Features of Selbaie’s operation appeal to many specialists

Recently Quebec-based Les Mines Selbaie, 61.1% owned by BP Canada (TSE), celebrated its 10th anniversary of production. For the specialist, there are numerous interesting and unusual features of Selbaie’s underground operation.

In fact, for the specialist in many disciplines, geophysics, structural geology, mineralogy (there is supergene enrichment here, unusual for the Pre-Cambrian) and for the metallurgist, this operation is probably one of the top half dozen in Canada for commanding interest, not only for what may be seen at the mine and in the vicinity, but for the manner in which problems have been tackled and resolved.

Exploration of the property began in 1973. There was little to go on except a few scattered outcrops. Selco was looking for felsic volcanic rocks, the same family of rocks that paid off so handsomely for the company with the South Bay mine near Red Lake, Ont.

Overburden is deep in this low, swampy region of Quebec, 180 km from the shores of James Bay, but once having located the potential ore-bearing host rock, geophysical work started in earnest.

According to chief geologist Jean-Jacques Bouillon, practically every geophysical technique in the book was tried, and fortunately, because the overburden was sand rather than clay, the orebodies were ultimately discovered. Had the overburden been clay (that is, electrically conductive), the mineralization would more than likely have been missed.

There are two ore-bearing zones. The B zone is primarily copper with small but significant values in gold and silver; it has a low zinc content. About 1.5 km distant is the A zone, which is essentially zinc mineralization with low values in copper, and again, with small but significant values in gold and silver.

There is no connection between the two zones and each has its own distinct metallurgical characteristics that necessitate separate milling circuits. The B zone was the first to be exploited because of its high copper content and has been mined exclusively from underground. It takes the form of a sinuous lens about 500 metres in strike length, 10-40 metres in width and reaches from the 240-metre level to the sub-outcrop, 40 metres from surface. Overburden reaches a depth of 40 metres in this area. The dip of the orebody varies from 40 degrees to 55 degrees.

Much of the wall rock is strongly altered to kaolin (up to 20% clay), and to compound the problem for the mill, there is a strong fault along the footwall of the ore containing 1-2 metres of mud.

The B zone is practically exhausted and the remaining reserves, as of December, 1990, amounted to 760,000 tonnes grading 3.55% copper, 0.22% zinc, 0.9 gram gold and 26.6 grams silver per tonne.

The A zone is a more complex zone of mineralization and comprises four main lenses and a number of smaller sulphide pods. Strike lengths range from 20 metres to 200 metres, widths to 40 metres and dips vary from 30 degrees to 70 degrees.

Additional challenges for both geologist and miner are the disparate strikes of the A lenses; there can be a change of up to 90 degrees in the direction of one lens compared with that of its neighbor. And, as if this were not enough, the whole is cut by joints and small faults displacing the ore from a few metres to tens of metres.

These lenses collectively known as the A zone are worked both from underground and they form the staple for the principal tonnage producer, the open pit. Of interest to the geologist and of considerable interest to the shareholder is the marked zoning of the mineralization. Thus, the near-surface ore of the open pit is dominantly zinc, while at depth the same zones change primarily to copper.

The underground ores bottom at the 310-metre level and currently delineated reserves are expected to be exhausted by the end of 1993. On the other hand, quoted open pit reserves are sufficient for nine years at the pit’s present production rate. In addition, and of more than passing interest, three diamond drills were noted on the fringe of the pit and there is considerable optimism that the pit limits will be extended.

Quoted open pit reserves to December, 1990, were 17.5 million tonnes grading 2.06% zinc, 0.86% copper, 0.15 grams gold and 27.9 grams silver. Reserves for the same zones at depth and to be worked from underground stood at 979,000 tonnes grading 1.49% zinc, 2.77% copper and 1.0 gram gold and 14.8 grams silver.

Access to the mine is by a single, vertical, hoisting shaft sunk in the footwall of the B zone with four main levels at 60 metres, 120 metres, 180 metres and 240 metres. Only the 240-metre level carries through to the A zone, 1.5 km to the west.

Both the A and B zones are mined from sublevels at 20-metre intervals with each sublevel connecting to a through-going ramp.

The lowest limits of A zone are mined by a level at the 310-metre elevation, opened by a 10% ramp from the arterial 240-metre level.

Because of the mine’s geometry with the single-hoisting shaft at the extreme end of the network, all production from the A zone must be trucked a minimum 1.4 km (and more than 2 km including a 10% ramp if it comes from the 310-metre level).

If our reserves had justified it, said Christian Marti, mine manager, BP would have installed electric truck haulage, but the tons were not there. Mining is by blasthole open stoping with delayed fill carried in 20-metre lifts from the bottom up. If pillars justify recovery, the rock fill is consolidated with cemented hydraulic fill and if not, the cement is omitted. The mining system is actually cut-and-fill with an extended cut cycle.