The joint I'm on at the moment pulls 40Mtpa ore and 120Mtpa total movement. Big gear and big mining blocks. Much of the discussion this week has been around dilution and ore loss. When a competent open pit mining engineer designs a bench in a pit, the geologist's block model is re-blocked to a Smallest Mining Unit (SMU) size.
The geologist estimates ore grade at a block size that reflects the size of the orebody,ideally half the drill spacing in length and some width by some vertical depth. Typically this might be 25m x 10m x 5m. These blocks are divided into sub-blocks at the same grade that allow the shape of the mineralization to be followed. These models are typically huge and the block size unsuitable for mine planning purposes.
The mining engineer's SMU's are imposed on the geologist's model and grade re-estimated. A competent mining engineer will use an SMU size that reflects a unit of production - the volume of rock that is mined an a shift, a day, a week, whatever. This assists scheduling and reconciliation. Typically, the SMU may be 20m x 20 x the mine bench height, say 12m.
In doing this, the SMU incorporates some dilution, and ore loss because it hasn't got the resolution of the geo's model. Some waste is included in the block and some ore is excluded. This is normal mining behavior and the degree of dilution and ore loss should mimic that experienced in real life. Ya gots a 2m bucket on ya excavator, mining a 1.5m mineralized vein, ya gets dilution. In t' pit, dilution and ore loss can be reduced by grade control, particularly if there is a colour difference - ore be brown, waste be grey, for example.
However, mining engineers like to blast big benches, which are more efficient. They also like the big trucks and diggers that go with 'em. This is disliked by the guessologist, because it increases dilution and ore loss. So a compromise is reached - the engineer can have the 12m benches, but the geo mines them on 3m flitches. The grade control drilling (usually blast holes) is sampled at 3m intervals, which informs the dig plans. The digger mines the uppermost flitch and then returns to succeeding lower flitches and all is well in the world of grade control and reconciliation.
The joint I'm looking at right now has whacking great rope shovels mining 15m benches in one hit using 15m x 15m x 15m SMU's. I don't like putting shovels of any size in front of anything greater than, say 8m, for safety and for grade control. So the suggestion is that the geos take 2 x 7.5m samples and the engineers re-block their SMU's to 7.5m flitches. This actually increases production, because it is easier to dig a 7.5m face (which becomes something like 10m after blasting) and the machines work slightly more quickly. It also improves grade control, because the geos are better able to model the mineralization in their dig plans.
So if you're investing in any entity, ask questions about how the orebody is to be mined and whether this will have implications for head grade.
Here endeth the lesson.
