Kettle River a jewel for Echo Bay, Crown

The Kettle River project is the newest operating mine in Washington State, and owners Echo Bay Mines (TSE) and Crown Resources (VSE) appear to be pleased at how smoothly the operation came on stream. Echo Bay, with a 70% interest, and Crown expect the mine to produce about 110,000 oz. of gold per year for the first two years, and then drop to about 85,000 oz. per year for the remaining 7-year mine life.

Cash costs are estimated at US$185 per oz. of gold for the first two years, rising to the US$200- level in subsequent years.

The decision to go ahead with the project was made in September, 1988, and financing was secured in November. Echo Bay owned 60% of the project before it purchased a further 10% from Crown for US$10 million in early 1988. It also agreed to finance Crown Resources’ portion of the US$55-million capital cost.

The loan is to be paid back out of 50% of the project’s cash flow attributable to Crown. Interest, set at prime plus 1%, has been capitalized until payments begin in March.

Construction began in April, 1989, and culminated with the first dore bar poured Feb. 3.

John Zigarlick, president of Echo Bay, seemed almost surprised by the absence of problems during the construction and development of this gold mine, near the town of Republic in Ferry Cty. Echo Bay’s extensive experience in developing mines partly accounts for the project’s success to date.

On a recent tour of the operations, it seemed almost an aside to Echo Bay, the operator, to mention the beauty of the gold deposit itself.

The low operating costs can be attributed primarily to the nature of the main orebody, the Overlook deposit. One of the four deposits that will feed the mill over the life of the operation contains an estimated 3.3 million tons grading 0.17 oz. gold per ton, plus a small amount of silver.

The Overlook itself can be divided into four zones. Zone 4, the deepest and also the “sweetest,” contains an estimated minable reserve of 850,000 tons grading 0.30 oz. per ton based on a mining recovery of 95%.

The gold is associated with a flat- lying, massive magnetite/sulphide zone measuring about 700×350 ft. The zone ranges in thickness from 10 ft. to 90 ft. with an average thickness of about 40 ft.

The geometry of the deposit lends itself well to bulk-mining methods, the key to the low-operating costs.

Accessible by decline, zone 4 is being mined by a rib-and-pillar method. This involves driving a heading into the ore, benching out the ore below the heading, and then filling the area with a cement-rock fill. The process is then repeated, driving a heading on the remaining ore above the fill.

The method ultimately leaves a number of parallel pillars about 24 ft. wide, separated by the cement- rock fill. These pillars are then removed in much the same way, and the mined-out area is subsequently filled with an uncemented rock mixture.

Zones 1 to 3, higher up in the stratigraphy of the Overlook deposit, are estimated to contain a minable reserve of about 2.5 million tons grading 0.13 oz. (A mining recovery of about 75% is used.)

Gold in these zones is associated with a clastic-hosted flat-lying quartz/sulphide vein swarms. Zone 1 is by far the largest of the three, containing about 2.2 million tons. Described as somewhat dome- shaped, it reaches a thickness of 120 ft. and measures about 1,200×750 ft. at its largest. Zones 2 and 3 are relatively thin in comparison, ranging from 20 ft. to 40 ft. in thickness.

The mining method used in these upper zones is different from the room-and-pillar method of zone 4. A straight rock fill with no cement is used since the grade of the material does not warrant retrieving pillars.

Ore from the two areas is blended on surface and sent to the mill, 2.5 miles away, by 15-ton Pacific dump trucks.

Besides the Overlook, the Kettle River project contains three other mineral deposits including the Kettle, the Key East and the Key West.

The Key East and West are small deposits containing a total of about one million tons grading about 0.18 oz. gold. The two deposits, in the same general area as the Overlook, are near surface and may be mined by open pit.

Brian Labadie, general manager of the Kettle River joint venture, said no specific mine plan had been formulated although the two deposits could be mined by open pit methods with a strip ratio of about 6.5:1.

Labadie said parts of the Overlook extend toward Key East and West, and he noted there was the potential for mining part or all of the two deposits by underground methods.

Since they are not scheduled to come on stream until about 1994, a definitive mine plan can wait until the relationship between the Key East and West and the Overlook deposit becomes clearer.

Labadie also stressed that since Zones 1 and 4 are still open, underground reserves at the Overlook will likely be expanded, pushing back the development of the two Key deposits.

The Kettle deposit, about 22.5 miles away, contains a proven probable minable reserve of 350,000 tons grading 0.25 oz. gold. The deposit consists of a number of parallel, near-vertical quartz veins and quartz-vein breccias of 2-8 ft. wide for the quartz zones, and 8-15 ft. for the breccia zones. Labadie noted that higher up in the orebody there are areas that reach widths of up to 40 ft.

Although there is little potential to increase reserves on the known vein systems, Labadie said the potential of finding other systems in the area is estimated to be good, particularly in the immediate vicinity to the west of the known systems.

The joint venture expects to spend about US$400,000 on exploration in the area in the coming year.

Mining of the Kettle ore is expected to begin in 1991. Ore will be sent by trucks, most likely by a contractor, to the mill.

The Kettle River mill is rated at 1,500 tons per day and can easily be expanded to 2,000 tons per day later in the mine’s life as the Key East and West deposits are brought on stream. This increased throughput will make up for the lower grade ore from the Key East and West pits.

Since the open pit ore is generally softer, an increased crushing and grinding throughput is not expected to be a problem although a further leach tank may be required.

The process plant utilizes 2-stage crushing, rod and ball mill grinding, carbon-in-leach cyanidation, leaching and zinc precipitation for precious metal recovery.

The jaw and cone crushers, the grizzly, as well as the two ball mills and two rod mills are all refurbished second-hand equipment. The cone crusher and the ball and rod mills were purchased from Pine Point Mines following its shutdown.

Process control in the mill is provided by a distributed control system which monitors the entire system. The system is believed to enhance recoveries by constantly monitoring and optimizing the numerous variables such as feed rates and reagent concentrations. Operators can graphically display the process variables on “pages” corresponding to a specific plant area and process function.

Gold recovery in the mill is running at about 88% while silver recoveries run significantly lower. Silver, a byproduct, is essentially ignored until the pour takes place.

At this point, quite understandably, the less the silver content in the resulting bar, the better. A sigh of relief could almost be imagined at the company’s first pour as the bar cooled, revealing its deep gold color. The bar weighed in at 1,012.99 oz. and had a purity of 80.846% gold.