Showing papers on "Ankerite published in 2001"

Carbon and Oxygen Isotope Halo in Carbonates Related to the McArthur River (HYC) Zn-Pb-Ag Deposit, North Australia:Implications for Sedimentation, Ore Genesis, and Mineral Exploration

Abstract: A study of carbon and oxygen isotope ratios in fine-grained sedimentary dolomite in the Barney Creek Formation of the McArthur basin demonstrates the presence of an extensive isotope halo surrounding the giant stratiform HYC Zn-Pb-Ag deposit. Dolomite within the halo shows an 18O-enriched and 13C-depleted isotope signature ( δ 18O = 23 to 26‰ SMOW, δ 13C = –2 to –3.5‰ PDB), relative to normal Proterozoic sedimentary dolomite beyond the halo ( δ 18O = 20–23‰ and δ 13C = 0 to –2‰). The C-O isotope halo within the dolomitic siltstones extends at least 15 km southwest of the HYC deposit and approximately coincides with a previously defined lithogeochemical halo of elevated Fe, Mn, Zn, Pb, and Tl. Dolomitic siltstone lamellae within the stratiform Zn-Pb-Ag ores at HYC exhibit an isotopic range similar to that of the halo dolomites, suggesting that the ore and halo equilibrated with the same hydrothermal fluid. Modeling of isotopic exchange accompanying fluid-rock interaction suggests that the halo dolomites equilibrated with low-temperature fluids (50°–120°C), which were enriched in 18O ( δ 18O = 5 ± 5‰) but with an average crustal carbon isotope signature ( δ 13C = –6 ± 1‰). Our preferred interpretation is that the oxygen and carbon isotope halo at HYC is related to the development of an extensive brine pool. This pool was deepest in the vicinity of the HYC deposit adjacent to the Emu fault, and it became shallower to the southwest away from the fault. Using the carbon isotope fractionation equation between dolomite and HC\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(O_{3}^{{\mbox{--}}}\) \end{document}, it is possible to estimate the temperature variation at the base of the brine pool during the accumulation of the Barney Creek Formation. Brine pool temperatures were highest in and adjacent to the HYC deposit (40°–70°C) and decreased to values of 17° to 30°C remote from the deposit. These temperatures are similar to those recorded in the Red Sea brine pool associated with the Atlantis II metalliferous sediment deposit. Based on our work at HYC and Lady Loretta, strata-bound 18O-enriched carbonate lithogeochemical halos may be a characteristic of the Proterozoic stratiform Zn-Pb-Ag deposits of northern Australia. These halos are more extensive than the narrow 18O-depletion halos recorded in dolomites surrounding skarn, Mississippi Valley-type and Irish-style Zn-Pb deposits. This fundamental difference in the isotopic halo characteristics of these groups of zinc deposits is probably related to the low-temperature, synsedimentary brine pool origin of the North Australian SEDEX deposits in contrast to the various replacement and open space fill origins of skarn, Mississippi Valley-type, and Irish-style deposits.

Hydrothermal Alteration and Volatile Element Halos for the Rosebery K Lens Volcanic-Hosted Massive Sulfide Deposit, Western Tasmania

