FT-IR and XRD analysis of coal from Makum coalfield of Assam

High sulphur coal sample from Ledo colliery of Makum coalfield, Assam, India was studied using FT-IR and XRD methods. FT-IR study shows the presence of aliphatic -CH, -CH₂ and -CH₃ groups, aliphatic C-O-C stretching associated with -OH and -NH stretching vibrations and HCC rocking (single and condensed rings). XRD pattern of the coal shows that it is amorphous in nature. Function of Radial Distribution Analysis (FRDA) indicates that coal is lignite in type and there is no evidence of graphite-like structure. The first maximum in the G(r) plot of FRDA at r = 0.14 nm relates to the aliphatic C-C bond (Type C-CH=CH-C), the second maximum at r = 0.25 nm relates to the distance between carbon atoms of aliphatic chains that are located across one carbon atom. The curve intensity profiles obtained from FRDA show quite regular molecular packets for this coal. The coal was found to be lignite in nature.     

Environmental geochemistry and quality assessment of mine water of Jharia coalfield,India

A long mining history and unscientific exploitation of Jharia coalfield caused many environmental problems including water resource depletion and contamination. A geochemical study of mine water in the Jharia coalfield has been undertaken to assess its quality and suitability for domestic, industrial and irrigation uses. For this purpose, 92 mine water samples collected from different mining areas of Jharia coalfield were analysed for pH, electrical conductivity (EC), major cations (Ca²⁺, Mg²⁺, Na⁺, K⁺), anions (F⁻, Cl⁻, HCO₃ ⁻, SO₄ ²⁻, NO₃ ⁻), dissolved silica (H₄SiO₄) and trace metals. The pH of the analysed mine water samples varied from 6.2 to 8.6, indicating mildly acidic to alkaline nature. Concentration of TDS varied from 437 to 1,593 mg L⁻¹ and spatial differences in TDS values reflect the variation in lithology, surface activities and hydrological regime prevailing in the region. SO₄ ²⁻ and HCO₃ ⁻ are dominant in the anion and Mg²⁺ and Ca²⁺ in the cation chemistry of mine water. High concentrations of SO₄ ²⁻ in the mine water of the area are attributed to the oxidative weathering of pyrites. Ca–Mg–SO₄ and Ca–Mg–HCO₃ are the dominant hydrochemical facies. The drinking water quality assessment indicates that number of mine water samples have high TDS, total hardness and SO₄ ²⁻ concentrations and needs treatment before its utilization. Concentrations of some trace metals (Fe, Mn, Ni, Pb) were also found to be above the desirable levels recommended for drinking water. The mine water is good to permissible quality and suitable for irrigation in most cases. However, higher salinity, residual sodium carbonate and Mg-ratio restrict its suitability for irrigation at some sites.     

Petrology of oligocene coal, makum coalfield, assam, Northeast India

There are five workable coal beds in the Tikak Parbat Formation of the Barail Group in the Makum coalfield, Tinsukia District, Assam. Two of these beds, 18 and 6 m thick, are persistent across the field. The coal is high volatile bituminous B/C, has excellent coking properties, and is of great importance as a blending coal to improve the coking properties of the lower-quality Gondwanan coals.These coals are bimacerites as vitrinertite or trimacerites as duroclarite. Virtrinite predominates with minor amounts of other macerals and minerals. The high percentage of vitrinite indicates that the bark and woody tissues were the dominant contributors to the precursor peat. These peats were strongly decomposed under anaerobic conditions as indicated by the abundance of the collinite type of vitrinite.     

