British Columbia Mine Reclamation Symposia

Planning for abandonment at the Willa gold-copper project of Northair Mines Ltd. Brodie, John B. 1988

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th  Proceedings of the 12 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1988. The Technical and Research Committee on Reclamation  63  th  Proceedings of the 12 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1988. The Technical and Research Committee on Reclamation  64  Planning for Abandonment at The Willa Gold-Copper Project of Northair Mines Ltd.  J.B. Brodie, P.Eng. Abstract  Development of the Willa gold-copper project, in the Slocan Lake area of British Columbia, has presented some challenging design problems associated with the potential for acid generation from mine waste rock and tailings. Solutions to these problems are available but require early planning and implementation in order to guarantee effectiveness. Important abandonment concepts developed for the Willa project include the following: creating an impervious tailings basin by pond sealing in order to facilitate operation of a permanently flooded system utilizing natural inflows of ground and surface water; encapsulating waste rock in the tailings embankment such that the rock is isolated from air, thus depriving it of a critical reactant in the oxidation process; and modifying the acid/base ratio of tailings deposited in the final months prior to mine closure. Reclamation of the tailings pond as a wetland environment will provide for long-term environmental protection and will also produce an aesthetically pleasing marsh habitat suitable for waterfowl. This type of habitat is currently absent in this region of the Slocan Valley. Three rules are presented for successful abandonment planning when acid mine drainage is a possibility: 1) consider abandonment strategies during initial planning and feasibility studies rather than leaving them until mine shutdown is imminent; 2) provide defence-in-depth for potential problems; and 3) plan for abandonment with the assumption that you or your company will be financially liable for any failures. Introduction  In 1988, Northair Mines Ltd. proposes to place into production its Willa gold-copper property in the Slocan area of British Columbia. The project is a joint venture of Northair (75 percent) and BP Resources Canada Limited/Rio Algom Exploration Inc. (together 25 percent). With a predeyelopment expenditure exceeding $10 million, the Willa Project has already had a significant economic impact on the Slocan Valley. A construction capital expenditure of approximately $10 million plus annual outlays of $5 million for operating supplies, contractual services and payroll will provide a welcome stimulus to the area which currently experiences a nearly 15 percent unemployment rate. The project is located near Aylwin Creek on the east side of Slocan Lake within a short distance of the small communities of Silverton, New Denver and Slocan (Figure 1). Nakusp to the north and Kaslo to the east are also easily accessible from the Willa site. It is expected that the majority of the workforce will be derived from these communities which have historically been strongly dependent on mining. The larger centres of Nelson and Castlegar to the south are accessible via Highway 6 within approximately 1 hour's drive. The Willa project has posed some challenging design problems for the Company and its consultants. Two aspects have required particular attention: 1) the close proximity of the mine to several families occupying rural-residential acreages in the valley, and 2) the  th  Proceedings of the 12 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1988. The Technical and Research Committee on Reclamation  th  66  Proceedings of the 12 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1988. The Technical and Research Committee on Reclamation  potential acid-generating nature of the waste rock and tailings. In contrast with many mining operations which give serious consideration Io abandonment only near the end of their operating lives, abandonment planning al Willa required careful design from the very early slaves. Indeed, the closure concepts have an important bearing on the economic viability of the project, its acceptability to the mine's neighbours and the likelihood that approval will be granted by government review agencies. The purpose of this paper is to outline the concepts which have been proposed Io manage wastes al Willa, with particular emphasis on the reclamation and abandonment aspects. One mailer which had Io be dealt with early was the wish by the B.C. Ministry of Environment and Parks for the Company to study the feasibility and environmental impact of tailings disposal to the deep waters of Slocan Lake. Lake disposal was suggested as one means of dealing with potentially acid-generating tailings, and was based unfavourable experience with subaqueous tailings disposal in the Kootenays. However, federal legislation has changed in the years since these mines were placed under permit and it is now very difficult (though not impossible) to obtain permission under Fisheries Act Regulations for tailings discharges Io lakes containing fish. Sufficient encouragement was not forthcoming from Environment Canada to warrant the expense and delays associated with the intensive studies that were deemed to be necessary. Moreover, strong opposition from environmental groups within the Slocan Valley was anticipated. Consequently, the lake disposal option was not studied, and Northair concentrated all its efforts towards developing an environmentally secure land disposal scheme. It should be emphasized