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Mario Rojo - One of the best experts on this subject based on the ideXlab platform.
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Trace element geochemistry of magnetite from the Cerro Negro Norte iron oxide−apatite deposit, northern Chile
Mineralium Deposita, 2019Co-Authors: Eduardo Salazar, M Leisen, Gisella Palma, Rurik Romero, Martin Reich, Fernando Barra, Adam Simon, Mario RojoAbstract:Kiruna-type iron oxide−apatite (IOA) deposits constitute an important source of iron and phosphorus, and potentially of rare earth elements (REE). However, the origin of IOA deposits is still a matter of debate with models that range from a purely magmatic origin by liquid immiscibility to replacement of host rocks by hydrothermal fluids from different sources. In order to better constrain the origin of Andean IOA deposits, we focused on the Cretaceous Cerro Negro Norte deposit located in the Chilean Iron Belt, northern Chile. The Cerro Negro Norte magnetite ore is hosted in andesitic rocks and is spatially and genetically associated with a diorite intrusion. Our results show that the deposit is characterized by three main mineralization/alteration episodes: an early Fe–oxide event with magnetite and actinolite followed by four stages that comprise the main hydrothermal event (hydrothermal magnetite + actinolite; calcic–sodic alteration + sulfides; quartz–tourmaline and propylitic alteration) and a minor supergene event. Based on textural and chemical characteristics, four different types of magnetite are recognized at Cerro Negro Norte: type I, represented by high-temperature (~ 500 °C) magnetite cores with amphibole, pyroxene, and minor Ti–Fe oxide inclusions; type II, an inclusion-free magnetite, usually surrounding type I magnetite cores; type III corresponds to an inclusion-free magnetite with chemical zoning formed under moderate temperatures; and type IV magnetite contains abundant inclusions and is related to low-temperature (~ 250 °C) hydrothermal veinlets. Electron probe and laser ablation ICP-MS analyses of the four magnetite types show that the incorporation of Al, Mn, Ti, and V into the magnetite structure is controlled by temperature. Vanadium and Ga concentrations are relatively constant within each magnetite type, but are statistically different among magnetite types, suggesting that both elements could be used to discriminate between magmatic and hydrothermal magnetite. However, our results show that the use of elemental discrimination diagrams should be coupled with detailed textural studies in order to identify superimposed metasomatic events and evaluate the impact of inclusions on the interpretation of microanalytical data. The presence of a distinct textural and chemical variation between magnetite types in Cerro Negro Norte is explained by a transition from high- to low-temperature magmatic-hydrothermal conditions. The microanalytical data of magnetite presented here, coupled with new δ^34S data for pyrite (− 0.5 to + 4.3‰) and U–Pb ages of the diorite (129.6 ± 1.0 Ma), are indicative of a genetic connection between the diorite intrusion and the magnetite mineralization, supporting a magmatic-hydrothermal flotation model to explain the origin of Kiruna-type deposits in the Coastal Cordillera of northern Chile.
Eduardo Salazar - One of the best experts on this subject based on the ideXlab platform.
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Trace element geochemistry of magnetite from the Cerro Negro Norte iron oxide−apatite deposit, northern Chile
Mineralium Deposita, 2019Co-Authors: Eduardo Salazar, M Leisen, Gisella Palma, Rurik Romero, Martin Reich, Fernando Barra, Adam Simon, Mario RojoAbstract:Kiruna-type iron oxide−apatite (IOA) deposits constitute an important source of iron and phosphorus, and potentially of rare earth elements (REE). However, the origin of IOA deposits is still a matter of debate with models that range from a purely magmatic origin by liquid immiscibility to replacement of host rocks by hydrothermal fluids from different sources. In order to better constrain the origin of Andean IOA deposits, we focused on the Cretaceous Cerro Negro Norte deposit located in the Chilean Iron Belt, northern Chile. The Cerro Negro Norte magnetite ore is hosted in andesitic rocks and is spatially and genetically associated with a diorite intrusion. Our results show that the deposit is characterized by three main mineralization/alteration episodes: an early Fe–oxide event with magnetite and actinolite followed by four stages that comprise the main hydrothermal event (hydrothermal magnetite + actinolite; calcic–sodic alteration + sulfides; quartz–tourmaline and propylitic alteration) and a minor supergene event. Based on textural and chemical characteristics, four different types of magnetite are recognized at Cerro Negro Norte: type I, represented by high-temperature (~ 500 °C) magnetite cores with amphibole, pyroxene, and minor Ti–Fe oxide inclusions; type II, an inclusion-free magnetite, usually surrounding type I magnetite cores; type III corresponds to an inclusion-free magnetite with chemical zoning formed under moderate temperatures; and type IV magnetite contains abundant inclusions and is related to low-temperature (~ 250 °C) hydrothermal veinlets. Electron probe and laser ablation ICP-MS analyses of the four magnetite types show that the incorporation of Al, Mn, Ti, and V into the magnetite structure is controlled by temperature. Vanadium and Ga concentrations are relatively constant within each magnetite type, but are statistically different among magnetite types, suggesting that both elements could be used to discriminate between magmatic and hydrothermal magnetite. However, our results show that the use of elemental discrimination diagrams should be coupled with detailed textural studies in order to identify superimposed metasomatic events and evaluate the impact of inclusions on the interpretation of microanalytical data. The presence of a distinct textural and chemical variation between magnetite types in Cerro Negro Norte is explained by a transition from high- to low-temperature magmatic-hydrothermal conditions. The microanalytical data of magnetite presented here, coupled with new δ^34S data for pyrite (− 0.5 to + 4.3‰) and U–Pb ages of the diorite (129.6 ± 1.0 Ma), are indicative of a genetic connection between the diorite intrusion and the magnetite mineralization, supporting a magmatic-hydrothermal flotation model to explain the origin of Kiruna-type deposits in the Coastal Cordillera of northern Chile.
