Types of Sampling in Open Pit Grade Control
Estimating block grades from sampling data is an activity that is fraught with difficulty. Most classical statisticians would regard the data forany grade control problem as dangerously inadequate. A 10m x 10m RC grade control grid equates to sampling of about 1/25,000th
of each 10m x 10m area. The small volume sampled is better appreciated if we express this as 0.004%.
The main decisions made about allocating material to ore or waste (or intermediate stockpiles) are taken at the grade control stage. The quality of the grade control will thus impact profoundly on the economics of the mine.
Delineation of ore boundaries above a given cut-off is a difficult problem for the following reasons:
1. The grade measured in one hole can be well correlated or poorly correlated to the grade of thesurrounding mineralisation (‘the block within which our sample is located’). This correlation between sample
and block grades is measured by the variogram and in particular the nugget effect and any short-range structures (these points are detailed further, later in the course).
2. The grade we have from our sampling may not actually represent the grade of thesampled material very well. This is generally due to a poor sampling protocol and has two implications:
o Contributing to an incorrect grade estimation of the
surrounding block.
o Increasing the proportion of nugget effect on the variogram,
because the increased sampling variance due to poor sampling protocol adds to the nugget. This always decreases the precision of estimation, in extreme cases (‘pure nugget’) it renders local (block) estimation unfeasible.
3. The resultant error will always overestimate the higher grade blocks and underestimate lower grade blocks (‘conditional bias’).
In fact, it is a general case that grade control cuttings are either not geologically logged, or are logged in a very cursory manner (often by inexperienced geologists). So it is the usual situation that ore-waste boundary recognition is highly reliant upon the values obtained from ‘sampled material’. The quality of this sampling is thus a key factor in determination of the overall quality of grade control.
The most common approaches to open pit grade control sampling are:
1. Sampling of blast hole (BH) cuttings.
2. Sampling of purpose-drilled production reverse circulation (RC) drilling.
2 Types of ‘Lie’ 2 Types of ‘Lie’
There are two ways in which a sample grade can mislead us. The first is that it will be imperfectly correlated to the grade of surrounding material; the second is that the sample may not represent the sampled material in any case.
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These two approaches are not equivalent in terms of sampling quality, and in particular, there are a range of methods for sampling the cuttings from RC and BH from which widely differing data quality can be obtained. In some circumstances, other approaches (Ditch Witching 9 or channel sampling may be considered). We
look at each of these methods in detail, below. But first, we consider how to compare different approaches.
Testing Different Approaches to Sampling Testing Different Approaches to Sampling
Before deciding to adopt a particular approach to grade control it is advisable to make quantified tests of each method considered. The main steps in such a test are:
1. Select a characteristic (or several characteristic) test areas. In general, these should be large enough to allow proper statistical and geostatistical assessment of the results. An area of at least, say, 50m x 50m will be required, usually a larger area is needed.
2. Sample the area by the different methods at a closely spaced grid (the closest grid we can envisage for production grade control). Attempt to implement both methods on the same spacing, but off-set (e.g. “paired” holes), if possible.
3. Compare statistics and variograms for the different data.
4. Assess the selectivity on the basis of estimated block models for the different methods. We consider this later in the course.
5. Rank each method according to the “conventional benefit” returned in each case. Subsequent discounted cash flow analysis (DCF) can complete the process and allow us to make an economic decision.
6. Use the variogram to determine appropriate drill spacing (again, we consider this later on).
7. Use the variogram to perform tests of different estimation strategies. Now we consider the main methods available for collecting grade control data. Blast Holes (BH)
Blast Holes (BH)
Blast holes areopen holes , generally drilled by a percussion rig. By ‘open hole’ we mean that the sample is returned via the annulus between the hole walls and the drill rod. The potential for contamination, hole collapse and recovery difficulties is significant in this type of drilling.
