Heap leaching on west coast Golden Bear gold project on

A mountainous region of steep valleys is an unlikely location for a developing heap leach project. Nevertheless, North American Metals (VSE) is forging ahead with leaching plans here in northwestern British Columbia.

North American (NAM), which is 81.4% owned by Wheaton River Minerals (TSE), expects to begin stacking crushed ore from its Kodiak A deposit in late September or early October.

The Northern Miner recently visited the project, where gold recovery to the end of the calender year is projected at 27,000 oz. Total production from the 9-month-long first phase of the Kodiak A deposit is projected to be 46,000 oz., at a cash production cost of US$225 per oz.

Kodiak A is one of three deposits that make up the Golden Bear project, which NAM plans to operate as a heap-leach mine over 2.5-3 years.

The decision to employ heap leaching marks a major change for the mine, which has hitherto operated as a refractory milling operation.

Kodiak A, B and C are situated 3 km north of the Main Bear mine and mill. The four deposits are all associated with the Ophir Break, a major regional fault that has been traced for about 20 km along a north-south trend. All production to date has come from open-pit and underground mining on the Main Bear deposit. The company recently pulled out its underground crew at Main Bear because of bad ground conditions, leaving behind about 5,000 tonnes of the remaining reserve.

At the time of the site visit, mining crews were cleaning up some lower-grade surface material in the pit area, and the last ore was expected to be run through the mill in mid-September.

From startup in 1990 to the end of June, 1994, production from Main Bear totaled about 419,000 tonnes averaging 15.4 grams gold per tonne. Mill recovery during the period averaged 89%, netting a total of 210,998 oz. gold. The reopening of Golden Bear’s 365-tonne-per-day refractory mill will depend on the success of ongoing exploration of the Grizzly zone, which is below and south of the Main Bear deposit as well as being adjacent to and just above the mill.

In spite of the zone’s ideal location, drilling data are limited. The third of three surface holes returned a 15.5-metre intersection grading 14.4 grams beginning at a depth of 532.8 metres. The true width of the intersection is estimated at 7-8 metres. The second hole, about 200 metres south of the third, graded 8.5 grams over a true width of 0.95 metres, while the first hole, 65 metres farther south, returned 1.68 grams over a true width of 3.87 metres.

Further surface drill testing on the zone was discontinued because of drilling problems associated with a boulder field on the mountainside above the Grizzly zone.

However, the company “bit the bullet” and is now driving a 15 decline parallel with, and about 75 metres into, the hangingwall of the Grizzly structure. The decline will be about 800 metres long, bringing its end roughly perpendicular to the high-grade drill intersection.

Underground drilling plans include fan holes on 100-metre sections. The fan holes are designed to intersect the Grizzly on a 75-metre spacing. Based on the drilling results and geological interpretation, the Grizzly can be said to have the shape of an inverted tear drop thinning out along its periphery.

“We could easily have another deposit the size of the Main Bear,” NAM President Vic Jutronich said.

The decline is more than half-finished at the 460-metre point and a trip underground revealed that the first 200 metres is no picnic. The entire length is timbered, and a visual inspection of the back reveals numerous caved areas that require cribbing well above the top of the timbers. Ground conditions have improved markedly and only strapping is currently being used. Jutronich said the decline is advancing at 7-8 metres per day and should be completed in time to allow drilling to start in early November. While work continues on the Grizzly decline, the company is pushing ahead on pad and plant construction in the Kodiak area, 3 km to the north. Unlike the Main Bear deposit and Grizzly zone, the Kodiak deposits are, by and large, leachable, with only a portion being refractory. Leaching tests to date have returned excellent recoveries, with values of greater than 80% within 5-10 days.

“It’s some of the cleanest ore I have ever dealt with”, said Michael Cassiday of Nevada-based Kappes, Cassiday & Associates, NAM’s metallurgical and leaching consultant.

Cassiday does not expect to encounter any difficulties while leaching during the winter months. Leach solution temperatures will be monitored and heated to 4C if necessary.

“Temperature is not really a major factor in leaching kinetics,” Cassiday explained, adding that leaching is actually helped by the higher oxygen content in colder solutions, which partly offsets the negative effect of lower temperatures on the chemical reaction.

