Showing papers on "Ankerite published in 2004"

Petrographic and geochemical evidence for hydrothermal evolution of the North Deposit, Mt Tom Price, Western Australia

Abstract: High-grade iron mineralisation (>65%Fe) in the North Deposit occurs as an E-W trending synclinal sheet within banded iron formation (BIF) of the Early Proterozoic Dales Gorge Member and consists of martite-microplaty hematite ore. Three hypogene alteration zones between unmineralised BIF and high-grade iron ore are observed: (1) distal magnetite-siderite-iron silicate, (2) intermediate hematite-ankerite-magnetite, and (3) proximal martite-microplaty hematite-apatite alteration zones. Fluid inclusions trapped in ankerite within ankerite-hematite veins in the hematite-ankerite-magnetite alteration zone revealed mostly H2O–CaCl2 pseudosecondary and secondary inclusions with salinities of 23.9±1.5 (1σ, n=38) and 24.4±1.5 (1σ, n=66) eq.wt.% CaCl2, respectively. Pseudosecondary inclusions homogenised at 253±59.9°C (1σ, n=34) and secondary inclusions at 117±10.0°C (1σ, n=66). The decrepitation of pseudosecondary inclusions above 350°C suggests that their trapping temperatures are likely to be higher (i.e. 400°C). Hypogene siderite and ankerite from magnetite-siderite-iron silicate and hematite-ankerite-magnetite alteration zones have similar oxygen isotope compositions, but increasingly enriched carbon isotopes from magnetite-siderite-iron silicate alteration (−8.8±0.7‰, 1σ, n=17) to hematite-ankerite-magnetite alteration zones (−4.9±2.2‰, 1σ, n=17) when compared to the dolomite in the Wittenoom Formation (0.9±0.7‰, 1σ, n=15) that underlies the deposit. A two-stage hydrothermal-supergene model is proposed for the formation of the North Deposit. Early 1a hypogene alteration involved the upward movement of hydrothermal, CaCl2-rich brines (150–250°C), likely from the carbonate-rich Wittenoom Formation (δ13C signature of 0.9±0.7‰, 1σ, n=15), within large-scale folds of the Dales Gorge Member. Fluid rock reactions transformed unmineralised BIF to magnetite siderite-iron silicate BIF, with subsequent desilicification of the chert bands. Stage 1b hypogene alteration is characterised by an increase in temperature (possibly to 400°C), depleted δ13C signature of −4.9±2.2‰ (1σ, n=17), and the formation of hematite-ankerite-magnetite alteration and finally the crystallisation of microplaty hematite. Late Stage 1c hypogene alteration involved the interaction of low temperature (~120°C) basinal brines with the hematite-ankerite-magnetite hydrothermal assemblage leaving a porous martite-microplaty hematite-apatite mineral assemblage. Stage 2 supergene enrichment in the Tertiary resulted in the removal of residual ankerite and apatite and the weathering of the shale bands to clay.

Fluid Inclusion Characteristics and Genesis of the Fluorite-Parisite Mineralization in the Snowbird Deposit, Montana

Abstract: The Snowbird fluorite-rare earth element (REE) deposit, located in western Montana, is unusual in that it is characterized by very coarse grained hydrothermal quartz, calcite, and fluorite. Also, the rare mineral parisite-(Ce) [Ca(Ce,La,Nd,Pr,Y)2(CO3)3F2], which is typically associated with ankerite in the deposit, occurs as exceptionally large (up to 24 cm in length) and abundant crystals. Snowbird is one of a number of ankerite-bearing deposits hosted by the Belt Supergroup metasedimentary rocks that have been linked genetically to mineralization in the Coeur d’Alene district. Fluid inclusions in quartz, fluorite, ankerite, and parisite from the Snowbird deposit were investigated using petrography, microthermometry, bulk leachate, Raman spectroscopy, and gas chromatographic analyses. Based on field and petrographic observations, we conclude that ankerite and parisite are cogenetic and not restricted to a late stage in the development of the deposit, as had been suggested previously. The fluid inclusions most closely associated with parisite and ankerite contain aqueous liquid, one or two carbonic phases, and a halite crystal at room temperature. The fluid from which parisite was deposited was highly saline (33–50 wt % NaCl equiv) and dominated by Na and Ca with Ca/(Na + Ca) = 0.1 to 0.2. The carbonic component was dominated by CO2, with minor CH4 and N2, and the bulk XCO2 was less than or equal to 0.11. The minimum temperature and pressure of parisite deposition is estimated to be 400° to 500°C and 200 to 300 MPa, respectively. Later fluorite was precipitated from similar fluids but with more variable CO2 contents and lower salinities (down to ~29 wt % NaCl equiv). Fluorite deposition was followed by the infiltration of lower salinity aqueous fluids. Our paragenetic and fluid inclusion observations combined with previously published radiometric age dates, Sr isotope data, and mineral chemistry are most consistent with a model in which the entire Snowbird deposit was formed in the Cretaceous from fluids derived from the Idaho batholith and that, at least partially, equilibrated with the Belt Supergroup metasedimentary rocks. Furthermore, these data suggest that Snowbird is unrelated genetically to the base metal and silver mineralization of the Coeur d’Alene district, which generally have been interpreted to have been formed from dominantly metamorphic fluids.