Mineralogical and ash geochemical studies of coal-mine shale and its hydrocarbon potential: A case study of shale from Makum coalfield,Northeast India

The mineralogy and geochemical studies of the coal-mine shale collected from the Tirap opencast coal-mine (Makum coalfield, Northeast India) are reported in this paper. Thermo-chemical conversion (pyrolysis) of coal-mine shale has been studied to see its hydrocarbon potential. A combined approach using X-Ray diffraction (LTA-XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray spectrophotometer (EDS), X-ray fluorescence spectrometry (XRF), thermogravimetry-derivative and differential thermogravimetric (TG-DTG and DTA) analysis is made to obtain new information on the mineralogical and geochemical studies of a coal-mine shale (CMS) sample. Gas chromatography-mass spectrometry (GC-MS) analysis is performed to evaluate the quality of the liquid fraction (tar) obtained after pyrolysis at 600°C. The shale sample is dominated by quartz, clay minerals (kaolinite and illite), sulphate bearing phase like gypsum with minor proportion of anatase, probably as artifacts of the plasma-ashing process. GC-MS analysis illustrates the presence of highly oxygenated organic components (M.W. around 94-108) and high molecular weight (M.W. 256) cyclic sulphur (e.g. octathiocane with molecular formula S₈) compounds along with the complex N-containing organic sulphur compounds (M.W. around 255-486) in the tar produced.     

Eco-restoration of a high-sulphur coal mine overburden dumping site in northeast India: A case study

Eco-restoration of mine overburden (OB) or abandoned mine sites is a major environmental concern. In the present investigation, an integrated approach was used to rejuvenate a high-sulphur mine OB dumping site in the Tirap Collieries, Assam, India, which is situated in the Indo-Burma mega-biodiversity hotspot. A mine OB is devoid of true soil character with poor macro and micronutrient content and contains elevated concentrations of trace and heavy metals. Planting of herbs, shrubs, cover crops and tree species at close proximity leads to primary and secondary sere state succession within a period of 3 to 5 years. A variety of plant species were screened for potential use in restoration: herbs, including Sccharum spontaneum, Cymbopogon winterianus Jowitt (citronella), and Cymbopogon flexuosus (lemon grass) cover plants, including Mimosa strigillosa, M. striata, and M. pigra; shrubs, including Sesbania rostrata (dhaincha) and Cassia streata (cassia); and tree species, including Gmelina arborea (gomari) and Dalbergia sissoo (sissoo). Amendment with unmined soil and bio-organic matter was required for primary establishment of some plant species. Management of these plant species at the site will ensure long term sustainable eco-restoration of the coal mine-degraded land.     

A preliminary study of mineralogy and geochemistry of four coal samples from northern Iran

This study is related to four Jurassic-age bituminous coal (0.69–1.02 Ro%) samples collected from coal mines from the west, central and east of central, Alborz in northern Iran. Geological settings played key roles in determining the geochemistry and mineralogy of coals from the central Alborz region of northern Iran. The mineralogy of coals from the eastern part of the region is dominated by kaolinite; halloysite; and carbonates such as calcite, dolomite/ankerite, and siderite. The coals were deposited in a lacustrine environment. In the western part of the region, where the depositional setting was also lacustrine with volcanic input and tonstein deposition (glass shards present), the coal primarily contains kaolinite (68%) and fluorapatite (26%). In contrast, coal from the central part of the region, which was deposited in a terrestrial environment and on eroded limestone and dolomite rocks, is dominated by dolomite (98%) with little input by kaolinite. These coals have low sulphur (0.35–0.70 wt.%), which is mostly in the organic form (0.34–0.69 wt.%). Pyritic sulphur is detected only in one coal and in small quantities. The boron contents of these coals range from 9 to 33 mg/kg, indicating that deposition occurred in a fresh water environment. Coal with higher concentrations of Ba, Sr, and P contain fluorapatite and goyazite–gorceixite series [BaAl₃ (PO₄)₂ (OH)₅, H₂O] minerals, which indicates volcanoclastic input. Compared to world coal averages, these coals exhibit low concentrations of elements of environmental concern, such as As (1.3–5.9 mg/kg), Cd (< 0.02–0.06 mg/kg), Hg (< 0.01–0.07 mg/kg) Mo (< 0.6–1.7 mg/kg), Pb (4.8–13 mg/kg), Th (0.5–21 mg/kg), Se (< 0.2–0.8 mg/kg) and U (0.2–4.6 mg/kg). Two of the northern Iranian coals have concentrations of Cl (2560 and 3010 mg/kg) that are higher than world coal average.     