that a production decision regarding the Willa Project has not yet been made nor has approval-in-principle been received from the Mine Development Steering Committee which is currently reviewing the Stage I environmental assessment The concepts discussed in this paper have been outlined in more detail in Northair's Stage I Report. Geology and Reserves  Mineralization at Willa is found within several distinct zones of volcanic and metamorphic rocks within and adjacent to an intrusive pipe of heterolithic breccias and porphyrys. West Zone geological reserves, which constitute the initially mineable orebody, are estimated al 544,000 tonnes grading 7.5 g/t gold, 1.04 % copper and 9.5 g/t silver. Considerable potential also exists for developing additional reserves based on other zones currently under exploration. Current West Zone reserves provide an initial mine life of approximately 3 years, however, a 5-year operation has been planned based on the considerable potential of the property. Sulphides consist of pyrite with lesser amounts of chalcopyrite and pyrrholite. Gold is present as particles of native gold and electrum in association with the sulphides. Mining and Milling Process Underground mine production will be obtained by a combination of sloping methods necessitated by the irregular nature of the orebody. Open blasthole slopes, cut-and-fill slopes and possibly room-and-pillar slopes will be utilized depending on the configuration, orientation and size of the mineralized zones. Ore will be moved to the mill by Granbytype ore cars dumping into a coarse ore bin above the mill. The gold, copper and silver values in the ore are amenable to recovery by simple gravity and flotation operations (Figure 2). Cyanide will not be used. Ore will be processed at  th  Proceedings of the 12 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1988. The Technical and Research Committee on Reclamation  th  68  Proceedings of the 12 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1988. The Technical and Research Committee on Reclamation  450 tonnes per day, using mill equipment already on site. Dore bullion will be produced from the jig and table concentrate and a copper-gold concentrate will result from a selective flotation of sulphides. Mill reagents will be Aerofroth 65 (polypropylene glycol) and Aerophine 34 ISA (sodium diisobutylthiophosphinate). Overall recovery is expected to be 87 percent for gold and 88 percent for copper. Mill tailings will be discharged to a tailings impoundment, with pond water returned to the mill for reuse. Environmental Setting  The Willa claims comprise about 25 square kilometres on the northwestern flank of Mount Aylwin (el. 2530 m), a peak in the Selkirk Mountain Range. The minesite is located on steep terrain adjacent to Aylwin Creek between the 1000 and 1100 m elevations. In many respects the environment at Willa is not particularly sensitive. Minesite area streams are too steep and turbulent to support fish populations. The aquatic productivity of Slocan Lake is also low due to limited nutrients and deep waters. Although small populations of deer are present at the site, no other important game species are represented. Agricultural activity is non-existent except for small garden plots within privately-owned lots. Some logging is carried out in the region, though its economics appear marginal at the high-elevation sites. However, these positive factors do not imply that environmental constraints are absent at Willa. Although not discussed in this paper, site selection for the mill and tailings disposal has proven to represent the most contentious aspect of the development. This conflict has arisen because of the proponents' need for topographically suitable and economic development sites within a reasonable distance of the mine. With flat, habitable land in relatively short supply in this area, human settlement has tended to concentrate in those areas also most desirable for construction of mill and tailings disposal facilities. The Willa mill is to be located between the minesite and Aylwin Creek at the 900-1000 m elevation. Tailings will flow by gravity to a 10 hectare site on the west side of, and below, Highway 6. The tailings pond site is a shallow, flat valley at the northern extremity of the drainage catchment of Brahms Creek. This small brook flows in a southwesterly direction across a flat bench overlooking Slocan Lake. The basin receives groundwater-derived flows originating upslope and consequently a small, but steady flow is present throughout the year. Characteristics of Waste Materials  Gold, silver and copper values at Willa are associated with sulphide minerals, predominantly pyrite, chalcopyrite and pyrrhotite. Sulphides not recovered with the flotation concentrate will report to the tailings solids and be discharged to the tailings pond. Acid generation potential tests have been carried out on tailings samples obtained from bench scale studies. A deficiency of calcite compared with the 5 to 6 percent sulphur content of the tailings results in this material demonstrating a potential to produce acid, according to acid/base accounting procedures (Table 1). Waste rock at Willa may be classified as volcanics, heterolithic breccia, feldspar porphyry, and lamprophyre. Sulphur content of waste rock is extremely variable but typically ranges up to 3 or 4 percent. Except for lamprophyre, these materials have also been shown in the laboratory to have a potential to generate acid (Table 1).  