CURRAGH HITS YUKON PAYDIRT A SECOND TIME; Now well into

Curragh Resources’ second Yukon operation, Sa Dena Hes, is approximately 45 km north of Watson Lake or about an hour by air south from Faro. (Sa Dena Hes is a joint venture between Curragh, 80%, and Hillsborough Resources, 20%). After mining and stockpiling 90,000 tonnes of open pit ore, Curragh began developing the underground portion of the Sa Dena Hes Jewelbox deposit in November, 1990. Kilborn Engineering did the engineering, procurement and construction management for the joint venture. It was officially opened in September, 1991.

The Jewelbox orebody is described by Chief Geologist Roland Crossley as a set of ore-bearing underground zones “stacked on top of each other.” Each ore zone sits in an envelope of marble surrounded by phyllites and all ore zones lie at or near the marble/phyllite contact.

“The Chimney (zone) is the one exception. It runs from one contact to the other,” Crossley said. Going into production in late 1991, the total Sa Dena Hes mineral reserve was 4,862,500 tonnes grading 3.9% Pb, 12.68% Zn, and 58.8 g/t Ag.

Structurally, the Jewelbox has undergone intensive folding, and tensional forces have created boudinage (from the French word for sausage, boudin), which in cross-section appears, logically enough, as sausage-like structures. “We say here that it’s either an interesting or an entertaining orebody,” Crossley noted wryly. Of course, this presents quite a challenge to the mine department and mining contractor Canadian Mine Developments (CMD).

Access is by way of a portal at the 1,408-m elevation (above sea level). A ramp, roughly 4.5 m by 4.5 m, provides access to the various levels. At the time of our visit, millfeed was being drawn from the Chimney zone (25%), the JB1 Pod (25%), and the JB2-West and JB2-East zones (50%).

Each zone requires a different mining method. For example, Chimney, because of its near-vertical dip and relatively consistent width, is a longhole operation. A converted jumbo, a Tamrock single-boom, diesel/hydraulic, drills off a 10- to 11-hole ring. The 2 1/4-inch holes are drilled with a 4-ft. burden and 5-ft. spacing. The mining sequence begins with a 5-m-high sill, which is then benched out another 5 m. “By doing that we save having to develop another sub-level,” said Doug Thompson, Mine Manager. Long hole drilling then takes out the remaining 10-15 m to the next sublevel in the Chimney zone. Sub-levels have been excavated at 1420, 1395, 1370, and 1350. The drawpoint is at 1350.

Wagner scooptrams (two 8-cu.-yd and a 3.5-cu.-yd.) load a 13-tonne and three 26-tonne trucks, which haul the muck to the portal. Rockbolts are few and far between in the ramp, “because the marble (through which it passes) is the best ground we have,” said Thompson. At the portal, the ore is loaded by a Caterpillar wheel loader on to 27-tonne, 6-wheel-drive, articulated Moxy trucks that haul the ore to the primary crusher, a distance of 1.8 km down the mountainside. (A 35-tonne Moxy is also used.)

In spite of the relatively short downhill haul, it was not feasible to have the underground haulers deliver ore to the crusher. The underground trucks with their slicks and the difference in mine versus outdoor winter temperatures made that cost-saving tactic impossible. The portal-to-crusher haul is contracted to Northland-Kaska Corp., a joint venture of Northland Enterprises and Kaska Corp., a private company established by the Kaska Dena Nation. (Please see accompanying story for more on the native group called Kaska Dena and its involvement with Curragh Resources.)

In terms of productivity, the Chimney likely is the closest this mine comes to “bulk mining.” Of course, Mike Hoffman, Canadian Mine Development’s Chief Mining Engineer, hopes more Chimney-like zones are identified within the Jewelbox deposit. “Any other mine like this has numerous chimneys. We haven’t found any more yet. We’re looking.” Curragh decided to go the contract-mining route because CMD had “the depth of miners” to draw on. “We needed a diversified workforce,” said Thompson. “With the kinds of ore zones and orebodies we had through the startup period, it made sense.”