Hydrogeochemical characteristics of acid mine drainage and water pollution at Makum Coalfield,India

Acid mine drainage (AMD) is one of the severe environmental problems that coal mines are facing. Generation of AMD in the northeastern part of India due to the coal mining activities has long been reported. However detailed geochemical characterization of AMD and its impact on water quality of various creeks, river and groundwater in the area has never been reported. Coal and coal measure rocks in the study area show finely disseminated pyrite crystals. Secondary solid phases, resulted due to oxidation of pyrite, occur on the surface of coal, and are mainly consisting of hydrated sulphate complexes of Fe and Mg (copiapite group of minerals). The direct mine discharges are highly acidic (up to pH 2.3) to alkaline (up to pH 7.6) in nature with high concentration of SO₄²⁻. Acidic discharges are highly enriched with Fe, Al, Mn, Ni, Pb and Cd, while Cr, Cu, Zn and Co are below their maximum permissible limit in most mine discharges. Creeks that carrying the direct mine discharges are highly contaminated; whereas major rivers are not much impacted by AMD. Ground water close to the collieries and AMD affected creeks are highly contaminated by Mn, Fe and Pb. Through geochemical modeling, it is inferred that jarosite is stable at pH less than 2.5, schwertmannite at pH less than 4.5, ferrihydrite above 5.8 and goethite is stable over wide range of pH, from highly acidic to alkaline condition.     

Braunite mineralogy and paragenesis from the Kajlidongri mine,Madhya Pradesh,India

The Kajlidongri manganese deposit, Madhya Pradesh, India contains braunite belonging to at least two different parageneses, i.e. a fine-grained, metamorphic type and a younger, coarser-grained, hydrothermal type. Microanalyses of braunite from these paragenetic types indicate considerable variations in iron content from zero to 32.7% Fe₃O₃, which the writers consider is due to varying proportions of braunite I and bixbyite modules in polysomatic braunite.     

Temporal transition analysis of coal mine fire of Jharia coalfield,India, using Landsat satellite imageries

Coal is dominant energy source of world at large and India in particular for several decades and expected to being continued for foreseeable future. However, fire in this fossils fuel is still a global catastrophe for the major coal-producing countries. There are several direct and cascading distress of coal mine fire has been encountered by environment, economy, safety and society. Without knowledge of actual status and extent of coal mine fire, any scope for its better managements may be futile. In India, the coal mine fire of well-known Jharia coalfield (JCF) has been continued since last ten decades and still spreading towards newer areas. The aim of this paper is to highlight the chronological net lateral spatial changes in coal mine fire areas of JCF using Landsat satellite thermal imageries. The mapping of coal mine fire area of JCF during 1988–2013 has been carried out with consecutive 5-year intervals. The lateral changes in surface and subsurface fire areas along with propagation towards new area have been studied. The study also included to evaluate changes of status and extent fire during this time span and effect of its management efforts. On the basis of aforementioned analysis, the study concludes a slow reducing trend in coal mine fire area during last 25 years.     

Petrographic and geochemical characterization of coal from Talcher coalfield,Mahanadi Basin,India

In the present study an attempt has been made to characterize the coals of Talcher coalfield employing petrographic and geochemical techniques on a large number of coal samples. Pillar coal samples were collected from all the six workable coal seams, which occur in the Karharbari (Seam-I) and the Barakar (Seams II, III, IV, V and IX) formations.     

