Ankerite

About: Ankerite is a research topic. Over the lifetime, 859 publications have been published within this topic receiving 23960 citations.

Geochemical and Mineralogical Characteristics of the Middle Jurassic Coals from the Tongjialiang Mine in the Northern Datong Coalfield, Shanxi Province, China

Abstract: There is limited information available on the minerals and elements present in the Jurassic coals from Datong Coalfield. This paper investigates the geochemical and mineralogical characteristics of the Middle Jurassic coals from the Tongjialiang Mine using X-ray powder diffraction (XRD), X-ray fluorescence spectrometry (XRF), inductively coupled plasma mass spectrometry (ICP-MS), and scanning electron microscopy in combination with energy-dispersive X-ray spectrometry (SEM-EDS). No.12 coal is a low-medium volatile bituminous coal and is characterized by low ash yield content, low moisture content, and ultra-low sulfur content. Compared with Chinese coals, the Tongjialiang coals have slightly higher average percentages of MgO and P2O5, and lower average percentages of the other major oxides, including SiO2, TiO2, Al2O3, Fe2O3, CaO, MnO, Na2O, and K2O. Compared with the World hard coals, Be, Cr, Co, Ni, Ge, Sn, Ta, and W are slightly enriched in the Tongjialiang coals. The concentrations of Li, F, Sc, V, Cu, Ga, Se, Sr, Zr, Nb, Hf, Pb, Th, and U are close to the average values of the world’s hard coals. The minerals in No.12 coal mainly include quartz, kaolinite, siderite, and ankerite, along with smaller amounts of pyrite, illite, calcite, and rutile. The formation of syngenetic siderite in No.12 coal is related to the weathering of biotite in the gneiss of the Yinshan Upland. The modes of occurrence of ankerite indicate that the coals may be affected by the injection of low temperature hydrothermal fluids. It is noteworthy that a portion of epigenetic ankerite may be a product of metasomatism between syngenetic siderite and the epigenetic Fe-Mg-Ca rich hydrothermal fluids. The ratios of Al2O3/TiO2, REY (rare earth elements and yittrium) enrichment patterns, the modes of occurrence of siderite and ankerite, as well as the enriched lithophile and siderophile elements indicate that the No.12 coal may have originated from the Yinshan Upland and may also have been influenced by low temperature hydrothermal fluids that might have circulated in the coal basin.

Nb-Rich Baotite in Carbonatites and Fenites at Haast River, New Zealand

Abstract: Baotite occurs as an accessory mineral in carbonatites, fenites, and carbothermal veins associated with a lamprophyre dyke swarm in the Haast River area of south Westland, New Zealand. Carbonatites are petrogenetically evolved, with assemblages dominated by ankerite, siderite and Ba-Sr-REE carbonates. Microprobe analysis indicates baotite compositions more Nb-rich than previously recorded, with compositions close to Ba4[Ti3(Nb,Fe)s]Si4028C1. Ti must be partially replaced in both crystallographically-independent octahedral sites. Compositional zoning, and stoichiometric considerations suggest that the dominant octahedral substitution is the same as that described in rutile, namely 3Ti 4§ ~ 2Nb 5+ + Fe 2+. Contrary to previous suggestions, Fe in the octahedral site should, therefore, be dominated by Fe 2+. The presence of baotite further documents the involvement of halogens in carbonatite magmas. In the New Zealand occurrences it is suggested that the chlorine originates from associated phonolitic magmas and is partitioned into carbonatite during liquid immiscibility.

Structural, mineralogical and geochemical studies of the Paleoproterozoic Omai gold deposit, Guyana

