Sampling and drilling

— This is the first of a series of excerpts from the ninth edition of Mining Explained, recently published by The Northern Miner. The book sells for $30, plus $3 for shipping, and is available by calling 1-800-668-2374. E-mail: northernminer2@northernminer.com

Sampling is the process of taking a small representative portion of a larger mass. By analyzing the sample to determine the concentration of metal it contains, the potential value of the larger mass can be determined.

The first samples taken from a mineral showing are called grab samples. Prospectors and geological field crews gather grab samples from outcrops, road cuts, trenches or river beds. These rocks are selected specifically because they appear to contain a significant amount of metal, so they are not considered representative of the outcrop or road cut from which they come.

In the field, grab samples are gathered, their original location is recorded, each rock is labeled, and the most promising ones are sent to a lab for metal analysis.

If worthwhile or significant amounts of metal are present in such grab samples, channel sampling may be warranted. In this sampling technique, bedrock where the sample was taken is exposed as fully as possible, typically by using a backhoe or some such piece of earth- moving equipment.

Next, the outcrop is hosed down with water and, if a zone of mineralization is revealed, representative surface samples are taken at regular intervals across the exposed zone. These samples are usually cut with a portable circular saw equipped with a diamond-studded blade, leaving a linear channel across the outcrop.

The surface channel is the most desirable type of sample. It is normally cut about 10 cm (4 inches) wide and 2 cm (3/4 inch) deep across the supposed ore zone. The chips of rock removed are carefully collected, marked and bagged for analysis.

Chip samples are sometimes taken by the geologist or engineer for a quick approximation of contained value. Random pieces are quickly knocked off the outcrop with a hammer and chisel, with an effort made to take representative amounts. Chip samples cannot be relied upon fully, so they generally do not enter final mathematical calculations of possible reserves.

It is highly desirable, but not often practical, to space surface channels at regular intervals along the mineralized zone. This obviates one mathematical calculation in the interpretation process.

In certain circumstances, particularly when sampling kimberlite rock for diamonds, it is useful to collect a bulk sample, which may range from a few hundred kilograms to several tonnes in weight. It is important for a bulk sample to be representative of the zone, since this material can be used later for definitive metallurgical test work and grades.

Diamond drilling

Just as one should not judge a book by its cover, surface sampling gives no definitive indication of how tremendous — or how mediocre — a deposit lies underfoot. Thus, after surface sampling, which indicates a possible concentration of valuable mineral, diamond drilling is undertaken.

The only way to ascertain the quantity (tonnage) and concentration (grade) of a deposit is to make a circular cut in the rock and extract the continuous cylindrical core sample from the centre of the cut. To do this, a special type of drill has been developed — one with a rotating core barrel that grinds down through the bedrock. At the end of the core barrel is a cylindrical bit studded with the hardest of natural substances — diamonds.

The size of diamond drill core varies with the size of the machine used, hole depths and material being drilled. However, the most common sizes are the following:

— Size A — core diameter 27 mm, hole diameter 48 mm;

— Size B — core diameter 36.5 mm, hole diameter 60 mm;

— Size N — core diameter 47.6 mm, hole diameter 75.5 mm;

— Size H — core diameter 63.5 mm, hole diameter 96 mm.

Mechanically, the diamond drill consists of a power unit rotating a tubular steel bit, on the face of which are set diamonds. This bit and attached core barrel are rotated under controlled pressure by means of hollow steel rods. Water is pumped through these rods to cool the bit and remove the rock cuttings.

Under ideal drilling conditions, and once the drill bit is lowered to the bottom of the hole and the drill started, the bit will cut a core, consisting of a cylindrical piece of rock. Rotating at a high rate of speed, the bit is forced downward by the action of hydraulic cylinders on the drill. As it moves through the rock, it pushes the core up into the core barrel.

The rods are withdrawn at intervals of 1.5 or 3 metres (5 or 10 ft.) and the core removed from the core barrel for examination and storage. The core presents a tangible and accurate record of the various rock formations through which the bit has passed.

Sludge samples are also sometimes taken while drilling is under way. These consist of the cuttings made by the drill, and are useful as checks against the drill core samples.

Wireline drilling

Most drilling today is done using the wireline method, which was introduced in the 1960s. In this method, an inner tube containing the core is detached from the core barrel assembly when the core barrel is full or a blockage occurs. The tube and core contained in it are pulled to the surface by a wire dropped down the string of drill rods. A latch, or “overshot assembly,” which snaps on to the top of the inner tube, is used for this purpose. The inner tube is then rapidly hoisted to surface within the string of drill rods. After the core is removed, the inner tube is dropped down into the outer core barrel, and drilling resumes. Thus, the core is retrieved without having to pull all of the rods.

Large drills have recovered cores of up to 100 mm in diameter from depths of more than 4,500 metres (15,000 ft.).

Core is carefully placed in sequential order in boxes or trays and taken to a geological field camp or office for examination by geologists. Interesting sections are split along their length by a core-splitter, with half of the section being returned to its place in the core box and the other sent to the assayer for analysis. The trays or boxes are usually stored on racks in a core shack.

Sometimes it is desirable to obtain a second intersection of a particular geological structure or ore zone from a single hole. This is done by placing a wedge at some point above the intersection to deflect the bit in another direction. This wedging procedure is used frequently in deeper holes, where it can save considerable time and money.

The reverse-circulation drilling process is being used successfully in areas where rocks are deeply weathered, as in many tropical locales, or when drilling in glacial overburden. In this method, a tri-cone bit and dual-tube drill pipe are employed. Drilling fluid or air, or a combination of the two, is pumped down between the dual tubing and returned up the inner tube, bringing cuttings from the bit to surface.

Sonic drilling, in which a sonic vibration device pushes the drill rod into the ground, can be used for soil investigation.

Sometimes explorers don’t need a core sample, as when an open hole allows instruments to be lowered. A standard rotary drill equipped with necessary compressors and drill pipe is all that is needed for this type of drilling.

Where applicable, cost reduction is significant, together with greatly improved productivity. However, this method is not as reliable as collecting the full core by diamond drilling.

Diamond drilling is also important to an established mine. It is used to:

— explore for new ore or outline and map known orebodies;

— investigate rock types and their structure;

— locate orebodies displaced by faults and folds;

— put out pilot holes to direct drifts and stopes to the proper location;

— establish drain holes, grouting and ventilation holes; and

— drill undisturbed holes to allow placement of rock mechanics instrumentation for measuring stresses in the rock.

Underground diamond drills are smaller and lighter than surface drills, and normally run on electricity, not diesel fuel.

Typical modern underground drills have features that allow for fast extraction of rods from the hole.

While most underground drills are now hydraulic types powered by electric motors, some sizes are also available with air and diesel power for when electricity is not available. Diesel-powered drills, however, need extra ventilation and diesel exhaust scrubbers.

Having a drill underground permits the drilling of holes at any angle, which is advantageous to the mine geologist.