Wet RC sampling is not recommended. The main problem is that you will introduce significant bias in your results due to fine and lighter materials being washed away.

Are we talking about wet samples or just drilling below the water table where the air from the RC rig can keep the sample dry (apart from rod changes).

Why is the sample that wet? The RC process, if set up correctly, should keep it dry enough to sample.

I've seen wet RC (air core) used extensively for mineral sands work in WA. The system involves injecting water to make slurry as the drill advances and a RSD is used take increments from the stream. The method seems to work OK as long as not drilling too far below the ground water table (there you run into serious cavitations problems) - sample mass can be used to monitor the quality. I've also seen similar systems for deep iron ore holes - again the trick is to use a vezin device to cut the stream. Here diamond core holes (1 in10) were used to confirm that the RC was producing similar results to coring.

We are studying a way to do RC in an underground mine, but to do that is necessary the use of water. As I´ve read the comments above all of you are against it, when working with water. I think the problem we found was where to stock the sample (cloth bag), because we lose fines in order to dry it. While drilling there is a cone splitter where the sample is stocked until the maneuver is over, once it´s done there is a flush which 50% goes to the cloth bag and 50% is waste. We haven´t conclude the feasibility of the RC in the underground and haven´t tested the bag and the gold content. But what is your opinion?

I am told the decision at the time was to not try to keep the samples dry as the high air pressure necessary was blowing out the RC collar. Is the main concern a non-representative sample or also smearing down hole?

Mark for iron ore you saw the use of a stream cutter like a slurry sampling device in a process plant? That is an excellent idea, but it would be difficult again to get a representative sample in a nugget gold environment.

Perhaps the paper by Clark & Carswell is of use: Clark, F and Carswell, J T, 2014. Using reverse circulation drilling to improve sampling in a complex underground gold operation – an innovative approach to underground grade control at the Sunrise Dam Mine, Western Australia, in Proceedings Ninth International Mining Geology Conference 2014 , pp 143–152 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Have you ever considered Sonic Drilling for this job? You will get high quality samples, also below the ground water table. We have many customers who use our systems and are successful. The different samplers make the method suited for all overburden formations. The drilling process is not as quick as RC but the quality will make it possible to give a better indication of what depth the gold can be found. This will result in lower mining costs.

Can you give some more information? What are the typical sampling depth, formation and start of groundwater?

We had significant problems with dilution and mixing across zones with RC. We've already positively considered Sonic for our next program. How deep can your sonic drills go?

Anglo Gold's leadership into underground RC drilling is admirable and a function of open pit success. The issue is not whether it should be done but how to guarantee safety. Current steps ensure safety (water) and that helps establish a platform to build on. But this is only the start.

I am reviewing a previous practice and not contemplating a drill program. However the water table was basically at surface and the holes were routinely drilled to a nominal 150m within a volcanic sequence.

I think the comments above give you a fairly clear consensus. I generally don't like wet RC, particularly for gold. If you can't keep the air pressure up enough to keep the hole dry, time to change methods. The risk is not only loss of fines, but smearing/random recovery in a coarse gold environment, particularly as the hole gets deeper. You'd need a fair bit of QC via twinning with core to convince yourself that the data was representative.

Application for GC is possibly slightly different inasmuch as holes are shorter, close spaced patterns, and you can possibly give a little on individual sample quality so long as there is not a rank bias. Just like blastholes-not ideal, but if that's what you've got to play with.

Water injection for dust suppression is also a slightly different issue as that can happen post exit of hole (if it really must be done).

Assuming you can recover close to all of the material produced during the drilling process, the next question is do you run the material through a sampling device in its raw, unmixed state - straight from the drill (continuous sampling) or do you mix the sample material, prior to running it through a sampling device. Up to 20% of the R.C. material may be rock chips ranging between 2 mm to 10 mm.