Percussion drill rigs are based on having a relatively slowly turning rock chisel or hammer hit the rock face, with cuttings being brought to the surface by the return
9 A Ditch Witch is a modified trenching devices srcinally designed for laying telecommunications and
other cables. Selecting a Test
Selecting a Test Area
Area
This is probably the most difficult step. Choosing a ‘typical’ area is difficult and requires careful geological and geostatistical consideration.
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air. The down-the-hole hammer, if fitted, operates by air. Figure 2.6 shows different types of non-core drilling.
Figure 2.6 Drilling methods. (After Hartley, 1994, figure 7).
General Characteristics of BH Samples General Characteristics of BH Samples
The general characteristics of blast hole samples may be summarised as follows:
1. Generally large samples, depending on hole size (although in some small-pit operations, BH hole diameters are less than standard RC holes). BH holes vary from less than 100mm up to greater than 300mm in large operations.
2. Vertical holes: angling is usually not an option. This is a concern if the mineralisation in sub-vertical in “grain”.
3. Relatively rapid drilling (of the order of several hundred metres per shift).
4. No additional drilling cost (BH have the primary purpose of blasting, of course).
5. The drilling pattern will be dictated by blasting requirements.
6. Pronounced sampling difficulties: samples are often collected from spoil heaps, or less frequently via dedicated riffle splitters. However, such riffle splitters are usually not correct in design or implementation.
7. Potential biases by virtue of differential recovery, sample loss, “sub- drilling” etc.
8. Generally only one sample is taken, representing the full depth of the BH hole. We have little chance to take metre/metre samples with this approach, so if vertical selectivity is critical, BH sampling can be a poor option. This has the additional down-side that the variogram often cannot be defined properly in the vertical (down-hole) direction from bench-height samples.
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9. We do not have data from benchesbelow the current mining bench: this reduces the precision of estimation in most cases.
The sampling problems with BH are daunting. The material in the spoil pile is always granulometrically segregated, i.e. there is a stratified variation in particle sizes. Further, the spoil pile is commonly asymmetric, which causes further sampling difficulties. The spoil pile is exposed to the climate and this may result in wind-blown winnowing of finer granulometric fractions. Rain can wash the pile
causing additional degradation of representivity.
The profile of the spoil pile is thus not symmetric, with cuttings collecting in the pile based on random patterns which are influenced by changes in drilling pressures. The grain-size typically coarsens downwards, with the coarsest fragments tending to be deposited at the base of the pile and the fines at the top. This results in a profile which does not facilitate representative sampling. These problems are all exacerbated when the mineral phase of interest is relatively dense, e.g. gold, base metals, uranium.
The sub-drill material (i.e. the material from the next flitch) generally accumulates on the top of the spoil pile, which is particularly dangerous if “grab” sampling is performed.
Approaches to BH Sampling Approaches to BH Sampling
Most larger BH holes are sampled by:
1. “Grab” samplingfrom the pile. This is invariably disastrous due to the granulometric segregation just discussed: a grab sample over samples the fines, usually. It will also be biased because of sub-drilling. A marginally more sophisticated approach is to attempt to scrape offthe sub-drill prior to taking a grab sample: but this is easier said than done and all the other problems inherent in BH sampling are not solved by this.
2. Sector pan sampling. A “pie” shaped sampling dish is placed at the collar prior to drilling. This is better than a grab (or manual “section”) sample, but still does not alleviate the difficulties associated with non- representivity of the cuttings themselves. Furthermore, a very large sector sample is often required, which has significant OH&S implications for those handling the samples. Because large samples must be collected, sampling preparation costs are increased. The authors have seen sector pans used in incorrect ways: the pan must always be radial to the spoil pile.
3. Pipe sampling: a spear or chute of some type is pressed into the drill spoil pile. Often a number of such samples are taken and then re- combined (this is called an “incremental sampling strategy”). Again, problems of representivity of the cuttings aren’t addressed. In general, such approaches can be very poor if there is a significant degree of heterogeneity within the spoil pile (e.g. gold, phosphorous in an iron
BH Sampling BH Sampling
In most cases it is impossible to collect unbiased, precise samples from blast hole cuttings.
