Добірка наукової літератури з теми "Calc-silicates"

Skarn-like calc-silicate rocks are reported in spatial association with the Archean Cu–Zn–Ag massive sulphide deposits at the Manitouwadge mining camp, Ontario. Calc-silicates in the footwall of the Willroy mine occur as matrix to breccia fragments of garnetiferous quartzo-feldspathic gneiss and as lenses within garnetiferous quartzo-feldspathic gneiss and are composed of clinopyroxene, garnet, calcic amphiboles, wollastonite, plagioclase, K-feldspar, epidote, quartz, calcite, magnetite, and minor sulphides. Calc-silicates within the main orebody of the Geco mine are characterized by clinopyroxene, calcic amphiboles (Cl–K-rich hastingsitic and ferro-edenitic hornblende, ferro-edenite (up to 4.7 wt.% Cl); and ferroactinolite (6.7 wt.% MnO)), garnet, epidote (including an epidote rich in rare-earth elements and Cl), calcite, quartz, and abundant sulphides. Calc-silicates within the basal 4/2 Copper Zone of the Geco mine contain garnet, gahnite, sphalerite, ferroactinolite (8.5 wt.% MnO), epidote, quartz, biotite, plagioclase, chlorite, muscovite, K-feldspar, and pyrosmalite (with Mn/(Mn + Fe) ratio ranging from 0.21 to 0.61, and up to 3.9 wt.% Cl). The calc-silicates probably represent metasomatic remobilization of dispersed Ca (and Cl) from sea-floor hydrothermal alteration of mafic to intermediate volcanic rocks and are only indirectly related to the hypothesized syngenetic ore-forming processes for the associated base metal sulphide deposits. The calc-silicates formed initially at about 600 °C and 3–5 kbar (1 kbar = 100 MPa) in a mildly reducing environment (from 1 log unit above to 1 log unit below the fayalite–magnetite–quartz buffer) during the upper-amphibolite- to granulite-facies regional metamorphism and were altered subsequently at lower temperatures (<500 °C).

The section between Tal to Talekhu of Manang District lacks the detailed geological study. The geological mapping in the scale of 1:50,000 followed by the preparation of geological cross-section and lithostratigraphic column has been done in the present study. The studied area lies partially in the Higher Himalayan Crystalline and the Tibetan Tethys Sequence. The units of the Higher Himalayan Group from Tal to Talekhu consists mainly of vigorous to faintly calcareous gneiss, migmatitic gneiss, quartzite, granite, etc. They are named as the Calc. Silicate Gneiss and Paragneiss and the Orthogneiss and Granite units. The lowermost part of the Tibetan Tethys consisted of metamorphosed calcareous rocks containing silicates and feldspar, so this unit is termed as the Marble and Calc. Gneiss. The section is about 9 km in thickness and is highly deformed with presence of igneous rocks at many places.

The section between Tal to Talekhu of Manang District lacks the detailed geological study. The geological mapping in the scale of 1:50,000 followed by the preparation of geological cross-section and lithostratigraphic column has been done in the present study. The studied area lies partially in the Higher Himalayan Crystalline and the Tibetan Tethys Sequence. The units of the Higher Himalayan Group from Tal to Talekhu consists mainly of vigorous to faintly calcareous gneiss, migmatitic gneiss, quartzite, granite, etc. They are named as the Calc. Silicate Gneiss and Paragneiss and the Orthogneiss and Granite units. The lowermost part of the Tibetan Tethys consisted of metamorphosed calcareous rocks containing silicates and feldspar, so this unit is termed as the Marble and Calc. Gneiss. The section is about 9 km in thickness and is highly deformed with presence of igneous rocks at many places.

In this paper, we have reported small bands of Manganiferous quartzite at S. Gopalapuram, Tudi, .Some other small occurrences also observed as float ore/boulders and/or small manganese ore observed north of Nandala-meta, Tangidi and Rayavalasa The pockets of Mn ores are lithologically controlled but the disposition of manganese bands are structur-ally controlled, localized near the hinge zones and at some parts in the limb areas. The Mn-rich silicates are converted to manganese ore due to leaching or oxidation. The relict phases of silicate with Mn-rich rim and inclu-sions indicate that they were derived during leaching. Some features such as colloform bands, relict silicates and rare pseudomorphs, indicate its secondary nature formed due to the alteration of manganiferous silicates. Braunite, and psilomelane are represented as the primary minerals. secondary minerals are made up of psilomelane, cryptomelane, py-rolusite, and goethite and are formed in the low grades of metamorphism. Analytical val-ues of manganese (Mn) in bedrock samples collected from the Mn-enriched horizons of calc-granulite and khondalite rock types range from 0.16% to 39.70%. The analytical results of float ore samples have maximum values of upto 23.64%

