Georges Beaudoin

Trace elements in magnetite as petrogenetic indicators

We have characterized the distribution of 25 trace elements in magnetite (Mg, Al, Si, P, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Nb, Mo, Sn, Hf, Ta, W, and Pb), using laser ablation ICP-MS and electron microprobe, from a variety of magmatic and hydrothermal ore-forming environments and compared them with data from the literature. We propose a new multielement diagram, normalized to bulk continental crust, designed to emphasize the partitioning behavior of trace elements between magnetite, the melt/fluid, and co-crystallizing phases. The normalized pattern of magnetite reflects the composition of the melt/fluid, which in both magmatic and hydrothermal systems varies with temperature. Thus, it is possible to distinguish magnetite formed at different degrees of crystal fractionation in both silicate and sulfide melts. The crystallization of ilmenite or sulfide before magnetite is recorded as a marked depletion in Ti or Cu, respectively. The chemical signature of hydrothermal magnetite is distinct being depleted in elements that are relatively immobile during alteration and commonly enriched in elements that are highly incompatible into magnetite (e.g., Si and Ca). Magnetite formed from low-temperature fluids has the lowest overall abundance of trace elements due to their lower solubility. Chemical zonation of magnetite is rare but occurs in some hydrothermal deposits where laser mapping reveals oscillatory zoning, which records the changing conditions and composition of the fluid during magnetite growth. This new way of plotting all 25 trace elements on 1 diagram, normalized to bulk continental crust and elements in order of compatibility into magnetite, provides a tool to help understand the processes that control partitioning of a full suit of trace elements in magnetite and aid discrimination of magnetite formed in different environments. It has applications in both petrogenetic and provenance studies, such as in the exploration of ore deposits and in sedimentology.

Hydrothermal alteration and microbial sulfate reduction in peridotite and gabbro exposed by detachment faulting at the Mid-Atlantic Ridge, 15°20′N (ODP Leg 209): A sulfur and oxygen isotope study

Whole rock sulfur and oxygen isotope compositions of altered peridotites and gabbros from near the 15°20′N Fracture Zone on the Mid-Atlantic Ridge were analyzed to investigate hydrothermal alteration processes and test for a subsurface biosphere in oceanic basement. Three processes are identified. (1) High-temperature hydrothermal alteration (∼250–350°C) at Sites 1268 and 1271 is characterized by 18O depletion (2.6–4.4‰), elevated sulfide-S, and high δ34S (up to ∼2 wt% and 4.4–10.8‰). Fluids were derived from high-temperature (>350°C) reaction of seawater with gabbro at depth. These cores contain gabbroic rocks, suggesting that associated heat may influence serpentinization. (2) Low-temperature (<150°C) serpentinization at Sites 1272 and 1274 is characterized by elevated δ18O (up to 8.1‰), high sulfide-S (up to ∼3000 ppm), and negative δ34S (to −32.1‰) that reflect microbial reduction of seawater sulfate. These holes penetrate faults at depth, suggesting links between faulting and temperatures of serpentinization. (3) Late low-temperature oxidation of sulfide minerals caused loss of sulfur from rocks close to the seafloor. Sulfate at all sites contains a component of oxidized sulfide minerals. Low δ34S of sulfate may result from kinetic isotope fractionation during oxidation or may indicate readily oxidized low-δ34S sulfide derived from microbial sulfate reduction. Results show that peridotite alteration may be commonly affected by fluids ± heat derived from mafic intrusions and that microbial sulfate reduction is widespread in mantle exposed at the seafloor.

Carbon sequestration kinetic and storage capacity of ultramafic mining waste.

Mineral carbonation of ultramafic rocks provides an environmentally safe and permanent solution for CO(2) sequestration. In order to assess the carbonation potential of ultramafic waste material produced by industrial processing, we designed a laboratory-scale method, using a modified eudiometer, to measure continuous CO(2) consumption in samples at atmospheric pressure and near ambient temperature. The eudiometer allows monitoring the CO(2) partial pressure during mineral carbonation reactions. The maximum amount of carbonation and the reaction rate of different samples were measured in a range of experimental conditions: humidity from dry to submerged, temperatures of 21 and 33 °C, and the proportion of CO(2) in the air from 4.4 to 33.6 mol %. The most reactive samples contained ca. 8 wt % CO(2) after carbonation. The modal proportion of brucite in the mining residue is the main parameter determining maximum storage capacity of CO(2). The reaction rate depends primarily on the proportion of CO(2) in the gas mixture and secondarily on parameters controlling the diffusion of CO(2) in the sample, such as relative saturation of water in pore space. Nesquehonite was the dominant carbonate for reactions at 21 °C, whereas dypingite was most common at 33 °C.