Tercon Contractors is building the pad and plant as well as mining, crushing (to 80% minus 19 mm) and stacking the initial phase of the Kodiak A deposit. The upper bench of the deposit sits at the top of the ridge, 250 metres above the pad area in the valley bottom below (at an elevation of about 1,700 metres).

Kodiak A contains an estimated minable diluted reserve of 473,000 tonnes grading 4.6 grams. First-phase plans call for Tercon to mine, crush and stack 325,000 tonnes grading 4.6 grams at a stripping ratio of 2.73-to-1. Stacking should begin in late September, and Tercon expects to have the 325,000 tonnes on the pad within two months, in order to beat the full onslaught of winter. The heap is projected to produce about 27,000 oz. before the end of the year, plus a further 19,000 oz. in 1996. Based on a mining cost of $21 per tonne, a processing cost of $15 per tonne and a gold recovery of 89%, the company expects the cash cost of production to average US$225 per oz.

The capital cost of the first heap-leach phase is estimated to be $13 million (including working capital needs) and Jutronich said payback should occur before year-end.

The second heap-leach phase runs from May, 1995, to May, 1996, and will involve mining the remaining 143,000 tonnes grading 4.55 grams in the Kodiak A deposit at a stripping ratio of 0.96-to-1. This phase also includes underground bulk mining of 280,000 tonnes grading 8.3 grams from the Kodiak B deposit and heap leaching all but 30,000 tonnes of higher-grade refractory material. This material will be stockpiled for future processing in the mill. Open-pit costs are expected to be equivalent to the first phase of work, while underground mining, crushing, stacking and leaching costs are projected to total $64 per tonne.

Gold output in calendar 1994 is projected to be 27,000 oz., with a further 81,000 oz. expected in the remainder of the second phase through to May, 1996. A further third phase includes the bulk underground mining of the Kodiak C deposit, subject to completion of a positive feasibility study. Kodiak C contains an estimated 276,000 tonnes grading 7.9 grams and preliminary metallurgical work indicates heap-leach recoveries in the region of 91%. The third phase runs to May, 1997, and NAM plans a fourth, running from May, 1997, through to May, 1999.

The fourth phase, which is also subject to completion of a feasibility study, includes what the company calls the “east low-grade stockpile.” The stockpile was generated by previous open-pit mining on the Bear Main deposit. The material was considered waste during previous operations and now sits in a dump down the mountainside above the mill and below the Main Bear. NAM estimates the pile contains 2.4 million tonnes grading in the order of 1.34 grams. That grade is based on sampling across the face of the pile, as well as trenching across the top. Further sampling will need to be done to confirm the grade, and the company is looking at possibly using a sonic drill to retrieve samples from inside the pile. Leach tests on the material are also in progress.

Looking farther into the future, NAM is trying to develop additional reserves along the Ophir Break.

Drilling is under way on two new zones in the Kodiak North area, 800 metres north of Kodiak A (Kodiak B and C are south of Kodiak A). The initial program includes drilling 1,300 metres to follow up some trenching values which the company regards as encouraging.

Two trenches, situated 50 metres apart on one zone, returned 11 metres grading 3.8 grams and 5 metres grading 2.1 grams. The other zone, which was exposed by one trench, returned 4.2 grams across 8 metres.

Repadre Capital (VSE) bought a royalty on the property in mid-1993. As a result, it is entitled to a 7.5% royalty until it has received $1 million; thereafter, the royalty decreases to 2%. Jutronich expects Repadre to have received its initial $1 million by the end of the year.

In addition, NAM sold Homestake Mining (NYSE) a 2% net smelter royalty in 1992 for $100,000. The royalty did not include any ore reserve areas known at the time. As a result, the royalty is in effect for the Kodiak A and Kodiak North zones, but not for Kodiak B and C. The royalty would also apply to any reserves in the Grizzly zone above the 850-metre level, which is about 50 metres below the high-grade drill intercept.

Wheaton River is lending NAM $10.5 million to cover first-phase expenditures, on top of the $62 million already owed to it by the company.