Genetic Issues of Some of the Non Metallic Minerals in
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Abstract:
A brief account of the representative and workable industrial minerals namely magnesite, talc and barite in Lesser Himalaya, is presented here emphasizing their genesis. Deposits of magnesite and talc are found associated with Neoproterozoic, plateform type, shelf-slope limestone-dolomite host rocks from inner Lesser Himalayan sequences. Field, textural, geochemical signatures and fluid inclusions trapped in dolomite and magnesite reveal within basin processes, in an increased burial- diagenetic environment responsible for formation of magnesite replacing dolomite. Talc is formed at the expense of magnesite and silica, and with limited dolomite involvement at transition conditions from diagenetic to metamorphism. Barite deposit is hosted within Neoproterozoic Nagthat quartzite rocks of outer Lesser Himalaya, wherein its textures, fluid inclusion, sulfur and strontium isotopic studies helped in genetic understanding.Keywords:
Magnesite
Talc
Metasomatism
Cite
Magnesite
Rare-earth element
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Magnesite
Metasomatism
Dolomitization
Protolith
Carbonatite
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This study was undertaken to determine the source of iron in Comus Formation sedimentary rocks that were sulfidized during deposition of gold in the Megapit area of the Twin Creeks Carlin-type deposit. Sedimentary rocks in and near the Megapit contain ferroan dolomite, largely as overgrowths on iron-poor dolomite. Iron to form these overgrowths appears to have been released from mafic volcanic rocks that are interlayered with the sedimentary rocks. These igneous rocks have undergone two stages of hydrothermal alteration. The first stage involved formation of albite and iron-rich chlorite, possibly caused by interaction with seawater. The second stage involved destruction of the iron-rich chlorite by illite or sericite, which released iron to form ferroan dolomite in the sedimentary rocks. Comparisons show that transfer of iron from the igneous rocks to the sedimentary rocks can account for the present distributions of iron in these rocks. Relative to basalts, Comus Formation igneous rocks are enriched in iron and potassium. These results suggest that ferroan dolomite in sedimentary rocks is not solely a product of diagenetic processes and can form when iron is released from adjacent iron-bearing igneous rocks. Recognition of this additional mechanism for formation of ferroan dolomite expands the range of geologic settings that can be favorable for formation of gold deposits formed by sulfidation.
Ironstone
Illite
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Felsic
Magmatic water
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Ankerite
Magnesite
Protolith
Metasomatism
Devonian
Recrystallization (geology)
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Metasomatism
Carbonatite
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The Thalanga polymetallic massive sulfide deposit is hosted by felsic volcanics of the Cambro-Ordovician Mount Windsor subprovince in North Queensland. The sheetlike sulfide deposit consists of several semiconnected lenses, totaling 6.6 million metric tons (Mt), which lie at a single stratigraphic level between underlying subaqueously emplaced rhyolite and overlying dacitic to andesitic lavas, sills, and volcaniclastic rocks. The ores and host rocks have been deformed and metamorphosed to upper greenschist facies. Associated with the West Thalanga orebody are strata-bound lenses composed of chlorite, tremolite, dolomite, and calcite that previously have been interpreted as exhalites. They extend up to a few hundred meters along the mineralized horizon and locally also exist in the stratigraphic footwall around the fringes of the deposit.
Rhyolitic volcanics beneath the deposit are extensively altered. A pervasive sericite ± chlorite ± pyrite zone that extends greater than 200 m into the footwall envelops semistratiform proximal quartz + pyrite ± chlorite stringer zones. Major and trace element geochemical data indicate that Zr, Ti, and Al remained essentially immobile in the footwall rocks despite intense hydrothermal alteration involving significant mass transfers of mobile elements.
The chlorite + tremolite + carbonate assemblages associated with ore lenses have immobile element ratios identical to those of the altered rhyolites. Carbonate-rich assemblages retain some relict spheroidal and rhombic textures in dolomite, which suggest formation at an early hydrothermal stage in a matrix of chlorite or in open spaces.
The chlorite + tremolite + carbonate rocks probably represent metamorphic equivalents of hydrothermal chlorite + quartz + dolomite + calcite (or Mg smectite + quartz + dolomite + calcite) assemblages. The formation of tremolite by metamorphic reaction of quartz and dolomite is consistent with regional metamorphic conditions. With allowance for metamorphic decarbonation, carbon and oxygen isotope data from dolomite and calcite are consistent with precipitation of primary dolomite at 170° to 250°C from seawater-dominated fluid.
The chlorite + tremolite + carbonate rocks at Thalanga are consequently reinterpreted as altered rhyolitic volcaniclastics, containing variable proportions of hydrothermal carbonate. They probably formed in a zone of mixing of hydrothermal fluid and seawater, in a permeable substrate close to the paleosea floor. Siliceous ironstones that are present in distal parts of some Thalanga ore lenses have very low immobile element contents and are probably true exhalites.
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Carbonatite
Xenolith
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Carbonatite
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Allanite
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Ankerite
Breccia
Rhodochrosite
Carbonate minerals
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