International Journal of Innovative Research and Development

D. Majumdar , P. Dutta

Abstract

Petrography and rare earth element (REE) geochemistry of representative anorogenic granite samples of Pan-African origin from the Kathalguri-Lungsung-Lalpahar area in western Mikir Hills of NE India were analyzed in an attempt to determine their origin, evolutionary status and tectonic setting. Petrographic study indicates appearance of REE enriched accessory minerals like zircon, monazite, apatite, sphene, allanite, xenotime etc., thereby, forcing reasonably high absolute REE concentrations. Chondrite normalized REE patterns suggest richness in LREE (194-2l38 ppm; av. 688.05 ppm) and low HREE content (27-65 ppm; av. 44 ppm). The LREE and HREE distribution patterns maintain consistent parallelism in the studied samples suggesting comagmatic origin, although, two samples, however, show minor deviation in cases of La and Eu values only. The ratios ofCe_N/Yb_N (7.66-35.35); La_N/Yb_N (3.18-16.67); La_N/Sm_N (2.72-5.27) and Th_N/Yb_N (0.9l-l.87) suggest differentiation and REE fractionation during magma production and derivation from mantle sources under variable degree of contamination with crustal material. Evaluated Eu/Eu^∗ (0.26-0.91 av. =0.47) ratios over a consistent level indicate moderate but less fluctuating f₀₂ level of magma at the source.

Keywords

Pan-African, Mikir Hills, LREE, HREE, Differentiation, Fractionation

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P. Bhattacharyya , B. K. Tamuli , D. Majumdar

Abstract

Sulphide mineralization in a thin shear of about 25 mts wide locating in the Potin - Yazali road section of Subansiri district, Arunachal Pradesh has been reappraised. The mineralization is structurally controlled. Structural elements include shear fractures and shear foliations; lithological variant is confined to garnetiferous quartz-biotite-schist. A comprehensive EPMA analysis of host minerals viz., garnet and biotite has been evaluated and P&T condition for the host rock has also been determined by garnet-biotite geothermometry. Ore mineralogical study revealed dominance of chalcopyrite and pyrrhotite with minor quantities of sphalerite, occasional patches of arsenopyrite and isolated finely disseminated bodies of pyrite. From the EPMA analysis of sphalerite, sphalerite geobarometry has been evaluated. The equilibrium temperature estimated for the peak metamorphic assemblages are 480°c ± 24°c while the pressure value might be higher than estimated value of 2.5 Kbar.

Keywords

Sulphide Mineralization, Potin-Yazali Area, Shear Zone, Structural

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