Geology, evaporative salt accumulation and geoecology at Springvale historic gold mine, Central Otago, New Zealand

Abstract

The Springvale placer gold mine was developed in distal Pleistocene fan deposits that are dominated by auriferous gravel. The gravel was originally deposited in channels in Miocene mudstone. Mining has removed the gravel, exhuming the mudstone. The mudstone has since become an impermeable substrate for accumulation of evaporitic encrustations, with substrate slurry conductivities up to 20,000µS. The predominant evaporitic minerals (halite, gypsum and calcite) are derived from marine aerosols in rain with some dissolution of soluble minerals in the sediments. Post-mining erosion of mudstone during rain events has formed muddy and silty outwash pans, which are repeatedly replenished from runoff and accumulate evaporitic salts. The pans have been the principal site of halophyte plant establishment where conductivities exceed 1000 µS. Substrates with conductivity <1000µS have generally been colonised by adventive plant species. Mudstone outcrops have remained unvegetated and continue to contribute sediment and salts to saline pans.