Atlantic Report Low recoveries cause layoffs at Coxheath

Mine grades have averaged 0.5 oz per ton since production started in early February, according to President Michael Riddell, but the mill has recovered less than 20% of the gold in the form of bullion. Tailings losses have been less than 15%, but a substantial portion of the balance is tied up in the mill circuits or is recovered in the form of sulphide concentrates.

Until recoveries improve, Coxheath will shut down the mill every two weeks for cleanup. Each cleanup recovers approximately 200 oz of gold from within the mill circuits for furnace feed. “We think this situation could last from six weeks to two months, after which we except to be able to operate at full production again, running 200 tons per day through the mill,” Riddell says.

“The reason the gold isn’t coming out is that the crusher doesn’t have a circulating load in it, so a substantial amount of coarse material ends up in the mill,” Riddell says. “This makes it impossible to raise the densities in the mill to the required 75%.” The resulting low densities in the mill, combined with the extremely coarse nature of the gold (nuggets of up to two ounce in size) means that the gold can’t be ejected from the mill. Once gold gets into the mill, it is either pounded into the liners and thin flat flakes come out, which are extremely hard to separate from arsenopyrite, or the gold stays with the sulphides and ends up in a concentrate bag.

Coxheath has been able to get only 20% recovery as furnace feed.

“By cleaning the mill every two weeks, we get 40% recovery in furnace feed,” Riddell says.

The cure for the problem is another story, according to Riddell. It involves a recirculating load in the crusher so that 100% of the mill feed is less than 3/8 inch in size. In addition, the final crushed product will be subjected to preconcentration by either jigs or sluices before the material passes through the mill.

Coxheath plans to remove coarse gold from the material before it reaches the mill, crush the remaining material finer, and then feed it through the mill.

“Our recoveries should increase to 60% furnace feed and 25 to 30% in concentrate bags,” Riddell says. “That’ll permit the company to operate all of its stopes again.”

Coxheath’s run-of mine grade is 0.5 oz per ton. Calculated head grades have averaged 0.23 oz per ton recently, “but the method of sampling the head grades means that none of the coarse gold has been picked up, so it’s not a not a true head,” Riddell says.

The concentrate grades 20 to 30 oz per ton. The concentration ratio — the tons of rock to make a ton of concentrate — varies daily. Coxheath normally produces 1.5 ton of concentrates a day with an ore sulphide content of about 1%. Tailings losses are as the company expected, running .008 to .009 oz per ton.

1988 was a record year for mineral exploration in Newfoundland and Labrador, with 26,000 new claims staked, 230,000 m of drilling completed, and exploration expenditures exceeding $40 million. The preliminary value of production for 1988 was $938 million, according to the provincial mines department. This figure is composed of $776 million for iron ore and $162 million for other products, including gold production at Hope Brook and a full year’s production of zinc at Daniel’s Harbour. Non-metals such as asbestos, gypsum, fluorspar, dolomite, silica, pyrophyllite and peat, and structural materials such as brick, cement, stone, sand and gravel, totalled $72.5 million in 1988.

Newfoundland will have its first mineral exploration exposition, Mineralex `89, from May 25 to 27 at the Springdale Recreation Complex.

Stratabound Minerals (ASE) of Calgary has received additional assay results from its current 20-hole drilling program on the CNE massive sulphide property near Bathurst, N.B. Hole 89-3 carried 0.31% copper, 1.73% lead, 9.53% zinc, 3.65 oz silver per ton and 0.014 oz gold per ton between 16 and 21 m. Hole 89-1 assayed 0.04% copper, 5.04% lead, 11.51% zinc, (16.55% Pb+Zn), 3.35 oz silver and 0.01 oz gold between 15 and 22 m.

Assays announced previously included 16.06 to 19.95% combined lead-zinc and 4.89 to 10.82 oz per ton silver across widths of 3 to 4 m. Stratabound is drilling 20 holes totalling some 1,300 m, according to President Stan Stricker. The drill program has been directed toward outlining open-pit reserves to a vertical depth of about 25 m within a strike length of 150 m. Preliminary metallurgical and mineralogical studies are under way at the Research and Productivity Council in Fredericton, N.B.

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