Abstract: A detailed study of alteration mineralogy, mineral chemistry, and lithogeochemistry in the host rocks surrounding the A-B and K lenses at the north end of the Rosebery mine has revealed a series of overlapping alteration halos with characteristic mineralogy and geochemistry. The study involved logging and sampling from nine drill holes spaced at varying distances from the A-B and K lenses. The stratiform Zn-Pb-Cu ore lenses have a sheetlike morphology and are hosted by a mixed sequence of rhyolitic to dacitic massive mediumgrained quartz-porphyritic pumice breccia, black mudstone, and crystal-rich volcaniclastic sandstone, overlying a thick homogeneous sequence of rhyolitic pumice breccia. The major alteration minerals at Rosebery are arranged in a complex series of zones passing away from the deposit-quartz-sericite zone, Mn carbonate zone, chlorite zone, and outer sericite zone. The chlorite zone is best developed in the immediate footwall below the copper-pyrite-rich ore lenses, whereas the strongest Mn carbonate alteration occurs in the immediate hanging-wall volcanics or lateral to the ore lenses. The outermost visible sericitic alteration extends about 60 to 100 m into the footwall, 10 to 20 m into the hanging wall, and over 500 m along the upper contact of the footwall pumice breccias. Thallium and antimony form the most extensive trace element halos related to the mineralization. Thallium forms a halo that extends 200 to 300 m into the overlying volcanics and 60 to 100 m into the footwall. Anomalously high thallium also occurs over 500 m along strike marking the contact between the footwall pumice breccias and the overlying volcaniclastic sandstones. Proximal to the ore lenses Tl and Sb values range from 10 to 100 ppm, compared to the halo zone where they vary from around 1 to 10 ppm. Studies of alteration mineral chemistry at Rosebery have revealed some important relationships that may assist exploration. The Mn content of alteration carbonate increases toward ore, both along strike and across strike. Close to ore, alteration carbonates contain >20 mole percent MnCO3 (kutnahorite, rhodochrosite, Mn siderite, and Mn ankerite), whereas at distances of 40 to 60 m across strike the mole percent MnCO3 in carbonate drops to below 10 percent. At greater than 80 m, the carbonates are Mn-poor calcites and are commonly located in synmetamorphic structures. White mica composition changes with stratigraphy and alteration assemblages and may be related to the mineralizing event, although this has not been convincingly demonstrated. Proximal white mica contains minor Ba substituting for octahedral Al. However, except for their Ba content, these phengetic white micas are similar to those found in nonmineralized areas of the Mount Read Volcanics. Sodic white mica with up to 0.35 Na/(Na + K) and a low phengite content (<0.5 Fe + Mg cations) occurs in a zone of volcanic sandstones and black slates overlying the ore deposit. This research has lead to the development of a series of proximal, medial, and distal vectors useful for both regional and mine-scale exploration. The most useful vectors, listed from proximal to distal, include Zn, Ba content of white mica, Na2O, K2O, Ishikawa alteration index (AI), S/Na2O, Ba/Sr, Mn content of carbonate, Tl, and Sb.

Analysis of mineral trapping for CO2 disposal in deep aquifers

Abstract: Analysis of Mineral Trapping for CO 2 Disposal in Deep Aquifers Tianfu Xu, John A Apps, and Karsten Pruess Earth Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720 Abstract CO 2 disposal into deep aquifers has been suggested as a potential means whereby atmospheric emissions of greenhouse gases may be reduced However, our knowledge of the geohydrology, geochemistry, geophysics, and geomechanics of CO 2 disposal must be refined if this technology is to be implemented safely, efficiently, and predictably As a prelude to a fully coupled treatment of physical and chemical effects of CO 2 injection, we have analyzed the impact of CO 2 immobilization through carbonate precipitation A survey of all major classes of rock-forming minerals, whose alteration would lead to carbonate precipitation, indicated that very few minerals are present in sufficient quantities in aquifer host rocks to permit significant sequestration of CO 2 We performed batch reaction modeling of the geochemical evolution of three different aquifer mineralogies in the presence of CO 2 at high pressure Our modeling considered (1) redox processes that could be important in deep subsurface environments, (2) the presence of organic matter, (3) the kinetics of chemical interactions between the host rock minerals and the aqueous phase, and (4) CO 2 solubility dependence on pressure, temperature and salinity of the system The geochemical evolution under both natural background and CO 2 injection conditions was evaluated In addition, changes in porosity were monitored during the simulations Results indicate that CO 2 sequestration by matrix minerals varies considerably with rock type Under favorable conditions the amount of CO 2 that may be sequestered by precipitation of secondary carbonates is comparable with and can be larger than the effect of CO 2 dissolution in pore waters The precipitation of ankerite and siderite is sensitive to the rate of reduction of ferric mineral precursors such as glauconite, which in turn is dependent on the reactivity of associated organic material The accumulation of carbonates in the rock matrix and induced rock mineral alteration due to the presence of dissolved CO 2 lead to a considerable decrease in porosity The numerical experiments described here provide useful insight into sequestration mechanisms, and their controlling geochemical conditions and parameters