Variations in the sulfur isotope composition of troilite from the Cañon Diablo iron meteorite

Abstract Canon Diablo troilite (CDT), although accepted as the reference for the sulfur isotope scale, is not a Standard Reference Material manufactured for international distribution, and its sulfur isotope composition is not well characterized. We report high-precision sulfur isotope analyses of troilite from three different samples of Canon Diablo meteorite, and interlaboratory comparison of one CDT sample and of a reference SF6 gas. The sulfur isotope composition of CDT displays a range in δ 34 S values of 0.4%., significantly larger than our analytical uncertainty (0.05%.). CDT should not be used for calibration of the sulfur isotope scale. These results support the definition of a new sulfur isotope scale relative to a hypothetical Vienna-CDT, as recently proposed by the International Atomic Energy Agency.

CO2-depleted warm air venting from chrysotile milling waste (Thetford Mines, Canada): Evidence for in-situ carbon capture from the atmosphere

We have discovered diffuse warm air vents at the surface of a chrysotile milling waste heap at the Black Lake mine, Thetford Mines, Quebec, Canada. The venting areas are inconspicuous, except in winter when the vents form snow-free areas of unfrozen ground, each with a surface area of 1–15 m 2 . The temperature and chemical composition of the warm air vents have been monitored from March 2009 to July 2010. The temperature of the warm air and ground surface at the venting sites ranged from 6.6 to 20.0 °C, whereas that of the ambient air ranged from −13.2 to 20.0 °C. The difference between atmospheric and vent air temperatures is greater in cold-weather months. The warm air has low CO 2 content, but has otherwise normal atmospheric gas composition. Warm air volumetric flow varies from 2.1 to 19.9 L/m 2 /s in winter, when it contains between 2 . In summer, the venting areas are more diffuse, with volumetric flow rates ranging from 0.5 to 1.5 L/m 2 /s, and are less depleted in CO 2 (260–370 ppm). Frozen ground is likely focusing airflow in winter compared to summer. We present a conceptual model in which air enters the steep flanks of the chrysotile milling waste heap, into which CO 2 reacts with Mg-rich minerals, stripping CO 2 from air by exothermic mineral carbonation reactions. Considering the surface area of summer and winter venting areas, flow rates, and concentration of CO 2 in warm air vents, we estimate that the Black Lake mine heap passively captures at least 0.6 kt CO 2 per year.

Silver-lead-zinc veins and crustal hydrology during Eocene extension, southeastern British Columbia, Canada

Abstract The Kokanee Range, southeastern British Columbia, Canada, contains more than 370 Ag-Pb-Zn vein and replacement deposits hosted by the Middle Jurassic Nelson batholith and surrounding Cambrian to Triassic metasedimentary rocks. All deposits are in the hanging wall of the Slocan Lake Fault, an east-dipping low-angle normal fault that reaches the Moho, and along which the Valhalla metamorphic core complex has been unroofed during the Eocene. Fluid inclusions in sphalerite, quartz, or siderite indicate a temperature and pressure of trapping of approximately 300°C and 0.17 GPa. The fluid inclusions display a large range of salinities from 0 to 18 wt% NaCl eq. Assuming lithostatic pressure at the time of mineralization, the depth of mineralization was 6 ± 3 km. δ34Ssulfide values display a 20‰ range and a regional zonation that correlates with the distribution of sulfur isotopic compositions of host rocks. In contrast, gd13Csiderite values are homogeneous ( −7.1 ± 0.5‰ ) over an area of 1200 km2. It is concluded that S was derived from local country rocks whereas C was derived from mantle CO2 degassing. The δ18Osiderite and δ18Orock values display regional zonations revealing fluid flow paths of a large, ancient hydrothermal system. Three different fluids are identified from salinity, δ18O, and δD values: 1. (1) a high-salinity, deep-seated and isotopically crust-equilibrated fluid; 2. (2) a low-salinity, upper crustal fluid of evolved meteoric origin; 3. (3) meteoric water. Mixing between thermally similar fluids having different salinities is proposed to have been the dominant mineralizing process. Regional isotopic zonations are controlled by the Slocan Lake Fault which channelled deep-seated hydrothermal fluids and mantle CO2 to higher crustal levels where mixing occurred with upper crustal fluids that leached local sulfur. Meteoric water invaded the fracture network in the waning stages of the hydrothermal system.

Silver-lead-zinc veins, metamorphic core complexes, and hydrologic regimes during crustal extension

The Kokanee Range Ag-Pb-Zn-Au vein and replacement deposits formed during Eocene crustal extension and unroofing of the Valhalla metamorphic core complex, in the hanging wall of the transcrustal Slocan Lake fault. The δ34Ssulfide, δ18Oquartz, δ18Osiderite, and galena Pb isotopic ratios for vein minerals display regional zonations revealing fluid-flow paths of a large, fossil hydrothermal system. Sulfur was derived from local country rocks, but carbon was derived from a deep-seated reservoir, possibly the upper mantle. Lead is a mixture of local, upper-crustal Pb with lower-crustal and depleted upper-mantle Pb. Regional isotopic zonations were controlled by deep fracture zones, such as the Slocan Lake fault, which channeled lower-crustal and mantle Pb and mantle CO2 to higher-crustal levels, where mixing occurred with highly evolved meteoric waters that had leached local sulfur and upper-crustal Ph. This is the first Ag-Pb-Zn vein district for which a genetic link between mineralization and metamorphic core complex unroofing is demonstrated.

