OVERCOMING THE NUGGET EFFECT: Extracting all the gold contained in

The infamous “nugget effect” can be bad news for promoters of gold properties. Gold is unevenly distributed in such deposits, and so sampling can have the effect of either over-or under-estimating the total gold content. But nuggets need not be a headache for promoters or exploration geologists. To get around the thorny issues of whether to cut assays or leave them uncut and whether these assay results are representative of a deposit as a whole, all that is required are slightly more expensive sampling techniques. Splitting samples prior to taking a fire assay can be avoided entirely by using such techniques.

Typically, when gold samples are assayed, a 10-kg sample, say, is crushed and a 2.5-kg portion is taken as an accurate representation of the whole. This is fine, provided the gold is evenly distributed throughout the sample. But if it isn’t, as is the case with nuggety gold deposits, then a 30-g sample, eventually assayed from the original 10 kg, would not represent the whole. That’s elementary enough.

But how do explorationists determine whether a deposit has a nugget effect? In the Meguma-type ores of Nova Scotia, examination of a hand specimen is often sufficient to detect a nugget effect. But in other deposits, the effect can be more subtle. The nuggety nature can be caused by the presence of clusters of very fine gold, which occurs in numerous ores, but is not always evident. One way to detect the nugget effect is to run a lab-scale cyanidation test on a sample and plot calculated head assay versus leach resident time. A homogeneous gold deposit will show no increase in assay over time, whereas a nuggety gold deposit will show a steady increase up to a certain point after which it levels off.

There are at least two ways to determine the total gold content of a rock sample from a deposit which shows a nugget effect. One is a chemical leaching method, the other is a physical method.

Both involve extracting all the gold from the sample without splitting the sample. Because large volumes of potential ore are treated using these methods (compared with traditional methods), total gold extraction can be expensive. But testing labs can be set up in such a way as to mitigate those costs.

Casmyn Research & Engineering of Oakville, Ont., has done just that. The company opened its laboratory and pilot plant in August but already has four clients: Noranda Inc., Derry Michener Booth & Wahl, RJK Mineral Corp. and Monte Carlo Gold Mines. Samples are crushed to, say, 80% minus 200 mesh, then leached in a strong cyanide solution in small mechanically agitated vessels, one per sample. Tests are monitored for such details as evaporation to ensure accuracy. At the end of the leach cycle, the slurry is filtered, Casmyn president Amyn Dahya explains. The gold content is ascertained by atomic absorption and the solids residue by fire assay. The head assay of the sample is then derived by a summation of the gold content of the solid and solution phases.

“The leaching technique provides a reliable measure of the total gold content of a sample as the precious metal can only report to the solid and solution phases, which can both be accurately assayed,” Dahya says.

Casmyn is currently testing 3,000, ninety-pound samples from reverse circulation drill holes put down on two gold deposits (the Box and Athona deposits) in northern Saskatchewan.

“Both these deposits displayed a strong nugget effect, since regular sample splitting and fire assay produced erratic results, ranging from very low to exceptionally high values,” Dahya says.

In 1988, Casmyn conducted on-site pilot-scale leach tests at the Box and Athona properties. Results were so good (within a variance limit of less than 10%) that RJK Mineral Corp decided to collect 3,000 reverse circulation samples. The results, which took five months to compile, indicate a significant increase in gold reserves for the deposits.

KMS Concentrator

Another method of recovering all the gold in a sample is to use a non-destructive physical separation technique that uses only water. Such a method has recently been developed by M-Tech of Halifax, N.S.

Successfully tested by Westminer Canada and M-Tech on Nova Scotia Meguma-type gold mineralization, the technique has demonstrated such high recoveries at the labratory scale, the company is considering scaling up the apparatus this year to create a new gold ore-processing machine. The company hopes to test the scaled-up version this summer.

President Mike Kuryluk invented the apparatus, which has been called the KMS Concentrator.

It works by creating a velocity gradient in two opposing streams of water. High specific gravity particles overcome an upward flow of water and are discharged out the bottom of the concentrator while lighter specific gravity particles remain at the top and are forced out of the unit through a waste port.

Samples must first be crushed to minus 10 mesh by three crushers: a jaw crusher, a cone crusher, then a roll crusher. They are then screened into four size fractions: plus 20 mesh, plus 60 mesh, plus 100 mesh and minus 100 mesh. The first three size fractions run through the concentrator separately and the concentrates of each run are combined with the minus 100 mesh size fraction and this fraction of the total sample is fire assayed to produce a gold bead.

The efficiency of the concentrator depends on the uniformity of the particle size being separated, Kuryluk says. This is the largest drawback of the technique since considerable time is required for sample preparation and handling. The equipment is being modified to overcome this drawback.

More than 20,000 samples have been tested by the concentrator. The confidence level of the data produced is about 95%, Kuryluk says. (This is checked by assaying the float material as well as the concentrate.) Roughly 3,000 samples weighing 30 kg each can be processed in about two months.

Westminer and Seabright Explorations have been using the unit for about three years to assay samples from their various gold properties in Nova Scotia and Australia.