Petrographic, mineralogy, and geochemistry of coals of Pabedana, Kerman Province, Central Iran

This paper discusses the result of the detailed investigations carried out on the coal characteristics, including coal petrography and its geochemistry of the Pabedana region. A total of 16 samples were collected from four coal seams d2, d4, d5, and d6 of the Pabedana underground mine which is located in the central part of the Central-East Iranian Microcontinent. These samples were reduced to four samples through composite sampling of each seam and were analyzed for their petrographic, mineralogical, and geochemical compositions. Proximate analysis data of the Pabedana coals indicate no major variations in the moisture, ash, volatile matter, and fixed carbon contents in the coals of different seams. Based on sulfur content, the Pabedana coals may be classified as low-sulfur coals. The low-sulfur contents in the Pabedana coal and relatively low proportion of pyritic sulfur suggest a possible fresh water environment during the deposition of the peat of the Pabedana coal. X-ray diffraction and petrographic analyses indicate the presence of pyrite in coal samples. The Pabedana coals have been classified as a high volatile, bituminous coal in accordance with the vitrinite reflectance values (58.75–74.32 %) and other rank parameters (carbon, calorific value, and volatile matter content). The maceral analysis and reflectance study suggest that the coals in all the four seams are of good quality with low maceral matter association. Mineralogical investigations indicate that the inorganic fraction in the Pabedana coal samples is dominated by carbonates; thus, constituting the major inorganic fraction of the coal samples. Illite, kaolinite, muscovite, quartz, feldspar, apatite, and hematite occur as minor or trace phases. The variation in major elements content is relatively narrow between different coal seams. Elements Sc,, Zr, Ga, Ge, La, As, W, Ce, Sb, Nb, Th, Pb, Se, Tl, Bi, Hg, Re, Li, Zn, Mo, and Ba show varying negative correlation with ash yield. These elements possibly have an organic affinity and may be present as primary biological concentrations either with tissues in living condition and/or through sorption and formation of organometallic compounds.     

Environmental geochemistry and mineralogy of lead at the old mine area of Baccu Locci (south-east Sardinia,Italy)

About a century of exploitation of the galena-arsenopyrite deposit of Baccu Locci in Sardinia (Italy) has caused a severe, persistent arsenic contamination that extends downstream from the mine for several kilometres. Differently from As, the contamination of lead in surface waters is only localised in the upper part of the mine despite very high Pb concentrations in geologic materials (waste rocks, tailings, stream sediments, soils) over the whole Baccu Locci stream catchment. The aqueous and solid speciation of Pb in various environmental media of the Baccu Locci system was determined by means of a combined analytical (ICP-MS, SEM-EDX, TEM-EDX, chemical extraction) and thermodynamic approach (PHREEQC). The study has pointed out that relatively little Pb (up to 30 µg/L) is initially released to surface waters (pH = 7–8, Eh = 0.4–0.6 V) very rapidly due to dissolution of anglesite that is the first product of galena oxidation. Subsequently, Pb is removed (down to 0.6 µg/L) by probable sorption onto hydrous ferric oxides (e.g. ferrihydrite) and/or possible precipitation of As-containing plumbojarosite that is the main secondary Pb-bearing phase in stream sediments/tailings along the Baccu Locci stream course. The latter hypothesis is controversial since it is reported from the literature that plumbojarosite is formed under acidic conditions, although there is contrary field evidence as well. Bearing in mind the uncertainties introduced from thermodynamic and analytical data, the solubility calculations indicate strong undersaturation of surface waters with respect to plumbojarosite (SI: ? 19.9 to ? 3.7). On the contrary arsenatian plumbojarosite is at or close to saturation (SI: ? 0.6 to 3.2) in most surface waters and beudantite is clearly above saturation (SI: 4.1 to 12.7). This suggests that the incorporation of As might increase the stability of plumbojarosite, extending it up to near-neutral conditions. As a consequence, Pb is prevented from being released downstream to surface waters, and dissolved Pb concentrations remain definitely below the Italian and WHO limits for drinking waters (50 µg/L and 10 µg/L, respectively).     