Mineralogy of granulated wood ash from a heating plant in Kalmar, Sweden

Abstract: The central heating plant of Kalmar, Sweden produces 200–300 tons wood ash every year. A stabilised material for nutrient recycling is produced by adding water and dolomite to the wood ash and granulating the mixture. Combined mineralogy and chemistry can be used to interpret the transformation processes that occur during hardening and weathering of the granules, thus leading to a possibility to refine the production process and final characteristics of the granules. Mineralogy was separately studied in the wood ash, dolomite, self-hardened wood ash and granules by X-ray diffraction. Magnesium- and calcium-containing minerals are most common in the ash materials in the present study. The amounts of portlandite and calcite present in self-hardened and granulated ash samples are clearly higher than those in the untreated ash, showing that these minerals are formed during the treatments. Additionally, one potassium-containing secondary mineral, syngenite, is formed during the self-hardening of wood ash. Quartz, dolomite and the Fe–K–Mg-silicate in the granules originate from dolomite. The secondary minerals gypsum and calcium silicate hydrate are present in the granules. Portlandite occurs only in control granules in the field study. This suggests that hardening of granules continues in the field and portlandite is transformed into calcite. After up to 3 years on forest soil, the crystalline compounds dolomite, calcite, quartz, ankerite, albite and alumohydrocalcite are present in granules, of which alumohydrocalcite is formed as a secondary mineral in the field. These results suggest that the dissolution of granulated wood ash is strongly delayed compared with untreated wood ash and self-hardened wood ash because of the formation of less soluble compounds during the granulation process.

Mineralogical and Geochemical Study on Carbonate Veins of the Salem-Attur Fault Zone, Southern India: Evidence for Carbonatitic Affinity

Abstract: Carbonate veins up to few metres thick have been recognised in the E-W striking Salem-Attur deep fault zone which represents the eastern extention of the Moyar fault in the southern granulite terrane of India. X-ray diffraction, FTIR spectroscopy and chemical analyses show calcite, ankerite, siderite and benstonite as dominant minerals besides minor apatite and green biotite. Whole rock compositions show enrichment of light REE, Ba, Sr and Nb. C and 0 isotopic data determined for calcite (δ 13 C= -4.9 to -5.1%0 V-PDB and δ 18 0 = 9 to 9.2%0 V-SMOW) points to a mantle origin. The chemical characteristics therefore suggest that the carbonate veins have carbonatitic affinity. The FTIR spectra provide evidence for presence of H 2 0 and CH 4 in addition to CO 2 in the carbonate minerals of these veins.

Oxygen and Carbon Isotope Study of Calcite and Dolomite in the Disseminated Au-Ag Telluride Bulawan Deposit, Negros Island, Philippines

Abstract: The disseminated Au-Ag telluride Bulawan deposit, Negros island, Philippines, is hosted by dacite porphyry breccia pipes which formed in a Middle Miocene dacite porphyry stock. Electrum and Au-Ag tellurides occur mostly as grains intergrown with or filling voids between sphalerite, pyrite, chalcopyrite, galena and tennantite. Calcite, quartz and rare dolomite are the principal gangue minerals. Four types of alteration were recognized in the deposit, namely; propylitic, K-feldspar-sericitic, sericitic and carbonate alteration. Carbonate alteration is correlatable to the gold deposition stage and occurs mostly along fault zones. The δ18O and δ13C compositions of calcite and dolomite in propylite zone and ore-stage dacite porphyry breccia were determined. The δ18O values of calcite in propylitized andesite range from +12.2 to +14.7%, and their δ13C values range from -6.1 to -1.0%. The δ18O values of calcite and dolomite in sericite- and carbonate-altered, mineralized dacite porphyry breccia and dacite porphyry rocks range from +15.1 to +23.1%, and the δ13C values of calcite and dolomite range from -3.9 to +0.9%. The δ18O and δ13C values of the hydrothermal fluids were estimated from inferred temperatures of formation on the basis of fluid inclusion microthermometry. The δ18O values of hydrothermal fluid for the propylitic alteration were calculated to be +8.5 - +9.5%, assuming 375°C. On the other hand, the δ18O values of ore solutions for base metal and Au mineralization were computed to be +13.6 - +14.6%, assuming 270°C. The hydrothermal fluids that formed the Bulawan deposit are dilute and 18O-enriched fluids which reacted with 18O- and 13C-rich wallrocks such as limestone.

