Form and distribution of gold mobilized into surface waters and sediments from a gossan tailings pile,Murray Brook massive sulphide deposit,New Brunswick,Canada

Stream waters and sediments draining a gossan tailings pile at the Murray Brook massive sulphide deposit were collected to investigate Au mobility. Weathering of the massive sulphides at Murray Brook during the Late Tertiary period resulted in the concentration of Au in the gossan cap overlying the supergene Cu and unoxidized massive sulphide zones of the deposit. The gossan was mined between 1989 and 1992, and Au and Ag were extracted using a cyanide vat leach process. Although stream sediments prior to mining had Au<5 ppb (the detection limit), sediments collected in 1997 had Au contents ranging up to 256 ppm with values up to 6 ppm more than 3 km downstream from the deposit. Dissolved Au contents were similarly anomalous, up to 19 μg/L and in excess of 3 μg/L 3 km downstream. The elevated Au contents in the waters and sediments are interpreted to reflect complexation of Au (as Au(CN)₂^?) by cyanide hosted within the gossan tailings pile. Precipitation recharges through the tailings pile with groundwater flow exiting to Gossan Creek. Degradation of cyanide along the flow path and within Gossan Creek allows colloidal Au to form via reduction of Au(I) by Fe²⁺, consistent with SEM observations of Au as <1 μm subrounded particles. In the surface waters, the majority of the Au must be in a form <0.45 μm in size to account for the similarity in Au contents between the <0.45 μm and unfiltered samples. The very elevated stream sediment Au values close to the headwaters of Gossan Creek near the tailings indicate that upon exiting to the surface environment, Au(CN)₂^? complexes are rapidly destroyed and Au removed from solution. However, the high Au_{<0.004 μm}/Au_total in the headwaters and the extended Au dispersion in Gossan Creek waters and sediments suggest that Au(CN)₂^? complexes persist for the full length of Gossan Creek. The decrease in aqueous Au which is less than 0.004 μm indicates that Au is converted from a complexed form to a colloidal form with increasing distance downstream, consistent with dissolved NO₃^? contents which decrease from 5210 μg/L near the headwaters to 1350 μg/L at the lower end of the stream.