|Title||Arsenic mobility in mildly alkaline drainage from an orogenic lode gold deposit, Bralorne mine, British Columbia|
|Author||Desbarats, A J; Parsons, M B; Percival, J B|
|Source||Applied Geochemistry vol. 57, 2015 p. 45-54, https://doi.org/10.1016/j.apgeochem.2014.11.015|
|Alt Series||Earth Sciences Sector, Contribution Series 20140134|
|Media||paper; digital; on-line|
|NTS||92; 93; 102|
|Area||Bralorne mine; British Columbia|
|Lat/Long WENS||-124.0000 -122.0000 51.0000 49.0000|
|Subjects||environmental geology; geochemistry; hydrogeology; arsenic; heavy metals contamination; arsenic geochemistry; heavy metals geochemistry; water geochemistry; environmental analysis; environmental
studies; groundwater; groundwater geochemistry; groundwater pollution; Bralorne Mine|
|Illustrations||location maps; plots; tables|
|Program||Environmental Geoscience, Tools for
environmental impacts and adaptation for metal mining|
|Abstract||The historical (1932-1971) Bralorne mine produced over 87 million grams of Au from an archetypal orogenic lode gold deposit in southwest British Columbia. High concentrations of As in mine drainage,
however, represent an on-going environmental concern prompting a detailed study of effluent chemistry. The discharge rate at the mine portal was monitored continuously over a fourteen-month period during which effluent samples were collected on a
quasi-weekly basis. Water samples were also collected on synoptic surveys of the adit between the portal and the main source of flow in the flooded workings. Total concentrations of As in the mildly alkaline (pH = 8.7) portal drainage average 3034
lg/L whereas at the source they average 5898 lg/L. As emergent waters from the flooded workings flow toward the portal, their dissolved oxygen content and pH increase from 0 to 10 mg/L and from 7.7 to 9, respectively. Near the emergence point,
dissolved Fe precipitates rapidly, sorbing both As(III) and As(V). With increasing distance from the emergence point, dissolved As(III) concentrations drop to detection limits through sorption on hydrous ferric oxide and through oxidation to As(V).
Concentrations of dissolved As(V), on the other hand, increase and stabilize, reflecting lower sorption at higher pH and the lack of available sorbent. Nonetheless, based on synoptic surveys, approximately 35% of the source As load is sequestered in
the adit resulting in As sediment concentrations averaging 8.5 wt%. The remaining average As load of 1.34 kg/d is discharged from the portal. Partitioning of As(V) between dissolved and particulate phases in portal effluent is characterized by a
sorption density of 0.37 mol As (mol Fe) 1 and by a distribution coefficient (Kd) of 130 L/g HFO. The relatively high sorption density may reflect co-precipitation of As with Fe oxyhydroxides rather than a purely adsorption-controlled process.
Results of this study show that the As self-mitigating capacity of drainage from orogenic lode gold deposits may be poor in high-pH and Felimited settings.|
|Summary||(Plain Language Summary, not published)|
High concentrations of arsenic in drainage from ¿orogenic¿ gold deposits represent an important environmental challenge for the Canadian mining industry.
In order to better understand processes involved in the release, transport and fate of arsenic in mine drainage, NRCan researchers investigated the Bralorne mine in British Columbia, a typical example of this class of deposits. Arsenic is released
through the oxidation of arsenopyrite, an arsenic-bearing sulphide mineral, in the presence of water. Iron precipitates resulting from sulphide oxidation are usually effective at scavenging arsenic from drainage. However, results of this study show
that the effectiveness this process decreases when drainage is more alkaline due to an excess of dissolved carbonate minerals. Results also show that the natural arsenic self-mitigating capacity of drainage may be poor when iron precipitates are
limited by a low abundance of primary iron sulphide minerals in the host rocks.