Characterisation of arsenic geochemistry in mine tailings from a mesothermal gold deposit

摘要

Macraes mine was opened in 1989, and extracts mesothermal gold from a shearing zone in the Otago schist. The gold occurs in pyrite and arsenopyrite, which comprise 1 % of the ore. In September 2015 an 80 m core was drilled through the northern end of the Mixed Tailings Facility (MTF) that received flotation and sulfide concentrate tailings from 1990 to 2014. Adaptations to ore processing occurred over this period, including the introduction of pressure oxidation in 1999. Preliminary study of the core has involved characterisation of a section at 44 m depth, and a section at 77 m depth. The shallower sample has brown tailings, indicative of iron oxide minerals produced during pressure oxidation. The deep sample is grey, similar to the original colour of the schist. The tailings porewater at both depths contains 2 ppm As. Arsenic in the tailings is associated predominantly with iron oxide minerals at 44 m depth, and with sulfide minerals at 77 m depth. A sequential extraction showed promise as a way to investigate mineral associations in the MTF, and preliminary geochemical modelling suggests that sorption equilibria are a key process in determining As concentrations in pore water of the tailings facility.