CIM Bulletin, Vol. 2, No. 5, 2007
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Pressure exerted on gold producers by regulators, project financiers, and civil society in the wake of the Baia Mare tailings spill in Romania during 2000 prompted most responsible gold companies to sign up to the International Cyanide Management Code for the Manufacture, Transport and Use of Cyanide in the Production of Gold (the Code). The Code stipulates a ‘cradle to grave’ approach more comprehensive than that applied to any other mining reagent, and poses particular challenges for operators who use cyanide-bearing tailings for backfill. This paper examines some of the challenges for code compliance facing deep underground gold mines that employ backfill technology, most particularly the requirement to demonstrate that the potential migration of cyanide (and its degradation products) from emplaced backfill will not have an unacceptable impact on worker health and safety or the receiving environment, either during mine life or after closure.
Although the Code may initially appear to be almost silent on cyanide risks associated with backfill (with only a single clause making specific reference to backfill), more careful consideration confirms that many Code requirements are relevant to backfill operations. Most sections of the Code that deal with tailings management are potentially applicable to mines where tailings material is used for backfill, as are certain other principles and standards that deal with operations, worker safety, emergency response, training, and dialogue with stakeholders.
Backfill use in underground mines is undertaken for one (or a combination) of the following reasons): rock stabilization, direct in-mine heat load reduction, improvement in ventilation air utilization and/or reduction in the volume of mine waste to be disposed of on surface. For the ultradeep gold mines of the Witwatersrand that extend to depths of over 3 km, the decision to backfill is primarily motivated on the basis of stability considerations, which safeguard not only worker health and safety, but also minimize disruption to production. Gold tailings contain significant concentrations of cyanide and are generally not detoxified prior to emplacement as backfill.
For gold mines of the Far West Rand, the most significant cyanide-related risks associated with backfill operations relate to worker health and safety associated with potential exposure to cyanide-bearing seepage and hydrogen cyanide gas (which may be liberated where locally high sulphide concentrations in the orebody and host rock generate acid drainage and drop the pH of the tailings). Although there are a range of engineering interventions such as tailings thickening or detoxification that may serve to reduce this risk profile, the most effective (and least costly) risk mitigation strategies would appear to relate to behaviour-based safety systems that empower the workforce to identify and mitigate cyanide-related risks and to take appropriate action should such risks eventuate.
Even though the gold-bearing Witwatersrand Formation is overlain by the most significant aquifer in South Africa—the Malmani Dolomite - the environmental risks associated with cyanide in backfill on the Far West Rand are considered to be less significant than the health and safety risks to the workforce. The potential environmental impact of cyanide migration from backfill is considered to be relatively low due to large-scale, mining-related dewatering that has drawn down regional groundwater levels as much as 1,000 m below pre-mining elevations, compounded by the fact that backfilling takes place at depth well below significant aquifer horizons. This risk is further mitigated by the absence of pathways to facilitate the migration of water from the deep workings to the (near) surface environment under operational conditions. Although groundwater levels will recover post-closure, the vast volume of water in storage in the dolomitic aquifer is likely to result in massive dilution of any cyanide that may be mobilized from backfill.
However, it is possible that in other regions, where backfilling takes place at or close to surface and where different geological and hydrological controls come into play, the environmental impacts associated with cyanide-bearing seepage from backfill material could be potentially significant and would warrant specific management intervention.