Blast Holes and RC drilling have different objectives and use different metrics. Blast holes are very short hole on a very small grid (short spacing) compared to RC drilling. But the most important aspect is that Blast holes are oriented to meet explosive and other mine planning objective. RC drilling is designed to meet geological objectives. RC will be oriented to cross mineral bearing structure. Blast hole are often design to be parallel to mineral bearing structure. Hence Blast holes may miss or hit one specific mineral at a time. This will increase sampling variance but the large number of samples in blast holes may make up for the lack of "geological control" in the sampling.Mine planning is generally based on core and/or RC drilling.Ore control is generally based on Blasthole data.Whether the results can be compared fairly or not requires some knowledge about the geology.

A common question that is raised amongst the mining geology community! I have always been a strong believer in RCGC, because of the low chance of contamination, better geological/grade definition, and the ability to have grade information several benches ahead of mining. This is especially relevant in narrow ore bodies and/or flitch mining operations. However saying that, large bulk tonnage, disseminated ore bodies on a cost-benefit scenario may be suitable for blast hole sampling. As a basic first step, you could do a trial by drilling out a defined area with both BH and RC, interpreting the ore separately and see what the difference is in defined ore vs. cost. The ultimate test would be a batch test through the mill, but unfortunately at most mining operations convincing management to do this is difficult.

The drilling of Blasthole has its origin in equipment designed to perform only holes for blasting, cutting generated by this equipment has a form and amount that is difficult to sample, even today, and there is a technique that guarantees their representativity. Sampling errors are usually around 15% from the original sample and duplicate, but it is difficult to ensure accuracy, the only way is to increase the mass and increases, to approach the real grade. The Blasthole negative has the following elements:

Sample contaminated pasadur
Sample contaminated by irregular walls of the hole
Geometry of the cone Blasthole Irregular
Irregular size distribution, cone Blasthole
Poor sampling technique, with errors of delimitation, extraction, etc.
The reverse circulation drilling, remove all items negatives, listed above.

The Blasthole drilling is a free control laws and reverse circulation drilling has a cost that is hard to take because of its high cost.

Some mining companies are conducting studies to change the process Blasthole a reverse circulation in grade control, this study should account for the profits that each contribute to the process and see what that adds more value.

The main issue is the cost. The RC is just for a geological purpose and is an extra cost with good results, on the other hand, the BH could be used in grade control sampling at the same time that is used for the blasting process. If you want to have a good sample, you have to control the contamination related to the drilling rig and related to little deposits in the surface generated for mobile equipment and you have to collect several increments for each hole to increase precision (more than 12).

I have a powerpoint presentation that explores the cost impacts of Blast Hole sampling to RC Grade Control to Diamond Core drilling that I could send you. The numbers are quite staggering. I struggle to justify blast hole sampling on any level for many reasons, some of which have already been mentioned but there are many more.

A complex question! There are papers and case studies galore. I have worked with both, and in my experience (in narrow vein gold) the main negative for blast hole sampling was the contamination from an open hole, and the loss of fines leading to biased sampling. In terms of using the results, vertical blast holes did not sample perpendicular to steeply dipping ore bodies therefore the variography was poor quality for grade estimation.
I am sure there are many people out there who have just as many examples where blast hole sampling works well.

There is a mine manager who thinks he can mine without a geologist. In such a mine, blast hole sampling will far outweigh the RC sampling, but can we scarify quality with cost? I think if we want to control grade at this level, then i will strongly advocate for RC drilling for the above reasons.

I cannot agree more that the bias of collected sample from Blast Hole compared to RC is great to say the least. Contamination of open hole drilling alone is a concern but when you also consider sample recovery it must surely prove that blast hole drilling should be used for its purpose and that is to drill a blast hole.

I actually had a major mining company that converted to RC grade control tell me that, as a result of it, the geology and production departments now get on with one another. Point being though, that the mill superintendent now receives ore grade that the geologist tells him/her they will. They are also now able to selectively mine based on the commodity price of the day.

I've had a thought, in regard to the powerpoint presentation, I'll put a link to download it from the Progradex website (http://is.gd/Xux8qG). I've got a section on the site called "Did You Know" and will put it in there

That sounds great. As for production and geology getting on I'll believe it when I see it!

Very good question that has been going around in the mining industry for some years! Personally I've blasthole sampling to be a total disaster in some instances (usually gold) and a very successful grade control system in others (usually iron ore).

