Consolidation and incipient oxidation of alkaline arsenopyrite-bearing mine tailings, Macraes Mine, New Zealand

Fine grained (ca. 15 mu m), arsenopyrite-bearing mine tailings have been ex posed to drying and oxidation for 4 a pending relocation. The tailings are still partly covered by a pond of decanted pore waters. The water table in drying tailings has lowered by 1-3 m and desiccation cracks up to 2 cm wide have formed on the 1 m scale, extending through the unsaturated zone. Tail ings in the unsaturated zone have similar pore water contents to saturated tailings: typically 16-32 wt% water. Saturated tailings retain alkaline pH (ca, 10) from the mine cyanidation plant, but pH lowers progressively towar ds ca. 7 near the surface, or near desiccation cracks, in the unsaturated z one. The redox state of the tailings changes in parallel with pH, with an e mpirical relationship: Eh(mV) = -55 pH + 290. Water in the remnant decant p ond reflects this relationship also. Unsaturated tailings have variable but low permeabilities, typically 10(-3) to 10(-4) m/day, and more permeable h orizons have allowed incursion of oxygenated air and/or rain water from des iccation cracks. Sulphide grains in all tailings examined are unaltered. Su lphides and solutions in the tailings are out of thermodynamic equilibrium predicted from the redox-pH conditions, due to kinetic constraints. Incursi on of rain water locally facilitates deposition from pore waters of insolub le Fe oxide and arsenate minerals, thus fixing As in the dry unsaturated ta ilings. (C) 1999 Elsevier Science Ltd. All rights reserved.