Shaun L.L. Barker

Isotopic disequilibrium during uptake of atmospheric CO2 into mine process waters: implications for CO2 sequestration.

Dypingite, a hydrated Mg-carbonate mineral, was precipitated from high-pH, high salinity solutions to investigate controls on carbon fixation and to identify the isotopic characteristics of mineral sequestration in mine tailings. δ(13)C values of dissolved inorganic carbon content and synthetic dypingite are significantly more negative than those predicted for equilibrium exchange of CO(2) gas between the atmosphere and solution. The measured δ(13)C of aqueous carbonate species is consistent with a kinetic fractionation that results from a slow diffusion of atmospheric CO(2) into solution. During dypingite precipitation, dissolved inorganic carbon concentrations decrease and δ(13)C values become more negative, indicating that the rate of CO(2) uptake into solution was outpaced by the rate of carbon fixation within the precipitate. This implies that CO(2) gas uptake is rate-limiting to CO(2) fixation. δ(13)C of carbonate mineral precipitates in mine tailings and of DIC in mine process waters display similar (13)C-depletions that are inconsistent with equilibrium fractionation. Thus, the rate of carbon fixation in mine tailings may also be limited by supply of CO(2). Carbon sequestration could be accelerated by increasing the partial pressure of CO(2) in tailings ponds or by using chemicals that enhance the uptake of gaseous CO(2) into aqueous solution.

Subarctic Weathering of Mineral Wastes Provides a Sink for Atmospheric CO2

The mineral waste from some mines has the capacity to trap and store CO(2) within secondary carbonate minerals via the process of silicate weathering. Nesquehonite [MgCO(3)·3H(2)O] forms by weathering of Mg-silicate minerals in kimberlitic mine tailings at the Diavik Diamond Mine, Northwest Territories, Canada. Less abundant Na- and Ca-carbonate minerals precipitate from sewage treatment effluent deposited in the tailings storage facility. Radiocarbon and stable carbon and oxygen isotopes are used to assess the ability of mine tailings to trap and store modern CO(2) within these minerals in the arid, subarctic climate at Diavik. Stable isotopic data cannot always uniquely identify the source of carbon stored within minerals in this setting; however, radiocarbon isotopic data provide a reliable quantitative estimate for sequestration of modern carbon. At least 89% of the carbon trapped within secondary carbonate minerals at Diavik is derived from a modern source, either by direct uptake of atmospheric CO(2) or indirect uptake though the biosphere. Silicate weathering at Diavik is trapping 102-114 g C/m(2)/y within nesquehonite, which corresponds to a 2 orders of magnitude increase over the background rate of CO(2) uptake predicted from arctic and subarctic river catchment data.

Use of laser spectroscopy to measure the 13C/12C and 18O/16O compositions of carbonate minerals.

The stable carbon and oxygen isotope compositions of carbonate minerals are utilized throughout the earth and environmental sciences for various purposes. Here, we demonstrate the first application of a prototype instrument, based on off-axis integrated cavity output laser spectroscopy, to measure the carbon and oxygen isotope composition of CO(2) gas evolved from the acidification of carbonate minerals. The carbon and oxygen isotope ratios were recorded from absorption spectra of (12)C(16)O(16)O, (13)C(16)O(16)O, and (12)C(16)O(18)O in the near-infrared wavelength region. The instrument was calibrated using CaCO(3) minerals with known δ(13)C(VPDB) and δ(18)O(VSMOW) values, which had been previously calibrated by isotope ratio mass spectrometry relative to the international isotopic standards NBS 18 and NBS 19. Individual analyses are demonstrated to have internal precision (1 SE) of better than 0.15‰ for δ(13)C and 0.6‰ for δ(18)O. Analysis of four carbonate standards of known isotopic composition over 2 months, determined using the original instrumental calibration, indicates that analyses are accurate to better than 0.5‰ for both δ(13)C and δ(18)O without application of standard-sample-standard corrections.

Determination of carbonate vein chemistry using portable X-ray fluorescence and its application to mineral exploration

The composition of carbonate minerals infilling hydrothermal veins can vary as a function of factors including fluid chemistry, temperature and pressure. If effectively quantified, carbonate vein chemistry can represent an important tool in mineral exploration. We present a method for determining carbonate vein chemistry using portable X-ray fluorescence analysis (pXRF) utilising matrix matched certified reference materials (CRMs) to assess pXRF analyser performance, before establishing strict QA/QC procedures, along with linear calibration equations for Mg, S, Mn, Fe, Zn, Sr and Pb. However, low concentrations of mineralization-related elements in commercially available CRMs meant raw S, Zn and Pb concentrations determined for case study samples could not be reliably corrected. Our new workflow was then applied to carbonate vein samples collected from Mount Isa, Northwest Queensland, Australia. Though complexity of this system limited the interpretation of results, the case study showed that pXRF can also be used to identify veins with multiple generations of carbonate infill. Portable X-ray fluorescence analyses, used in conjunction with robust QA/QC processes is a powerful tool that can quickly and cost-effectively inform decisions during exploration programs. Supplementary material: Results of pXRF analyser performance for six reference materials (S1) and results from analysis of carbonate veins collected from Mount Isa, Australia (S2). All tables are available at https://doi.org/10.6084/m9.figshare.c.3831385

