I have personal experience with the disastrous consequences of believing in personal immortality and thinking I understand how to drill rocks. Rocks can be very nasty substances at times.

There are many different types of drills and all have their place in the universe - if only to enrich the people who develop and build them. There are diamond drills which produce a continuous core of rock (in theory); reverse circulation drills which produce chips of rock which can be sampled under the assumption that they come to the surface in the order in which they were produced; augur drills which are used for semi-consolidated soils and produce a core (hollow core) or loose samples (solid core); churn drills which are used to sample gravels by pounding a steel pipe into the ground and then pulling out the material trapped inside the pipe; and sonic drills which use sound waves to consolidate wet deposits like tailings ponds and capture the soils in a tube. By far the most common are diamond drills and reverse circulation drills.

Diamond drills are so-named because they use a hollow bit which is impregnated with diamond bits. By applying pressure on the bit and rotating it a core of rock is carried up into the core barrel. Every couple of meters, the core is broken away from the rock at the bottom of the hole and the core barrel is pulled to the surface where the core is removed and stored in a core box. Like most things, this sounds pretty simple and as always, the devil is in the details. There are a million things that can go wrong while diamond drilling which leaves the geologist scratching his head. No wonder they drink so much. Job tenure is on the line when the company spends $100 to drill a meter of rock only to get a few small pebbles. This is not the place to describe the myriad of things that can bedevil a perfectly planned and executed drill program. But suffice to say that you should always ask your brother-in-law what the core recovery was in his drill program. He will always say, “On the average we got over 85% recovery” which is pretty good. But you are not interested in the average but in the particular - like what was the recovery of core where the gold is supposed to be. If he hums and stammers you know there is a potential problem.

The other primary drilling technique is by reverse circulation drilling. In this instance a very large truck is loaded with a tower, drill head, compressor and motor. The drill bit is usually of a tri-cone construction (that’s right - 3 cone shaped bits) and a bunch of air blown into the hole to capture the rock chips. This type of drilling is usually a fraction of the cost of diamond drilling but there is controversy surrounding the validity of the samples that are obtained and, because it is truck-mounted, you must have a road to get to where you want to get and building such can be very expensive. The term “reverse” is used in the drill name because someone forgot how to spell “perverse”. Not really. In this case, air is blown down the outside of the drill steel (between it and the wall of rock) and the air and rock chips are carried to the surface on the inside of the drill steel. As the air exits the drill with great billows of dust the rock chips are captured and put in bags for subsequent assaying. Unfortunately there is no way of knowing what the recovery of chips is.  Usually it is over 100% because the wall of the drill hole caves back a bit and extra rock chips are created and who knows where they are from. If this was an oil well drilling operation someone would invent a method for continuously measuring the diameter of the drill hole to determine the extent of sloughing. But this is the mining industry so everyone just says, “Bugger it... close enough!”

The other types of drilling are for fairly specialized cases and because a solid sample is obtained directly can be quite accurate. Of course it often happens that the sample won’t come out of the pipe or if it does it comes with a whoosh that carries it off the sample catcher and you are reduced to picking up as much as you can and putting it back in the sample box. But this causes contamination you say. Yes, it does and no one likes it but it sometimes happens. As an investor you had better dial this bit of reality into your evaluation of the press releases.

As has been stated previously, the mining industry lives and dies on the accuracy of the samples taken. And the most important samples are taken by drills so it is important to understand the drilling process ensuring that good questions can be asked. As with anything in life it is best to find out a bit about the company doing the drilling to make a determination as to the veracity of the sampling results. If the drilling company is named “JimBob’s Drilling and Cattle Ranch” you might be wise to question the results. Having just come off a very frustrating diamond drill program I am currently very partial to reverse circulation drilling but I might be biased.  

Now that there is a lot of data and it seems to point to some interesting mineralization it is time to try and represent the data accurately in space. So we come to the concept of the geological model.