Metal redistribution in historic mine wastes, Coromandel Peninsula, New Zealand

Abstract

Abstract Mine wastes from historic gold mining on the western Coromandel Peninsula have variable base metal (Cu, Pb, Zn) contents, from near background (10–100 ppm) to several weight percent. Arsenic occurs as a minor element in some sulphide minerals, and is typically at c. 10% of the base metal levels in mine wastes. Decomposition of primary sulphide minerals is slow because the moist climate keeps the wastes saturated with water, so as to exclude oxygen, and alteration zones are on the millimetre scale after 100 years. Waters in the mine wastes have pH varying from 1.5 to 8, depending on host‐rock composition and permeability to flushing rainwater. Carbonate‐rich tailings can neutralise any acidity generated by sulphide oxidation. Without carbonate, pH drops due to pyrite decomposition. Iron oxyhydroxide from pyrite decomposition forms localised cements in mine wastes, and this cement incorporates some other metals. Base metals can be fixed in the iron oxyhydroxide at high pH (6–8), but arsenic is soluble and is flushed from the rock. At pH 2‐A, base metals are largely flushed in solution, whereas arsenic is locally fixed in iron oxyhydroxide precipitates. Iron oxyhydroxide‐rich spring precipitates (ferricrete) from the Monowai mine form at pH near 2, but co‐precipitation of iron phosphate(s) and other phosphatic material incorporates dispersed metals, especially arsenic, which prevents their discharge into the environment.