While the Chimney zone is 75 m high and roughly 35 m by 50 m, the JB1 Pod zone, mined by mechanized cut-and-fill, is approximately 35 m high, with an areal extent of 30 m by 30 m. Potential ground problems ruled out an attempt at longholing this zone. The JB2-West and JB2-East run between 5 m and 20 m thick and are mined by room-and-pillar. Most development headings, other than the ramp, are in ore.

ICI provides the explosives. “Where the ground is vuggy, we use 40 mm gendyne and where it is competent we use Amex,” explained Lance Flewelling, CMD’s Project Superintendent.

Because the wallrock is a competent marble, there is no backfilling. In other parts of the mine, unconsolidated waste rock provides the backfill. Pillars are sometimes given extra support, such as split sets, screen and the like.

Three 75-hp fans set up at the bottom of a ventilation raise draw in air at the rate of 120,000 cfm. The air is heated by propane and exhausted out the ramp to surface.

The entire Sa Dena Hes operation is powered by four generators — two 2,000-kw GM units and two 1,000-kw Detroit diesels. A fifth unit is on order.

Geology

The Sa Dena Hes property covers most of a 9-km block of Lower Cambrian sedimentary rocks that has been upfaulted into sedimentary rocks of Silurian and Triassic age. The predominant rock type on the property is a grey to black phyllite. Limestone, which hosts the mineralization, occurs in units up to 100 m thick, but makes up less than 5% of the exposed strata.

Prominent alteration zones, up to 2 km long and 1 km wide, occur in four locations — Jewelbox Hill, Gribbler Ridge, Atilla and Burnick. A nearby target that has yet to be explored in detail, known as Porcupine Hill, also is prominently altered. From current information, the alteration pattern appears to be similar in all five areas. The phyllite is altered to a mica hornfels over a large vertical distance — at least 300 m. Limestone beds are converted to marble, in most cases, and more locally, to an actinolite, diopside or garnet skarn. The skarn zones occur usually at the limestone-phyllite contacts. Zinc/lead/silver mineralization is directly associated with the skarn zones.

The sulphide mineralization is reasonably consistent throughout the Sa Dena Hes property. It consists of medium- to coarse-grained sphalerite and galena with essentially no iron sulphides. The Pb:Zn ratio is generally in the order of 1:2, with the exception of the Burnick zone which has a ratio of 1:30. Silver values are associated with galena; no separate silver minerals have been identified.

An exploration drift in the Jewelbox deposit is being driven in ore in ground that is open to the south of the existing ore zones. Crossley explained that the drift, now 150 m long, is a “semi-definition and semi-exploration” drift. Diamond drill stations will be set up every 25 m and perhaps every 12.5 m where required for infill drilling. “We know we have ore out there, but we don’t have a good handle on it yet.” Jewelbox is open to the west as well.

Ideally, a major extention of the Jewelbox would delight Thompson. “Our major challenge will be to decide which orebodies to bring on first. There is so much potential (not just in Jewelbox, but in Burnick, Gribbler and other known deposits), yet nothing is major yet. We can’t sit back for five years and just mine. The best of all worlds would be to extend the Jewelbox,” he said, during the tour by The Northern Miner Magazine.

Milling

The concentrator is designed to process 1,500 tonnes per day at a 22% combined Pb/Zn grade. At lower grades the mill can handle up to 2,500 tonnes. In fact, mill throughput has run as high as 2,200 tonnes so far. According to Vic Rozon, Director, Processing and Maintenance, the projected head grade this year is 20%.

Underground and open pit ore, stored initially in stockpiles at the 1408 adit, is transported 1.8 km downhill to the crusher plant where a 42×48 jaw crusher reduces the ore to minus 20 cm. The crushed ore is then carried by conveyor to a 200-tonne coarse ore bin or, depending on the level in the ore bin, diverted to a storage pile.

