Internal Structure and Preferential Flow of a Waste Rock Pile from Geophysical Surveys

Abstract

The authors have carried out a geophysical investigation on a waste rock pile at the Laronde gold mine (Agnico‐Eagle Ltd, Quebec). GPR, low‐induction number EM and 2D resistivity tomography measurements have been conducted during consecutive field surveys in 2002 and 2003 on top of the pile to map its internal structure. Repeated infiltration tests and geophysical monitoring from ground surface have also been performed to map preferential flow paths. Using the field data, the authors propose here a three‐layer model for the first 5–6 m of the pile, mainly based on electrical conductivities. Electrical conductivity increases with the degree of oxidation of the sulphidic minerals contained in the waste rock. The contact between unoxidized‐oxidized waste rocks is typically marked by a well‐defined resistive‐conductive boundary, as shown by digging a 2.5 m‐deep trench. Integration of geophysical and hydrogeological information indicates that there is a lateral conductivity variation, which may be due to the deposition method, mechanical alteration caused by machinery, and by increased oxidation of sulphides near the external boundary of the pile. GPR monitoring during infiltration tests seems to indicate preferential flow paths towards a zone of low sulphide content materials and low oxidation by‐products. The resistivity and GPR methods appear to be efficient geophysical methods to characterize the internal structure of the Laronde waste rock pile.