Environmental geochemistry of the Mt Perry copper mines area, SE Queensland, Australia

Abstract

Intermittent mining and processing of sulphide ore (pyrite–chalcopyrite) at the abandoned Mt Perry mine from the 1870s has resulted in numerous mine workings, waste dumps, a local drainage system contaminated by acid mine drainage (pH 3.3–4.5) and degradation of local soil and vegetation. Weathering of sulphidic ore, oxidized waste material, tailings and slags (median of 32 samples: 0.65 ppm Au, 17 ppm Ag, 48 ppm As, 3090 ppm Cu, 3.5 wt% Fe, 138 ppm Mn, 19 ppm Mo, 55 ppm Pb, 39 ppm Zn) has led to the formation of secondary minerals (e.g. goethite, hematite, Cu carbonates, gypsum, jarosite, natrojarosite, chalcanthite) on waste materials, stream sediments and stream banks. Waters emanating from underground workings and draining waste dumps have pH values as low as 3.3 and contain elevated contents of As, Cu, Fe, Zn and SO₄. Dilution from uncontaminated tributaries and groundwaters causes decreasing Cu and Zn concentrations in stream waters, whereas Fe and As precipitate close to the mine site due to adsorption and coprecipitation processes. While much of the dissolved metal load derives from the mine site, Cu is also dissolved out of contaminated stream sediments. Soils in the mines area have elevated Cu and Fe values compared to background soils. The soils acquired their metal enrichments by hydromorphic dispersion from the dissolution of metal-rich particulates, transport of dust and physical erosion of metal-rich particulates from the dumps, and perhaps atmospheric fall-out from smelting plant emissions. Levels of Ag, As, Cu, Fe, and Zn in the fine fraction of stream sediments exceed background values by one to three orders of magnitude. This enrichment has been caused by physical erosion of metal-bearing minerals from abandoned ore and waste dumps and adsorption and coprecipitation of dissolved metals and metalloids in the stream bed. Areas disturbed by mining are characterized by lack of native vegetation, selective and non-selective bioaccumulation of metals and metalloids into plants (gold fern 'Pityrogramma austroamericana'; hickory wattle 'Acacia aulococarpa'; bunch speargrass 'Heteropogon contortus'; wiregrass 'Aristida benthamii', 'A. caput-medusae'), and partial recolonization by metal-tolerant plant species (gold fern; bunch speargrass; couch grass, 'Cynodon dactylon'; swordsedge, 'Lepidosperma' sp.).