Recon #8 of 17. When Geos Have a Blast or How to Account for the Blast Movement

Hi everyone and thanks for your interest in mining geology!

The assumptions regarding Ore Loss and Dilution (OL and DIL) typically come into play during the preparation of the planning polygon, a visually appealing in-situ shape directly derived from the grade control block model. However, two significant sources of unplanned OL and DIL often have the potential to significantly disrupt our planning process and distort reconciliation results. These sources are blasting and mining. In this post, we will address the former, and we'll delve into the latter in subsequent posts related to the mining control process in a few weeks.

Depending on the continuity of your mineralization, as well as the physical characteristics of your rock, the additional/unplanned OL and DIL introduced by blasting can vary. In the case of a large, continuous bauxite deposit with a minimal swelling factor in the weathered material, the movement and volume change resulting from blasting may be negligible (if blasting occurs at all). However, for a highly variable nuggety gold stockwork hosted in structurally deformed, yet otherwise competent diorite rocks with a choked center lift blast that swells like a pop-corn or free-face one with a lot of movement, the picture that you observe after the blast will most likely be non-comparable with the insitu narrative.

From the projects I've seen and audited in the past, I can highlight three major strategies that we can use to account for blast movement when adjusting our mining perimeter for operations. Since there may be additional methods, I encourage you to share your approaches in the comments and discussions.

1.?Using the Insitu Polygons for Mining

This strategy is quite straightforward: we simply assume that the blast hasn't altered anything and proceed by providing our pre-blast perimeter for dispatch and marking it on the blasted ground. While this approach may suffice if the blast movement is minimal, if you anticipate significant material displacement, there are more effective methods to address this issue.

2.?The Sensors to Measure the Blast Movement

This methodology involves the use of small sensors, typically ball-shaped, to measure the movement caused by a blast. These sensors are placed into designated blast holes, where they record their position. After the blast, detectors are used to locate the sensors, and specialized software is employed to calculate the displacement of each sensor. This information is then used to adjust the 2D polygon shape, taking the movement into account. This additional step adds to the workload for the geology team and may potentially slow down the process of miners receiving the perimeter. Nevertheless, when aiming for higher quality, investing time is often necessary ??

3.?3D Methodologies

In this approach, we eliminate the need for extra work and special devices. Instead, we rely on obtaining topographic surveys for both pre and post-blast assessments. By using this data along with the blasting pattern configuration, timing contours, powder factor, and the grade control block model, algorithms are employed to reverse-engineer the direction of the blast movement in three dimensions and fill the extra volume between the pre and post-blast topo. Through this process, we calculate swelling and adjust the density and location of the blocks, ultimately leading to the development of the 3D blasted block model. Then the model is used to generate the updated polygon shapes.

In all scenarios involving blast movement, my personal preference is to utilize method #3. Not only does this methodology eliminate the need for running around with beeping detectors on the blasted material, but it also enables control of the process in three dimensions. While this approach may not significantly aid in 3D grade prediction (typically, the block size equals the bench size), accurately accounting for volume shift and density heterogeneity is invaluable, particularly in free-face blasts where the heave height can vary from 0.1 to 2 times the bench height, depending on if you are on the periphery or in the center of the blast.

One other thing worth mentioning is that the process of polygon conversion cannot be fully automated and will always require the input of the production geologist. Developing a simple one-page QAQC checklist to address the most commonly observed mistakes (for instance a simple tonnage-grade check between pre and post shapes) and ensuring that this process is peer-reviewed and documented blast-by-blast will significantly reduce the variability of outcomes among different team members and ensure consistent quality in their work.

This was a quick overview of the methodologies available to us right now for improving our ability to predict the most accurate location of our mineralized material. Please feel free to share your experiences of the blast adjustments in the comments. In the next post, we'll dive into the topic of how to properly calculate the post-blast reconciliation and include it in your regular monthly cycle of reporting.

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