th  Proceedings of the 12 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1988. The Technical and Research Committee on Reclamation  69  Table 1 Acid Generation Potential of Willa Ore, Tailings and Waste Rock  These findings underscore the need to handle waste materials carefully at Willa in order to prevent acid runoff or seepages from occurring. However, it is interesting to note that in contrast with the laboratory findings, waste rock has existed on surface at Willa for nearly 3 years with no evidence of significant oxidation or acid production. This suggests that while acid production from the waste rock may ultimately occur, the rate of reaction will likely be sufficiently slow that controls may be implemented without haste. These proposed measures are discussed in the following section. Water draining from the mine portal at Willa has also been closely monitored for signs of acid production. While sulphate concentrations have been elevated (1000 mg/L), there has been no evidence of levels increasing significantly since monitoring commenced three years ago. The pH has also remained consistently above 7 during this time and dissolved iron concentrations have been extremely low. The sulphate data indicates that some oxidation of sulphide mineralization has occurred, as would be expected from a sulphide orebody; however, it appears that any acidic reaction products have been subsequently neutralized prior to exiting from the mine. Thus far, at least, acid mine drainage in any form has not manifested itself at Willa.  th  Proceedings of the 12 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1988. The Technical and Research Committee on Reclamation  70  Tailings Pond Design  The tailings pond is to be constructed in a shallow valley of somewhat ideal configuration, allowing the impoundment of the mine reserves of tailings using a low, small volume dam across the valley mouth of upper Brahms Creek (unofficially named Willa Creek). With the gravity and flotation mill process employed, which only require the use of low toxicity reagents, it would not normally be necessary to ensure a water-tight impoundment. However, in response to concerns of residents regarding possible effects of pond seepage on groundwaters and surface waters downstream of the pond, Northair made the early commitment to line the impoundment. While this vail have the desired effect of essentially eliminating underseepage, it will also create difficulties in maintaining water balance; accordingly, a discharge from the impoundment will be necessary, in spite of the planned construction of diversion ditches above the pond. Discharge of surplus water to Aylwin Creek will occur via the mill reclaim system. Aylwin Creek is not used by residents for potable supply or any other purposes. Figure 3 illustrates the water balance for the mill and tailings pond during normal conditions. Soils at the site are predominantly gravelly, sandy till, with silty alluvium overlain by peat present in the valley bottom. Figure 4 shows the proposed dam cross section. An upstream and downstream shell will be constructed from sand and gravel till locally available within the impoundment area. Where the downstream portion of the dam overlies soft, weak alluvial foundations, a stabilizing berm of random sand and gravel will be necessary. A similar stabilizing berm will be needed upstream. An impervious core will be constructed from bentonite modified silt. A liner will be created over the entire area of the impoundment by addition of bentonite to the sandy soils. The bentonite will be mixed into the soil using a tractor-towed disc harrow. It has been calculated that the tailings and liner 7acting together will have an equivalent vertical hydraulic conductivity of 5 x 10' cm/s. By this means the underseepage for the entire impoundment will be reduced to a negligble quantity, particularly considering the hydraulic gradient imposed by artesian pressure. Reclamation and Abandonment Concepts  A critical question which had to be addressed at the outset was what should be done with mine waste rock which poses a long-term potential for acid mine drainage. Even if the waste rock did not develop acidity during the operating life of the mine, it could not simply be abandoned as might be done in other circumstances. A solution to this dilemma is available at the project design stage, but becomes increasingly difficult if abandonment is not addressed until mine shutdown is being contemplated. It was decided that the existing inventory of waste rock from underground development work should be moved to the tailings pond site and used in the embankment. Waste rock is to be incorporated into the upstream zone of the dam in such a way that it is totally encapsulated between impervious material on the base of the pond, the impervious dam core and tailings. By this means, sulphides in the waste rock will be isolated from the air and thereby deprived of an essential component for oxidation. As new waste rock is generated during on-going mining operations, it will be stockpiled until it can be incorporated into the next dam lift, scheduled roughly every 3 years. Waste rock which is surplus to dam building requirements will simply be dumped into the pond on the upstream side of the dam so that it sinks and is covered by tailings. This procedure would also be followed if stored material showed signs of acid generation before the next scheduled dam lift. Although an expensive technique, the transfer of waste rock from the  th  Proceedings of the 12 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1988. The Technical and Research Committee on Reclamation  th  Proceedings of the 12 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1988. The Technical and Research Committee on Reclamation  th  Proceedings of the 12 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1988. The