Carbon and oxygen isotopic survey of diagenetic carbonate deposits in the Agardhfjellet Formation (Upper Jurassic), Spitsbergen: preliminary results

Abstract: Diagenetic carbonate deposits (concretions, cementation bodies and cement− stone bands) commonly occur in organic carbon−rich sequence of the Agardhfjellet Forma− tion (Upper Jurassic) in Spitsbergen. They are dominated by dolomite/ankerite and siderite. These deposits originated as a result of displacive cementation of host sediment in a range of post−depositional environments, from shallow subsurface to deep−burial ones. Prelimi− nary results of the carbon and oxygen isotopic survey of these deposits in southern Spitsbergen (Lagkollane, Ingebrigtsenbukta, Reinodden, and Lidfjellet sections) show the 13 C values ranging between -13.0‰ and -1.8‰ VPDB, and the 18 O values between -16.0‰ and -7.7‰ VPDB. These results suggest that the major stage of formation of the carbonate deposits occurred during burial diagenesis under increased temperature, most probably in late diagenetic to early catagenic environments. Carbonate carbon for mineral precipitation was derived from dissolution of skeletal carbonate and from thermal decom− position of organic matter.

Covariance of C- and O-isotopes with magnetic susceptibility as a result of burial diagenesis of sandstones and carbonates: an example from the Lower Devonian La Vid Group, Cantabrian Zone, NW Spain

Abstract: The burial diagenesis of sandstones, limestones, and dolostones of the Lower Devonian La Vid Group in the Cantabrian Zone (NW Spain) reveal a covariance of carbon and oxygen isotope values with magnetic susceptibility. Also, strontium isotopes, and to a minor degree Fe, follow this trend. The main carriers of the magnetic susceptibility appear to be diagenetic Fe-carbonates, i.e., siderite, ferroan dolomite, and ankerite, which occur as cements in primary and secondary voids, as well as in fractures. In some layers, especially at the top of the succession there occurs additionally secondary Fe-chlorite and pyrite. The Fe-carbonates were formed during upward migration of a reducing, iron-bearing, petroliferous fluid that was depleted in 13C and carried radiogenic Sr. Similar geochemical covariance and/or correlations can be expected in other sedimentary successions affected by the migration of petroliferous formation fluids.

The Onaman Prospect, Ontario: An Unusual Occurrence of Cu-bearing Au Mineralization in a Shear Zone

Abstract: The Onaman prospect is an unusual example of mesothermal gold mineralization in which native gold, electrum, and minor chalcopyrite are disseminated in metavolcaniclastic rocks in shear zones or concentrated in felsite dikes that intrude the metavolcaniclastic rocks. Mineralization was contemporaneous with carbonatization and involved early replacement of iron-bearing minerals in the wall rock by ankerite and pyrite and later replacement of pyrite by arsenopyrite, pyrrhotite, chalcopyrite, electrum, and native gold. Oxygen isotope ratios of ankerite suggest a metamorphic origin for the fluid, and carbon isotope ratios are similar to those of large mesothermal gold deposits in the Superior province for which a mantle origin has been proposed. Sulfur isotope ratios correlate positively with gold grades and may reflect decreasing f O2 during mineralization. A model is proposed in which deformation of the volcanic pile led to the formation of a shear zone and created dilational zones allowing dike intrusion. Metamorphic fluids rose along the shear zones and the dikes. Pyritization of the host rock was the major control on gold and chalcopyrite deposition, reducing a H 2 S and destabilizing the bisulfide complexes of Au and Cu. The Onaman occurrences provide an uncommon example of a mesothermal system in which pH was relatively low, and the correspondingly low solubility of copper allowed the fluid to saturate with and deposit chalcopyrite. In most other mesothermal gold systems, copper is too soluble to saturate the fluid with a copper mineral and is therefore flushed through the system.