Geochemistry of ochreous precipitates from coal mine drainage in India

The deposition of ochreous is common by a consequence of acid mine drainage (AMD). The ochreous precipitated from the AMD sites around Tertiary coalfield of Assam, India were collected and characterized by X-ray diffractometry (XRD), Fe to S molar ratio, ammonium oxalate acid (pH 3.0) extraction, fourier-transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The ochreous mainly consists of goethite, schwertmannite, ferrihydrite and jarosite. Mineralogy of ochreous was controlled by the pH whereas formation of ferrihydrite was favored at high organic carbon content. Role of bacteria for the formation of secondary minerals was observed. Mobility of metals was controlled by the ochreous, and they were also retained during the process of phase transformation of poorly ordered iron-oxyhydroxysulfates into the stable forms.     

Subsurface variations in arsenic mineralogy and geochemistry following long-term weathering of gold mine tailings

Variations in arsenic (As) mineralogy and geochemical controls on its mobility were evaluated in subsurface tailings at the historical Montague and Goldenville mine sites in Nova Scotia, Canada. Tailings at these sites contain some of the highest As concentrations in Nova Scotia and are located in close proximity to local communities. Pore water in the subsurface tailings is characterized by circumneutral to alkaline pH (6.2 to 8.7) and mildly reducing to oxidizing redox conditions (+130 mV to +347 mV). Bulk chemistry, scanning electron microscopy, and synchrotron micro-X-ray diffraction analyses showed As mineral hosts differ with depth. The deepest tailings (max. 2 m) are in direct contact with partially decomposed vegetation, which supports reducing conditions and the precipitation of authigenic As and Fe sulfides. Under reducing conditions, dissolved As concentrations are also controlled by desorption of As from dissolution of Fe and Mn oxides and the sorption or co-precipitation of As with carbonates. These geochemical controls differ from those influencing dissolved As concentrations under oxidizing conditions. In the near surface, As mobility is controlled by oxidative dissolution of primary arsenopyrite, precipitation of secondary Fe arsenates, Fe oxyhydroxides and Mn oxides, secondary Ca-Fe arsenates, and sorption onto Fe oxyhydroxides and gangue minerals. Some of these mineral species are stable under different conditions yet occur in close association, indicating the importance of microenvironments. The results of this study show that the weathering characteristics of these tailings vary with depth, leading to the formation of new As hosts that are distinct from those observed in the near surface. Identification of these As hosts provides an understanding of current controls on As mobility and has implications for future reprocessing and/or remediation efforts.     

阳泉新景矿高煤级煤的孔隙结构分形特征

分形理论是宏观上定量评价储层非均质性的有效手段。以阳泉新景矿高煤级煤样品压汞数据为基础,建立分形几何模型,定量描述了孔隙结构。实验结果表明：样品孔隙以纳米孔为主,孔径、比表面积及孔容也集中分布在纳米孔段。煤样孔径65 nm以上的孔隙具有显著的分形特征,分维值分布范围为2.89~2.99,体积增量呈现阶段式的变化,孔隙结构复杂;孔径65 nm以下孔隙几乎无分形特征,比表面积增量与孔径在对数坐标中呈线性关系;基于分形特征及分子运动规律,将储层孔隙以孔径65 nm为界划分为扩散孔和渗透孔2个大类6个小类。分维值与体积孔隙中值半径、总孔隙体积呈负相关,与孔径65 nm以上的孔隙体积、比表面积呈正相关,与孔隙度无相关性。分形分维值对储层结构具有较全面的表征能力,可以作为综合指标在煤储层孔隙研究中加以应用。     

Abundance of sulfur in Eocene coal beds from Bapung, Northeast India

At 625°C the percentage of sulfur in the non-volatile portion of the Bapung coal beds ranges from 1.08 to 3.10 wt.%, with the pyritic sulfur ranging from 0.02 to 0.32 wt.%, the sulfate sulfur ranges from 0.02 to 0.30 wt.%, and the organic sulfur from 0.88 to 2.49 wt.%. The total sulfur in the Bapung coal ranges from 2.50 to 12.44%. Organic sulfur appears to be more abundant among the different sulfur species. However, the pyritic sulfur increases quickly as the total sulfur content increases at 925°C, the percentage of sulfur in non-volatile portion ranges from 1.27 to 2.54 wt.%, with pyritic sulfur ranging from 0.02 to 0.56 wt.%, sulfate sulfur ranges from 0.04 to 0.16 wt.%, and the organic sulfur from 1.15 to 2.03 wt.%.     

