Discrete Selenide Phases from the Uraniferous Mahadek Sandstones of Domiasiat and Wahkyn South, West Khasi Hills District, Meghalaya
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Occurrences of discrete selenide phases in the geological environment are rather rare and are confined to a few sulphide deposits and sandstone-type uranium deposits such as those of the Colorado plateau, USA. The occurrence of discrete selenide phases viz., clausthalite (PbSe), ferroselite (FeSe 2 ) and Co-penroseite (CoNiCuSe 2 ) are reported for the first time from the Uraniferous Mahadek sandstones of Domiasiat and Wahkyn South, West Khasi Hills District, Meghalaya. The selenide phases are associated with pyritiferous organic matter, pitchblende and other ore minerals such as marcasite and sphalerite.Keywords:
Selenide
Khasi
Uraninite
Marcasite
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The upper part of the Precambrian Dolomite Series is mineralized with fluorite and minor lead-zinc. The deposits show similarities to the Mississippi Valley type but were subsequently metamorphosed in the contact aureole of the Bushveld Igneous Complex. The ore locations are mostly controlled by paleoporosity of the dolomite such as vuggy horizons and paleokarst. On a larger scale the fluorite district is localized on a paleorelief which acted as an ore trap. Several genetic hypotheses are proposed; the most attractive one is precipitation from solutions which derived fluorine from the Dolomite Series and/or Pretoria Series during diagenesis. Economically the fluorite district appears to be one of the largest in the world.
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DOI:10.17014/ijog.2.3.167-184Strong mineralized carbonate rock-bearing Pb-Zn-Cu-Ag-(Au) ores are well exposed on the Latong River area, Madina Regency, North Sumatra Province. The ore deposit is hosted within the carbonate rocks of the Permian to Carboniferous Tapanuli Group. It is mainly accumulated in hollows replacing limestone in the forms of lensoidal, colloform, veins, veinlets, cavity filling, breccia, and dissemination. The ores dominantly consist of galena (126 000 ppm Pb) and sphalerite (2347 ppm Zn). The other minerals are silver, azurite, covellite, pyrite, marcasite, and chalcopyrite. This deposit was formed by at least three phases of mineralization, i.e. pyrite and then galena replaced pyrite, sphalerite replaced galena, and pyrite. The last phase is the deposition of chalcopyrite that replaced sphalerite. The Latong sulfide ore deposits posses Pb isotope ratio of 206Pb/204Pb = 19.16 - 20.72, 207Pb/204Pb = 16.16 - 17.29, and 208Pb/204Pb = 42.92 - 40.78. The characteristic feature of the deposit indicates that it is formed by a sedimentary process rather than an igneous activity in origin. This leads to an interpretation that the Latong deposit belongs to the Sedimentary Hosted Massive Sulfide (SHMS) of Mississippi Valley-Type (MVT). The presence of SHMS in the island arc such as Sumatra has become controversial. For a long time, ore deposits in the Indonesian Island Arc are always identical with the porphyry and hydrothermal processes related to arc magmatism. This paper is dealing with the geology of Latong and its base metal deposits. This work is also to interpret their genesis as well as general relationship to the regional geology and tectonic setting of Sumatra.
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Strata-bound dolomite-hosted lead-zinc deposit. Crusts of colloform sphalerite, galena, pyrite, and marcasite (stage I mineralization) were deposited on the surfaces of the karst-produced solution cavities. Reduced sulfur was produced by sulfate-reducing bacteria within the karst system. A second stage of mineralization consisting of coarsely crystalline sphalerite, galena, pyrite, and marcasite occurs in veins and as the matrix for dolomite breccias.--Modified journal abstract.
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An ash-flow tuff sequence characterizes the rocks of the Moctezuma district, Sonora. Within the sequence, a 50-m-thick unit, the Arenillas Formation, consists of volcaniclastic sedimentary rocks and limestone lenses. Tensional deformation of the region has produced basin-and-range structure. A strong north-trending normal fault forms the western boundary of the principal range of the district, normal faults on the east side of the range define a horst, and the sequence is tilted 10 degrees to 30 degrees east. At the Oposura mine, a Zn-Pb skarn deposit in the limestone of the Arenillas Formation contains galena, sphalerite, magnetite, bustamite, Mn-bearing andradite, hedenbergitic pyroxene, and strong late chloritic alteration. Elsewhere in the district, steep quartz veins contain three characteristic associations: (1) Au with Te, Ag, and barite, (2) Ag with galena, sphalerite, tetrahedrite, and high values of Sb, As, and Mn, and (3) galena, sphalerite, chalcopyrite, and ankerite with low Ag and Au values. Structural reconstructions suggest that these vein deposits developed at different depths, with the first association shallowest, the third association deepest. The skarn may have formed deeper than all the others.
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Abstract New Cu isotope data obtained on chalcopyrite from the Black Mountain and the Broken Hill deposits in the medium‐ to high‐grade metamorphic Aggeneys‐Gamsberg ore district (South Africa) require a revision of our understanding of the genesis of metamorphic Broken Hill‐type massive sulphide deposits. Chalcopyrite from both deposits revealed unusually wide ranges in δ 65 Cu (−2.41 to 2.84‰ NIST 976 standard) in combination with distinctly positive mean values (0.27 and 0.94‰, respectively). This is interpreted to reflect derivation from various silicate and oxide precursor minerals in which Cu occurred in higher oxidation states. Together with the observation of a typical supergene base metal distribution within the deposits and their spatial association with an unconformity only meters above the ore horizon, our new data are best explained by supergene oxidation of originally possibly SEDEX deposits prior to metamorphic sulphide formation, between the Okiepian (1,210–1,180 Ma) and Klondikean (1,040–1,020 Ma) orogenic events.
