Following a solid performance in the recent first quarter,
The two mines cranked out a total of 27,820 oz. gold at cash costs of US$285.89 per oz. during the quarter, compared with a projected 25,000 oz. at US$295 per oz.
The Con mine produced 25,072 oz. from 78,559 tons of ore grading 0.358 oz. gold per ton. Cash costs at the operation amounted to US$286.04 per oz.
The Giant mine produced 2,748 oz. from 13,141 tons of ore grading 0.273 oz. gold. Cash costs there were US$280.20 per oz.
“I am pleased to see the Con and Giant mines continue to exceed expectations,” said Miramar President Anthony Walsh, who spoke with The Northern Miner at the Con mine site. “This is the third consecutive quarter in which the Con mine has met or surpassed our forecast. The Giant mine is off to a good start and the autoclave continues to operate well.”
Miramar attributes its successful quarter to higher grades of its free-milling ores and to the startup of the refractory circuit and autoclave.
“I’m confident we will meet our 2000 forecast of 130,000 oz. gold at cash costs of US$265 per oz.,” said Walsh, adding that Miramar should be cash flow positive by the third quarter.
Con mine
The Con mine poured its first gold bar in 1938; Giant started up 10 years later.
In October 1993, Miramar acquired the Con and continued operations until production was suspended, in May 1998, owing to a strike action. Operations resumed in July 1999 following the resolution of the labour dispute. The strike cost Miramar $5.3 million in 1998 and $3 million during the following year.
In December 1999, Miramar acquired the Giant mine and related assets for an undisclosed nominal sum from the creditors of bankrupt Royal Oak Mines. The mine was reopened as a satellite operation during the first quarter of this year.
The mill circuit combines two cycles: free-milling and refractory. At Jan. 1, the Con mine’s proven and probable reserves of free-milling ore weighed in at 962,000 tons grading 0.35 oz. per ton, or 336,800 contained ounces gold. The refractory portion of the reserves were pegged at 261,000 tons grading 0.31 oz. per ton, or 82,000 oz. The estimates are based on a gold price of US$325 per oz. and restricted to already-developed material in the core area of the mine.
Over its entire history, the Con has produced more than 5.5 million oz. gold from 11.5 million tonnes of ore.
The Con and Giant deposits are hosted by 2.5-to-2.7-billion-year-old volcanic and sedimentary rocks of the Yellowknife greenstone belt. Gold primarily occurs in quartz-bearing shear zones that crosscut the original rocks.
Gold at the Con was originally discovered in the north-southerly striking Con shear zone. Mining currently is focused on the Campbell shear, which also strikes north-south and dips toward the west at about minus 55. The shear zone extends from under the city of Yellowknife to the north and continues to the south for more than 3 km.
The host rocks are dominantly mafic volcanics, and the shear zone itself is a chlorite schist. Alteration around mineralized zones is dominantly composed of sericite and carbonate.
Above the 3100 level at Con, gold mineralization is refractory; below, it is free-milling. “This may be the result of the overlap of different mineralizing events,” said Mine Manager John Staard. “It could also be, in part, the result of chemical changes in the mineralizing fluid as it came up from depth.”
Miramar is developing two exploration platforms on the 5900 level in order to drill for free-milling ore. The company plans to drill about 45,000 ft. before the end of the year to delineate a structure it has dubbed the Lower North Ore trend.
“We also expect to add ore below the current workings,” said Staard, adding that Miramar intends to explore the Campbell shear at depth and that the refractory ore reserves above the 3100 level could potentially be increased. “In addition, we are looking at changing the waste pass to a refractory ore pass, which would allow us to operate from one shaft instead of from two.” This development project is expected to reduce mining costs by $7-10 per ton.
When it was constructed in 1937, the Con mill had a 100-ton-per-day capacity. It has been upgraded and expanded several times to its current daily capacity of 1,200 tons.
After being crushed, the free-milling ore goes directly to a cyanide leach; the product is then filtered and the pregnant solution removed. The material is then subjected to the Merrill-Crowe process and refined. Whatever is left over is sent to the tailings pond, or back underground, to be used as fill. Recovery rates from the free-milling ore are pegged at 93%.
The refractory ore goes through a flotation circuit to create a concentrate. The concentrate is then fed into the autoclave with arsenic sludges and calcine. The feed contains 40% sulphide concentrate, 30% arsenic sludge and 30% calcine. The autoclave oxidizes the ore and neutralizes the arsenic to arsenic trioxide, an environmentally stable product. The concentrate is then neutralized with lime and sent through a cyanide-leach Merrill-Crowe process to recover the remaining gold.
The autoclave handles 80-100 tons of concentrate per day and recovers about 80% of the gold from the refractory ore. Miramar hopes to increase recoveries to 88% by year-end.
Miramar has retained engineering consultant Fluor Daniel Wright to examine alternatives that would allow further increases in the Con refractory processing capacity, such as the addition of a pre-oxidation circuit, and the expansion of the existing autoclave vessel.
As part of the company’s reclamation plan, the Con’s arsenic sludge wastes are being processed through the autoclave at the rate of 20 tons per day. The mine contains an estimated 85,000 tons of arsenical wastes grading 0.47 oz. gold per ton.
Miramar’s abandonment and restoration plan for the Con mine includes the removal of surface facilities, as well as any arsenic-impacted soils. Revegetation of the site and water treatment are also planned.
Giant mine
Over its 50-year history, the Giant mine has produced in excess of 7.1 million oz. gold from 17.4 million tons of ore. At last report, the mine contained proven and probable reserves of 229,000 tons grading 0.36 oz. gold per ton, or 83,000 contained ounces. The estimate is based on a gold price of US$300 per oz.
In addition, there are gold resources outside the currently producing Supercrest area, and these could potentially extend the mine’s operating life. This resource totals 5.7 million tons grading 0.23 oz. gold, or 1.31 million oz.
Average cash costs of US$265 per oz. this year are expected to decrease to US$260 per oz. in 2001. Miramar anticipates that the Giant mine will produce 300 tons of ore per day grading 0.34 oz. gold per ton over the next two and a half years.
The geology at Giant is complex in comparison with the Con mine. There is a higher density of shear zones, which are folded and dip to the west. As at Con, the gold is hosted by quartz-carbonate veins in the shears.
In order to take advantage of the incremental capacity available at the Con mill, Miramar is trucking Giant ore to the Con mill, where it is processed in the flotation-autoclave circuits.
In addition, Miramar is evaluating the economics of reprocessing the tailings from the Giant mine. The resource contained in tailings at Giant is estimated at 11.6 million tons grading 0.055 oz. gold, or 640,000 contained ounces.
Previous attempts to reprocess the tailings through leaching failed, owing to the refractory nature of the gold. However, recent tests indicate that 50-55% of the gold can be recovered into a low-weight/high-gold-grade iron oxide concentrate that can then be processed in the Con autoclave. All of the ore at Giant is refractory.
“This would be very low-cost material to process, as the tailings are easily accessible and require no crushing or grinding,” said Brian Labadie, Miramar’s senior vice-president of operations. “Processing such a concentrate is straightforward and inexpensive because the material is already largely oxidized.”
The operating cost for the tailings reprocessing is estimated at US$200 per oz. of gold recovered. Extraction and processing could begin as early as spring 2001, provided study results are favourable.
Miramar has budgeted $1 million this year for exploration at the Con and Giant mines.
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