Difference between diamond and RC reverse circulation drilling?

Difference between diamond and RC reverse circulation drilling?

 
A diamond rig should produce intact rock core which can be examined and orientated for geological features, whereas an RC rig just produces rock chips. For geo information a diamond rig is far more reliable, but more expensive.

RC drill rigs break up the core so it comes up as rubble, while with diamond drill rigs the core comes up intact (it cuts out a cylindrical slice intact). Preservation of the core in intact form gives the geo's additional information about the ore bodies and can also help them to better understand the angles/orientation of the structures they are drilling into. This is also more useful for metallurgical studies as well. As I understand it diamond drill rigs can also drill a lot deeper than RC rigs and are better for certain types of rock/conditions. I believe diamond drilling is typically more expensive than RC drilling.

RC drilling is basically a percussion drilling method, utilising a rotating bit and high-pressure air (in the order of 700-800psi). Sample quality is usually not an issue, as the chips are forced up an inner sleeve inside the rotating drill rods, and their velocity slowed considerably upon entering the cyclone. The offsider working the cyclone then collects a bulk sample in a bag every meter which are stored on the ground in 20m rows, and also collects a smaller sample from the same meter for lab analysis in a calico mining bag.

The Geo will then wet-sieve every meter, and chip tray and geologically log the entire hole.

Diamond Drilling is very useful for eliciting structural information about the dip and plunge of the ore bodies etc, (provided the drillers have orientated the core properly so you know which is the "bottom" of hole) as mentioned above, but also makes geologically logging the hole much easier as you can "see" the geology in front of you (a bit like working underground I guess!). Costs can vary from about $170-200 per meter, so yes, it's pricey.

When I've logged core, the structural Geo will usually come along with various tools and obtain alpha and beta measurements, which basically can be worked up into 3-d visuals of structures in space, such as shears, veins, faults etc. Very useful when you want to know what that damned ore body is doing at depth.

Until fresh rock is reached, it's usually not possible or practical to extract whole core, as the oxide layer is mostly clays and highly weathered bedrock. The drillers can "tri-cone" or "clore" their way to depth until competent rock is reached and they can start coring, and until then the ejected material from the hole is not logged. Sometimes RC rigs are brought in to drill to the base of partial weathering, and then a diamond rig brought in to re-enter the hole and start coring straight away, this is called diamond-tailing.

For both RC and Diamond drilling, it is very important to get the holes down-hole-surveyed post-drilling, which specialist contractors can do, to see how much the holes have deviated by. Drill holes are never straight, they can bend a hell of a lot and this has as much to do with the skill of the driller as it does to the local ground conditions. I've heard stories of very deep diamond holes actually daylighting at the surface 1-1.5km away from where they started drilling.