CHARACTERIZATION OF ALUMINUM-RICH PHASES IN HEAP-LEACH PADS AT THE LANDUSKY GOLD MINE, MONTANA, USA

摘要

Hydrothermal alteration, pre-mining weathering, mining, metal extraction by heap leaching with cyanide, and post-mining alteration of rock at the Landusky gold mine, in Montana, have resulted in repeated transformations of Al-bearing minerals and their eventual dissolution to form Al-rich aqueous solutions. The primary aluminosilicate minerals in the Landusky syenite are K-feldspar and sodic plagioclase. Detailed studies of clay mineralogy and geochemistry have shown that these feldspars were partially altered to illite during hydrothermal alteration of the syenite, probably coincidental with the gold-mineralization event. Kaolinite was formed by weathering of illite and feldspars. Then alkaline conditions (pH >10) during cyanide leaching (1979–1996) caused the hydrothermal illite to become laced with a smectite (the K-dominant analogue of beidellite), which is stable under alkaline conditions. After the mine closed, oxidation of pyrite decreased the pH, and the smectite became unstable, forming kaolinite. Over time, further increases in acidity resulted in the dissolution of kaolinite, thereby generating elevated concentrations of Al in the drainage water (up to 100 mg/L). Precipitation of amorphous aluminum hydroxides and oxyhydroxides from this water resulted in the blockage of drainage pipes, and a reduced efficiency of a treatment plant installed to remove cyanide, selenium and nitrate.