LETTER TO THE EDITOR — Drill spacing imperative for accuracy

In the mid- to late-1980s, my apprenticeship as a geologist at the Dome and Paymaster gold mines, and more recent exploration work in the Hoyle Pond belt of the Timmins-Porcupine camp, afforded me the opportunity to observe reconciliation between diamond drill-indicated results and drift and muck sampling in relation to actual stope production.

This experience has provided me with some insight into the plight of the exploration geologist who is responsible for auriferous vein resource estimates based solely on diamond drill information.

In my opinion, geostatistics is another tool for estimating grade along the vein structure, between drill-hole pierce points that are perpendicular to the plane. It is a useful tool in the hands of a competent user and extremely effective with large databases. However, in the situation of a narrow gold structure intersected by fewer than, say, 100 drill intercepts, I much prefer employing basic, practical techniques for interpretation leading to ore resource estimation.

I agree with Marcel Vallee, who, in a letter printed in your Nov. 2-8 edition, stated that insufficient sampling density has contributed significantly to the overestimation of gold resources. Many individual vein structures, though crudely plate-like in form, consist of shorter strike-length oreshoots. Grade thickness contouring, structural composite contouring and creative, selective sampling techniques should be employed to establish what controls gold mineralization in these shoots, and where. Core orientation, routinely utilized in Australia, may be of critical assistance in resolving orientation subtleties. Once the scale of the shoot is established, an appropriate drill spacing can be applied.

If a vein is unexposed on surface or has not undergone development underground, a small test grid on tight spacing can be used, as mentioned by Vallee.

Priority concerns with regard to economic viability or prefeasibility studies should include: minimum mining widths of 6-8 ft. for shrinkage and cut-and-fill mining and 10-15 ft. for blast-hole stoping; hangingwall competency or dilution factor (30-50%); cutoff grade of a weighted average of 3-4 grams per tonne; and a cutting factor of 0.5-1 oz. per tonne. These are ballpark figures for underground mines.

My first experience with ore reserve estimates at the Dome mine involved planimeter measured areas and arithmetic averages of detailed sampling using conservative, clearly defined reserve categories. There, a reserve is a zone — that is, an auriferous vein structure — constrained by development above and below, with channel, chip and muck sampling and demonstrated vertical continuity. A sampled raise development constituted proven reserves.

A zone defined only by drift or sill development above and below, with continuity between supported by diamond drilling alone, was catalogued as a probable reserve. Reserve blocks defined by development on one side with drill-indicated continuity were classified as possible reserves. Blocks outlined by diamond drilling alone were separated as a possible geological resource. Assayed diamond drill intercepts were not included in the reserve estimates.

Diamond drilling was used strictly to define the location of the vein structure for development purposes. Dean Rogers, chief geologist at Dome for 16 years, studied the relationship between drill results and stope production, and concluded that for both individual vein sets and stringer/stockwork zones, between 20-60% of the drill results severely underestimated the resulting stope grade. A disseminated nugget effect was the contributing factor. Therefore, beyond the assay data, the true width, sulphide content, wallrock alteration (carbonatization and sericitization), accessory vein mineralogy (magnesium-tourmaline dravite, fuchsite or ankerite) and internal vein texture (sheeted texture) may aid in qualifying a vein intersection as fertile or barren.

A significant contributor to overestimation of the resource is insufficient drill spacing density for resolution of oreshoots — that is, spreading better intersections too far along a vein structure.

On the other hand, diamond drilling that underestimates grade also results from insufficient drill spacing, as low-grade results will likely kill an exploration program prematurely.

What is the likelihood that many poorly drilled prospects exist, having returned lacklustre preliminary intercepts because of the nugget effect? The true test, if surface exposure is unavailable, is vein development with detailed sampling. Unfortunately, abuse of flow-through financing has negated underground development as an exploration expenditure. Once again, we are our own worst enemy!

Dean Crick

Exploration Geologist

Boliden Westmin

Toronto