Jamie Cudby, J. Scott, D. Craw, Mark Hesson, C. Rufaut
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Surficial redistribution of gold and arsenic from the Rise and Shine Shear Zone, Otago, New Zealand
ABSTRACT
The historic Come-In-Time gold mine was developed in a ∼100*100 m area of the >7 km long orogenic Rise and Shine Shear Zone in Otago Schist. The oxidation of hydrothermal ankeritic and auriferous arsenopyrite and pyrite-bearing Otago Schist beneath a regional Miocene unconformity resulted in replacement of sulphides and ankerite by arsenic (As)-bearing ferric oxyhydroxide. The As-bearing ferric oxyhydroxide is mainly concentrated in and near the original mineralisation sites, especially shears, but mobilisation into post-mineralisation joints and fractures has redistributed As on a metre scale. Gold was liberated from sulphides during oxidation and aggregated by supergene oxidation processes into a smaller number of larger particles (mm scale) that are intimately intergrown with As-bearing ferric oxyhydroxide, especially in shears. Eluvial gold in proximal immature debris includes variably rounded supergene particles eroded from nearby, and some externally derived detrital flakes from previously overlying Miocene sediments. Some eluvial gold is trapped in fine sediments by lichen on outcrops. The hydrothermal As-Au relationship has therefore been broadly maintained through oxidation of rocks at the metre scale but Au concentrations have been locally enhanced in the surficial environment.
期刊介绍:
Aims: New Zealand is well respected for its growing research activity in the geosciences, particularly in circum-Pacific earth science. The New Zealand Journal of Geology and Geophysics plays an important role in disseminating field-based, experimental, and theoretical research to geoscientists with interests both within and beyond the circum-Pacific. Scope of submissions: The New Zealand Journal of Geology and Geophysics publishes original research papers, review papers, short communications and letters. We welcome submissions on all aspects of the earth sciences relevant to New Zealand, the Pacific Rim, and Antarctica. The subject matter includes geology, geophysics, physical geography and pedology.