TEMPORAL AND SPATIAL CONTRIBUTIONS TO ELEMENT RELEASE FROM A LOW SULFIDE WASTE ROCK PILE IN THE CANADIAN ARCTIC

An experimental waste-rock pile (0.053 wt. % S) was constructed at the Diavik Diamond Mine, in the Northwest Territories, Canada to evaluate the generation of acid-rock drainage and the seasonal and annual release of sulfide oxidation products into solution. Measurements of ground thawing index, ground freezing index, and mean annual temperature were used to divide the pile into core and batter drainage subsystems where effluent from the subsystems was collected by basal drains for measurements of flow and water chemistry. Comparison of geochemical loading of water from within the core, batter and aggregated basal drainage system (2007 to 2012) allowed for an evaluation of the temporal and spatial contributions to element release over six annual freeze-thaw cycles. Distinctly different core and batter subsystems resulted from contrasting temperature and flow regimes developed during the annual freeze-thaw cycle. Rapid spring snowmelt may contribute up to 59% to flow in the batter subsystem; whereas snowmelt does not contribute significantly to the core subsystem. Mean concentrations in the core effluent (Ni: 16 mg L^-1; SO₄: 2400 mg L^-1; Co: 3.2 mg L^-1; Fe: 0.60 mg L^-1) were elevated relative to the batter effluent (Ni: 3.0 mg L^-1; SO₄: 890 mg L^-1; Co: 0.60 mg L^-1; Fe: 0.10 mg L^-1). Between 2007 and 2012, flow from the core accounted for 13 % of the total drainage volume; whereas 45 %, 29 %, 44 % and 45 % of Ni, SO₄, Co and Fe loads were attributed to this zone. By 2012, the cumulative release of pyrrhotite oxidation products from the < 5 mm reactive grain-size fraction of waste rock was 9.0 %, 5.1 %, 7.2 % and < 0.1 % for Ni, S, Co and Fe respectively. Depletion of primary oxidation products was significantly lower in the core – relative to the batter – due to decreased flow and variations in element mobility and transport controls. Results indicate that a robust conceptual model describing the physiochemical dynamics governed by thermal cycling must be defined before evaluating the seasonal and annual release of metals from waste rock.