Characterization of Geochemical and Mineralogical Controls on Metal Mobility in the Prairie Creek Mine Area, NWT

Abstract

The Prairie Creek Mine, NWT, is a non-producing Zn-Pb-Ag mine located within the Mackenzie Mountains. The 320 km2 area is surrounded by the Nahanni National Park Reserve, and is found approximately 40 km upstream of a UNESCO World Heritage Site. Geochemical characterization of weathered material in the area is useful from both an exploration geochemistry and environmental geochemistry perspective. This investigation attempted to characterize geochemical and mineralogical controls on metal mobility in natural and manufactured environments. Surface waters and sediments were sampled, analyzed, and interpreted. Detailed mineralogical analyses were also performed, including SEM, XRD, and synchrotron-based µXRF and µXRD. Prairie Creek contains sulphide and supergene mineralization hosted in carbonate rocks, which supply the area with a high pH, as well as a wealth of alkalinity and buffering capacity. This study aims to provide some insight as to the geochemical and mineralogical controls on the weathering processes which may enhance or inhibit mobility of metals downstream of known mineralization. Results show that the metals are found in much higher concentration in the stream sediments than in the stream waters. Pb and Zn show the highest concentrations, and Zn appears to be more mobile than Pb. Based on water versus sediment chemistry, it may be possible to differentiate between mineralization types. Dissolution textures and alteration of detrital grains indicates that chemical weathering does occur within the streams and releases metals to the aqueous environment, though they are likely quickly attenuated through adsorption or co-precipitation. The mine site hosts a historic ore stockpile and waste rock pile, established in the early 1980’s. Geochemical characterization of the reactions occurring within these piles and of the mineralogical controls on metal mobility can contribute to the mitigation of risk from leachate. Metal concentrations are variable and do not support a trend, which indicates that metal mobility within these piles may be attributed to micro-environments. There appears to be a trend in increasing alkalinity and decreasing sulphate towards the bottom of the piles. Therefore, metals appear to have limited mobility in the studied environments, although small scale chemical reactions are occurring which may release and attenuate metals.