A microbial pathway for the formation of gold-anomalous calcrete |
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| Authors: | Frank Reith Steven A Wakelin Adrienne L Gregg Andreas Schmidt Mumm |
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| Institution: | 1. Institut für Mineralogie, TU Bergakademie Freiberg/Sachsen, Brennhausgasse 14, D-09596 Freiberg, Germany;2. Institut für Geographie und Geologie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany;1. School of Earth Sciences, The University of Queensland, St. Lucia, Queensland 4072, Australia;2. CMC-XOR-Sector 9, Advanced Photon Source, Argonne Laboratory, Argonne, IL 60439, USA;3. SRK Consulting, Saskatoon, Saskatchewan S7L 6M6, Canada;1. MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Science, Beijing 100037, China;2. School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083, China;3. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 10009, China;4. University of Chinese Academy of Sciences, Beijing 100049, China;1. School of Earth and Environmental Sciences, Centre of Tectonics, Resources and Exploration (TRaX), The University of Adelaide, North Terrace, SA 5005, Australia;2. South Australian Museum, Department of Mineralogy, Adelaide, SA 5000, Australia;3. Martin Luther University Halle-Wittenberg, Institut für Biologie/Mikrobiologie, DE-06120 Halle, Germany;4. Rimfire Pacific Mining NL Suite 411, 530 Little Collins Street Melbourne, VIC 3000, Australia;5. Australian Synchrotron, 800 Blackburn Road, Clayton, VIC 3168, Australia;6. School of Biological Sciences, Faculty of Science and Engineering, Flinders University, Adelaide, SA 5001, Australia;7. School of Chemical and Physical Sciences, Faculty of Science and Engineering, Flinders University, Adelaide, SA 5001, Australia;8. School of Applied Sciences, RMIT University, PO Box 71, Bundoora, VIC 3083, Australia;9. CSIRO Land and Water, Environmental Biogeochemistry, PMB2, Glen Osmond, SA 5064, Australia |
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| Abstract: | The formation of pedogenic carbonate (calcrete) in terrestrial environments is commonly mediated by microorganisms. In Australia, Au-anomalous calcrete is an important sampling medium for geochemical exploration, but current models describing its formation do not include a confirmed microbial component. This study demonstrates that bacterial communities in calcareous sands from dunes overlying the Barns Gold Deposit in semi-arid South Australia, are capable of mediating the biomineralisation of Au-anomalous carbonates. Bacterial enrichment cultures obtained from calcareous sands at three depths (0.1, 0.64 and 2.1 m, plus abiotic control) were incubated in urea and Ca2+-containing growth media (pH 8), unamended and amended with Au (100 parts-per-billion, ppb) as Au–aspartic-acid complex. During the incubation of the enrichment cultures urea was turned over to NH4+ within 96 h to 220 h. The solution pH increased concurrently by approximately 1.2 units, and Au-anomalous Ca-carbonate crystallites were precipitated on cells, which functioned as nucleation sites; no carbonate precipitation was observed in abiotic controls. Compared to the medium, Au was strongly enriched in these carbonates and appeared to be uniformly dispersed in the individual crystallites, as shown using LA-ICP-MS; a similar distribution is present in naturally occurring Au-anomalous calcrete. Phylogenetic 16S rRNA PCR DGGE analyses, shotgun cloning and functional microbial analyses (BioLog, ureC quantitative PCR) demonstrated that naturally occurring and culture-enriched bacterial communities were dominated by alkaliphylic, halotolerant Bacillus spp. The indigenous bacterial communities were capable of utilising amino acids (including l-aspartic acid) and urea, which appears to lead to the destabilisation of the Au–amino acid complexes and concomitant co-precipitation of Au in the Ca-carbonates. In conclusion, a model combining geomicrobial– with evapotranspiration– and plant-based components is likely to best describe the formation of (Au-anomalous) calcrete in semi-arid and arid zones. |
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