Technical and Research Committee on Reclamation  73  mine area to the tailings pond provides a permanent solution to potential acid generation. The possibility that the tailings themselves might oxidize and produce acid has obviously been an equally important consideration which is discussed below. It was originally planned to add limestone to the mill feed in sufficient quantity to neutralize any possible acid produced by sulphides in the tailings. However, subsequent testwork indicated that the quantity or limestone required for this purpose would be excessive and represent a prohibitive operating cost. Another strategy was therefore sought. There has been much discussion over the benefits of submerging mining wastes to prevent acid generation. This method, proposed for Willa tailings, requires an environment in which the tailings pond will remain flooded under natural conditions, without human intervention for an indefinite period after mine closure. The basin selected for tailings deposition at Willa is a natural groundwater discharge area which experiences a small flow throughout the entire year. Yet the area is not subject to large flows, as it occupies an upper corner of the drainage catchment of Brahms Creek. It would probably be possible to maintain a positive water balance in the tailings impoundment even without a liner considering that the tailings will be ground to a very fine size (90-95 % minus 200 mesh). Material of this size has a very low permeability. However, the use of a liner will provide an extra degree of confidence in the ability of the system to hold water. The tailings pond will be operated as a flooded system in order to keep tailings covered with water continuously. This measure will prevent both the tailings and the waste rock from producing acid. During operation, the tailings will be spigoted from several points around the pond perimeter. The fine size of the solids will cause them to flow to the bottom of the pond without creation of an exposed beach. The supernatant reclaim rate will be adjusted continuously in order to maintain the pond water at its desired level. When the system is abandoned, the diversion ditch will be breached, thereby allowing natural runoff to enter the basin as it did before operation. Water balance calculations have shown that there will be a surplus of water even during the 100-year return period annual low flow. The water level will be controlled by a spillway 1.5 m below the dam crest and approximately 1 m above the final solids level. With this depth of water over the solids, it would be possible for the system to go without precipitation or runoff input for an entire year or more without drying up and exposing the solids. A final line of defence against acid generation in the unlikely event of drying of the pond surface is the addition of limestone to the tailings during the final months of deposition before shutdown. This will be accomplished by adding limestone to the mill feed in sufficient quantity that the top layer of tailings will have a positive net neutralization potential. Upon mill shutdown, the tailings pond will be reclaimed as a wetland habitat, suitable for waterfowl. This type of habitat is presently almost totally lacking in the area. Deposits of peaty soil, excavated and stockpiled during the dam construction phase, will be bulldozed into the water's edge around the pond perimeter. Cattails, bulrushes, sedges and other bog species will be introduced into this zone (Figure 5). The cleared area around the pond will be seeded with grasses and legumes. When reclamation is complete, the area should be attractive and environmentally secure.  th  Proceedings of the 12 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1988. The Technical and Research Committee on Reclamation  th  Proceedings of the 12 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1988. The Technical and Research Committee on Reclamation  These concepts were developed with full confidence in their ability to provide long-term environmental protection without human intervention. However, in order to gain a final measure of security, the Company has offered to develop a contingent liability trust fund out of operating revenue. This fund would provide for annual inspection and maintenance costs as well as revenue for remediation facilities, should they ever be required. Conclusion In recent years, some valuable lessons have been learned about the hazards and liabilities of acid mine drainage. Although it is not always possible to predict with certainty which mine wastes will produce acidic drainages, prudent planning dictates a cautious approach. Often, solutions to potential problems are available if implemented at the appropriate time. The key to prevention of acid mine drainage is the development of abandonment strategies prior to operation, when the development concepts are being formulated. This approach would not have been considered even ten years ago but represents common sense today. The solutions available at the time of project inception sometimes represent an added cost of development, but these costs are insignificant compared with the expense of acid mine drainage treatment in perpetuity or the impairment of the environment. In summary, three important principles are embodied in abandonment planning: 1. Plan early, at the project feasibility stage. 2. Provide defence-in-depth for potential problems. 3. Plan with the knowledge that under current and future legislation you or your company will be held responsible for the cost of rectifying environmental transgressions.  75  


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