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ore mine, etc.). May be acceptable in lower nugget situation (porphyry Cu).
4. Riffle splitting: certainly the best approach, but in the case of blast holes, especially larger diameter blast holes, the mechanics of mounting such a splitter are not simple. When riffle splitters are in use, BH sample cuttings are normally delivered to a cyclone via a stuffer-box at the collar. Often, a Jone’s riffle splitter (which itself may be well- designed) is then mounted directly under the cyclone. This effectively means that only the central slots of the splitter are being used, and, as we will explain later, this results in potentially very serious biases andimprecision.
Other Considerations Other Considerations
There are some other significant aspects to using blast holes for grade control. Unlike Ditch Witching or production RC drilling, BH drilling does not interfere with the mining schedule. However, the time frames are consequentlyset by the
mining schedule. These may place enormous pressures on work efficiencies in a grade control system, especially if adequate (read: time-consuming) sampling and grade control practices are performed.
The time-gap between BH drilling and ore-waste allocation can be very short. In some cases, as mine geologists may have experienced, it can contract tonothing ! The main problem with BH, then, is the inherently poor representivity of the
cuttings and the difficulty of correct sampling. If the sampling error is too high, ore/waste misallocation may increase to unacceptable (perhaps economically calamitous) levels. In this case, the sample becomes “dangerous” in the sense that it incorrectly represents the sampled ground.
Reverse Circulation Drilling Reverse Circulation Drilling
RC drilling is where the drilling fluid travels down the outsides of the rods and returns with cuttings on theinside of the rods. Dual tube RC—which is what is meant by the everyday use of the term RC—generally means that the fluids pass down to the face in the annulus between the drive rod and an inner, light-weight rod. The cuttings are returned to surface inside the inner rod.
The principle advantage of RC over open hole methods is that the possibility of smearing is significantly reduced. Originally, RC systems were used with conventional down-the-hole hammers (DTHH), which required a cross-over sub immediately above the hammer. This ‘sub’ is a connection that took clean air from the annulus to the centre of the hammer and then diverted the air and cuttings returningoutside the hammer back to the inside tube. This meant that, in effect, standard circulation occurred for approximately 1.5m back from the drill-face (Hartley, 1994).
However, in recent years ‘face sampling hammers’ have quickly gained popularity. In these systems, reverse circulation is maintained to the drill-face.Unless there are
RC Drilling RC Drilling
Is usually significantly better than BH sampling, but we must justify the additional cost and possible interference in mining schedule.
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significant problems with ground conditions face-sampling systems are to be preferred over “conventional” RC.
General Characteristics of RC Samples General Characteristics of RC Samples
The general characteristics of RC samples may be summarised as follows:
1. Typically, smaller samples than BH, depending on hole size (although in some small-pit operations, BH hole diameters are less than standard RC holes). The industry standard for RC is about 5.5” or 140mm.
2. We can drill RC at angles from about 45 degrees to vertical. This is a benefit if the mineralisation in sub-vertical in ‘grain’.
3. Relatively rapid drilling (of the order of several hundred metres per shift).
4. Extra drilling cost. On a particular project, we calculated this as about +15% relative to BH in terms of the total cost of grade control for a given drill spacing.
5. Unlike BH, where the drilling pattern will be dictated by blasting requirements, we can plan our RC holes taking geology or other grade control factors into account.
6. Fewer sampling difficulties compared to BH. Samples are generally collected by dedicated riffle splitters, but unlike the case of BH, such riffle splitters are more frequently correct in designand
implementation. We discuss this further, later.
7. Potential biases by virtue of differential recovery, sample loss, “sub- drilling” etc. are reduced dramatically compared to BH.
8. We have the opportunity to take metre/metre samples with this approach, so if vertical selectivity is critical, RC sampling can be a much better option than BH. This has the additional advantage that the variogram can thus be defined properly in the vertical (down-hole) direction, unlike BH.
9. We can have data from benchesbelow the current mining bench. This increases the precision of estimation in most cases, and allows better planning, scheduling and geological modelling. It also gives us valuable forward information for designing mining or drilling contracts.
