Mineralogy and geochemistry of mine wastes, Macraes Mine, East Otago

Mains, Dusk.

This study examines environmental issues related to tailings at Macraes gold mine, East Otago. To do this, information was compiled and integrated on mine ore, processing system, and tailings impoundment structure and hydrogeology. The focus of this study was on arsenic mobility and stabilisation, but also on physical stabilisation of the tailings using plants. An experiment was set up with different plant species and amendments. Leachates were collected to test whether addition of phosphate fertiliser caused As mobilisation. These results were compared with a laboratory test tube experiment. Examined processing plant mineral scales are geochemically similar to historic mine wastes. These scales and mine wastes were examined to see if they could be used to predict future mineralogical changes in the tailings, especially with respect to the formation of scorodite, FeAs04.2H20. A circumneutral pH means that scorodite is unlikely to form in the Macraes mine tailings. Arsenic is most likely to be held up in, or adsorbed to, amorphous iron oxyhydroxide, which has a solubility two orders of magnitude greater than scorodite . .c- Mine waters passing through tailings are distinguished by high sul,.J>hate and magnesium content. Arsenic content in sump and chimney drain waters from Southern Pit Tailings Impoundment has increased dramatically over the past two years. This dramatic increase has not occurred in Mixed Tailings Impoundment waters. Contrasting As concentrations in the discharge waters of the two impoundments is best explained by differing tailings depositional history. Iron oxyhydroxide forming in chimney drains and sumps is only partially saturated with respect to As. A temporary tailings surface stabilisation method (using vegetation as a ground cover) was tested. Plants grew best with fertiliser, and the best performing plant species was Rye Corn. The addition of amendments (fertiliser) did not cause any additional As leaching. High levels of As uptake in plants highlights that this method is not suitable as a long term rehabilitation method. Arsenic mobility was found to be hindered or stopped by increasing spatial scale.

xiii, 171 leaves : col. ill., tables ; 30 cm.

2004Mains