Textural changes of graphitic carbon by tectonic and hydrothermal processes in an active plate boundary fault zone, Alpine Fault, New Zealand
Martina Kirilova, Virginia G. Toy, Nick Timms, Angela Halfpenny, Catriona Menzies, Dave Craw, Olivier Beyssac, Rupert Sutherland, John Townend, Carolyn Boulton, Brett M. Carpenter, Alan Cooper, Jason Grieve, Timothy Little, Luiz Morales, Chance Morgan, Hiroshi Mori, Katrina M. Sauer, Anja M. Schleicher, Jack Williams, Lisa Craw, 2018. "Textural changes of graphitic carbon by tectonic and hydrothermal processes in an active plate boundary fault zone, Alpine Fault, New Zealand", Characterization of Ore-Forming Systems from Geological, Geochemical and Geophysical Studies, K. Gessner, T.G. Blenkinsop, P. Sorjonen-Ward
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Graphitization in fault zones is associated both with fault weakening and orogenic gold mineralization. We examine processes of graphitic carbon emplacement and deformation in the active Alpine Fault Zone, New Zealand by analysing samples obtained from Deep Fault Drilling Project (DFDP) boreholes. Optical and scanning electron microscopy reveal a microtextural record of graphite mobilization as a function of temperature and ductile then brittle shear strain. Raman spectroscopy allowed interpretation of the degree of graphite crystallinity, which reflects both thermal and mechanical processes. In the amphibolite-facies Alpine Schist, highly crystalline graphite, indicating peak metamorphic temperatures up to 640°C, occurs mainly on grain boundaries within quartzo-feldspathic domains. The subsequent mylonitization process resulted in the reworking of graphite under lower temperature conditions (500–600°C), resulting in clustered (in protomylonites) and foliation-aligned graphite (in mylonites). In cataclasites, derived from the mylonitized schists, graphite is most abundant (<50% as opposed to <10% elsewhere), and has two different habits: inherited mylonitic graphite and less mature patches of potentially hydrothermal graphitic carbon. Tectonic–hydrothermal fluid flow was probably important in graphite deposition throughout the examined rock sequences. The increasing abundance of graphite towards the fault zone core may be a significant source of strain localization, allowing fault weakening.
Supplementary material: Raman spectra of graphite from the Alpine Fault rocks is available at https://doi.org/10.6084/m9.figshare.c.3911797
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Economically viable concentrations of mineral resources are uncommon in Earth’s crust. Most ore deposits that were mined in the past or are currently being extracted were found at or near Earth’s surface, often serendipitously. To meet the future demand for mineral resources, exploration success hinges on identifying targets at depth. Achieving this requires accurate and informed models of the Earth’s crust that are consistent with all available geological, geochemical and geophysical information, paired with an understanding of how ore-forming systems relate to Earth’s evolving structure. Contributions to this volume address the future resources challenge by (i) applying advanced microscale geochemical detection and characterization methods, (ii) introducing more rigorous 3D Earth models, (iii) exploring critical behaviour and coupled processes, (iv) evaluating the role of geodynamic and tectonic setting and (v) applying 3D structural models to characterize specific ore-forming systems.