Spatial and temporal variations in the Ruttan Mine Tailings, Leaf Rapids, Manitoba, Canada.

摘要

The Ruttan Cu-Zn mine produced approximately 30 million tons of fine-grained tailings over 30 years. Since the closure of the mine in 2002, the tailings have been systematically dewatered through trenches which drain into Ruttan Lake and into the open pit and underground mine workings. This study evaluated the evolution of the tailings of Cell 3, which was underwater in 2002, and Cell 2, which was already dry and oxidized.;Very fine grained material from the milling process, found in fresh, reduced tailings was absent after one year of oxidation and may be the initial cause of mobile metals. During the summer months, evaporative conditions favour the formation of evaporite materials such as melanterite, which temporarily attenuate metals in a thin crust on the surface of the tailings. These evaporites are quickly dissolved during precipitation events and their metals are re-released into surface and groundwaters. To track the movement of metals in the tailings, water samples were collected from the tailings pore water, groundwater, and adjacent surface waters. In 2004, pore water in Cell 3 had a pH of 4.4 at 10 cm depth, increasing to 7 by 30 cm. Dissolved constituents decrease between 10 and 30 cm depth, sulphate (36,000 to 1,500 ppm) Fe and Zn from 18,000 and 9,550 ppm respectively to below detection limits. By 2005, pH had dropped to 3.2 above 10 cm. Sulphate concentration was unchanged but Fe had increased to 47,000 ppm. Pore water from the oxidized upper 20 cm of Cell 2 had the lowest pH (1.8) in 2004 and the highest sulphate (230,000 ppm), 65,000 ppm Fe, 1,100 ppm Cu, and 9,800 ppm Zn. By 2005, compositions were unchanged but the acidic front had moved from 120 cm depth to 250 cm. In shallow groundwater in Cell 3, there was a progressive decrease in pH (7 to 2) and an increase in Fe (14 to 5,870 ppm), Zn (5 to 327 ppm) and sulphate (1860 to 32,600 ppm) from 2004 to 2005. Surface pools on the tailings had a constant pH of 3+/-0.5 and high sulphate (13,000 ppm), Fe (2,700 ppm) and Zn (420 ppm). Ruttan Lake is acidic (pH 2.9) but with a constant composition despite Tailings Cell 3 draining into the lake. Sampling in 2007 comprised surface and groundwater samples that were compositionally similar to those of 2005.;In conclusion, dewatering the submerged tailings in Cell 3 resulted in measureable changes in acidification in pore and shallow groundwater from rapid oxidation of very fine grained sulphides. Depth profiles of the concentration of metals in dissolved and solid fractions, when compared to other studies, suggest that the tailings in Cell 2 and Cell 3 are in an early stage of oxidation and that these tailings will produce low pH, metal laden water for years to come.;The mining process at the Ruttan mill removed most of the Zn and Cu sulphides during beneficiation, leaving the tailings dominated by pyrite and pyrrhotite. Iron oxides and oxyhydroxides are formed during oxidation of the Ruttan Tailings. Sequential extraction results show that there is a considerable fraction of metals precipitated in secondary Fe-oxide/hydroxide minerals: Up to 50 wt. % of the total Fe at the top of Cell 2. In Cell 3, the amount of Fe in crystalline oxyhydroxide phases was lower than Cell 2, but there was a notable increase in Fe in this fraction from 11 wt. % in 2004 to 23 wt. in 2005. Ni and Zn dissolved in stagnant pools on the tailings surface, were found to precipitate with schwertmannite, and Cu in jarosite. Additionally, Fe was found to precipitate with lepidocrocite from more neutral groundwaters.