Changes in geochemistry and mineralogy of thermally altered coal, Upper Hunter Valley, Australia

Igneous intrusions thermally and geochemically alter coal, commonly causing economic and safety problems for many coal mines. The effects of two dykes on the inorganic content of the Late Permian bituminous Upper Wynn seam were determined from analyses of 44 samples that were collected along transects approaching the intrusions. Petrographic and XRD data were used to determine sample mineralogy, and INAA and XRF spectrometry were utilised to determine the contents of 57 elements. The mineralogy of the unaltered coal, altered coal and dyke is dominated by carbonates, particularly dawsonite, which formed by epigenetic precipitation at a late stage, after thermal alteration. Ankerite and siderite are the products of thermal alteration and are restricted to the altered coal and dykes.Principal component analysis, correlations and compositional trends approaching the intrusions were used to subdivide the elements into groups and to identify the mineralogical affinity of each group. Geochemical data are more sensitive than mineralogy for defining the size of the alteration halo, and three zones ranging from unaltered coal, through altered coal to highly coked coal are recognised as each intrusion is approached.The alteration halos differ in extent (9.5 and 56 m wide), but their size is not a multiple of the size of the causative intrusion. At the contact between coal and intrusion, concentrations of elements with affinities to some aluminosilicates, oxides, carbonates, sulphides and organic components are enriched, while other aluminosilicate-related elements are depleted. In the altered coal towards the edge of the alteration halo, some aluminosilicate elements are enriched and oxide elements are depleted.     

Mineralogy and geochemistry of Al-hydroxide/oxyhydroxide mineral-bearing coals of Late Paleozoic age from the Weibei coalfield, southeastern Ordos Basin, North China

This paper mainly describes mineralogy and geochemistry of coals from the Weibei coalfield in the southeastern Ordos Basin, North China. A number of Al-hydroxide/oxyhydroxide minerals were detected in the Late Carboniferous coals (Nos. 5, 10 and 11 coals), especially in the No. 10 coal. Aluminum-hydroxide/oxyhydroxide minerals (nordstrandite, boehmite and diaspore) in the No. 10 coal are associated with kaolinite, suggesting that these minerals are derived from the breakdown of kaolinite. A model in which Al-hydroxide/oxyhydroxide minerals form from the incongruent dissolution of kaolinite is presented. Nordstrandite and boehmite mainly occur as massive lenses (<500 μm in length). Diaspore appears as massive aggregates and as single euhedral crystals (<50 μm in length) in a kaolinite matrix. The presence of high temperature quartz, and zircon indicates that there was input of felsic volcanic debris during accumulation of the Late Carboniferous coals. These volcanic materials have also had a significant influence on the enrichment of certain trace elements including Li, Be, Ga, Zr, Nb, Mo, Sn, W and U in the Late Carboniferous coals (Nos. 5, 10, and 11 coals). SEM-EDX results show that Ga in the No. 10 coal (whole coal average 33.4 μg/g; n = 2) mainly occurs within Al-hydroxide/oxyhydroxide minerals (nordstrandite, boehmite, and diaspore), kaolinite and organic matter.     

Tsumoite (BiTe) and associated Ni-PGE mineralization from Gondpipri mafic-ultramafic complex,Bastar Craton,Central India: mineralogy and genetic significance

