Structure and geochronology of Sargur schist belt, Western Dharwar Craton, southern India

<p>The Sargur schist belt (SSB) - one of the oldest supracrustal belt (>3.4 Ga) - occurs as discontinuous band along the south-eastern part of Western Dharwar Craton of Indian peninsula. It is a 320 km long belt present in form of lenses, sheets, enclaves, pockets, patches and disrupted layers within the peninsular gneisses, tectonically interleaved, deformed and metamorphosed together with the associated supracrustal rocks (Janardhan et al., 1978; Srikantappa et al., 1984, 1985; Bidyananda and Mitra, 2005; Jayananda et al., 2008). The SSB shows a wide variation in lithology ranging from metapelites, metamafites, metaultramafites, quartzites, calc-silicates etc. with a varying metamorphic grade from greenschist to granulite facies. The major rock types in the study area include garnet-biotite±muscovite±staurolite schist, talc-tremolite-chlorite schist, banded magnetite quartzite, micaceous quartzite, hornblende-biotite±garnet gneiss, amphibolite schist, pyroxene granulites, foliated/deformed granite etc. The fabric in schistose rocks is mainly defined by the shape preferred aggregates of biotite-muscovite (in metapelites) and tremolite-talc-chlorite/amphibole (in metamafites/ultramafites). Whereas the gneissic fabric is defined by the quartzo-feldspathic rich leucocratic layers and biotite-garnet-amphibole-pyroxene rich melanocratic layers.</p><p>In the northern part, the SSB trends roughly N-S but towards the southern part the fabric orientation changes to E-W, whereas the dip is nearly vertical through-out the belt. The belt has undergone at least three phases of deformations. In the northern part the most penetrative fabric is a crenulation cleavage S<sub>1</sub>. The S<sub>1</sub> fabric describes open asymmetric folds having sub-vertical N-S and NNE-SSW axial plane (S<sub>2</sub>). The F<sub>2</sub> fold plunges gentle to moderately towards NNE to SSW. A set of E-W trending shears (S<sub>3</sub>) truncating the S<sub>2</sub> axial zones are zonally developed. In the southern part, as the E-W trending Moyar shear zone approaches, the early fabrics are obliterated or brought into parallelism with the E-W trending penetrative S<sub>3</sub> fabric. U-Th-total Pb dating of texturally controlled metamorphic monazites have yielded mainly two different age peaks at 2.2-2.3Ga and 2.4-2.5Ga with few older ages of ~2.7Ga ages along the northern part while the sample from the southern part (near to the E-W trending Moyar shear zone) gave younger ages ranging from 700-850 Ma and 500-600 Ma.</p><p>From the integration of structural and chronological data the D<sub>2</sub> deformation corresponds to the E-W shortening during the East and West Dharwar Craton accretion is syn- to post-tectonic with respect to the 2.4-2.6 Ga monazite growth. The 700-850 Ma and 500-600 Ma monazite growths post-tectonic with respect to the D<sub>3</sub> deformation indicates that the Neoproterozoic accretionary events affected the whole Southern Granulite Terrain and recrystallize the monazites present in the Moyar shear zone.</p>

The Ambatolampy Group, central Madagascar, a Neoproterozoic rift-basin sequence?

<p>The crystalline basement of central Madagascar is composed of the Neoarchaean, high-grade metamorphic Antananarivo Domain, made up of granulite to upper-amphibolite orthogneisses and paragneisses, and intruded by Tonian igneous rocks of the Imorona-Itsindro suite (Archibald et al. 2016). Along its southern, western and northern margins several terranes were accreted between the Paleoproterozoic and the Neoproterozoic (Tucker et al. 2014) before Madagascar was affected by the collision of East- and West-Gondwana at the end of the Ediacaran.</p><p>Within the Antananarivo Domain, a more than 700 km long and up to 80 km wide belt of supracrustal amphibolite-facies rocks forms te Ambatolampy Group. It is characterized by abundant monotonous biotite schists and gneisses that are locally migmatised. The schists contain biotite, sillimanite, garnet and locally thick graphite-rich layers. Associated paragneisses are also biotite-rich and commonly carry sillimanite or hornblende. White quartzites ranging from thick-bedded ridge-forming units to fine, cm-scale interbeds are coarse-grained and contain often sillimanite. Dark quartzites rich in magnetite and heavy minerals occur as cm-thin layers throughout the whole group. Small bodies of pyroxenite, pyroxene-amphibolite, amphibolite ±garnet, and pyroxene gneiss are common, especially close to the base of the group.</p><p>The age of the Ambatolampy Group is highly controversial. A group of researchers from BGS and USGS reported a youngest detrital zircon age of 1054 Ma, whereas Archibald et al. (2016) assumed a Mesoproterozoic age, based on their youngest zircons of roughly 1.8 Ga. We present new near-concordant U-Pb detrital zircons ages as young as 800 Ma, indicating a sedimentary input from igneous rocks of the Imorona-Itsindro suite. Sedimentation must have ceased before 630 Ma which is constrained by the U-Pb zircon age of an intruding leucogabbro.</p><p>About half of Madagascar’s known 1050 gold occurrences are lying within the Ambatolampy Group. Fine-grained disseminated gold appears to be concentrated within relatively narrow stratigraphic intervals of the Ambatolampy Group, defined by the occurrence of boudinaged or fractured magnetite quartzite. In general, the gold grades in fresh rocks are below economic interest, the highest gold tenors were recorded in an up to 30 meter thick laterite zone above the basement. Another important commodity related to the Ambatolampy Group is graphite which had seen a mining boom in the 1910s and 1920s. The graphite is flaky with crystal diameters between 0.5 and 5 mm and contents of graphitic carbon between 6 and 15 %. Individual seams are up to 12 m wide and can be tracked for several kilometers.</p><p>We interpret the Ambatolampy Group as a mainly siliciclastic fill of a continental rift basin during a phase of crustal extension occurring contemporaneously with the intrusion of the Imorona-Itsindro Suite. The gold mineralization is most likely related to fluvial deposits from surrounding gold-bearing Archean basement.</p><p> </p><p><strong>References</strong></p><p>Archibald, D.B. et al. 2015. Tectonophysics 662, pp. 167-182.</p><p>Archibald, D.B. et al. 2016. Precambr. Res. 281, pp. 312–337.</p><p>Tucker, R.D. et al. 2014. J. African Earth Sci. 94, pp. 9-30.</p>