Lateral variability of sedimentology, mineralogy, and geochemistry in the HYC Zn-Pb-Ag deposit, Northern Territory, Australia; implications for ore genesis

Abstract: An understanding of the lateral variability of sedimentology, mineralogy and geochemistry at HYC has the potential to contribute to improved ore genesis models, and to facilitate better mine planning and grade control at the McArthur River Mine. HYC is the largest of the north-Australian Proterozoic sediment hosted stratiform base metal deposits (230 mT @ 12%Zn), and is hosted by the reduced sub-wave base marine carbonate-siliclastic Barney Creek Formation. Macroscopic planar sulphide laminae consist of variable intergrowths of two distinct sphalerite and pyrite phases, galena, arsenopyrite, quartz, ankerite and dolomite, and disguise significant microscopic textural complexity. Sphalerite 1 (spl) and pyrite 1 (pyl) are paragenetically early, fine grained and volumetrically dominant in the deposit, whereas sphalerite 2 (sp2) occurs as late stage, relatively coarse grained replacement of carbonates, and pyrite 2 (py2) is coarse grained, volumetrically minor, and is the last sulphide phase to form. The deposit fringe is characterised by systematic changes of microscopic spl texture with concommitant stratigraphic thinning and declining base metal grade that is unrelated to the distribution of iron. Characteristic anastamosing microscopic textures are explained by pelagic fallout of pyrite crystallites from the water column (pyl) with simultaneous seafloor precipitation ofbasemetal sulphides (spl). Sheet-like mass flow deposits that separate ore lenses and dilute ore at HYC exhibit rapid lateral transition from erosive boulder-bearing debris flows to non-erosive normally-graded turbidites. Turbidites formed by elutriation of fines into a turbid flow that followed and outran the primary debris flows. Plastically deformed sulphidic intraclasts and sulphidic matrices of these breccias are texturally and isotopically identical to in-situ laminated sulphide ore, and constrain mineralisation to the upper few metres of the unconsolidated marine sediments. Nodular carbonates occur on all preserved fringes of the deposit, and are the direct lateral equivalent of the mineralised stratigraphy. The SIC ratio of unmineralised siltstone reveals primary differences in shallow diagenetic processes between the nodular and laminar sulphide ore facies. Bacterially triggered carbonate precipitation probably took place at shallow depth during base-metal mineralisation, and resulted in Fe-Mn-calcite nodule formation in a zone concentric about the local depocentre. Dolomitisation and partial replacement by sphalerite (sp2) occurred shortly after nodule formation, as modified ore fluids permeated the sediment pile. The ()13C of these carbonates is consistent with that of detrital carbonates in the host unit, which indicates that oxidised organic carbon (a major product of inorganic sulphate reduction) was not incorporated into the nodular carbonates. The o34S values of spl and pyl are heavier in the deposit centre (5-8%) than in the deposit fringe (0 to -2%). This suggests a gradient of 34S fractionation concentric around the focus of mineralisation, interpreted to be largely due to bacterial processes that were limited spatially by physicochemical gradients in a stratified water body. Fine grained pyrite textures closely resemble contemporaneous microfossils and microbially laminated pyritic sediments elsewhere, and confirm the presence of a prolific benthic microbial fauna spatially related to the mineralising system. Sp2 defines a 835 population (mean= 9.3%) that is distinct from, and always heavier than coexistent spl, interpreted to arise from partial closed-system sulphate reduction in the sediment pile. The textural, geochemical and sedimentological data demonstrate that the stratiform HYC Zn-PbAg deposit at McArthur River is a vent-distal sedimentary exhalative deposit, in which the contemporaneous benthic microbial fauna played an important role in mineralisation. The HYC sub-basin was probably not significantly more extensive than the deposit, and restriction of the water body, including 'plugging' of the northern end by a fanglomerate wedge, were crucial to establishment of a stratified aqueous environment amenable to extreme concentration of metals. Metalliferous fluids were introduced into the basin via hydrothermal pluses from a structural conduit related to the Emu Fault.

