Ankerite

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

The Effect of Differing Hydrogeological Regimes on Sandstone Diagenesis: Brent Group Oilfields, U.K. North Sea

Abstract: Deltaic sandstones of the Middle Jurassic Brent Group, northern North Sea, have a complex diagenetic history. The paragenetic sequence can be simplified to; siderite - feldspar dissolution - vermiform kaolinite - calcite - feldspar dissolution - blocky kaolinite - quartz - ankerite - illite. Petrographic and stable isotopic studies indicate that Fe-rich siderite precipitated from meteoric water which flushed through the Brent Group during the northward progradation of the delta system. Later Ca and Mg rich siderites precipitated from sea water during the subsequent marine transgression which eventually drowned the delta. Later diagenesis is related to depth of burial and hence depends upon the subsidence history of individual oilfields. In shallow buried oilfields ( 3.0km; >80°C), continued subsidence meant that the reservoirs were sealed-off from the influx of surface-derived meteoric fluids. Illite cement, and greater volumes of quartz cement (>5%) then precipitated in a closed geochemical system. The unusually high "palaeotemperatures" of the fluid inclusions in the quartz overgrowths are false and are a product of resetting during continued subsidence, and do not indicate that influx of hot fluids occured. Finally pore-waters evolved to present day isotopic compositions (delta18O= 0 to +2‰) due to water-rock interaction.

Method of producing ultrafine carbonate powders

Abstract: FIELD: chemistry. ^ SUBSTANCE: invention can be used in chemical engineering of inorganic substances and materials. The method of producing ultrafine carbonate powders involves carbonation of an aqueous suspension of powders of natural carbonates with particle size not greater than 100 mcm under excess carbon dioxide pressure with simultaneous mechanical agitation of the suspension. Siderite is used to obtain iron carbonate, magnesite is used to obtain magnesium carbonate, dolomite is used to obtain calcium-magnesium carbonate and ankerite is used to obtain calcium-iron-magnesium carbonate. The carbonation process is carried out at temperature between 4 and 50C and high carbon dioxide pressure ranging from 0.8 to 2.5 MPa without additional stabilisation of acidity of the medium. Further, carbon dioxide CO2 pressure is reduced to atmospheric pressure and ultrafine carbonate powders are precipitated from saturated solutions of unstable hydrocarbonates. ^ EFFECT: invention allows for producing ultrafine carbonates from mineral material using simple technology. ^ 3 ex

Mineralogical and Fluid Inclusion Evidence for Reworking of Au Mineralization by Ag-Sb-Base Metal-Rich Fluids from the Bytíz Deposit, Příbram Uranium and Base-Metal Ore District, Czech Republic

Abstract: This mineralogical and fluid inclusion study was conducted on an Au-bearing quartz–sulfide vein encountered in the deep parts of the Bytíz deposit in the Příbram uranium and base-metal district, Bohemian Massif, Czech Republic. The samples were taken where the Au-bearing vein is crosscut by the common base-metal Zn-Pb ore vein Bt23C. The early mineralization of the Au-bearing vein is composed mainly of quartz (Q-1 to Q-3), illite–muscovite, Fe-Mg chlorite, arsenopyrite, and Au-Ag alloys, showing a wide range of compositions (4–69 at. % Ag) and a decrease in Au/(Au + Ag) ratios during vein evolution. Younger hydrothermal processes led to the crystallization of nests and veinlets composed of late quartz (Q-4), carbonates (siderite, dolomite–ankerite and calcite), base-metal sulfides (galena, sphalerite, chalcopyrite, and tetrahedrite), a suite of Ag and Bi-tellurides, and acanthite. The input of Sb is manifested by the partial to complete replacement of some gold grains by aurostibite and an unnamed (Ag,Au)-Sb oxide with a composition close to AuSbO3. The fluid inclusion study, combined with chlorite thermometry and arsenopyrite thermometry, showed that the early mineralization crystallized from progressively cooled (from 300 to 400 °C down to ca. 180 °C), diluted (1.2–7.0 wt. % NaCl eq.) aqueous solutions. The late portion of the mineralization formed from aqueous fluids with highly variable salinity (0.2–23.4 wt. % NaCl eq.) and homogenization temperatures decreasing from ca. 250 °C to < 50 °C, which compare well with the base-metal mineralization of the vein Bt23C and other base-metal veins of the Příbram ore area. Our study illustrates the nature and intensity of the processes of the reworking of the early gold mineralization mediated by the younger Ag,Sb-rich base-metal fluids, giving rise to Příbram’s typical late-Variscan vein Zn-Pb mineralization.

