Talc deposits of Rema area in the Kumaun Inner Lesser Himalaya are hosted within high magnesium carbonates of the Proterozoic Deoban Formation. These deposits occur as irregular patches or pockets mainly within magnesite bodies, along with impurities of magnesite, dolomite and clinochlore. Textures represent different phases of reactions between magnesite and silica to produce talc. Petrography, XRD and geochemistry reveal that the talc has primarily developed at the expense of magnesite and silica, leaving dolomite largely un-reacted. Early fluid inclusions in magnesite and dolomite associated with talc are filled with H 2 O+NaCl+KCl± MgCl 2 ± CaCl 2 fluids, which represent basin fluid system during diagenesis of carbonates. Their varied degree of re-equilibration was although not pervasive but points to increased burial, and hence requires careful interpretation. H 2 O-CO 2 fluid with XCO 2 between 0.06 and 0.12 was equilibrated with talc formation. The reaction dolomite+quartz o talc was not extensive because T-XCO 2 was not favourable, and talc was developed principally after magnesite+quartz.
Primary gold mineralization occurring in the Lungri Khola region is observed in the Pre Cambrian green schist facies and Lower Paleozoic micaceous marble and limestone. The Pre-Cambrian green schist facies comprising mainly of sericite and chloritic quartzite, chlorite schist, quartz-chlorite schist, chlorite phyllite and schistose pebble beds include a discontinuous auriferous zone which persists laterally for about 30 km extension from the Gajul khola in the west to the upper reaches of Gam khola in the east. The auriferous host rocks are found confined close to the contact with the overlying Lower Paleozoic carbonates. Eight distinct auriferous hosts are encountered in the Pre-Cambrian green schists facies. Occurrence of auriferous host in the Lower Paleozoic limestone and marble is rather irregular and insignificant So far minor quartz-carbonate veins have indicated the presence of gold. Finely disseminated gold with minor amount of pyrite, chalcopyrite and rarely arsenopyrite occurs in the above hosts. Visible gold ranging from 0.08 to 2 mm dimension has been recovered. Gold content of upto 6.7 gm/ton has been recorded but in general they show 0.1 gm/ton gold which is significantly 30 to 60 times higher in magnitude than the background value. Depending upon the type of the hosts, three possible models, namely (1) volcanogenic (2) hydrothermal and (3) syn-sedimentary, could be suggested for the origin of the primary gold mineralization which has later undergone remobilisation after the initial deposition.
The pegmatites of Hyakule, and to a lesser extent, of Phakuwa area. Sankhuwa Sabha district. Eastern Nepal, have been a source for gem quality tourmaline since about 60 years. Only 4 pegmatite bodies, 2 each in Hyakule and Phakuwa, out of about 40 dikes altogether, yielded this precious stones despite at least 5 larger openings and probably some smaller ones in other dikes. The Phakuwa and Hyakule area occupies the eastern limb of the Arun anticline and consists of high grade metamorphic rocks of possibly Precambrian age belonging to the basal part of the Higher Himalayan crystalline. These rocks thrust south ward along the Main Central Thrust over the rocks of the Nawakot complex belonging to the Lesser Himalaya. The metamorphic sequence, about 2500 m thick, consists mainly of gneisses, garnet-kyanite and game-staurolite mica schist, quartzites and marbles with intercalated minor calc-silicate rocks. Pegmatites occur in the mica schist and interbedded calc-silicate schist sequence (approximately 1000 m thick) as scattered lenses or possibly dikes of a few min thickness, which cut discordantly through the host rocks. Internal zoning has developed generally into an upper blocky feldspar zone, a central quartz core with tourmaline and beryl (partly as aquamarine) and a lower fine grained, garnet bearing muscovite-quartz feldspar zone. Schorl is quite frequent as is biotite, the latter is often more abundant than muscovite. Uraninite and Ta-Nb minerals (tantalite, wodginile, pyrochlore) as well as cassiterite have been found in heavy mineral concentrates of a few investigated pegmatite samples. The current paper deals with several questions: what is the type of these pegmatites in terms of a classification? Do all pegmatites belong to the same generation and/or stage of evolution? Why are gem quality tourmalines known only from a few dikes? Is there an exploration method to distinguish between barren and gem tourmaline pegmatites in these areas? Could this method possibly be applied to other pegmatite areas in Nepal? We used trace and some major element characteristics of 24 coarse grained muscovite samples from 19 pegmatite dikes in Phakuwa and Hyakule and from localities nearby to answer these questions. Additional 4 muscovite samples from gem tourmaline bearing and barren pegmatites in the Ilam district have been included for comparison. The trace element characteristics, some main elements and some element ratios of coarse grained muscovites in Hyakule and Phakuwa distinguish clearly between a group of gem producing and barren pegmatites. The most powerful distinguishing geochemical features are the concentration ranges of Rb, Ba, MnO and MgO and the element ratios K/Rb and Li/Mg. Less useful are the concentrations of Cs, Sn and Ta because of a somewhat irregular behaviour. Surprisingly, Li is rather low in all samples and does not discriminate very distinctly between the two pegmatite groups. The results of this study can probably be used as a rapid exploration tool in sorting out barren pegmatites from pegmatites with a potential for gem quality tourmaline.
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