Eric H. Oelkers - One of the best experts on this subject based on the ideXlab platform.
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on the effect of aqueous ca on Magnesite growth insight into trace element inhibition of carbonate mineral precipitation
Geochimica et Cosmochimica Acta, 2016Co-Authors: Guntram Jordan, Jacques Schott, Ulfniklas Berninger, Michael Lindner, Alexander Reul, Eric H. OelkersAbstract:Abstract Motivated by the strong effect of aqueous Mg on calcite growth rates, this study used hydrothermal atomic force microscopy (HAFM) and hydrothermal mixed-flow reactor (HMFR) experiments to explore the effect of aqueous Ca on Magnesite growth kinetics at 100 °C and pH ∼7.7. Obtuse step velocities on (1 0 4) surfaces during Magnesite growth were measured to be 4 ± 3 nm/s at fluid saturation states, equal to the ion activity quotient divided by the equilibrium constant for the Magnesite hydrolysis reaction, of 86–117. These rates do not vary systematically with aqueous Ca concentration up to 3 × 10 − 3 mol/kg. Magnesite growth rates determined by HAFM are found to be negligibly affected by the presence of aqueous Ca at these saturation states and are largely consistent with those previously reported in aqueous Ca-free systems by Saldi et al. (2009) and Gautier et al. (2015). Similarly, Magnesite growth rates measured by HMFR exhibit no systematic variation on aqueous Ca concentrations. Rates in this study, however, were extended to higher degrees of fluid supersaturation with respect to Magnesite than previous studies. All measured HMFR rates can be accurately described taking account the combined effects of both the spiral growth and two dimensional nucleation/growth mechanisms. Despite the lack of a clear effect of aqueous Ca on Magnesite growth rates, Raman spectroscopy confirmed the incorporation of up to 8 mol percent of Ca 2+ into the growing Magnesite structure.
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an experimental study of Magnesite precipitation rates at neutral to alkaline conditions and 100 200 c as a function of ph aqueous solution composition and chemical affinity
Geochimica et Cosmochimica Acta, 2012Co-Authors: Giuseppe D Saldi, Jacques Schott, Oleg S Pokrovsky, Quentin Gautier, Eric H. OelkersAbstract:Abstract Magnesite precipitation rates were measured at temperatures from 100 to 200 °C as a function of saturation state and reactive fluid composition in mixed flow reactors. Measured rates were found to increase systematically with increasing saturation state but to decrease with increasing reactive fluid aqueous CO 3 2 - activity and pH. Measured rates are interpreted through a combination of surface complexation models and transition state theory. In accord with this formalism, constant saturation state BET surface area normalized Magnesite precipitation rates (rMg) are a function of the concentration of protonated Mg sites at the surface ( > MgOH 2 + ) and can be described using: r Mg = k Mg - K CO 3 K OH K CO 3 K OH + K OH a CO 3 2 - + K CO 3 a OH - n 1 - Ω Mg n where k Mg - represents a rate constant, KOH and K CO 3 stand for equilibrium constants, ai designates the activity of the subscripted aqueous species, n refers to a reaction order equal to 2, and ΩMg denotes the saturation state of the reactive solution with respect to Magnesite. Retrieved values of n are consistent with Magnesite precipitation control by a spiral growth mechanism. The temperature variation of the rate constant can be described using k Mg - = A a exp ( - E a / RT ) , where Aa represents a pre-exponential factor equal to 5.9 × 10−5 mol/cm2/s, Ea designates an activation energy equal to 80.2 kJ/mol, R denotes the gas constant, and T corresponds to the absolute temperature. Comparison of measured Magnesite precipitation rates with corresponding forsterite dissolution rates suggest that the relatively slow rates of Magnesite precipitation may be the rate limiting step in mineral carbonation efforts in ultramafic rocks.
