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Sludge Holes and Mixing with Chip Sampling (6 replies and 1 comment)
Sludge’s i.e. cuttings from core drilling may be used as an indicator but they are notoriously unreliable. The light fine minerals are washed out and the gold should concentrate as in panning. I have used them and never found them very useful. They do not comply with any kind of QA/QC. Therefore, they are a waste of money for that. In other words, you should see the veins or structure bearing gold in the core.
One famous myth is that gold can be carried in the gouge found in fault and open fracture planes. The gouge is washed out during drilling and could be caught in the sludge, as they say. Well, if you project depends on that gold, you are in deep trouble. Sludge will also smear the legitimate gold value when they cross a vein, making the gold bearing structure look wider. I have seen people use this to tamper with the gold bearing structure volume.
All together, I would keep away from it in gold especially. It is a misleading indicator at best. Other than that, if you drill minerals that are higher grade like massive sulphide or iron ore, you could use it as an indicator. But the sludge sample will suffer the same shortcomings I already mentioned. And they are simply not required in those cases.
On the other hand, if you consider RC (Reverse Circulation) drilling or other whole core percussion drilling. That is a different business. RC is well known and tested. It is best used in shallow, dry and soft rock like laterites and saprolite in tropical countries.
Blast holes are a different business. Track drilling can be use for short samples at surface in a mine or not. Again, the entire rock is crushed down the hole and recovery is maximized for sampling. However, blast holes are not drilled with geology in mind. They can easily miss vertical narrow veins.
These holes are near horizontal, quite short and are targeting disseminated mineralization in some areas and vein in other. Thanks for the comments so far. Basically confirmed my initial thinking on the matter be very cautious when using them.
Basically this is just catching the cuttings from an underground jumbo or airleg. For grade calculation purposes the sample is poor and should not be relied upon. However they can be useful where there is some ambiguity in the geology interpretation, assuming you can visually determine between ore and waste. I am thinking of a situation where you have anastamosing narrow veins and may not be confident that the interpretation joins from level to level in a proposed stope. If you can confirm the continuity from a few sludge holes this is much better than blasting a stope that tries to join two unrelated lenses! My advice is that the geologist needs to be in attendance while the drilling is undertaken. You need an understanding driller. You need to have sorted out a sample collection device. (I believe I have some drawings somewhere) And be prepared to get very wet! So despite all the shortcomings I think it has a place but as a method of last resort.
I worked in a high-grade narrow vein deposit and we collected sludge samples from the long-hole production rig for grade control; as earlier said, we got wet! Because the ROM stockpile was one of 5 sources of blended feed to the mill we never really knew how well it performed; but with 20/20 hindsight these are the issues we never got under control
Contamination from the open hole
Bending of the hole (15m interlevel distance)
Clustering of the data near the hole collars (rings were fan shaped)
One thing that did work well for us was geophysically logging the hanging and footwall holes because the ore was so sulphide-rich.
I would agree with you using the sludges for assay results is a very dangerous practice. However, they are useful to help define the local geology (veining and alteration zones) in ore drives. Used in conjunction with drive mapping, wall sampling and diamond drilling they can help define the ore shape that the engineers require for stope design. Moreover, as mentioned earlier a suitable geological volunteer must attend to log the cuttings during drilling.
Busy reading an interesting report for a Narrow Vein Greenstone belt mine which uses 'sludge holes' for grade control and sampling purposes. To be honest, I have never encountered sludge holes before. I gather these are pneumatically drilled holes akin to blast holes with the resultant chips being collected for analysis. If this is the case, then I assume relatively small volumes are extracted with variable to poor sample integrity?