a long time from deposition of dolomite to the formation of magnesite. This range the salinity and density for these fluids are characteristic of basinal environment (Huraiova et al., 2002). It is proposed that the initial enrichment of Mg in the solution must have been supplied by the strong process of compaction suffered by the underlying pelites in the Lower Trikuta Formation (e.g. Morteani, et al., 1982). Further, during diagenesis to low grade or metamorphism, the Mg releasing reactions at depth have given rise to the excess magnesium in the basinal fluids to form a further Mg+2 rich solution with CO2 to form sparry magnesite. References Huraiova, M., Vozaarova,A., and Repcok, I. (2002). Fluid inclusion and stable isotope constraints on the origin of magnesite at Burda, Ochtina, Lubenik and Ploske deposits (Slovakia, Western Carpathians). Geologica Carpathica, 53 : 96-99. Joshi, M.N. (1995). Sparry Magnesite of lesser Himalayas: A review of various Genetic Models; BRJ Jour. Adv. Sci. Tech., 1(1) :1-9. Lugli,S, Torres – Ruiz, J., Garatj, G. and Olmedo, F. (2000). Petrography and geochemistry of the Eugai Magnesite deposit (Western Pyrenees, Spain): Evidences for the development of Zebra banding in dolomite replacement. Econ. Geol., 95: 1775-1791. Morteani, G., Moller, P. and Schley, F. (1982). The Rare Earth Element content and the origin of the sparry magnesite mineralization of Tux-Lancrsbach. Entachen Alm, Spiessnagel and Hochfilzen, Austria, and the lacostrien magnesite deposits of Aiani-Kozani, Greece, and Bela Stena, Yugoslawia. – Econ. Geol.,77: 617-631. Pohl, W. (1990). Genesis of the magnesite deposits, model and trends. Geologiosche Rundscau.79: 291-299.