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Magnesite growth rates as a function of temperature and saturation state
Geochimica et Cosmochimica Acta, 2009Co-Authors: Giuseppe D Saldi, Guntram Jordan, Jacques Schott, Eric H. OelkersAbstract:Abstract Magnesite growth rates and step velocities have been measured systematically as a function of temperature from 80 to 105 °C and saturation state in 0.1 M NaCl solutions using hydrothermal atomic force microscopy (HAFM). The observations indicate that at these conditions Magnesite precipitation is dominated by the coupling of step generation via spiral growth at screw dislocations and step advancement away from these dislocations. As these two processes occur in series the slowest of these dominates precipitation rates. At 100 °C Magnesite growth rates ( r ) determined by HAFM are consistent with r = k ( Ω - 1 ) 2 , where k is a constant equal to 6.5 × 10 −16 mol/cm 2 /s and Ω is the saturation index with respect to Magnesite. This equation is consistent with spiral growth step generation controlling Magnesite precipitation rates. Corresponding Magnesite precipitation rates measured using mixed-flow reactors are shown to be consistent with both the rates measured by HAFM and the spiral growth theory, confirming the rate limiting mechanism. Step advancement, however, is observed to slow far faster than step generation with decreasing temperature; the activation energy for step advancement is 159 kJ/mol whereas step generation rates have an estimated activation energy of ∼60 kJ/mol. As such, it seems likely that at ambient temperatures Magnesite growth is limited by very slow step advancement rates.
Bogdan Z. Dlugogorski - One of the best experts on this subject based on the ideXlab platform.
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Mg isotope fractionation during continental weathering and low temperature carbonation of ultramafic rocks
Geochimica et Cosmochimica Acta, 2019Co-Authors: H.c. Oskierski, Andreas Beinlich, Vasileios Mavromatis, Mohammednoor Altarawneh, Bogdan Z. DlugogorskiAbstract:Abstract The Mg-isotope systematics of peridotite weathering and low-temperature carbonation have not yet been thoroughly investigated, despite their potential to provide insights into reaction pathways and mechanisms of lithosphere-hydrosphere transfer of Mg and sequestration of CO2 in carbonate minerals. Here, we present new observations of the evolution of Mg isotope ratios during subtropical ultramafic rock weathering and associated Magnesite formation, including the lowest δ26Mg of Magnesite reported so far. At the investigated field sites in eastern Australia, the proximity of the ultramafic Mg source rocks and associated Magnesite deposits provides boundary conditions that constrain Mg isotope fractionation during low-temperature alteration. Saprolite samples from Attunga, New South Wales, show that weathering of serpentinite is accompanied by Mg loss and formation of secondary Mg-bearing clay minerals. Furthermore, Mg isotope ratios increase systematically with weathering intensity, indicating that incorporation of 26Mg into clay mineral structures controls Mg isotope fractionation during ultramafic rock weathering. The Mg-bearing clay formed by decomposition of serpentine minerals has a δ26Mg value of ∼0.35‰, which is up to ∼0.6‰ heavier than the ultramafic precursor. In contrast, nodular Magnesite hosted in ultramafic rock shows δ26Mg values between −3.26‰ and −2.55‰ that are significantly lower than those of Magnesite and dolomite formed by hydrothermal alteration of peridotite at higher temperature (δ26Mg = −0.69‰ and −0.62‰). The strong enrichment of 24Mg in nodular Magnesite does not reconcile with simple fractionation during direct precipitation from ultramafic host rock buffered meteoric fluids and instead suggests multiple formation steps involving dissolution and re-precipitation of pre-existing carbonate accompanied by fractionation between species of dissolved Mg. Our data highlight the potential of Mg isotope studies for distinguishing the formation pathways of low temperature Magnesite and for tracing Mg in low temperature alteration processes based on the distinct signatures of secondary silicate and carbonate minerals.
