RE: RC better explainedVery good message Sara but I wanted to add a few points
Reverse circulation (RC) drilling utilizes a solid bit (usually tri-cone) to produce a hole and delivers rock chips to the surface for subsequent analysis.
RC commonly uses a hammer bit which leads to its greater rates of penetration and hence lower costs of drilling.
The tri cone is used is softer rocks and or clay. Depending on drilling conditions the hammer can produce larger chips but tends to shatter many crystals. The tricone is less shattering but grinds the rock into smaller fragments.
Neither bit is particularly good for crystal survival.
So why use it? Hole size and sample volume. RC drills a normally 5" - 6" diameter hole. They are capable of drilling up to 24" (tricone only).
This type of drilling uses a large, rotary drill and a compressor to blow the rock chips produced by the bit to the surface. The air is blown down the annulus between the rock wall and the drill pipe or, more commonly, down a special annulus in the drill pipe.
When the air is blown down the pipe and comes up outside the the method is refered to as rotary drilling not reverse circulation. Reverse circulation uses a dual walled pipe which blows compressed air (or air fluid mix) down the outside of the dual walled pipe and is returned up the middle of the dual walled pipe.
The air and rock chips are then blown past the bit and up inside the drill pipe to the surface. Normally the air exhausts through a centrifugal classifier so that the rock chips and dust will be captured in a sampler.
The dust collection causes a small concern during the sampling. If the dust is lost, there is a minor weight loss of the total sample and that can affect tonnage calculations (very minor). It can also be a possible source of minor contamination between samples.
The reverse circulation drills are all truck mounted and so must have good road access to the drill site therefore are no good for remote drill camps.
That's not necessarily true. The drills and associated compressors have been skid mounted and dozers used to skid them in place. I came across one such unit in the UP of Michigan. Dow Chemical was fronting for Debeers and had moved in to drill a kimberlite from the String of Pearls intrusions. The drill and the 2 supporting D9 dozers were stuck. Bit of a mess.
More portable units are available that can be transported by helicopters. It is my understanding that the RC rig in use by MTX is helicopter supported/mobilized.
For this reason, may drill campaigns start with reverse circulation drills and move to diamond rigs if the mineralization warrants it.
I differ a little here. The two methods give different sample sizes. Therefore the desired end product determines the method used. In a dessimenated deposit the RC gives a vastly superior sample. In a vein type deposit the core gives a much more accurate (but limited in size) sample.
So Cantex's Al Hariqah deposit is most efficiently drilled by RC (because of the larger sample size).
The same sample size advantage applies to sampling kimberlites. The diamonds are dessimenated in the kimberlite not clustered or in veins.
The rock chips created by the drill bit are delivered to the surface generally in the order in which they are produced but not always. Therefore, RC drilling is not as accurate as diamond drilling. There is no way of knowing with certainty where the chips are from.
That's not an entirely accurate statement. Often a mud wall is used to follow the bit. It essentially eliminated any fall behind the bit. With the largers sample size any error introduced is minimized. The increased size with RC allows for a more accurate representation of the ore body. A larger sample equals a better sample, at least in this case.
Also there is a tendency for heavier minerals (eg gold) to fall to the bottom of the hole or be caught in elbows and curves in the drill pipe. As a result, assays from RC drill samples may understate the amount of mineralization present in the rock.
I wouldn't say there is a tendency, more of a remote possibility. It is possible that minor amounts of material could become "lost " in the system but again the size of the sample minimizes that error.
Different drilling conditions, primarily rock hardness and ground water within the hole are major factors and can cause either an underestimation or an overestimation. Again the larger size of the RC sample comes into play.
at greater depths (+400 meters?) or high water flow (+25 gal/min) core may be more accurate.
BP' Minerals drilling of the Ridgeway Mine in south Carolina is a good example of the advantages of RC over core drilling.
RC drilling has commonly been used for kimberlite/diamond evaluation purposes because of its larger sample size.