One rock chip, containing let’s say, five pieces of gold has a much better chance of going to reject than five separate pieces of gold from a rock chip which has been crushed. The sampling device takes a massive bet on the reject, so if you can separate (via crushing) and spread (via mixing) your bet - you minimize the possibility of the reject taking most if not all of the gold. So no, I would try to avoid continuous sampling in this case.

It sounds to me as though the driller is drilling for metres, not for quality samples. He should be adjusting his air, penetration rate, rotation, etc to ensure the integrity of the hole and consequent sample quality. Collaring a hole is not a difficult job to someone who knows what they are doing and is critical to ensure a quality sample. I've seen this time and time again, too much air pressure and too much air volume from surface. Get it wrong at the surface and you'll never recover the hole. Get it right from surface and you'll maintain a quality, dry sample, assuming the drill is equipped with all of the components required to drill properly in this application. Ask the driller if he is using a blow-down valve?

Continuous wet RC is always going to be poor - I remember attempts in the 80s to collect all the water and let it settle and it was a failure. If the driller has enough air he/she can usually clear the water away from the drill face and get passable (dry) samples until rod change and then they have to blast it clear again with the loss of the rod change sample and with some risk to the crew as mud and goo go everywhere.

It was the geologist's decision not to try and keep the samples dry. He/She believed at the time that it was the best way to maintain a constant type of sample in a very high water environment and reduce cross contamination by build up of wet and dry in the sample capture path.

Clark & Carswell is of use: Clark, F and Carswell, J T, 2014. Using reverse circulation drilling to improve sampling in a complex underground gold operation – an innovative approach to underground grade control at the Sunrise Dam Mine, Western Australia, in Proceedings Ninth International Mining Geology Conference 2014 , pp 143–152 (The Australasian Institute of Mining and Metallurgy: Melbourne)

Be aware in the US we are forced to increase water content due to OSHA etc rules regarding dust. However, IF the pumps on the rig lack adequate horsepower to produce sufficient air, the venturi devices are not properly installed, and the cyclone is not properly set up, forget getting any useful samples from RC drills. Excess water leads to smearing of samples, dilution of samples from richer zones, and inability to properly clear the rods at a rod change. IF groundwater levels are high, returns are going to be impossible.

We've tried RC (wet!) on locations where outcrop assays indicate high values and the RC holes indicate garbage. Both surface and underground adit/shaft samples (random) indicate economic ore where the RC holes did not. IMHO, RC drilling in any instance is a waste of money in any circumstance. Needless to say, we fired the RC contractor (for numerous reasons).

Core it, Sonic (sleeved core) drills it, or uses a mud-rotary (i.e., oil field set up) with a screen and "lag" the samples, rather than reverse circulation.

OSHA is becoming an increasing impediment to clean samples.

This is not a small gold mine. It produces over 1 million oz of gold per annum. The point to really understand is what is the correct set up for "balanced" RC drilling/sampling? Done properly, you will receive 100% collection efficiency for sampling, maintain continuous, dry & highly representative samples (even when drilling below the water table) and emit zero dust to atmosphere from anywhere around the rig. And no water is used to achieve any of this. Water is the enemy of samples. The fact is, for the same money, RC drilling will provide 20 times the volume of dry, highly representative, homogenised duplicate samples in half the time of diamond core. But, like anything, do it badly and you'll get bad results. The key is to understand how to do it well.

Is the main concern a non-representative sample or also smearing down hole?

Regarding your earlier comment I would say yes to both. I've seen some awful smearing and many high grade intersections which when drilled dry have evaporated to mildly anomalous numbers.

What measure of precision between RC field duplicates is acceptable e.g. HARD that the RC sample is Representative of the material in-situ. Dust loses may also introduce a systematic sampling bias that neither is nor measured by duplicate sampling. In the presence of coarse gold what size sample is acceptable to minimise the Possion Effect.

Depending on site conditions and targets, sonic drilling might be the clever answer to RC sampling problems. RC is all about cheap meterage. In some cases this may pay, but usually it is safer to get confidence in samples and results.