ENVIRONMENTAL MOBILITY OF ANTIMONY: A FIELD STUDY AT BRALORNE GOLD MINE, BRITISH COLUMBIA, CANADA

This study investigates the mobilization and attenuation of antimony in the environs of an orogenic lode gold deposit at Bralorne in the Bridge River mining district of British Columbia, Canada. Native gold is found in veins dominated by quartz and carbonates, with a low sulfide content of 1 to 3 wt%. Pyrite and arsenopyrite are the most abundant sulfides while stibnite (Sb₂S₃), the primary Sb-bearing sulfide mineral, can be significant locally. Such is the case at the Upper Peter mine where Sb in the adit discharge regularly exceeds the provincial water quality guideline for the protection of aquatic life. A field study was initiated to determine the change in solubility and solid-phase speciation of Sb in sediments collected at key locations along the Upper Peter mine drainage. Sediments were sampled within the adit, outside the portal, in a sedimentation pond upstream of a wetland and in the wetland itself for chemical and mineralogical analyses. The antimony oxidation state in the solid phase was determined using X-ray absorption near-edge structure (XANES) spectroscopy. Examination of the original rock material by scanning electron microscopy showed that stibnite occurred as isolated particles or attached to pyrite. Antimony mobilization was observed inside the adit due to weathering of exposed Sb-bearing minerals. Inside the adit at the far end, XANES speciation indicates that Sb(III)-S dominates in the sediments. Although Sb appears largely oxidized in the bulk samples collected closer to the portal, Sb(III)-S species are nevertheless present in the fine fraction (< 53 µm), suggesting a higher oxidation rate for stibnite in the coarser grains possibly due to galvanic interaction with pyrite. From the portal to the wetland located 120 m downstream, attenuation processes occurred along the flow path and dissolved Sb in the drainage dropped from 139 µg L^-1 to < 5 µg L^-1. Sediment settling in the pond upstream of the wetland and dilution within the wetland are two significant factors for attenuating the transport of Sb in the drainage system. Sorption/co-precipitation with Fe-oxyhydroxides and possibly sorption with organic matter along the flow path and in the pond also serve as sinks for dissolved Sb. Overall, Sb has a limited mobility at the watershed scale.