This paper reports the occurrence of Tsumoite (a bismuth telluride) in the Heti Cu-Ni-PGM prospect, Gondpipri mafic-ultramafic complex, Central India. The Gondpipri complex consists of several tectonically dismembered gabbronorite-gabbro-anorthositic gabbro — olivine gabbro -websterite disposed in ～10 km long tonalite-trondhjemitegranodiorite (TTG) and charnockite-enderbite suite of rocks. The mineralization occurs in the sulphide zone hosted by gabbro variants. The host rocks have been deformed and metamorphosed to granulite grade and subjected to various degrees of hydrothermal alteration. The mineralization comprises chalcopyrite, pentlandite, pyrrhotite, cubanite, millerite, and pyrite. In addition to these, occur (1) tsumoite (2) PGM in the form of moncheite, merenskyite, Pd-mellonite, and Pt-Pd-Te-Bi-Fe-S alloy. The present study indicates that the mineralization occurs in two stages related to: (i) magmatic and (ii) hydrothermal remobilization and transport of Cu-rich sulphides, tsumoite and PGM, and their re-deposition in hydrosilicate alteration zones. It is possible that the mineralization at Heti formed at different stages of bismuth activity under variable fS₂, T, and fTe₂ conditions due to change in total concentration of Te and S and /or cooling. Since the role of S is limited, Te and cooling are important factor influencing mineralogy and composition of tsumoite and associated mineralization. Mineralization occurs in two different modes of occurrences. The early mineralisation occur as blebs, specks and dissemination of sulphides, viz. pyrrhotite, chalcopyrite, pentlandite and minor pyrite ± PGM, whereas later mineralisation occur as stringers, minor veins of sulphides viz. pyrite, millerite, cubanite, sijenite, tsumoite and ± PGM. Mineral assemblages and textural relationships at Heti has indicated precipitation of tsumoite and associated PGM along fractures and secondary silicates, which confirms their hydrothermal origin.     

Evidence of Mid-ocean ridge and shallow subduction forearc magmatism in the Nagaland-Manipur ophiolites,northeast India: constraints from mineralogy and geochemistry of gabbros and associated mafic dykes

We discuss here the mineralogical and geochemical characteristics of mafic intrusive rocks from the Nagaland-Manipur Ophiolites (NMO) of Indo-Myanmar Orogenic Belt, northeast India to define their mantle source and tectonic environment. Mafic intrusive sequence in the NMO is characterized by hornblende-free (type-I) and hornblende-bearing (type-II) rocks. The type-I is further categorized as mafic dykes (type-Ia) of tholeiitic N-MORB composition, having TiO₂ (0.72–1.93 wt.%) and flat REE patterns (La_N/Yb_N = 0.76–1.51) and as massive gabbros (type-Ib) that show alkaline E-MORB affinity, having moderate to high Ti content (TiO₂ = 1.18 to 1.45 wt.%) with strong LREE-HREE fractionations (La_N/Yb_N = 4.54–7.47). Such geochemical enrichment from N-MORB to E-MORB composition indicates mixing of melts derived from a depleted mantle and a fertile mantle/plume source at the spreading center. On the other hand, type-II mafic intrusives are hornblende bearing gabbros of SSZ-type tholeiitic composition with low Ti content (TiO₂ = 0.54 wt.%–0.86 wt.%) and depleted LREE pattern with respect to HREE (La_N/Yb_N = 0.37–0.49). They also have high Ba/Zr (1.13–2.82), Ba/Nb (45.56–151.66) and Ba/Th (84.58–744.19) and U/Th ratios (0.37–0.67) relative to the primitive mantle, which strongly represents the melt composition generated by partial melting of depleted lithospheric mantle wedge contaminated by hydrous fluids derived from subducting oceanic lithosphere in a forearc setting. Their subduction related origin is also supported by presence of calcium-rich plagioclase (An_16.6–32.3). Geothermometry calculation shows that the hornblende bearing (type-II) mafic rocks crystallized at temperature in range of 565°–625 °C ± 50 (at 10 kbar). Based on these available mineralogical and geochemical evidences, we conclude that mid ocean ridge (MOR) type mafic intrusive rocks from the NMO represent the section of older oceanic crust which was generated during the divergent process of the Indian plate from the Australian plate during Cretaceous period. Conversely, the hornblende-bearing gabbros (type-II) represent the younger oceanic crust which was formed at the forearc region by partial melting of the depleted mantle wedge slightly modified by the hydrous fluids released from the subducting oceanic slab during the initial stage of subduction of Indian plate beneath the Myanmar plate.