Otago Schist as an aggregate source : background investigations at Macraes Mine

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Reyes, A.G.; Cox, S.C.; Harvey, C.C.; Soong, C.W.R. 2003 Otago Schist as an aggregate source : background investigations at Macraes Mine . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2003/20 69 p.

Abstract: Otago Schist is relatively weak but has been used locally for building and roading due to its ready availability. It is used to construct haul roads at Macraes mine, but its poor performance appears to have a significant impact on the economics of the mining operations. Schists were used unselectively for roading but minable variations, that may influence schist strength and aggregate performance, include: (1) massive and laminated schist lithologies; (2) folded and planar schist structure; (3) fresh and weathered schist; and (4) crushed and coherent schist. Massive schist is subjectively stronger than laminated schist and weathering and oxidation decreases schist strength. When subjected to heavy mine traffic, schist disaggregates due to both mechanical crushing and accelerated chemical weathering. Comminuted schist deteriorates to a soft, wet and cohesionless material (mud) on road surfaces after rain and traffic. It is bi-modal in size, with silt- and sand- to pebble-sized fractions having different mineralogy controlled by the schist protolith and grain size. Phyllosilicate-rich schist crushes with more finely sized (silt to clay) fraction. Crushing accelerates weathering, which initially involves pit-etching of albite and muscovite, followed by dissolution-precipitation processes that produce goethite, kaolinite and expanding clays (smectite, interlayered illite-smectite and chlorite-smectite). Expanding clays and fine silt to clay sized fraction diminish the relative strength of road material and contribute to waterlogging and increased road slippage. Potential solutions for improving aggregates sourced from schist include: (1) selecting stronger schist-types, (assuming any are sufficiently strong); (2) selecting source material to minimise unwanted chemical or physical by-products; (3) mitigating effects of weathering principally by neutralising the pH of weathering solutions or decreasing dissolution rates with inhibiting compounds. Although Macraes schists have some geochemical differences from most other Otago Schists, the processes of chemical weathering and mechanical crushing are accelerated in the mine environment and are likely to occur wherever the schist is exposed in the region. (auth)