Underground Grade Control Sampling
This is usually by one or more of the following methods:
1. Chip sampling of faces and/or walls.
2. Channel sampling of faces and/or walls.
3. Grab samples of blasted, broken materials.
4. Sampling of drilling sludges from underground blast holes.
5. Purpose drilled holes.
Channel Sampling Channel Sampling
Cutting adequate channels is extremely difficult, primarily because the channel we cut does not actually look like the channel we intended to cut. This is ‘delimitation error’.
CH 4 – SAMPLING CH 4 – SAMPLING
Quantitative Group Short Course Manual
Face Sampling Face Sampling
Dedicated samplers or geologists collect the face samples underground. Relying on miners requires judgment, but cannot be recommended in general.
Often only every second or third round or face is sampled. Intermediate rounds are sometimes assigned assumed grades from previous faces. Sometimes, when a face is not sampled it is because it cannot be accessed (is not rock bolted, for example). Rounds that are not face sampled may be ‘grab’ sampled off the broken dirt. We comment on grab sampling, below.
Taking line chips from faces or walls is an attempt to mimic channel sampling.Channel samples, when correctly cut, are superior samples. Since we know that face sampling is very difficult to do well, we need robust protocols, good samplers and excellent management/supervision of this process. It is rare that any type of interpolation is performed with underground face sampling data, though it may be feasible.
The samplers should record sample locations in a neat and practical format. These should always tie in (or be referenced to) good quality geological face mapping. Drilling Methods
Drilling Methods
Sampling of various kinds of underground drilling is also often used for grade control. Such sampling should be carefully observed. It usually consists of collecting a wet slurry in a tray or a bucket and scooping sample (by means of a hand-or literally by hand). It may strike a rubber mat at the collar and then free-fall into the collection tray/bucket. There is usually the attendant problem of material being splashed around and lost. Such losses are often differential losses with respect to granulometry and density, and can be expected to introduce biases. Sampling of underground drilling is almost always the drilling contractors responsibility
⎯
which can be problematic from a QC point of view.So we should be concerned about potentially serious bias and imprecision in underground drilling sampling: certainly we should not sample very shallow or ‘down’ holes. The collection system should be standardised and the procedure documented.
We need to measure the quality of sampling processes. This will necessitate duplicate sampling of a significant number of underground holes. It is suggested that entire rings be duplicate sampled (this way two ‘alternative images’ of the ore/waste boundaries are generated). The two samples need to be
⎯
as close as practically possible⎯
collected in identical fashion.Other Considerations Other Considerations
We should remain open-minded about ways to improve sampling because underground grade control sampling is very difficult to perform properly and the
Face Sampling Face Sampling
Even with the difficulties of collecting ‘ideal’ face samples, they are much superior to grab samples of broken dirt.
Measuring Measuring Performance Performance
We should always duplicate sampling to allow proper QA/QC procedures.
CH 4 – SAMPLING CH 4 – SAMPLING
Quantitative Group Short Course Manual
economics of most operations are highly sensitive to it. If technical innovation is required, we should investigate the alternatives.
We should avoid grab sampling broken dirt wherever we can. This type of sampling is a disaster for low-grade materials (with high nugget) like gold. If it means that we need more samplers, or we need samplers to work more shifts, then this is very cheap compared to the costs of misallocation caused by grab sampling. Sampling of broken ore lots where the top-size is much larger than about 25mm is a extremely difficult task, usually involving very large samples and incremental sampling strategies (i.e. the taking ofmany samples). Broken ore in underground mines may have a top-size that exceeds the dimensions of a Volkswagen Beetle11!
It’s simply not possible to sample such material in an equiprobable manner.
The Role of Geostatistics The Role of Geostatistics
The variograms of the underground sampling data should be calculated and reported and directional variography is also important. The variography of HW to FW composites (i.e. accumulation variables, see Chapters 6 and 7) can be very interesting and should be calculated. Often, underground grade control data is not adequately statistically assessed.
In fact, a first step in assessing the quality of existing U/G data should be to performrigorous variography of the data.
Once we have variogram models we can start to address, for each deposit, such issues as:
1. How can we measure improvements in sampling and assay (i.e. measure reduction in the contribution of these to the nugget variance)?
2. Do we need to sample every face/ring? How many do we need to sample?
3. Are we collectingenough samples on each face? Are they large enough?
4. How should we use this data to best estimate for ore allocation or stope design purposes?
5. Can the grade control data help us to establish appropriate drilling geometries/spacings for near-mine extensions or new resources? In particular, the variogram can allow us to calculate estimation variances to establish global confidence intervals on the mean grade of intersected mineralisation.
6. What can the grade control data tell us about the spatial distribution of grade at depth? In other words, how can we best integrate this data into our overall geological picture of each deposit?
11 That’s a ‘bug’ if you’re North American
Grab Sampling Grab Sampling
Sampling of broken ore is generally impossible. Unless the grade is very homogeneously distributed, we cannot get a reasonable
sample from this type of material.
Variograms Variograms
Variograms are dealt with in the next chapters. A variogram measures spatial variability and is a key tool in
CH 4 – SAMPLING CH 4 – SAMPLING
Quantitative Group Short Course Manual