THE CADILLAC MINES: DOYON: A MAJOR-LEAGUE PRODUCER

There have been mineral claims on the Doyon property for a very long time. Sporadic exploration produced sufficient proof of mineralization to tantalize various owners for well over 40 years. But it wasn’t until 1972 that sufficient capital became available to launch the costly and high risk exploration programs that were needed. The capital came in the corporate form of an equity-financed exploration company, Silverstack, and an exploration arm of the Quebec government, SOQUEM. They were acting together as a joint venture.

By 1979, sufficient reserves had been established to justify a production decision and the first open pit ore was mined the following year. Ore reserves at that point were fairly limited — 3.0 million tons grading 0.181 oz. gold per ton. Nothing more than a short-term operation was envisaged. Consequently, the ore was simply trucked to Noranda (to avoid the expense of building a mill) and sold to the smelter as silica flux.

Later, the ore was processed at Est Malartic mill and in 1982, Doyons own mill came on-line at a rated capacity of 1,650 tons (1,485 tonnes) per day. All of the expansions were spurred by increasing ore reserves, and it was not long before Doyon qualified for the major leagues of gold producers.

In 1983, underground exploration was initiated from a ramp collared at surface, drill-indicated reserves were confirmed and a major new discovery was made. This was subsequently called the West zone. The results of the program were so encouraging that the multi-million dollar expense of shaft sinking was approved and sinking of the main shaft was started in 1985. It bottomed at 2,050 ft. (615 m), giving access to eight levels.

In the meantime, Doyon had become the largest open pit gold mine in Canada, its two pits producing 1.4 million tons (1.3 million tonnes) in 1986 (pit millhead grade for this same year was 0.166 oz. per ton). This, however, was the peak year for the pits. In 1987, underground mining contributed 33% of the mill feed and this rang in 45% of that years gold production. By 1989, open pit tonnages had diminished to a level at which they were no longer a significant source of ore.

On the corporate side, LAC Minerals obtained control of Silverstack in 1977, and Cambior acquired most of SOQUEMs interests in a number of mining properties, including Doyon, in 1986. Doyon ownership is consequently split 50/50 between the two companies. LAC manages the operation. To complete the picture, SOQUEM remains the major shareholder of Cambior with a 20.33% interest.

The ore zones cover a strike length of 4,600 ft. (1,380 m) with the Main zone responsible for 3,000 ft. (900 m). The West zone picks up the remainder. The Main zone trends east-west and has a width of up to 130 ft. (39 m). It dips to the south at 60. Commercial mineralization is carried in a number of parallel and sub-parallel belts.

The wall rocks are felsic volcanics. They are sheared and hydrothermally altered to a greater or lesser extent and carry 6% to 10% disseminated pyrite. The disseminated pyrite was responsible for the Induced Polarization (IP) anomalies over the orebody; drilling the anomalies ultimately led to their discovery. As if to balance the books, the pyrite causes headaches for the operators.

Exposed to oxygen and atmospheric moisture, fine pyrite rapidly oxidizes, forming sulphuric acid with the dark red-brown hue of dissolved iron — acidic effluent of a most aggressive nature. It represents an expensive environmental cost for Doyon but one that is being resolved, technically if not financially.

West vein structure

The West zone is quite different in character and consists of an intersecting network of veins oriented roughly in east-west and north-south directions. They are steeply dipping and range in width from 5 ft. to 16 ft. (1.5 to 4.8 m). They are frequently continuous for more than 200 ft. (60 m) and carry quartz, sulphides and a proportion of visible gold. The wall rock in which they are enclosed is diorite of the zoned Mooshla stock. The Main and West zones make a physical connection and common use is made of the mines main haulage levels.

Doyon is a highly mechanized mine with a variety of diesel-powered equipment including 15 scoops, eight trucks of 15- and 26-ton capacity, nine development jumbos (electro-hydraulic with one and two booms) and a large number of service vehicles. A number of the scoops are fitted with remote control. The mine is extensive along strike, and could easily become more so if results from exploratory drilling are confirmed. To compensate, the 6th and 8th levels are laid out with heavy-duty rail haulage, and the diesel equiment is simply a gathering system to feed the haulage. About 80% of the ore and a portion of the waste are handled along the 6th level.

Mining methods are determined by ore width and continuity and wall rock strength. In the West zone where the ore is mined from relatively narrow veins, the two methods are shrinkage and blasthole.

The stopes are carried longitudinally. (As an aside, many mines would like to have the so-called narrow vein widths of Doyon. The term is entirely relative with no meaning beyond the bounds of the mine.)

