Formation of polymetallic ores in the metasedimentary rocks of Rangpo area, Sikkim Lesser Himalaya, India: Mineralogical and geochemical attributes.

Present work investigates the ore mineralogy, geochemistry of host metasedimentary rocks, and the mineral chemistry of the main sulphide minerals, namely, sphalerite, chalcopyrite, galena, and pyrite from the Rangpo sulphide mineralization, Sikkim. Based on the textural relationships of the oxide and sulphide minerals such as mutual boundary, exsolution blebs, and replacement features, three stages including two stages of primary ore mineral formation, and an alteration stage of secondary mineral development is established. Oxide and sulphide minerals predominantly magnetite, chalcopyrite, sphalerite, galena, pyrrhotite, and cobaltite occur as stage‐I ore minerals, and the sulphide minerals namely galena, pyrrhotite chalcopyrite, arsenopyrite, sphalerite, and pyrite are identified as stage‐II minerals. The alteration minerals malachite and azurite represent stage‐III. The geochemistry of host metasedimentary rocks is characterized by SiO2 in the range of 53.28 to 85.62 wt%, Mg# values between 25.48 and 49.60, MgO and K2O having a strong negative correlation with SiO2, high values of Zr/Sc (32.5) versus Th/Sc (3.44), and enrichment of trace elements like Cr (30–87 ppm), V (27–81 ppm), Ni (13–49 ppm), and Sc (3–12 ppm). The Th/Sc versus Zr/Sc, K2O/Na2O versus SiO2, and Th/Co versus La/Sc plots attribute that the host metasedimentary rocks are recycled sediments largely of passive continental margin with a felsic source. However, the Mg# versus SiO2 and K2O/Na2O versus SiO2 point to the incorporation of mantle material from the orogenic margin, possibly arc‐related and mafic in nature. The features such as the mineral assemblage, the simultaneous crystallization shown by mutual boundaries and exsolution blebs, and the mineral chemical data of ore minerals favour a high‐temperature hydrothermal environment of ore deposition. This inference is substantiated by general high Fe concentration in sphalerite and other ore minerals, 800 to 2,400 ppm Cd in sphalerite, Ag and Sb in galena from 1,000 to 2,500 ppm and 400 to 800 ppm respectively, and noticeable Ag in chalcopyrite. The inferred depositional and tectonic environment of host metasedimentary rocks, the petrographic features of ores, and their mineral chemical data altogether invoke that the Rangpo ore mineralization in metasedimentary rocks was formed at high temperature in an environment where mafic mantle material from the active arc was amalgamated with the passive margin sediments derived from the felsic source rocks. [ABSTRACT FROM AUTHOR]