Articles written in Journal of Earth System Science

    • Textures and compositions of cobalt pentlandite and cobaltian mackinawite from the Madan-Kudan copper deposit, Khetri Copper Belt, Rajasthan, India

      Abu Saeed Baidya Atlanta Sen Dipak C Pal

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      The Khetri Copper Belt (KCB), a part of the Proterozoic Delhi–Aravalli fold belt in western India, hosts several Cu deposits, which are known to contain considerable Au, Ag, Co and Ni. Although many Cobearing phases have been reported from the KCB and adjacent areas, detailed textural and geochemical data are either unavailable or scant except for mackinawite. In this study, we describe the textures and compositions (determined by EPMA) of two very rare Co-rich phases, namely cobaltian mackinawite (containing up to 12.68 wt.% Co, 1.90wt.% Ni and 2.52 wt.% Cu) and cobalt-pentlandite (containing up to 49.30 wt.% Co and 10.19 wt.% Ni), identified based on composition, from the Madan-Kudan deposit. To the best of our knowledge, neither cobalt-pentlandite nor such highly Co-rich mackinawite have previously been reported from this area. The common sulphide minerals viz. chalcopyrite, pyrrhotite and rare pyrite occur in chalcopyrite-pyrrhotite±pyrite-magnetite-chlorite-blue amphibole (Cl-rich hastingsitepargasite- sadanagaite)±marialitic scapolite±allanite±uraninite veins in amphibole-bearing feldspathic quartzite and garnetiferous chlorite schist. Cobaltian mackinawite is invariably associated with chalcopyrite and occurs as exsolution and inclusion within chalcopyrite or outside, but at the contact of chalcopyrite. On the other hand, cobalt-pentlandite is invariably associated with pyrrhotite and shows similar textural relation with pyrrhotite as that of mackinawite with chalcopyrite. Mineralogically diverse undeformed sulphide veins comprising Cl-rich amphibole and locally Cl-rich marialitic scapolite suggests epigenetic hydrothermal mineralization involving Cl-rich saline fluid in the Madan-Kudan deposit.Transport of metals, derived from a mafic source rock with high intrinsic Ni:Co ratio, by Cl-rich fluid can suitably explain the high Co:Ni ratio of the studied ore minerals. Presence of such highly Co-rich phases and other circumstantial evidences, enumerated in this work, are consistent with variants of Fe oxide(–Cu–Au) (IOCG) style mineralization, at least for some stages of mineralization in the Madan-Kudan deposit.

    • Variation of strain pattern and its influence on the geometry of the uranium mineralized body in Bangurdih area, western part of the Singhbhum shear zone, eastern India


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      The mineralized (U bearing) rocks, near Bangurdih area, in the western part of the Singhbhum shear zone (SSZ) are strongly deformed and metamorphosed. The host and country rocks are mylonitic in nature with strong linear and planar fabrics. Surface studies indicate that the mineralized bodies are discontinuous in nature and occur sporadically. The thickness of the individual mineralized bodies is also not uniform. Present work is aimed at studying meso- and micro-scale deformational structures, their bearing on regional scale structure and their relation with the occurrences of U-mineralization in and around Bangurdih area. The area comprises predominantly of quartz–chlorite schist/quartz–chlorite–sericite schist with intercalated quartzite, meta-conglomerate and feldspathic schist of varying thickness. Apatite–tourmaline–magnetite-rich quartzite, chlorite schist and conglomerate are commonly radioactive. The uranium mineralized body at Bangurdih shows pinch-and-swell structure. Stretched thin quartz veins, observed within the sheared quartzite layers, have undergone layer parallel flow resulting in bidirectional pinch-and-swell structure lying on YZ and XZ sections of strain ellipsoid with their axes parallel and perpendicular to the shear lineation, respectively. Oriented samples were collected from quartz veins and the country rocks to prepare thin sections parallel to XY, YZ and XZ plane of strain ellipsoid from each sample for microstructural study. Aspect ratios of strain ellipses of XY, YZ and XZ sections are measured using Fry analysis. Flinn diagram shows that the rocks of the Bangurdih were subjected to bi-directional flattening (average k = 0.77), which indicates that in addition to simple shear deformation there is a shortening component perpendicular to the shear plane in this area. To correlate the strain pattern of Bangurdih with nearby areas, deformed pebbles from conglomeratic horizons and oriented samples of other rocks from Bhurkuli–Bundu and Rangamatia–Simulbera area were studied. Flinn diagram, prepared using all data, reveals that while the strain type in Bangurdih area shows intense flattening type of deformation, the Rangamatia–Simulbera area shows constrictional type of deformation (k values range from 0.57 to 0.88 and 1.02 to 2, respectively). Other locations (Bhurkuli–Bundu) in between Bangurdih and Rangamatia–Simulbera show both constriction and flattening type of strain (k values vary from 0.67 to 1.35). Therefore, it is concluded that the intense flattening type of simple shear deformation in Bangurdih area leads the ore rich layers to be stretched and detached in two directions (X and Y), which in turn, causes the mineralized body to occur intermittently with systematic trend on the surface and possibly in subsurface conditions too.

    • Hydrothermal alterations and U-REE mineralisation in the Narwapahar uranium deposit, Singhbhum shear zone, India


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      The polymetallic mineral district in the Singhbhum shear zone (SSZ), eastern India, hosts some of the most important Palaeoproterozoic uranium, copper and apatite–magnetite deposits in India. Although the shear zone had been the locale of profuse hydrothermal alteration and there is general consensus about hydrothermal mineralisation, the alteration types vis-à-vis ore mineralisation are not amply clear. In this paper, we describe ore mineralogy and multiple alteration types vis-à-vis uranium-rare earth element (U-REE) mineralisation in the Narwapahar uranium deposit located in the SSZ. Uranium mineralisationis hosted by intensely deformed and metamorphosed tourmalinite, albite schist and chlorite schist, which are mineralogically heterogeneous showing extreme variations in constituent minerals and their modal abundances in the outcrop, hand specimen and microscopic scales often grading to nearly monomineralic rocks. Thorium-poor uraninite is an ubiquitous ore mineral of uranium. High uranium content is also noted in ilmenorutile and some unidentified U silicates. Allanite is the most abundant REE-bearing mineral whereas monazite and xenotime are subordinate. Low-Ti magnetite is nearly ubiquitous in the U ores. Based on locally preserved cross-cutting hydrothermal veins and frozen-in mineral replacement textures, the sequential metasomatic events deciphered from the Narwapahar deposit are B (boron)${\to}$ Na–Fe (±Ca)${\to}$ K–Fe (±Ca)${\to}$ K${\to}$ H$^+$. Uranium mineralisation is associated with B, Na–Fe and H$^+$ metasomatism and perhaps with K–Fe metasomatism too whereas REE mineralisation is associated with K–Fe metasomatism and resultant hydrothermally altered rocks. The alteration types, especially the well-known pervasive regional-scale alkali metasomatism, the alteration paragenesis as described here and metal association (Fe–Cu–U–REE–Co–Ni–Mo–Au) are akin to Fe oxide (Cu–U–REE) (IOCG)-type mineralisation. The presence of a substantial amount of uranium in ilmenorutile and very fine-grained inclusion of uraninite in magnetite may potentially affect the recovery of uranium during beneficiation leading to low leachability of some uranium ores. The close association of REE-bearing minerals with the U ores requires focused study to examine the potential of U ores as a source of by-product REEs.

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