Dips are practically vertical and the hard diorite walls promise a minimum of dilution. Where the ore is consistent in value, it is generally mined by blasthole stoping. Where values are erratic and there is the strong possibility of having to leave low-grade pillars, the more selective shrinkage will be used.

Stope widths vary up to 15 ft. (4.5 m) and, since these are veins that are being followed, pinches and swells are commonly encountered. Widths can drop to 4-6 ft. (1.2-1.8 m) very quickly.

In the Main zone, narrow widths are mined by longitudinal blasthole stopes, but they are quite different from those of the same designation in the West zone. Stopes in the Main zone are characterized by contemporaneous fill, while those in the West zone do not have a fill cycle and may be left open on completion.

When widths permit, transverse stoping is the principal method in the Main zone. Primary stopes are 20 ft. wide and 100 ft. high (6×30 m) and they open to the width of the mineralization, or 30-65 ft. (9-19.5 m). Secondary stopes are 40 ft. wide (12 m). The ore dips at approximately 60%.

An interesting feature of the stopes is that the primaries are drilled with parallel down holes, while the secondaries are ring-drilled from a central drift. This technique avoids silling out the upper level where parallel holes are to be drilled here also.

Cemented rockfill fills the void of the completed primary transverse stopes and unconditioned rockfill is put into the secondaries.

Either type of fill may be used in other types of stopes depending on circumstances. The cement slurry is 50/50 portland cement and fly ash and it makes up 5% of the fill mass.

The Main zone

delivers the bulk of

annual tonnages.

So far as relative importance of the two main ore zones and the several mining methods are concerned, gross mine output is 1.141 million tons per year (1.027 million tonnes) — equivalent to 3,126 tons per day (2,813 tonnes). The West zone produces 32%; the Main zone, the balance. West zone shrinkage is responsible for 9% of the gross and blasthole stoping, 17%. The Main zone longitudinal blasthole method produces 19% of the gross; transverse primary stoping, 15%; and transverse secondary stoping, 20%. The balance is made up of 19% development ore and remnants from the open pits and the low-grade stockpile.

In addition to ore, more or less of the development waste must also be hoisted. The amount broken approximates 438,000 tons per year (394,200 tonnes) or 1,200 tons per day (1,080 tonnes).

Milling

The engineering for Doyons mill was considerably facilitated by the experience gained from the treatment of ore at Est Malartic during the early years of the open pit operation. The criteria had been determined and those problems that had not been fully resolved were clearly identified.

The first mill at Doyon was rated at 1,650 tons pe
r day (1,485 tonnes) and turned over in 1982. The capacity was doubled to 3,300 (2,970 tonnes) and came on line in 1987.

The basics of the circuit are SAG milling (semi-autogenous grinding) followed by fine ball milling, partial CIL (carbon in leach) followed by CIP (carbon in pulp).

The partial CIL treatment takes the load off the CIP stage and thereby improves overall efficiency and recovery. In 1990, for instance, treating 3,086 tons per day (2,777 tonnes) at a grade of 0.207 oz. gold per ton resulted in a 95.4% recovery. In the previous year, the head grade was 0.196 oz. and the recovery 94.7%.

One of the biggest problems for Doyon is how to handle the substantial flow of acid effluent issuing from its two, open-pit waste dumps. One of these dumps contains 23 million tons (20.7 million tonnes) of pyritic rock, and it is situated close to the Bousqet River. A second dump contains a similar amount of material, but, fortunately, it is not in such a sensitive location.

Millions of dollars

have been spent on

acid generating rock.

And still the problem

isn’t licked

The rock not only contains pyrite, but is quite porous and air and water have easy access. It has been determined, at least theoretically from laboratory work, that some samples of this particular rock have the capability of generating up to 200 kg. in pure sulphuric acid per metric ton of rock. “This figure is a peak,” said Pierre Beaudoin, metallurgist. “It’s not an average by any means.” Doyons other rock types are relatively innocuous and the diorite from the West zone completely so.

After extensive tests, a $6.2-million treatment plant came into operation in the spring of 1990. While some of the technology is patented, the principle is that of neutralization with lime, oxidizing iron to the ferric state and adding a proprietary polymer prior to thickening.

The system is called HDS, high density sludge, and the thickeners produce a thick underflow at 40% solids. The density reaches 50% by the time the sludge is directed to its own lagoon. The plant is designed for a flow of 4,800 (U.S.) gal. per min. and will use 11,000 tons of lime per year when in full operation.

This is not the end of the acid effluent problem — far from it. Studies continue and it is hoped that some means will be found to deactivate the pyrite before it has a chance to oxidize.

The HDS plant treats the effluent that is generated and it will continue in this role as a stop-gap until a solution is found.