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sequestration of atmospheric co2 in a weathering derived serpentinite hosted Magnesite deposit 14c tracing of carbon sources and age constraints for a refined genetic model
Geochimica et Cosmochimica Acta, 2013Co-Authors: H.c. Oskierski, Bogdan Z. Dlugogorski, Geraldine JacobsenAbstract:Abstract The Attunga Magnesite deposit is texturally and geochemically distinct from other spatially associated, serpentinite-hosted Magnesite deposits in the Great Serpentinite Belt, New South Wales, Australia, such as the hydrothermal Piedmont Magnesite deposit or widespread silica–carbonate alteration zones. Cryptocrystalline Magnesite at Attunga predominantly occurs in nodular masses and irregular, desiccated veins that occupy pre-existing cracks and pore spaces resulting from fracturing and weathering of the host rock. Incipient weathering of the serpentinite host rock is accompanied by a decrease in volume and the mobilisation of MgO and CaO from the serpentinite. Pore spaces and permeability created during weathering and fracturing of the host rock provide access for CO2-, MgO- and CaO-bearing meteoric waters which led to an increase of volume during carbonation. SiO2 is only mobilised during more advanced stages of weathering and late stage infiltration of SiO2-bearing waters and precipitation of opal-A lead to local silicification of the serpentinite. Stable carbon and oxygen isotope signatures show that nodular Magnesite at Attunga has formed under near-surface conditions incorporating carbon from C3-photosynthetic plants and oxygen from meteoric waters. Radiocarbon concentrations in the Magnesite preclude subducted carbonaceous sediments as the source of carbon and, together with distinct stable carbon and oxygen isotope signatures, indicate that Magnesite at Attunga precipitated from low temperature, supergene fluids. Even though there is no direct geochemical and isotopic evidence, some textural observations and field relationships for weathering-derived Magnesite deposits suggest the prior existence of a possibly Early Triassic, hydrothermal Magnesite deposit at Attunga. The presence of a pre-existing Magnesite deposit may entail the localised formation of the weathering-derived Magnesite at Attunga, but the predominance of weathering-related textures and geochemical signatures indicate that weathering is the integral Magnesite mineralisation process at Attunga. Conventional radiocarbon ages of about 50 ka represent a maximum age constraint for the formation of the Magnesite deposit during Quaternary weathering. A significant amount of atmospheric CO2 has been sequestered via the biosphere and carbonation of serpentinite at Attunga.
M Leisen - One of the best experts on this subject based on the ideXlab platform.
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Trace element geochemistry of magnetite from the Cerro Negro Norte iron oxide−apatite deposit, northern Chile
Mineralium Deposita, 2019Co-Authors: Eduardo Salazar, M Leisen, Gisella Palma, Rurik Romero, Martin Reich, Fernando Barra, Adam Simon, Mario RojoAbstract:Kiruna-type iron oxide−apatite (IOA) deposits constitute an important source of iron and phosphorus, and potentially of rare earth elements (REE). However, the origin of IOA deposits is still a matter of debate with models that range from a purely magmatic origin by liquid immiscibility to replacement of host rocks by hydrothermal fluids from different sources. In order to better constrain the origin of Andean IOA deposits, we focused on the Cretaceous Cerro Negro Norte deposit located in the Chilean Iron Belt, northern Chile. The Cerro Negro Norte magnetite ore is hosted in andesitic rocks and is spatially and genetically associated with a diorite intrusion. Our results show that the deposit is characterized by three main mineralization/alteration episodes: an early Fe–oxide event with magnetite and actinolite followed by four stages that comprise the main hydrothermal event (hydrothermal magnetite + actinolite; calcic–sodic alteration + sulfides; quartz–tourmaline and propylitic alteration) and a minor supergene event. Based on textural and chemical characteristics, four different types of magnetite are recognized at Cerro Negro Norte: type I, represented by high-temperature (~ 500 °C) magnetite cores with amphibole, pyroxene, and minor Ti–Fe oxide inclusions; type II, an inclusion-free magnetite, usually surrounding type I magnetite cores; type III corresponds to an inclusion-free magnetite with chemical zoning formed under moderate temperatures; and type IV magnetite contains abundant inclusions and is related to low-temperature (~ 250 °C) hydrothermal veinlets. Electron probe and laser ablation ICP-MS analyses of the four magnetite types show that the incorporation of Al, Mn, Ti, and V into the magnetite structure is controlled by temperature. Vanadium and Ga concentrations are relatively constant within each magnetite type, but are statistically different among magnetite types, suggesting that both elements could be used to discriminate between magmatic and hydrothermal magnetite. However, our results show that the use of elemental discrimination diagrams should be coupled with detailed textural studies in order to identify superimposed metasomatic events and evaluate the impact of inclusions on the interpretation of microanalytical data. The presence of a distinct textural and chemical variation between magnetite types in Cerro Negro Norte is explained by a transition from high- to low-temperature magmatic-hydrothermal conditions. The microanalytical data of magnetite presented here, coupled with new δ^34S data for pyrite (− 0.5 to + 4.3‰) and U–Pb ages of the diorite (129.6 ± 1.0 Ma), are indicative of a genetic connection between the diorite intrusion and the magnetite mineralization, supporting a magmatic-hydrothermal flotation model to explain the origin of Kiruna-type deposits in the Coastal Cordillera of northern Chile.
