Here is the abstract from the most interesting paper in the CIM 2007 conference on backfill:

The Challenge of Cyanide: Can backfilling with gold tailings be consistent with the requirements of the International Cyanide Management Code?

By Mrs. C.L. Reichardt, School of Mining Engineering, University of the Witwatersrand

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 (the Code). The Code stipulates a ‘cradle to grave’ approach more comprehensive than that applied to any other mining reagent and is particularly challenging for operators who use cyanide-bearing tailings for backfill. This paper will examine some of the challenges for Code compliance facing both surface and underground operations 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.

This requirement is particularly challenging for gold mines on the Far West Rand of South Africa’s Witwatersrand, where the gold bearing reefs are overlain by dolomite which constitutes the most significant groundwater resource in the country. The dolomite also provides baseflow to the Vaal-Orange River system, which supplies water to millions of downstream users. The Far West Rand gold mines require dewatering on a massive scale to maintain safe conditions in ultra deep workings, which substantially modifies pre-mining groundwater levels, flow directions and gradients.

Some mines on the Far West Rand use a small percentage of their tailings as backfill for roof support. Those who are Code signatories will need to demonstrate that use of cyanide-containing tailings for backfill will not compromise the ecological function or the beneficial use of this highly significant groundwater resource during either the operational or closure phases. This paper will discuss some of the challenges confronting these operators in demonstrating that their potential impacts have been identified and are of an acceptable magnitude.

That most famous expert of cyanide, Terry Mudder, comments as follows on this piece:

Interesting thoughts on her part. It is true the Code does not specifically deal with these issues but its general principles and standards of practice do. If a pit is backfilled and a pond forms on top the WAD cyanide must be below 50 mg/L to be in compliance with the Code. If the pond formed after operations the standard that would apply would be based on the appropriate governmental regulations which in turn would be based upon the classification of the surface water body formed. If there were a discharge to the environment it would have to meet the Code guidelines during operations and potentially more stringent standards based on governmental regulations. Backfill underground would involve both air and water issues. In the case of the air, the HCN concentration would have to meet the Code and governmental regulations. There are no standards for cyanide in the collected water underground until it reaches the surface, where during operations it would be pumped back into the process solution inventory. After operations it would have to be discharged into the environment and again meet governmental regulations to protect whatever beneficial use was applied to the surface water.