Field-scale analysis of flow mechanisms in highly heterogeneous mining media

A large-scale injection and tracer test was conducted in highly heterogeneous Au ore under typical mining practices of heap leaching. A total of 24 zero-tension lysimeters were installed beneath an active Au heap leaching operation and irrigated for 90 d with a cyanide solution. While rapid breakthrough of the wetting front suggesting preferential flow was observed in some lysimeters, the majority of the fluid flow did not follow preferential pathways. Rather, those lysimeters showing the most rapid breakthrough also showed the lowest steady-state drainage flux, suggesting that while rapid flow through the 6- to 8-m profile could occur, only a small percentage of the applied fluid volume was transmitted through these rapid pathways. Electrical resistivity imaging from the surface showed qualitatively good correlation between observed resistivity and the onset of drainage in adjacent lysimeters and also appeared to track the reaction of the infiltrating fluids with the carbonate ore and a lime amendment. The observation of limited preferential flow supports the empirical observations that heap leaching of Au ore with run-of-mine, end-dumped placed rock is a surprisingly efficient reactor in spite of the large heterogeneity inherent in these materials that would be expected to produce preferential flow.