The
Waimumu area, 12 km southwest of Gore, is underlain by the Gore Lignite
Measures, an early Tertiary fluvial sequence approximately 500 m thick resting
on Triassic-Jurassic argillitic basement. The Waimumu Quartz Gravels (Pliocene)
have been deposited locally and are in part due to the localised reworking of
the Gore Lignite Measures. Structurally the area is dominated by the Dunsdale
Fault System, a steeply dipping series of NE trending reverse faults (Isaac and
Lindqvist, 1990). The Belle-Brook, Hedgehope and Parker Road sites sampled are
adjacent to the downthrown (east) side of the Hedgehope Fault.The authigenic
gold-sulphide association at Belle-Brook was discovered in 1987 (Falconer, 1987:
Clough and Craw, 1989). Despite a very limited amount of alluvial mining a
significant quantity of authigenic marcasite has developed. Authigenic sulphide
trace element analysis has shown anomalous amounts of As, Ni, Cr and to a lesser
extent Zn, Pb and eu (Table 1). It has been established that the nickel occurs
as a colloform banded Ni sulphide, possibly bravoite, within the marcasite
(Figures 1 & 2). Arsenic and nickel (re)mobilisation are well documented in
the literature. Generally As and heavy metals are mobilised in the groundwater
as a result of the oxidation of sulphides (ie as a result of mining). At
Belle-Brook it is reasonable to assume the authigenic sulphide trace element
composition reflects either localised groundwater contamination or hydrothermal
mineralisation. This poster presents some preliminary
findings.