Fluid Inclusions and Daughter Minerals of Taibai Gold Deposit, ShaanXi Province, China

Abstract: A discovery of daughter minerals in fluid inclusions of Taibai gold deposit, Shaanxi province has been focused on, which is a unique breccia-cemented gold-bearing system. The breccia zone strikes NWW-SEE, occurring in Devonian strata of Southern Qinling Mountains. The cement is mainly composed of ankerite, pyrite, calcite and quartz, which may be divided into four main tectonic-mineralizing stages. Gold mainly occurs in pyrite and ankerite of stage II and IV. It is found that three types of fluid inclusions can be distinguished: (1) aqueous inclusions (type B); (2) CO2-rich inclusions (type C); (3) daughter minerals-containing inclusions (type A). LRM (Laser Raman Micro-probe) analyses shows that the content of CO2 occupies 54.4－70.7% (mole fraction, so as the follows) in vapor phases of different type fluid inclusions. CH4 (5.2%－7.3%) and H2S (6.0%－12.7%) exist in both vapor and liquid phases. Many daughter minerals in fluid inclusions of ankerite and quartz have been found. Several kinds of daughter minerals, including ankerite, pyrite, arsenopyrite and halite, were determined by using SEM (scanning electron microscope) / EDS (energy dispersive spectrometer) technique. EPMA (electron probe micro-analysis) technique was also applied to study the daughter minerals exposed to the surface of polis hed thin sections.

Carbonate Globules from Spitsbergen, Norway: Terrestrial Analogs of the Carbonates in Martian Meteorite ALH84001?

Abstract: Pleistocene volcanic centers in NW Spitsbergen, Norway host one of the world's richest occurrences of mantle xenoliths. The xenoliths comprise varieties of spinel lherzolites and pyroxenites. Some of these xenoliths (and their host basalts) contain 10-100 micrometer globules of ankedtic-magnesitic carbonates (AMC). In composition, mineralogy and petrology the AMC globules from Spitsbergen are strikingly similar to the carbonate globules in ALH84001. The AMC globules occur within interstitial quenched glass and as fracture fillings, although we have not seen replacement fabrics analogous to carbonate rosettes replacing glass in ALH84001. Siderite/ankerite forms the core of these concentrically zoned globules while rims are predominantly magnesite. Clay minerals can occasionally be found within and around the globules. Aside from the clay minerals, the principal mineralogical difference between the AMCs and the ALH84001 carbonate rosettes is the presence of concentrated zones of nanophase magnetite in the rosettes, notably absent in the AMCs. However, carbonate globules containing nanophase magnetite have been produced inorganically by hydrothermal precipitation of carbonates and subsequent heating. We heated Spitsbergen AMC at 585 C in a reducing atmosphere to determine whether magnetite could be produced. Optical micrographs of the heated Spitsbergen AMC show dark concentric zones within the AMC. High resolution SEM images of those areas reveal 150-200 nm euhedral crystals that exhibit various morphologies including octahedra and elongated prisms. EDS analyses of areas where the crystals occur contain Fe, O, and minor Si, and P. However, the probe integrates over volumes of material, which also include the surrounding matrix. We have begun TEM observations of both the heated and unheated Spitsbergen AMC to characterize the microstructures of the carbonates, establish the presence/absence of magnetite and determine the relationship of the clay minerals to the carbonates and host rock.