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Ag-Sb deposits of a new economic district that occurs on the boundary of the southeastern Altay Republic of eastern Russia and northeast Mongolia are herein described. Mineralization of this region is divided into two periods—(1) Late Hercynian (Mo-W deposits) and (2) Mesozoic (Sn-W, as well as Ni-Co-As, Ag-Sb, and Sb-Hg deposits). The deposits of main economic interest, silver-antimony, consist of siderite-sulfosalt veins and vein zones that occur in contact-metamorphosed coal-bearing terrigenous rocks of Devonian age. These veins occur to depths of up to 900 m, and occur along strike for 3 to 3.5 km; their thicknesses range up to 12 m. The deposits are mineralogically and geochemically similar to silver deposits of the Yakutian, Pamir, Slovakian, Coeur D'Alene (USA), and Freiberg (Germany) ore districts. The main ore minerals are quartz, carbonate, and Ag-tetrahedrite, with minor chalcostibnite, chalcopyrite, bournonite, zinkenite, arsenopyrite, and pyrite. Sulfides are characterized by widespread isomorphism of bismuth and antimony. The deposits are characterized by a metal suite of Ag, Sb, Bi, Cu, Pb, Zn, As, and Hg, with the Ag content in the ores ranging from 250 to 600 g/t. Fluid inclusion studies demonstrate that the siderite-sulfosalt veins formed over a temperature interval of 200 to 70°C and pressures of 500 to 120 bar from highly concentrated NaCl-CaCl2-FeCl2 solutions, with CO2, CN4, and N2 dominating the gas phase. A vertical gradient of 9°C/100 m characterized the beginning of ore formation. Investigations of O, S, Pb, and Sr isotope compositions show that ore-bearing fluids were related to the final stages of crystallization of a deep magmatic center, with subsequent crystallization of Sn-W, Cu-Co-W, Ni-Co-As, and Ag-Sb ores.
Tetrahedrite
Bornite
Arsenopyrite
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Abstract The mineralogy and textures of specimens from a sulphide occurrence in a green schist at Waitahuna River are described, and their origin is discussed. The ore minerals are chalcopyrite, pyrite, marcasite, pyrrhotite, and minor sphalerite, and there is a small amount of supergene alteration. Marcasite, and the later of two main types of pyrite, are believed to have formed from pyrrhotite after the main stage of metamorphism, but are not supergene. The occurrence is thought to be a stratiform volcanic-sedimentary deposit, but the textures are mainly of metamorphic origin. Mineralogy, geological environment, and probably origin, are similar to those of the occurrence at Moke Creek, near Queenstown.
Marcasite
Supergene (geology)
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Massive sulfide deposits, gold deposits, and tin-bearing minerals occur in rocks of the Carolina slate belt in South Carolina and adjacent Georgia. The belt of greenschist metamorphic rocks in which the ore deposits occur is mostly Cambrian in age. It is divided into an upper metasedimentary unit and a lower unit of predominantly felsic metavolcanic and metavolcaniclastic rocks. The ore deposits occur in the lower unit chiefly near the contact between the two units. The rocks have been deformed into two maior sets of coaxial folds and at least two minor deformations. Coarse-grained unmetamorphosed Carboniferous, I-type granites, and abundant Triassic-Jurassic diabase dikes cut the slate belt and appear largely unrelated to the ore deposits. Remnants of Cretaceous and younger sedimentary rocks overlie the crystalline rocks near several of the larger mineralized areas.Massive sulfides, largely pyrite, and some polymetallic sulfides occur in mines opened for gold in the nineteenth century. The largest of these in South Carolina are the Haile, Brewer, and Dorn mines. These mines and the Little Mountain and Cedar Creek-Blythewood areas have many similar lithologic characteristics. These are: hydrothermally altered wall-rock consisting of quartz-sericite-kaolinite schist and quartz-sericite schist; abundant aluminous silicates such as kyanite, andalusite, pyrophyllite, and topaz; zones of iron-enriched rocks; and a suite of resistant heavy minerals that includes tin-bearing minerals. Heavy mineral concentrates from alluvium of small streams showed 20,000 ppm tin. Cassiterite and nigerite have been identified.At the Brewer mine, gold has been produced from altered felsic volcanic rocks that contain silicified breccia, massive topaz, abundant pyrite, minor enargite, and probably cassiterite. Massive pyrite and gold have been mined at the Haile mine. In the McCormick-Lincolnton area, a near-surface granitoid pluton is thought to be the source for volcaniclastic rocks that contain polymetallic massive sulfide deposits, gold, and associated deposits of kyanite, barite, and manganese. Tin was found there in heavy mineral concentrates, and rutile occurring with kyanite contains 1,000 ppm tin. At Little Mountain, cassiterite and hematite are present in rocks considered to be metamorphosed hot spring deposits. In the Cedar Creek-Blythewood area, nigerite is present, together with cassiterite, chrysoberyl, and seventeen other heavy minerals in concentrates panned from alluvium in streams draining an area of quartz-sericite-kaolinite schist.The deposition of massive sulfides, gold, and tin minerals is considered to be part of a continuum of volcanic activity that included alteration, deformation, and metamorphism. Altered rocks, which host the ore deposits, result from superimposed processes beginning with alteration syngenetic with the massive sulfides and ending with fracture-controlled alteration.
South carolina
Sulfide Minerals
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