THE CADILLAC MINES: HOW SWEET IT IS; Both LaRonde and Bousquet No.

It is nothing more than a mere printed line on a claims map and a surveyed picket line in the bush, yet as insubstantial as it seems this represents the boundary between LACs Bousquet No. 2 mine and Agnico Eagles LaRonde. It slices a choice gold orebody into two, and who has the best of it, Agnico or LAC, remains for history to reveal.

Currently quoted ore reserves favor LAC. The company has 55% more “contained” ounces, but mining has been underway for only a short time, and then, there is the Cadillac phenomenon known as “upgrading” to be taken into account. There is time for the numbers to modify.

Reserves at Bousquet No. 2 to January, 1991, are quoted at 8.7 million tons (7.8 million tonnes) grading 0.20 oz. gold per ton. LaRonde’s reserves to January, 1990, are 7.3 million tons (6.6 million tonnes) grading 0.154 oz. In each case, tonnage refers to all designations of ore, dilution has been added and all high values have been cut. (The procedures for calculating reserves are practically identical for both companies.)

The phenomenon of “upgrading” first came to light at the Doyon mine, 50% owned by Cambior and 50% by LAC. This operation shares similar geology to LaRonde and Bousquet No. 2.

It was evident from early on that Doyon mill head grades were consistently higher than calculated grades. The cause is essentially conservative grade estimation, i.e., high assays are cut, dilution is added at a lower grade than actually realized and, at Bousquet No. 2 and La Ronde, there is an unpredictable degree of natural sweetening. Thus, the ore zone is intersected by numerous, vertically dipping joints, or tight fractures, the planes of which are frequently shot with coarse gold.

In addition to being no more than cracks, the fractures have a north-south orientation and they are rarely if ever intersected by diamond drilling, which runs in the same direction. They are generous sweeteners, but estimating their value by conventional sampling is impossible. The only register is the mill return.

The orebody is a lensing body of massive sulphide with accompaniments of disseminated, breccia and stringer sulphides. The principal mineral is iron pyrite with minor copper sulphides, chalcopyrite, chalcocite and bornite, and traces of lead and zinc. Copper content of the ore is 0.45% for Bousquet and similar for its neighbor LaRonde. Zinc can reach nuisance concentrations in some areas of LaRonde. It co-precipitates with the gold in the milling process and fouls both the precipitating carbon and the final bullion.

Steeply dipping main lense

The main lense tops out at about 600 ft. (180 m) below surface at LaRonde and can be traced to a depth of 3,600 ft. (1,080m) at Bousquet. The lens plunges at approximately 55 to the west and the upper margin of the massive sulphide enters Bousquet ground 1,400 ft. (420 m) below surface, (stringer and disseminated ore have a greater spatial extent and, consequently, cross the boundary at a shallower level). Dips vary from 80 to vertical, both to the north and to the south. The strike length on LaRonde’s upper levels reaches 1,100 ft. (540 m) but, as is to be expected with a lensing body, tapers with depth. Normal ore widths range from 10 ft. to 50 ft. (3-15 m), and, as might have been predicted, some of the richest ore and the best mining widths straddle the boundary.

(Mining the boundary pillar systematically requires the close co-ordination of mining practices by both companies if all the ore is to be extracted with the least amount of unbalanced rock stresses. Both LaRonde and Bousquet are starting at the bottom of the ore and mining upwards and outwards in lock step. The overall mining face will form an upside down V, the apex of the V gradually moving upward. The mined-out area is backfilled with extra-strength cemented rock fill. The sequence has already begun.)

Milling at LaRonde started in May, 1988. Throughput is 2,000 tons (1,800 tonnes). Estimated gold production for 1991 is 120,000 oz.

Bousquet’s ore is trucked 23 miles to Est Malartic mill. Commercial production started October, 1990, at 1,000 tons per day. Throughput is now approaching 1,500 tons (1,350 tonnes) and an ultimate 2,500 tons (2,250 tonnes) per day is envisaged. Estimated gold production for 1991 is 169,000 oz.