The Tsäkkok Lens of the Seve Nappe Complex in the Scandinavian Caledonides comprises eclogite bodies hosted within metasedimentary rocks. These rocks are thought to be derived from the outermost margin of Baltica along the periphery of the Iapetus Ocean, but detrital records from the sedimentary rocks are lacking.Many metasedimentary outcrops within the lens expose both well-foliated metapelitic rocks and massive calc-silicates. The contacts between these two lithologies are irregular and are observed to trend at all angles to the high-pressure foliation in the metapelites. Where folding is present in the metapelites, the calc-silicate rocks are also locally folded. These relationships suggest metasomatism of the metapelites during the Caledonian orogenesis. Zircon U-Pb geochronology was conducted on sixty-one zircon grains from a calc-silicate sample to investigate if they recorded the metasomatic event and to assess the detrital zircon populations. Zircon grains predominantly show oscillatory zoning, sometimes with thin, homogeneous rims that have embayed contacts with the oscillatory-zoned cores. The zircon cores yielded prominent early Stenian, Calymmian, and Statherian populations with a subordinate number of Tonian grains. The zircon rims exhibit dissolution-reprecipitation of the cores or new growth and provide ages that span similar time frames, indicating overprinting of successive tectonic events. Altogether, the zircon record of the calc-silicate suggests that the Tsäkkok Lens may be correlated to Neoproterozoic basins that are preserved in allochthonous positions within the northern extents of the Caledonian Orogen.

A1-type granite xenoliths occur in alkali basalts erupted during Pliocene–Pleistocene continental rifting of Carpathian back-arc basin (Central Europe). The Pliocene (5.2 Ma) peraluminous calc-alkalic granite contains unusually high concentrations of critical metals bound in Nb, Ta, REE, U, Th-oxides typical for silica-undersaturated alkalic granites, and syenites: columbite-Mn, fergusonite-Y, oxycalciopyrochlore, Nb-rutile, and Ca-niobate (fersmite or viggezite). In contrast, it does not contain allanite and monazite—the main REE-carriers in calc-alkalic granites. The crystallization of REE-bearing Nb-oxides instead of OH-silicates and phosphates was probably caused by strong water deficiency and low phosphorus content in the parental magma. Increased Nb and Ta concentrations have been inherited from the mafic parental magma derived from the metasomatized mantle. The strong Al- and Ca-enrichment probably reflects the specific composition of the mantle wedge modified by fluids, alkalic, and carbonatitic melts liberated from the subducted slab of oceanic crust prior to the Pliocene-Pleistocene rifting.

La tsavorite, grossulaire vert à V-Cr-Mn, est contenue dans des gneiss et roches calco-silicatées graphiteux, souvent associés à des marbres dolomitiques, et appartenant à la ceinture métamorphique néoprotérozoïque mozambicaine. La tsavorite se trouve soit dans des nodules ou des veines de quartz (gisements primaires), soit dans des placers (gisements secondaires). L'étude minéralogique des tsavorites propose un nouveau protocole de certification de leur origine géographique, à partir du rapport V/Cr, de la teneur en Mn et du [delta]18 O. L'étude des gisements de Lemshuku et Namalulu en Tanzanie montre que le métamorphisme des protolithes sédimentaires riches en matière organique et évaporites s'est effectué à P = 7,0 ± 0,4 kbar et T = 677 ± 14°C, à 634 ± 22 Ma (datation U-Th-Pb sur monazite). Le bâti métamorphique s'est refroidi vers 500 Ma (datation 40Ar-39Ar sur muscovite). Deux stades de métasomatose sont reliés à la formation de la tsavorite : (i) une métasomatose de diffusion formant les nodules à P = 5,0-7,4 kbar et T = 580-691°C; (ii) une métasomatose calcique d'infiltration contemporaine de la formation des veines de quartz à P = 3,6-4,9 kbar et T = 505-587°C. Ces dernières sont datées in situ par la méthode Sm-Nd à 606 ± 36 Ma. Les évaporites continentales, déposées dans une sabkha de côte marine avec des sédiments silico-calcaires, sont transformées en tsavorite dans le cas des nodules, alors que les sels fondus sont associés à la formation des veines de quartz. Les minéralisations sont contrôlées par la lithostratigraphie et la tectonique
Tsavorite, a (V, Cr, Mn)-bearing green grossular, is hosted by graphitic gneisses or calc-silicates, often asssociated with dolomitic marbles, and belonging to the Metamorphic Neoproterozoic Mozambique Belt. Tsavorite is found either as nodules or in quartz veins (primary deposits), or in placers (secondary deposits). The mineralogical study of tsavorites suggests a new protocol to certificate their geographical origin, based on the V/Cr ratio, Mn content and delta18O. The study of the Lemshuku and Namalulu deposits in Tanzania has shown that the metamorphism of organic matter-rich and evaporites-rich sedimentary protoliths occurred at P = 7.0 ± 0.4 kbar and T = 677 ± 14°C, at 634 ± 22 Ma (U-Th-Pb dating on monazite). The metamorphic series cooled down at around 500 Ma (40Ar-39Ar dating on muscovite). Two metasomatic stages are linked to the formation of tsavorite : (i) diffusion metasomatism forming nodules at P = 5.0-7.4 kbar and T = 580-691°C; (ii) calcitic infiltration metasomatism forming quartz veins at P = 3.6-4.9 kbar and T = 505-587°C. These last have been dated in situ with Sm-Nd dating at 606 ± 36 Ma. Continental evaporites, deposited in a coastal marine sabkha with (Si, Ca)-bearing sediments, transformed into tsavorite in the case of the nodules, while the molten salts are associated with the formation of the quartz veins. The mineralisations are controlled by lithostratigraphy and structure