EROSIONAL AND GEOCHEMICAL CONTROLS ON NATURAL REVEGETATION AT AN UNREMEDIATED PLACER GOLD MINE, CENTRAL OTAGO, NEW ZEALAND

The abandoned Springvale placer gold mine sits in the toe of a complex Quaternary alluvial fan in the Central Otago rain shadow of the Southern Alps of New Zealand. Biogeochemical maps and SEM analysis of the sediments and evaporative salts quantify the relationships controlling the natural revegetation of the site over the past ~100 years. Historically, miners sluiced auriferous gravels down mine channels to extract gold. This left behind highly variable yet small-scale topography (net 20 m of relief) and impermeable mudstone exposed. On this mudstone, evaporative salts (calcite, gypsum, and halite) crystallize due to the semi-arid climate and a saline input dominantly from marine-derived aerosols in the rain. Repeated weathering events concentrate the salts down-slope into planar fans of mudstone runoff (1-10 m²). This runoff has conductivities generally >1,000 µS and reaching >20,000 µS. Salt-tolerant plants (halophytes) colonize these runoff fans without encroaching up the adjacent mudstone outcrops, which is partly due to the compaction of the mudstone versus the open microstructure of the recycled sediment. The halophytes are rare and endangered, so the New Zealand Department of Conservation protected the site as a scientific reserve. Adventive plants are outcompeted by the halophytes in the saline runoff fans and instead only colonize gravel-rich mine tailings (conductivity <1,000 µS).The sharp control imparted by the geomorphology and substrates leads to a natural and biodiverse revegetation of the site. The use of varied landscapes and substrates to support biodiversity should be applied during active remediation of other mine sites.