Abstract: The granitoid- and greenstone-hosted Omai gold deposit is located in the Paleoproterozoic Barama-Mazaruni greenstone belt, Guiana Shield. At regional scale, the emplacement of the deposit was controlled by the Makapa-Kuribrong crustal shear zone. At local scale, the gold-bearing quartz veins are associated mainly with a quartz monzodioritic intrusion (Omai stock, Fennell pit) and subvolcanic quartz-feldspar porphyry and rhyolite dikes, and subordinately, with intermediate volcanic flows and metasedimentary rocks (Wenot pit). Six gold-bearing undeformed subhorizontal and subvertical vein sets can be distinguished in the two ore zones. On the basis of their internal structures and textures, the veins can be classified as crack and seal, laminated, breccia, and open-space-filling veins. The geometrical and textural relationships between the vein sets suggest that they are broadly contemporaneous. The formation of most veins can be summarized by two filling stages and a late fracture-filling stage related to a protracted hydrothermal process. Although the metallic minerals represent less than 1 percent of the vein volume, their mineralogy is complex and consists of various sulfides, together with tungstates, native elements, tellurides, and sulfosalts. The metallic paragenesis is defined by the Au-Ag-Te-W-Bi-Pb-Zn-Cu-Hg-Mo assemblage. The nonopaque gangue includes mainly quart and subordinately carbonates, albite, sericite, chlorite, tourmaline, rutile, and epidote.Wall rocks of gold-bearing veins are affected by synmineralization alteration. The dominant alteration assemblages include carbonates, sericite, silica, chlorite, albite, epidote, argillic minerals, pyrite, and pyrrhotite. There is an obvious association between the alteration assemblages, the vein internal textures, and the chemical nature of the host rocks.Vein-forming scheelite has 87 Sr/ 86 Sr ratios between 0.7019 and 0.7021 and delta 18 O values between 3.8 and 4.3 per mil, which suggest both consistent temperature and isotopic composition of the hydrothermal solutions during its deposition. Oxygen isotopes measured in vein quartz vary between 13.2 and 14 per mil, similar to the delta 18 O values of carbonates (avg 13.8ppm for calcite and 14.4ppm for ankerite). The carbon isotopes of carbonates range between 1.7 and 4.7 per mil. The delta 18 O values of the mineralizing fluids vary between +5.6 and -2.7 per mil and the delta D values between -52 and +18 per mil. The isotopic composition of the hydrothermal fluids plots outside both magmatic and metamorphic water boxes, therefore suggesting a significant component of surface-derived water.The Omai deposit is late orogenic and its emplacement was controlled by the last brittle to brittle-ductile stages of the Trans-Amazonian orogeny. It can be considered as a Paleoproterozoic equivalent of the Archean epizonal orogenic deposits described in the Yilgarn (Wiluna, Racetrack) and Zimbabwe (Shamwa, Commoner) cratons.

Origin of "cannon−ball" concretions in the Carolinefjellet Formation (Lower Cretaceous), Spitsbergen

Abstract: Ball−shaped concretions ("cannon balls") commonly occur in a marine, or − ganic carbon−rich sedimentary sequence (Innkjegla Member) of the Carolinefjellet Forma− tion (Aptian-Albian) in Spitsbergen The sedimentologic, petrographic and geochemical investigation of these concretions in the Kapp Morton section at Van Mijenfjorden gives in− sight into their origin and diagenetic evolution The concretion bodies commenced to form in subsurface environment in the upper part of the sulphate reduction (SR) diagenetic zone They resulted from pervasive cementation of uncompacted sediment enriched in frambo− idal pyrite by non−ferroan (up to 2 mol% FeCO3) calcite microspar at local sites of enhanced decomposition of organic matter Bacterial oxidation of organic matter provided most of carbon dioxide necessary for concretionary calcite precipitation ( 13 CCaCO3 -21‰ VPDB) Perfect ball−like shapes of the concretions originated at this stage, reflecting isotropic per− meability of uncompacted sediment The concretion bodies cracked under continuous burial as a result of amplification of stress around concretions in a more plastic sediment The crack systems were filled by non−ferroan (up to 5 mol% FeCO3) calcite spar and blocky pyrite in deeper parts of the SR−zone This cementation was associated with impregnation of parts of the concretion bodies with microgranular pyrite Bacterial oxidation of organic matter was still the major source of carbon dioxide for crack−filling calcite precipitation ( 13 CCaCO3 -19‰ VPDB) At this stage, the "cannon−ball" concretions attained their final shape and texture Subsequent stages of concretion evolution involved burial cementation of rudimentary pore space with carbonate minerals (dolomite/ankerite, siderite, calcite) un− der increased temperature ( 18 OCa,Mg,FeCO3 -14‰ VPDB) Carbon dioxide for mineral pre− cipitation was derived from thermal degradation of organic matter and from dissolution of skeletal carbonates ( 13 CCa,Mg,FeCO3 -8‰ VPDB) Kaolinite cement precipitated as the last diagenetic mineral, most probably during post−Early Cretaceous uplift of the sequence Key wor ds:Arctic, Svalbard, Lower Cretaceous, "cannon−ball" concretions, diagenesis