Yulong Deposit, Eastern Tibet: A High-Sulfidation Cu-Au Porphyry Copper Deposit in the Eastern Indo-Asian Collision Zone

The Yulong ore body is the largest Cu deposit (6.22 million metric tons [Mt] at 0.99% Cu) in the 300 km long Himalayan porphyry copper belt, and is controlled by major Cenozoic strike-slip faults in the eastern Indo-Asian collision zone. It is associated with a steeply dipping, pipe-like multiphase (42-35 Ma) monzogranitic stock. The host rocks are potassic calc-alkaline or shoshonitic, and show geochemical affinities with adakites. They appear to have been derived from a thickened lower crustal source in East Tibet. The Yulong deposit consists of a ring-shaped, high-grade Cu-Au zone overlying and/or surrounding a porphyry-type Cu-Mo ore body. Cu-Mo mineralization produced a steeply dipping, pipe-like, veinlet-disseminated ore body within the stock. Associated hydrothermal alteration produced K-silicate and quartz-sericite assemblages within the stock, and contemporaneous propylitic alteration in the Upper Triassic sandy-slate wall rock. Fluid inclusion and δ18O-δD data indicate that the ore-forming fluid was supercritical, and exsolved from a high-level magma chamber at >620°C; it then separated into a hypersaline aqueous liquid and a coexisting low-salinity vapor at 340°-600°C. The high-grade Cu-Au zone (3 Mt at 4.74% Cu, and 4.5 g/t Au) is dominated by a supergene chalcocite-malachite blanket resting on an underlying supergene/hypogene sulfide transition unit and a hypogene pyrite-chalcopyrite sulfide unit. The Cu-Au zone was controlled by a subhorizontal or gently outward dipping breccia horizon developed along the marginal fracture zone near the roof of the stock, produced by hydrothermal brecciation due to regional uplift and/or fluid boiling. Alteration associated with the hypogene Cu-Au mineralization was texture-destructive advanced argillic alteration, characterized by associations of quartz, kaolinite, dickite, endellite, montmorillonite, hydromica, and minor alunite. It mainly developed within the breccia horizons, and partially over-printed the early-formed K-silicate zone and the quartz-sericite zone. Associated mineralization was of the high-sulfidation epithermal-type, characterized by chalcocite, tennantite, covellite, bornite, and minor pyrite, which formed the main ore body in the high-grade Cu-Au zone. Epithermal fluids also caused the dissolution of early-formed sulfides and remobilization of Cu-Mo, the latter transported into the intense advanced argillic alteration halo within the mineralized stock. This late-stage alteration and mineralization is attributed to a CO2-rich, low-temperature (<350°C), low-salinity (<12 wt% NaCl equiv.) meteoric fluid, involving input of magmatic fluid. Based on alteration, mineralization, fluid inclusion and stable isotopic data, a two-stage genetic history has been reconstructed for the Yulong deposit. It spans (1) a magmatic hydrothermal environment reflecting the emplacement of the monzogranite stock and Cu-Mo introduction through (2) hydrothermal fluid infiltration of breccia zones to epithermal overprinting.

High precision and spatial resolution sulfur isotope analysis using MILES laser microprobe

The MILES (Micro In situ Laser Extraction System) laser microprobe permits high spatial resolution (< 10−3mm3, or < 0.2 μmol S) in situ sampling of geological material for sulfur isotope analysis. Sulfides are combusted in F2 by absorption of CO2 laser radiation and converted to sulfur hexafluoride (SF6). The product SF6 is purified by cryogenic distillation. In combination with a high-sensitivity dualinlet isotope ratio mass spectrometer, sulfur isotope analyses of powders of pyrite, galena, and sphalerite yield δ34Scdt values with a high precision, ranging from 0.03 to 0.09%. The sulfur isotope ratios measured are accurate and exhibit no matrix-dependent sulfur isotope effects over the range of 62%. A minimum F2 pressure of 20 kPa (for MILES) is required to mediate against small isotopic fractionations between multiple sulfur species apparently caused by laser isotope separation and/or reaction with oxygen during analysis. The precision and accuracy of δ34Scdt values from in situ analyses are good (

Glarus overthrust: A major pathway for the escape of fluids out of the Alpine orogen

0.2‰), but isotopically homogeneous working standards or intercomparison materials are not available thus far. Sulfur isotope ratios derived by conventional-SO2 and laser-SF6 are well correlated (r2 = 0.99999), but a slope different from unity (m = 1.035) arises, probably due to inadequate corrections to SO2 data for oxygen isobaric interferences. Sulfur isotope isopleths in a large, cubic metamorphic pyrite porphyroblast, determined from 79 in situ analyses, are discordant to crystallographic zonation. Concordance between crystallographic and isotopic zonation needs be tested using high precision and spatial resolution analyses such as those described here. Sampling crystallographic zones in minerals can result in erroneous conclusions if isotopic and crystallographic zoning are not coincident.