Geology and geochemistry of the Yuanjiacun banded iron formation in Shanxi Province, China: constraints on the genesis

The Yuanjiacun banded iron formation (BIF) is hosted in lower Proterozoic metamorphic strata, and its structures are dominated by bands or streaks. Based on their differences in mineral compositions, the iron ores can be subdivided into haematite quartzite, magnetite quartzite, stilpnomelane magnetite quartzite and stilpnomelane haematite quartzite. The geochemical characteristics of the surrounding rocks show that the protoliths consisted of argillaceous and arenaceous sedimentary rocks. The predominant provenance was a high-maturity felsic sedimentary terrane. The absence of syn-depositional igneous rocks and the tectonic setting discrimination diagrams indicate that the Yuanjiacun BIF formed in a passive continental margin setting. Negligible terrigenous materials were involved in the precipitation of the Yuanjiacun BIF. The precipitation of the Yuanjiacun BIF was predominantly controlled by the mixing of seawater and hydrothermal fluids. Its metallogenic material originated from the leaching of mafic oceanic crust by hydrothermal fluids. The observed Ce anomaly deficiency and heavy Fe isotope enrichment indicate that the Yuanjiacun BIF formed in an anoxic marine environment. In a redox-stratified palaeo-ocean, the Yuanjiacun BIF formed in reducing seawater below the oxidation–reduction transition zone. The Si and O isotope compositions of quartz suggest that the formation of the Yuanjiacun BIF was closely related to submarine hydrothermal activity. The Si and Fe erupted from the seafloor and precipitated by supersaturation and biological oxidation under anoxic conditions, respectively.

Prospects for resuming the underground development of magnetite quartzites in Ukraine, mining, technological and economic aspects

Purpose. Substantiation of expediency and prospects for resuming the development of magnetite quartzites by underground method in Ukraine. Methodology. Analysis of literature sources, project documentation and practical data that contain information on the current state and conditions for the development of iron ore in Ukraine, as well as data on varieties and state of reserves of different types of these ores. Findings. The problem, faced by domestic iron ore mining enterprises in connection with reaching the large depths of mining operations and the emergence of a shortage of raw material resources, is described. The volumes of magnetite quartzites, which are contained in the dormant mines, operating mines, and those mines of Ukraine that are not currently in operation, are determined. The expediency and directions for resuming the development of these ores, as well as expanding the raw material base of the domestic iron ore mining industry, are justified. Originalty. The principal approaches to the implementation at a modern technological and technical level of the cyclic-flow underground mining technology for the development of magnetite quartzite reserves, which is capable of ensuring the economic efficiency of their extraction at depths where the open method of their development becomes unprofitable, are expanded. Practical value. Ensuring the economic efficiency of underground development of magnetite quartzite reserves in operating conditions of the iron ore mining enterprises of Ukraine leads to a significant expansion of their raw material base, which is currently constantly decreasing, as well as support of the production capacity of these enterprises for a long period of time, and allows Ukraine to remain one of the leaders in the iron ore mining industry in the world. Key words: prospects, underground mining, magnetite quartzites, mining, technological, economic aspects.

Recovery of magnetite from low grade banded magnetite quartzite (BMQ) ore

There has been a steady increase of iron ore demand in the last few decades. This growing demand could be countered by use of low grade iron ore after beneficiation. Banded iron formations (BIF) are one of the resources of such low grade iron ores. Banded magnetite quartzite (BMQ) is one such BIF and a source of iron phase mineral in the form of magnetite. In the present study a low grade BMQ ore containing around 25.47% Fe was beneficiated for recovery of magnetite. XRD study shows that quartz, magnetite, hematite, and goethite are the major minerals phases present in the low grade BMQ sample. Unit operations such as crushing, scrubbing, grinding, and magnetic separations were used for recovering magnetite. Based on the large scale beneficiation studies the process flowsheet has been developed for enrichment of magnetite. It was found that with the help of developed process flowsheet it is possible to enrich Fe value up to 65.14% in the concentrate with a yield of 24.59%.