If Bousquet was able to move from greenfield in 1986 to production in 1990, a remarkably short period of time, it was because the target had been spotlighted by LaRonde. But the history of the pioneer, LaRonde, was otherwise and comprised a dogged two decades of stop-go exploration until the discovery hole hit ore at 2,800 ft. in 1986.

Dumagami Mines was the original owner of the property and Agnico Eagle was Dumagamis main financial backer. The two companies were amalgamated in 1989. LaRonde’s main shaft was sunk to 1,420 ft. in 1983 giving access to four levels. The work was exploratory with the purpose of investigating a drill-indicated reserve of 2.7 million tons (2.4 million tonnes) grading 0.094 oz. Detailed evaluation showed the deposit was not economic.

In 1986, after the main orebody had been delineated by drilling, the shaft was sunk to its present depth of 3,185 ft. (955 m) and intensive development started.

The upper, narrower sector of the orebody is worked by sub-level caving. Widths average eight to 12 ft. (2.4-3.6 m), and most of the backfill is natural, caved host rock supplemented by sized waste from the open pit. The latter is delivered underground by a centrally located raise.

The method has been found to be satisfactory for that particular section of the orebody, said Eberhard Scherkus, mine manager, but there is only 70,000 tons to go and increased ore widths will be mined by transverse blasthole stoping with delayed fill.

While sub-level caving took place in the upper part of the mine from about 500 ft. to 1,200 ft. (150-360 m) below surface, most work is now on the lower levels, 2,300 ft. (690 m) and below. Here, transverse stoping is the norm, with primary stoping blocks 30 ft. long and 100 ft. high (9×30 m). Secondary stopes are 60 ft. long (18 m).

Slot raises are drilled out with Rogers raise borers and the most recent drills to arrive on the property are Tamrock Data Solos, computer-controlled units that drill 4-inch-diameter holes for the blasthole stopes. Primary stopes are backfilled with rockfill and 5% cement by volume (the cement is 50/50 portland cement and fly-ash) and the secondaries with straight rockfill. Mining practices at Bousquet are similar in principal but there is considerable variation in detail.

Milling

Milling practices also approximate one another. LaRonde’s mill is a brand new plant and employs a flotation circuit to produce a gold-rich, copper concentrate for smelting at Noranda’s Rouyn/Noranda smelter. The tailings from flotation are then cyanided in a CIP circuit to produce a cupriferous gold bullion.

Overall recovery is 91%-92% with a split of 65% and 35% between flotation and cyanidation. Copper concentrates grade 17% copper and 10-12 oz. gold per ton. Gold bullion assays have a significant spread. Accessory silver runs 25%-30% and associated copper, 10%-25%.

Bousquet No.2 ore is milled at Est Malartic. This mill was originally built to treat ores from the Est Malartic gold mine and it first started in 1938. The main mine and neighboring ore supplier, Barnat, closed in 1981. The mill continued in operation, however, first on custom ore and then on LAC’s Doyon ore until that operation built its own plant.

The mill has been in a continuous state of improvement and expansion for years and with the high copper content of Bousquet’s ore, there have been many changes in procedures and in equipment. More of both are planned. A gravity concentrate is produced by the mill and accounts for 21% of the recovered gold. Flotation accounts for 62% and cyanidation 17%, for an overall recovery of 96%.

The Bousquet No.2 flotation circuit, designed to produce a lower grade concentrate than LaRonde, averages 13%-18% copper and two to five ounces of gold per ton. The lower gold content is due to t
he free gold reporting to the gravity circuit. Merrill-Crowe precipitation is used for the pregnant liquor and the resulting low-grade precipitate is shipped (at present) to Johnson Matthey Ltd.; it assays 90% copper and 3%-4% gold.

A fourth gold product will be produced in the near future when LAC installs a cyanide regeneration system. This technology has been known for many years but has had very little application so far. The only known user is the Pachuca operation in Mexico.

CANMET has refined the process to suit Bousquet conditions and not only will cyanide be recovered for re-use (current consumption is eight pounds of cyanide per ton), but a rich copper precipitate suitable for smelting will also be produced at the rate of three tons per day. The process is known by the acronym, AVR, (acidification-volatilization and recovery).