Historical Mining Review
Stoping of the ore at Winston Lake initially planned to use a mechanised cut and fill method, however this was changed to the more productive methods of mechanised AVOCA stoping and where no development existed, Alimak stoping. Access on ore was achieved through a series of sublevels developed at 20m vertical intervals connected via a hanging wall ramp (gradient of 1 in 7) at a 4m x 4m profile. Once the ore was extracted from the stope, unconsolidated rock-fill material was then placed into the mined-out stope.
Ore mined from Pick Lake Deposit was transported via a rail system on the 615-access drive where it along with ore from Winston was hoisted to the surface via the Winston Shaft.
Dilution from past mining was caused through a number of factors:
• In Winston Lake dilution was impacted by the size of ore drives, a chert intrusion along certain parts of the hanging wall, and the use of the AVOCA mining method.
• Development of new ground support techniques, such as the use of cable bolting to minimise the hangingwall failures occurring. The research and development work completed at Winston on cable bolting was leading edge at the time and is now standard procedure in many underground mining methods to minimise dilution.
• The Alimak mining method used at both Pick and Winston, where there was minimal access development resulted in dilution occurring due to no backfill being used. This lack of access prevented Inmet from backfilling the mined out areas as a result hanging wall dilution occurred. It was in this context that Inmet recognised these impacts on stoping and performed a test stope in Lower Pick.
In 1998, Inmet carried out extensive studies on alternative mining methods to limit the effect of dilution and costs in the prevailing low-price zinc environment at the time.
As part of this strategy, Inmet completed a successful test using the longhole stoping method on the 1045 level of Pick Lake (Figure 8 & 9). The 1045 test stope had a strike length of 25m, an average width of 2.5m and a height of 15.5m. On completion of the stope, a cavity monitoring survey measured the void to calculate dilution from the hanging wall and footwall. As part of this test, ground monitoring was installed prior to mining to measure hanging wall movement. After 20 days no measured movement of the hanging wall was recorded (no fill was placed in the stope). At the successful completion of the test stope, dilution was calculated at 18% however, prevailing zinc prices at the time meant that the Project was not viable until there was a substantial and sustained increase in the zinc price and the decision was made to permanently close the operation.