There are always tradeoffs between:

The concentration of variable of interest (gold is very low, while in iron ore iron is very high)
The continuity (and orientation) of the geology and mineralisation (gold short and iron ore usually long)
The cost of the sampling (blast holes you get for free, RC drilling you must pay for).
The quality of the sample (small diameter holes in gold deposits often poor), large diameter holes in iron ore - not to bad as the iron ore miner have data to show they can do a pretty good job of predicting the grades of large stockpiles

No easy solutions - geology first, sampling method second and good the precision and bias critical

I've had a lot of interest in this presentation and just want to clarify something. We are not suggesting that conventional open blast hole drilling rigs be replaced with RC rigs to drill the blast holes. We are advising the use of RC rig(s) in conjunction with the existing blast hole fleet to carry out multi-bench, angled RC grade control holes ahead of the blast hole rigs. The RC rig identifies the various grades drilling, say, 3 benches deep (keeping data about 6 months ahead of production) and this information then guides the blast hole fleet into the next area to mine. It is becoming standard practice, particularly in gold, but more and more every mineral going, including iron ore, to great success where some companies have reported now being able to selectively mine based on the gold price of the day.

Drilling blast holes with RC rigs has been tried before and doesn't work. The presentation explains this better but to summarise:-

The first 2m of the bench is broken ground so recovery is poor whether it be drilled conventionally or with RC. On 10m benches 20% of your data is lost straight away, hence one of the reasons why the multi-bench RC hole is used.

RC drilling penetration rates are much slower than conventional. To keep up with production you would ultimately need more rigs anyway (about twice as many).

Blast holes have to be vertical and often miss vertical structures. To drill with RC but vertically is a missed opportunity that does not optimise its ability to vastly improve sample quality with angled holes alone.

Sampling data bench by bench cannot often keep up with the production demands to mine. The last bag sample on the bench leaves for the lab but the production superintendent is ready to blast. I've seen it so many times where mining commences before data is received. Everyone is out of sorts and hates one another. Multi-bench RC Grade Control eliminates this completely because the data is so far ahead of production. Geology really does then guide production, as it is meant to be. The mill then receives the right stuff as does the waste dump, profits rise through efficiency and everyone is happy. (It’s a beautiful thing and I'm tipping myself to be invited as Best Man at the first Geology/Production wedding).

Existing blast hole crews would need to be retrained completely plus new ones to take the extra load of the additional rigs that are required. There is a vast difference between blast hole drilling and RC drilling.

Often the biggest issue! Consider the political impact of such a change. It will meet operator and managerialresistance, particularly in an existing mine that has been operating in a certain way for a number of years.

So, my advice, do not consider drilling your blast holes with RC but do consider independent rig(s) to carry out angled RC grade control sampling ahead of production rigs. Geology is geology, production is production. Combine the two at your peril.

Your comments are really very valuable.
The one thing that does tend to be left out of these discussions is the deposit and operation style. It's hinted at but isn't discussed in detail.

If your deposit has a high nugget, the ore is tightly constrained geologically and you are using relatively small equipment such as narrow vein gold or shear hosted deposits, Blasthole Grade Control will result in misallocation of ore to waste and vice versa. RC drilling will pay for itself many times over.

If you've got a large deposit with low nugget, gradational contacts and are mining with large rope shovels or similar it is possible the cost of RC drilling may outweigh the benefit. It still has an important place for defining high grade zones, complex boundaries and infilling the reserve drilling where it is required and should be budgeted for, but a satisfactory result can be obtained for much less money.

The technique of the Blasthole sampling will also be a big factor in blasthole sampling success. As mentioned elsewhere in this group knowing your samplers is very important. Employ samplers just to fill that role under the geology departments supervision, relying on the drillers is not good enough as they are paid to drill holes not take samples (regardless of what the managers etc. say.) Test the repeatability of the sampling technique, ensure regular field duplicates are taken and checked constantly. And when you start, experiment. Test multiple sampling techniques such as on rig samplers, augers, spears, scoops, pie trays and anything else you can think of in as many different parts of your deposit as possible.

As a final note the first four lines of point 4 of yours are the gospel. Blasthole samples will stress your geos, production will push for turnaround regardless of the site protocols. The last thing you need is the Geologists deciding what material goes to the crusher and what waste is with the Production supervisor standing over them demanding the dirt.

I've worked on both sides and in RC operations after a blast all that is required is the ore block to have the ore movement measured and applied and the blast marked up. In a Blast Hole operation normally it involves chasing up the lab for results, checking QAQC, modifying domains, running the model, block out the ore, apply the movement and then mark it out.

From an operational point of view the down side of RC is ensuring that operations allow the drills to drill where they are needed ahead of time. Drill sites are a magnet for haul roads and infrastructure such as pipes, but this is usually much less of a headache than waiting on samples.