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Approaches to Sampling RC Approaches to Sampling RC
Good sampling of RC is generally possible, so long as samples are dry. A well- designed, properly-used, riffle splitter is obligatory.
In the case of wet ground, riffle splitters may clog and cause contamination problems. In such cases, due to the time-scales involved in grade-control, drying of samples followed by disaggregation and riffling is generally not feasible.
Some kind of rotary splitting or multiple pipe sampling approach may be necessitated. Be warned that these approaches are generally significantly inferior to riffling, and care should be exercised in choosing particular rotary splitters or in designing pipe sampling approaches.
Sampling and mining of wet ore creates many difficulties, and the best solution is to have adequate de-watering programs in place. Quality grade-control is an additional argument for implementing de-watering.
The higher quality of RC samples often means that a sparser sampling grid can be used, which can virtually negate the additional drilling costs.
Automation of Sampling Automation of Sampling
The sampling of RC can be automated with the right equipment, for example an integrated cyclone/tiered riffle system produced by Metalcraft and other manufacturers. This reduces costs and avoids the OH&S problems of handling large sector pans, etc. A diagram of the Metalcraft device is given in figure 2.7. RC for grade control is becoming a more widely used technique, especially in large, low-grade operations. Because high-quality data results in a better, more confident definition of the variogram, kriging will be more efficient than with BH data from the same deposit. In addition, this better knowledge of the variogram makes simulation approaches more feasible and reliable.
Wet RC Wet RC
As a basis for reliable resource estimates, wet RC is questionable. “Wet RC samples are natures way of telling us that we need a diamond rig”!
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Figure 2.7: Metalcraft automated splitting system.
Rules of a Good Riffle Splitter Rules of a Good Riffle Splitter
1. Even number of slots. Hard as it is to believe, riffle splitters with an odd number of slots do exist!
2. At least 12 slots.
3. Slots at least 1.5 x the 95% sieve pass diameter. This prevents excessive “hanging up” of chips.
4. Parallel slots.
5. Slots must be of equal width (watch out for narrow “end” slots).
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7. Keep splitters well-maintained: broken splitters allow communication of material between slots or result in spillage.
8. Keep it clean: filthy, muddy splitters are prime candidates for causing contamination.
Ditch Witch Sampling Ditch Witch Sampling
A summary paper on the subject of Ditch Witching is Bird and Archer (1987). Ditch Witching is suited to sampling of relatively soft materials, for example in open pit operations where ore is highly oxidised (saprolitic). The machine is tractor mounted and was srcinally designed to lay communication cabling.
Depending on ground conditions, trenching rates can vary from 0.5m/min to more than 2m/min. Size range of sample material ranges from very fine (10 micron) up to about 20mm, and is dependent upon rock competency. The material looks much like RC cuttings and has roughly the same kind of granulometry. Trenches can be cut to about 1m in depth, and are usually about 150mm wide. Therefore alarge sample mass is obtained, which is good.
Displacement of sample along the trench is characteristically minimal but can be monitored visually.
The trenching spoils run along either side of the excavated Ditch Witching trench. Sampling of these is by various methods, the best probably being an oblique, longitudinal sample along one of the spoil lines. Such a sample can be collected by a piece of halved polypipe (this was the method in use for sampling of lateritic ores at Boddington Gold Mines in Western Australia).10
The depth of the trench leads to the most important qualification about Ditch Witching: if there is a strong sub-horizontal control on mineralisation, the
geometry of sampling by Ditch Witching will be potentially inadequate. Channel Sampling for Open Pit Mining
Channel Sampling for Open Pit Mining
In some open pit grade control sampling situations, sampling of drillholes or Ditchwitch may not be feasible. An example of this is the Waihi Gold Mine in the North Island of New Zealand, where the large proportion of epithermal clay alteration minerals (some of which are ‘swelling’ clays) make the physical process of drilling holes very difficult. In fact, at Waihi, RC drilling has been tested and found