Mineral Sequestration of Carbon Dixoide in a Sandstone-Shale System

Abstract: A conceptual model of CO2 injection in bedded sandstone-shale sequences has been developed using hydrogeologic properties and mineral compositions commonly encountered in Gulf Coast sediments. Numerical simulations were performed with the reactive fluid flow and geochemical transport code TOUGHREACT to analyze mass transfer between sandstone and shale layers and CO2 immobilization through carbonate precipitation. Results indicate that most CO2 sequestration occurs in the sandstone. The major CO2 trapping minerals are dawsonite and ankerite. The CO2 mineral-trapping capacity after 100,000 years reaches about 90 kg per cubic meter of the medium. The CO2 trapping capacity depends on primary mineral composition. Precipitation of siderite and ankerite requires Fe+2 supplied mainly by chlorite and some by hematite dissolution and reduction. Precipitation of dawsonite requires Na+ provided by oligoclase dissolution. The initial abundance of chlorite and oligoclase therefore affects the CO2 mineral trapping capacity. The sequestration time required depends on the kinetic rate of mineral dissolution and precipitation. Dawsonite reaction kinetics is not well understood, and sensitivity regarding the precipitation rate was examined. The addition of CO2 as secondary carbonates results in decreased porosity. The leaching of chemical constituents from the interior of the shale causes slightly increased porosity. The limited information currently available for the mineralogy of natural high-pressure CO2 gas reservoirs is also generally consistent with our simulation. The ''numerical experiments'' give a detailed understanding of the dynamic evolution of a sandstone-shale geochemical system.

Padrões Diagenéticos em Arenitos de Sistemas de Sabkha Costeiros-Eólicos: Um Estudo Comparativo dos Reservatórios Juruá da Área de Urucu, Bacia do Solimões, AM

Abstract: Coastal-eolian sabkha sandstones from different ages and basins show similar diagenetic patterns, which understanding is important for their evaluation as geochemical systems and as hydrocarbon reservoirs. The Carboniferous sandstones of the Jurua Formation (Solimoes Basin) are one of the most important gas reservoirs of Brazil. The sandstones and interbedded mudrocks, evaporites and dolostones were deposited within a coastal sabkha environment with pervasive eolian reworking, under increasing marine influence, and hot and dry climate. Four stacked drying/wetting upward cycles were identified, with sabkha facies in the base overlain by eolian deposits, followed again by sabkha deposits, commonly eroded by the next cycle. Eolian dune and sandsheet sandstones are the best reservoirs. The diagenetic evolution and the relationships among diagenesis, depositional facies and stratigraphic unit boundaries show similarities with other coastal-eolian sabkha sandstones. The eodiagenesis is characterized by mechanical compaction, hematite and infiltrated clay coatings, framboidal pyrite, microcrystalline and blocky dolomite. Mesodiagenesis comprises chemical compaction, K-feldspar and quartz overgrowths, poikilotopic anhydrite, feldspar dissolution and albitization, illite and chlorite authigenesis, and late quartz, Fe- dolomite/ankerite, calcite and siderite. Localized telogenetic effects include oxidation of ferroan constituents and kaolinite precipitation. Blocky dolomite and quartz cementation, and chemical compaction through intergranular and stylolitic pressure dissolution are more abundant in the non-eolian sandstones. Microcrystalline pore-filling and pore-lining dolomite, and patchy, poikilotopic, post-compactional anhydrite cementation, mostly close to the contacts with interbedded evaporites, are more abundant in the eolian sandstones. These diagenetic patterns are similar to those of the Rotliegend Group in northern Germany and in the North Sea, of the Norphlet and Tensleep Formations in USA, of the Muschelkalk Formation in Spain, and of the Monte Alegre Formation from the Amazonas Basin, northern Brazil. The similarities among the diagenetic histories of these coastal-eolian sabkha sandstones are ascribed to their similar patterns of stratigraphic organization (intercalated evaporite and carbonate beds) and of composition and circulation of pore fluids.