Read today the news that the Brucejack Mine have been given an environmental assessment certificate by the British Columbia Ministry of the Environment. The regulators say construction can begin once they are sure that discharges from water treatment plants will not harm the Unuk River. Some of the tailings will go back underground; some will go to Brucejack Lake which is apparently “fishless.” Most of the waste rock will go to the lake.
I read the following in the June 2014 Feasibility Study and Technical Report Update for Pretium Resource Inc. by Tetra Tech.
Approximately 3.5 Mt of waste rock and 8.7 Mt of tailings are anticipated to be deposited in Brucejack Lake over the projected 18-year mine life. Stringent discharge criteria (based on the Metal Mining Effluent Regulations (MMER)) state that the total suspended solids (TSS) concentrations in the outflow at Brucejack Creek must be less than 15 mg/L.
The tailings deposition system has been developed to minimize the concentration of fine suspended solids in the outflow to Brucejack Creek by discharging near the bottom of the lake (at 80 m depth) and under the accumulations of tailings solids.
On the other hand, waste rock with a wide range of particle sizes is to be deposited in the lake by surface dumping from causeways raising the possibility that fine granular material will be introduced to the surface layer of the lake and to the outflow.
Hydrodynamic modelling of Brucejack Lake was carried out by Lorax (2013) to examine the likelihood of the migration of tailings solids into lake surface waters. The results indicated that the potential for elevated TSS levels in surface waters was unlikely if the minimum particle diameter was greater than or equal to 5 µm.
However, it will be necessary to control the TSS concentrations at the outlet of Brucejack Lake to meet the MMER regulations. The current design basis for control of suspended solids includes the following:
- install one or more lines of turbidity curtains at the outlet of the lake to contain suspended solids
- install a flow monitoring weir across Brucejack Creek downstream from the lake outlet to facilitate monitoring.
An allowance for site investigation and design of the outflow monitoring weir has been included in the capital cost estimate. As a contingency to the use of turbidity curtains, an outlet control structure was designed to allow storage and release of lake water in a controlled manner.
Review of the storage capacity versus lake level elevation for the outlet control structure indicates that flow from the lake could be stopped for a period ranging from several days (e.g. during freshet) to several tens of days (e.g. during the summer and early fall) depending upon runoff conditions in the lake catchment area.
If you have an idle weekend, you can spend plenty of time reading additional documents at this link. For me it is off to the MET opera, the Tales of Hoffman, so I will say no more than this seems good news: a BC mine advancing with a rational tailings and waste rock disposal system. Good for them. I hope they succeed.