Influence of freeze-thaw dynamics on internal geochemical evolution of low sulfide waste rock

•Annual freeze-thaw cycle in 15 m high test-scale waste rock pile is described.•Long-term pore water geochemistry in 0.053 wt.% S waste rock is characterized.•Long-term effluent geochemistry in 0.053 wt.% S waste rock is characterized.•Effluent geochemical trends are correlated with the freeze-thaw cycle.

Continuous monitoring of a 15 m high heavily instrumented experimental waste rock pile (0.053 wt.% S) since 2006 at the Diavik diamond mine in northern Canada provided a unique opportunity to study the evolution of fresh run-of-mine waste rock as it evolved over annual freeze-thaw cycles. Samples were collected from soil water solution samplers to measure pore water properties, from twelve 4 to 16 m2 basal collection lysimeters to measure basal leachate properties in the region underlying the crest of the pile (the core), and from basal drains to measure aggregate total pile leachate properties. By 2012, monitoring of pore water geochemistry within the core structure of the test pile revealed an apparent steady state with respect to weathering geochemistry, represented by (i) a flush of pre-existing blasting residuals and applied tracers, (ii) declining pH, (iii) a stepwise progression and subsequent equilibrium with acid-neutralizing phases (depletion of available carbonates; equilibrium with respect to aluminum hydroxide phases and subsequent iron (III) hydroxide phases), and (iv) concordant release of SO4, major cations (Ca, Mg, K, Na, Si), and trace metals (Al, Fe, Ni, Co, Cu, Zn). Distinct, high concentration 'spring flushes', characteristic of drainage in northern environments and primarily explained by a combination of fluid residence time and the build-up of oxidation products over the winter, were released from core drainage each season. Following the initial flush, the concentration of all dissolved constituents steadily declined, with distinct minimums prior to freeze-up. The opposite trend was observed in the cumulative pile drainage, in which early season leachate dominated by snowmelt and batter flow had low concentrations and late season leachate dominated by contributions from the core of the pile (indicated by season end merging of core and cumulative drainage geochemistry) had higher concentrations. Northern waste rock pile drainage geochemistry is strongly influenced by freeze-thaw cycling and varying core and batter subsystem contributions to total drainage. A comprehensive understanding of thermal cycling in waste rock piles is an important component of temporal predictions of drainage water composition based on up-scaling or reactive transport modeling.