The geochemical dynamics of oxidising mine tailings at Laver, northern Sweden

Abstract

The former Cu mine Laver in northern Sweden was selected for a study of the geochemical processes in mine tailings. The Laver mine was in operation between 1936 and 1946, producing approximately 1.2 Mt of tailings. This study of the tailings at Laver includes time series of groundwater and surface water chemistry, chemical and mineralogical studies of the tailings sand and geophysical investigations. The results allow a good understanding of the dynamic geochemical processes taking place within the tailings. Oxidation of sulphide minerals in the tailings has led to the development of a distinct chemical zonation. Metals released from oxidation and weathering of sulphide minerals are retained within the tailings impoundment. This retention mechanism is today very efficient and only 5–10% of the released masses of metals reaches the surface water system. Zn, Cd, Co and Ni are probably secondarily bound to the tailings with adsorption in a large mass of non-weathered tailings. Cu, on the other hand, is enriched in a distinct zone just below the front of oxidation. This distinct Cu-enrichment suggests precipitation of a secondary mineral phase, probably covellite. The downward movement of the front of oxidation advances at a rate of 2.8 cm/a, and displaces the secondary zone with Cu-enrichment at the same rate. When the oxidation front reaches the groundwater table, the oxidation of sulphide minerals will almost completely stop. The Cu-enrichment will, in this case, probably partly go into solution and be transported away by the groundwater movement. The seepage of acid water from the oxidation front will consume pH-buffering capacity of the tailings and the groundwater will eventually become more acidic. Such a low pH-environment in the tailings will force the adsorbed Zn, Cd, Co and Ni to go into solution and be transported away. A front of water with low pH and high concentrations of metals has been detected and is moving towards the outlets of the tailing impoundment. Due to the low hydraulic gradient and conductivity, the front of acid metal-rich water will be delayed in time by 10–30 years before it reaches the recipient.