Reconstruction and geochemical modelling of the diagenetic history of the middle Jurassic Oseberg sandstone reservoir, Oseberg Field, Norwegian North Sea

Abstract: A detailed multidisciplinary integrated study of the Middle Jurassic Oseberg reservoir in 13 wells of the Oseberg field, Norwegian North Sea, was carried out in order to (1) reconstruct precisely the timing, conditions and spatial variation of diagenetic transformations (2) characterize the nature and origin of the diagenetic fluids, and (3) develop a geochemical model of the observed diagenesis. The 20-60 in thick Oseberg Formation occurs at depths of 2.5 to 3.2 km, and at present temperatures ranging from 100 to 125{degrees}C. The detrital assemblage is mainly composed of quartz, K-feldspar, albrite, muscovite and lithic clay clasts, and is very homogeneous throughout the study area. The chronological sequence of diagenetic phases established from petrographic observations includes: minor siderite and pyrite, K-feldspar overgrowths, ankerite, feldspar dissolution, vermiform, kaolinite, quartz overgrowths, poikilotopic Fe-rich calcite, dickite. Diagenetic temperatures were determined from fluid inclusions in ankerite, quarts and calcite. Combination with modelled burial/thermal history permitted to constrain approximate ages and duration of major diagenetic events. Isotopic compositions of diagenetic cements indicate that meteoric water was (and still is) a major constituant of diagenetic fluids. Present formation waters are fairly similar chemically and isotopically at reservoir scale and represent mixing of three endmembers: seawater, meteoricmore » water and primary evaporative brine. Stability diagrams and chemical geothermometers suggest that formation fluids are close to equilibrium with the host sandstone at present reservoir temperatures. Geochemical modelling of the diagenetic evolution of water-reservoir interactions was carried out using the EQ3/6 code and the Allan{sup TM}/Neptunix integrated simulator system. Results emphasize the importance of circulations of large volumes of fluid within the reservoir throughout the diagenetic history.« less

Pyrite as a Microtextural and Geochemical Tracer of Ore-Forming Processes, Central Zone Orogenic Gold Deposit, Gabgaba District, Sudan

Abstract: Gold deposition in structurally controlled deposits is triggered by changes in the mineralizing fluid conditions. Recent research has demonstrated that in deposits with a well-established paragenesis, the processes that control the ore-forming fluid conditions, and thus the gold timing and deposition, can be inferred from the study of both textural and chemical characteristics of ore-bearing minerals such as sulfides, which are ubiquitous in almost every gold deposit type. In this contribution, we carried out a coupled investigation of (1) microscopic-scale expression of regional deformation, (2) textures of mineralized veins and pyrite generations, and (3) laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace element concentrations in pyrite from the Neoproterozoic Central Zone gold deposit, located in the poorly studied Gabgaba gold district, central Keraf suture, Sudanese Nubian shield. The Central Zone gold mineralization is associated with late-collisional Keraf strike-slip shearing. It is expressed by visible gold-bearing quartz-ankerite-(albite) conjugate veins hosted by metagranitoids surrounded by metasediments. Some structurally lattice-bound gold occurs in proximal As-Au-Ni–enriched pyrite associated with sericite-albite-ankerite alteration. Vein textures and proximal pyrite oscillatory zoning and geochemical signatures indicate that vein infilling occurred as a response to sudden pressure drops and boiling of the mineralizing fluid. We therefore interpret the Central Zone deposit as a typical orogenic gold deposit, with microtextural evidence and geochemical data supporting the existence of earthquake-induced fault-valve processes.