We are getting close to the conclusion of the checklist of parameters the consider when starting a new project. Review the previous posts​​​​​​​ and see which factors you should take into account when calculating a mineable ore reserve computation for a block cave mine. In this post we will end the checklist with residual material in multi lift situations and other considerations.

By Tony Diering, Ph.D. VP Caving Business Unit Dassault Systèmes, GEOVIA

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RESIDUAL MATERIAL IN MULTI-LIFT SITUATIONS

As more block caves are developed, there are increasing number of instances in which a second or multi-lift situation is encountered where the deeper lift is wholly or partly beneath a previously mined block. Within PCBC, there are three basic approaches to modelling this situation:

• Assume a simple low grade background value for the various grade elements in the mined out areas.
• Use zones of constant grade in the mined out areas with grade values decreasing upwards. This would generally be linked to the shut-off grade that was used during the earlier mining. If a draw point was shut off at 1% Cu, then it is not unreasonable to assume that the grade of the material left behind in the draw point would be close to but less than 1%.
• Simulate the historical mining as accurately as possible and then use the residual slice file to estimate the remaining material. There are custom tools within PCBC to facilitate this process and the approach is gaining in popularity. The challenge is to try to provide a material classification for this residual material. It is possible to drill into old mining areas to firm up on the confidence of the estimate.

When mining beneath a previous mining area, there are several factors to be taken into account:

• What was the shut-off grade in effect for the previous lift? With generally increasing metal prices, it is likely that the previous shut-off grade would have been high enough to leave economic material behind.
• What was the overall tonnage recovery from the previous lift? For example, if the reserve had been stated at 100Mt, but only 75Mt was recovered, then it seems reasonable to assume that the other 25Mt is still in the ground and can potentially be mined. Although this seems obvious, it often turns out that the residual material is substantially discounted or written off altogether from reserve statements.
• Are there areas where pre-mature collapse of the draw points took place with incomplete recovery,which would leave higher grade material for subsequent extraction?
• Was there incomplete recovery of the ore due to early dilution entry and non-interacting draw cones? In this case, the average grade of the residual material could locally be higher than the previous shut-off grade.

It is the opinion of the author that the quantification and categorization of residual material will play a more important role in the future. However, if the material cannot be given a resource or reserve classification (e.g. Measured, Indicated or Inferred), then the metal from that source should not be included in the calculations. Effectively working through this situation can be complex.

An example of a deposit where multiple lifts have been mined (or planned) is shown in Figure 5 (Diering 2010) for the Cullinan Diamond Mine in South Africa.

OTHER CONSIDERATIONS

Computational Considerations

When working on a project it is often necessary to work with many different grade elements. Consider an example where there are eight metallurgical domains with different metal recovery characteristics, five metals to be tracked (e.g. Cu, Au, Ag, As, Mo) and it is also required to track Measured, Indicated, Inferred and Other material in the reporting. Within PCBC, due to the material mixing process, individual particles are not tracked. Instead, fractions of blocks are merged to form the slice file and the mixing algorithms work with fractions of slices. Thus, we cannot track and report rock types as easily as for other mining methods. In the above example, up to 240 grade attributes could be required for fully comprehensive reporting. The previous limit in PCBC was 20 grade attributes. This has recently been increased to 40 and will be further increased to 100 in the near future. However, even if the program can track and report all the attributes, the actual process of collating and managing this larger number of attributes can be formidable. Care must be exercised to select an appropriate list of elements with which to work.

A second consideration arises with the treatment of the Inferred material. For planning purposes, it may be decided to include this material. However, for official reporting purposes to the stock exchange, it may be required to “zero out” the grades for Inferred material. While this can quite easily be done, it does add to overall complexity and work flow and is also prone to errors for the unwary.

A third consideration arises out of the treatment of mined out areas. Usually and currently, a geologist may prepare the in-situ resource block model. However, if previous mining has occurred, then the onus would likely fall on the planning engineer to make adjustments to the block model for the mined out areas. This can lead to complications in terms of accountability and sign-off responsibilities.

METAL BALANCE

It is recommended to perform metal balance calculations as part of any PCBC reserve evaluation process. This is a very useful check and there are three principal areas where this is particularly useful Assessment of what percentage of the geological resource has been converted into a mineable reserve.  

• Comparative studies of different alternatives. In the early stages of a project, many different scenarios will be evaluated in varying levels of detail. However, tracking the metal totals for the different scenarios is very useful for relative evaluation and weeding out less attractive alternatives
• Metal balance is particularly useful in multi-lift scenarios. The basic approach is to start with an in-situ resource A. If the mined metal is B, then metal remaining in the ground should be C where C = A-B.

AUDIT TRAIL

Many block cave projects evolve over a period of 10 years or more from concept through to construction. Then the mine may operate for an additional 30 years. It is really important to have thorough documentation of the various models and reserves statements generated during this extended time frame. Many tools exist within PCBC to facilitate this process, but instances still occur where a previously calculated reserve report cannot be replicated. This is usually due to factors such as changing block models, changing inputs such as metal prices and recoveries, user error and different versions of the software. A good audit trail should document full work flow of all key inputs with graphical evidence of the steps taken to generate final reserve reports.

Don't miss the last article of this series that will provide you with a summary of all the factors and the full downloadable article!

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