Evolution and sulphide mineral occurrences of an incipient nonmarine sedimentary basin, New Zealand

River sediments at Macraes Flat, east Otago have formed in a tectonically a ctive depression in the Otago Schist basement adjacent to an active northea st striking fault. The depression is the mast juvenile of a series of nonma rine basins currently forming during tectonic convergence. The sediments we re derived from rapid erosion of the rising fault scarp, with long-term ave rage sedimentation rates between 0.15 and 0.18 mm/year. The Hyde-Macraes Sh ear Zone crosses the depression, and this sheer zone hosts the Macraes gold mine. The shear zone contains sulphide minerals, mainly pyrite and arsenop yrite. Sulphide minerals were eroded and deposited in the Macraes Rat river sediments at least 28,000 years ago. The sulphide: minerals have not suffe red any detectable decomposition in the sediments, even though some oxygena ted water incursion has occurred. This sulphide longevity is due to general water saturation in the basin floor despite possible occasional drying. So me sands have remained unaffected by oxygenated waters, and authigenic pyri te deposition has occurred. The pore waters in recently-exposed outcrops of organic sands with authigenic pyrite are too oxidised to be in equilibrium with pyrite. The Macraes Flat example demonstrates that sulphide minerals can remain stable for tens of thousands of years, and authigenic sulphides can form locally, within only 5 metres of the surface in an oxygenated atmo sphere and only sporadic water saturation in a tectonically active nonmarin e environment. (C) 1999 Elsevier Science B.V. All rights reserved.