In this new series we give an overview of the factors affecting the calculation of a mineable ore reserve computation for a block cave mine. Our last post​​​​​​​ introduced the list of parameters to consider when starting a new project and presented the 1st parameter: "dilution". Discover in this post the 2 next parameters to have in your checklist:  the "ore body shape and aspect ratio" and the "flow mechanisms".

ORE BODY SHAPE AND ASPECT RATIO

An ideal ore body for block caving is large, uniform and would have sides close to vertical. The more the shape deviates from this ideal shape, the more likely it is that dilution sources would be problematic. The deviation from ideal in the horizontal plane would affect cave shape and hydraulic radius. The vertical to horizontal ratio will have a large effect on side dilution. For a narrow high ore body, the risk of the cave NOT propagating to conveniently follow the sides of the ore zone is high.

If the length to width ratio or height to width ratio exceeds 3:1 or 4:1, then side dilution is likely to be problematic. For example, if an ore body is 200m X 120m, and one hopes to extract a vertical column of 500m, then the height to width ratio would exceed 4:1 and the risk of poor recovery of the top of the ore zone is high. The side contact of the ore body is also significant. There are three cases:

1. A gradual reduction of grade/value as one moves away from the ore zone. This is moderately favorable. If the cave or mining extracts material outside the target ore zone, then the dilution material still may contribute metal.

2. Sharp reduction in grade/value, but without a sharp geological contact. This is unfavorable.

3. Sharp reduction in grade/value, but with a matching sharp clean geological contact (e.g.: a weak kimberlite pipe within stronger host rock).

From an ore recovery perspective, this is convenient, since the contact will help with the separation of ore and dilution.

FLOW MECHANISMS

Detailed description of flow mechanisms is beyond the scope of this paper. However, a list (Diering 2007) is as follows:

• Vertical mixing
• Horizontal mixing (similar to diffusion)
• Toppling
• Rilling
• Erosion
• Compaction
• Major surface movements (eg Pit Failures)
• Inclined flow along a contact

The various flow mechanisms are shown in Figure 1 (from Diering 2010).

The understanding and impact of these mechanisms dominates a large part of the dilution and ore reserve calculations in a block cave mine. The three primary ones are likely to be vertical mixing, rilling and erosion mechanisms.

From an ore reserve perspective, and also within the PCBC program, we start by constructing a column of material above each draw point for which properties are derived from the block model. This is referred to as a slice file (as separate and distinct from a block model). As the different mechanisms take place, material may either remain within a draw column or else be transferred to other draw points. This is important from the perspective of ore reserve definition, since confidence in extracting material is strongly related to how much it moves around within a cave. It is reasonable to suppose that material which moves extensively in an uncertain manner would have a lower confidence limit than material, which moves downwards directly into the underlying draw point. There are three cases to consider:

• Material moves, but remains within the same single draw column. (Vertical mixing and erosion and compaction. Inclined cones can still fall into this category).
• Material moves from one draw column to another. (Rilling and horizontal mixing)
• Material moves from outside the defined draw columns into or on top of the draw columns. (Toppling and pit failure).

This is summarized in Table 1.

The above may seem obvious when put into a table, but there are examples where material to the sides of a block cave layout have been included as part of the stated ore reserve but, have not been recovered leading after the fact to ore reserve write downs. This situation is, potentially, quite common since it can occur any time a block cave is initiated beneath an exhausted open pit. The above mechanisms are modelled within the PCBC software. In a typical study, the mechanisms may be turned on or off and the effects on ore / metal recovery studied as part of the overall risk assessment process.

In our next post, we will detail the next 6 parameters to have in your checklist. Stay tuned!

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