Converting the in-situ grade control (GC) model to the set of mining perimeters utilized for short-term planning, and in some cases, for active operations (when no blast adjustment is applied), may seem like a straightforward task for any production geologist. It appears to be a skill that can be mastered in a relatively short time frame. However, this apparent simplicity is deceptive and gradually diminishes as more factors influencing the definition of polygons are introduced. These factors include cut-offs, metallurgical variables, and deleterious elements.
In the upcoming comments, let's review 6 less apparent yet practical tips that enable us to build a robust polygon:
- It is crucial to develop a comprehensive decision-making tree for the ore types in use and update it as needed. In geology, you can have as many of these trees as necessary and then combine some for operations and planning purposes. With at least 2-3 production geologists involved in the process, any ambiguity in how we label this combination of grade/recovery/geometry needs to be eliminated from the process. Simple color coding with pre-agreed names usually does the trick.
- A similar approach is employed when addressing shape tonnage and mining geometry. It is unrealistic to anticipate a large 6060 selectively mining something as small as 2-3 kt, although for a 993 mining 5m flitches, it might be feasible in certain situations. Discussing these limitations with the mining team is crucial, as they will vary blast by blast and depend on mining directions. Establishing a clear set of rules on what can and cannot be selectively taken is essential. Spending time with each production geologist to review their work will also enhance the convergence of results.
- Once the GC model is finalized, it should serve as the sole source for polygon development. It's not uncommon for geologists to go back and forth to the GC drilling and attempt to "refine" shapes, which can be adversely affected by the change-of-support effect. If the nugget effect is low and the estimation is localized, discrepancies should be minimal anyway. However, if the nugget effect is high, expecting one hole to be representative of the block volume is unreasonable in the first place.
- Special care and vigilance are required when handling zones not anticipated to be taken completely, such as ramps or drop cuts. If the ramp is temporary, it is acceptable to use the full tonnage for the polygon (just don’t forget about it when you take it several months after). In the case of a permanent design, the tonnage should be adjusted. Depending on the shape, this adjustment may involve a straightforward mathematical calculation or a proportional evaluation of the ramp design from the block model.
- The comparative recon metric of Preblast/Grade Control Model offers valuable insights into the expected levels of planned ore loss and dilution (OL and DIL). This will be complemented by unplanned OL and DIL during post-blast adjustments. In general, a simple tonnes-grade-metal comparison provides an intuitive overview (my preference), grouping ore types based on the reporting selectivity level preferred. Another option is to directly calculate OL and DIL to ensure they fall within reasonable limits.
- If you predrill and prepare polygons for the full bench, it wouldn't hurt to conduct the same comparison with the GC Model on a bench scale. However, blast-by-blast analysis might be too variable to yield insightful results. The same applies to reporting and planning – for the full-size bench, it may be beneficial to help planners and code the polygons directly to the block model, ensuring each block inside the polygon reflects the polygon grade. When conducting a blast-by-blast update, this may be overkill, and you only need to keep an eye for the polygons do not overlap (which may result in one or two discussions with a drill-and-blast team on how they prepare the design sequence and final blasting perimeters that rarely match each other).
That was a brief overview of key considerations in preparing a robust and sensible preblast perimeter. Please feel free to share your insights and experience on the matter. What tips and tricks do you incorporate into your work?
In the next post, we will delve into automated optimization solutions for polygon development, exploring their advantages and weaknesses.
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CEO at DustAct Eltura Group | Making Mining Sustainable
9 个月Well explained! As someone who's been entrenched in the mining industry for years, I understand firsthand the intricacies involved in converting grade control models to mining perimeters.
Senior Geologist - Consultant
9 个月Thanks for the post, Alexandr. I believe decision tree is important. Still it can be changed from time to time to reflect the prioritization of criteria because of changing requirements. For diglines, I guess one should refrain from creating acute angles or sharktooth boundaries which could be practically hard to mine by equipment. I also suggest peer review of the diglines and decisions by another mine geo. If the ore-waste boundary is very sharp and grade difference is very high in a narrow zone, and cutoff grade is very low, block model grades smears towards the waste zone and give unrealistic false positive zones. I believe one should always check samples and understand the reason if there is a discrepancy between samples and block grades. Coding block model grades should be helpful for mine planners. However, partially operated volumes (undercut or bench faces where small portion of block is inside mined volume) may not be coded and that creates issues for monthly reporting and comparison to mine physicals. Instead, I prefer creating ore and waste volumes within monthly survey and use those volumes for physical and spatial reconciliation purposes. It is important to see avoidable and unavoidable markup oreloss-dilution.