Characteristics and variations of sinters in the Coromandel Volcanic Zone: application to epithermal exploration

ABSTRACT The Hauraki Goldfield of the Coromandel Volcanic Zone, North Island, New Zealand, hosts up to 19 known siliceous hot spring deposits, or sinters, some directly associated with Au–Ag ore bodies, in a Miocene–Pliocene, sub-aerial, calc-alkaline volcanic arc overlying Jurassic meta-sedimentary basement. Sinters are surface expressions of predominately adularia-sericite epithermal systems, potentially linked to Au–Ag mineralised conduits at depth. They provide a paleosurface marker to determine the level of erosion, and help determine zones of thermal fluid upflow and/or lateral outflow. Mapped sinters of the Coromandel Volcanic Zone are mainly affiliated with rhyolite domes, occur along structural corridors of the volcanic arc, and contain characteristic textures, mineralogy and pathfinder elements typical of near-neutral pH alkali chloride fluid discharge derived from convectively circulating magmatic fluids diluted by meteoric waters. Textures and trace element compositions of the Coromandel sinters are similar to those of the Taupo Volcanic Zone, New Zealand, and the Jurassic Au–Ag mining districts of the Deseado Massif, Argentine Patagonia. Thus, detailed mapping, textural analysis and trace-element studies demonstrate the prospecting potential of siliceous hot spring deposits for locating ore deposits, and in particular to help define proximal to distal positions relative to vent zones at the paleo-surface.

Precise Measurement of the Hydrogen Isotope Composition of Phyllosilicates by Continuous Flow Off-Axis Integrated Cavity Output Spectroscopy

New methodology is presented for analyzing hydrogen isotope ratios (D/H) in phyllosilicate minerals by laser absorption spectroscopy. D/H measurements were carried out using an off-axis integrated cavity output spectroscopy (OA-ICOS) instrument operated in a continuous flow configuration. Water was extracted from minerals in a high temperature quartz column and advanced to the analyzer in a dry air carrier gas stream. We report the first D/H measurements by a laser system for serpentine, muscovite, sericite, talc, and biotite. We also measure kaolinite, gypsum, and small volumes of water. Materials, excluding biotite, were calibrated to within 1.5‰ of IRMS-measured δDVSMOW values, with an average precision of 1.1‰. Biotite δD measurements were up to 10‰ more positive than established IRMS values, due to partial reduction of evolved waters by Fe in the high temperature column. We provide recommendations for overcoming redox interference for measurements of biotite, and other ferrous materials, by OA-ICOS. Rapid, precise, and accurate analyses were carried out on water volumes as low as 0.25 μL extracted from minerals. With the exception of talc, the time required for thermal dewatering and measurement is 140 s, which translates into a throughput of up to 6 mineral samples per hour, including replicates. By demonstrating high precision, rapid throughput, low cost, and ease of operation, we provide a tool that should enable researchers at institutions with limited funding to routinely measure D/H in hydrous minerals. The protocols presented herein should also be useful to commercial users seeking to produce high density isotope data sets relevant to exploration of hydrothermal ore deposits and geothermal fields.

Using portable XRF to infer adularia halos within the Waihi Au-Ag system, New Zealand

Waihi Au-Ag mineralization is vein hosted within a relatively homogeneous andesite body. Geochemical exploration targets adularia and pathfinder (As, Sb, Hg etc.) anomalies. Adularia is formed as an alteration product of feldspars during hydrothermal alteration and occurs in anomalous concentrations near Au-Ag mineralization. This paper aims to test the potential of portable X-ray fluorescence (pXRF) to infer and map adularia and As anomalies. The pXRF was first tested on standards (CRM and ILD), it yielded accurate and precise data for most of the elements of interest (r2 > 0.9 and APD < 10%). The second stage applied the pXRF directly on core, most elements were negatively affected, but still to a usable degree of accuracy. As was particularly effected due to its heterogeneous distribution (APD 46%). Lastly, we applied the pXRF to a suite of 300 Favona samples, their K/Al value were used as a proxy for adularia presence. XRD analysis carried out on 60 samples was used to validate adularias presence or absence. When combined with pXRF data we found a K/Al value above 0.4 generally indicated a sample contained adularia. Therefore, with further work, the pXRF could be independently used to infer adularia alteration in the Waihi setting.

Acceptance of the SEG Waldemar Lindgren Award for 2013

President Kinnaird, SEG members, and guests: I am humbled to receive the Waldemar Lindgren Award, which has been awarded to such highly regarded and prominent geologists before me. Many of the past winners I recognize, having read their papers as a student and research fellow, and it is an honor to be considered one of their peers. I am thankful to the Society for making me the 2013 recipient of the Waldemar Lindgren award. I came to economic geology via rather a circuitous route. My BSc(Hons) project at the University of Otago, in New Zealand, focused on the structural setting and geochemistry of pseudotachylyte veins, working under the supervision of Rick Sibson. Rick inspired an interest in ore deposits through his fantastic lecturing on faults, fractures, and fluid flow, which frequently cited …