The crushed product is fed by conveyor to an 18-ft.-diameter by 7-ft. semi-autogenous grinding (SAG) mill. Soda ash, zinc sulphate and cyanide are added with the ore to the SAG mill — the soda ash for pH and the other reagents for zinc mineral depression and selective flotation of lead minerals. The Jewelbox ore, Rozon said, offers a “relatively easy grind.” The SAG mill work index, for example, is 12.8. To the SAG mill product, xanthate and frother are added as it is sent to unit flotation cells that produce a flash lead concentrate. Approximately 30% of the Sa Dena Hes lead concentrate is recovered by the unit flotation cells and reports directly to the thickener, while the tailings report to the second stage of grinding.

An 11-ft.-diameter by 14-ft. ball mill is in closed circuit with cyclones. The ball mill discharge and unit cell tailing are pumped to these cyclones that overflow a product which is 60% passing 200 mesh (and ready for lead rougher flotation) and underflow material that needs additional grinding. To the lead rougher flotation feed, more xanthate and frother are added and a lead rougher concentrate is recovered. The tailings from the lead rougher cells form the feed to the zinc rougher and cleaner flotation circuit.

The lead rougher concentrate is reground in a 6-ft.-diameter by 7-ft. ball mill in closed circuit with cyclones. The cyclone underflow returns to the ball mill and the overflow is subjected to two stages of cleaner flotation to produce the final lead concentrate.

The lead rougher tailing is conditioned with lime, copper sulphate and more xanthate and frother, and a zinc rougher concentrate is recovered. The tailing from the circuit flow to the tailing pond where the solids are impounded and the effluent recycled to the process. The zinc rougher concentrate is subjected to two stages of cleaning to produce a final zinc concentrate.

Both concentrates are thickened in separate 5-m-diameter, high-capacity thickeners. Thickener overflows report to their sequential Lamella clarifiers, where the solids settle to the underflow and are returned to the respective thickener. The overflow water is recycled to the process.

Thickener underflow is de-watered in air pressure filters to a shipping moisture of 6% to 8%. The zinc concentrate filter capacity is 450 tonnes per day; the lead concentrate filter capacity, 250 tonnes per day. The filtered concentrates are stored in separate 100-tonne capacity loadout bins. The loadout facility is highly computerized and automated. Truckers, themselves, punch in the instructions and the correct weight is loaded into each transporting pod so the truck can leave the site fully loaded at a 72,576 kg. gross weight, as well as with the accompanying waybill.

The lead concentrate averages 70% (5% zinc and 300 to 600 g per tonne silver) at a metal recovery of 93%. The zinc concentrate contains an average 60% zinc at a metal recovery of 90%. The high recoveries result from an ore that has few impurities. “Copper is not a problem and the iron content is low,” Rozon said.

The lead and zinc minerals in the millfeed from Jewelbox are primarily sulphides; however, oxides do occur and they are soft, causing handling problems. The oxides, generally not recoverable, are sandy brown in color and visible underground, making it fairly easy to spot in diamond drill core and in broken rock.

Curragh spent $1.5-$2 million on sensitive instrumentation and computer control to optimize operations and efficiently modulate reagents and circuits for this high-grade ore. For example, a Fisher Provox distributed control system (DCS) and an Outokumpu Courier 30 on-stream analyzer (OSA) provide the mill crews with continuous analyses of various circuits within the mill. SAG mill monitors give continual readouts on such things as tonnage throughput, slurry density, bearing pressure, sound, and motor power draw. Should any of these indicators swing from pre-established set points, the operator, with the assistance of the central Fisher Provox computer, implements “control strategies” to regain the balance in the mill.

The OSA and DCS, said Rozon, are particularly helpful in controlling reagent levels. “Reagents are automatically added based on head grade and tonnage throughput.”

To set up and calibrate the instrumentation have taken about six months, Rozon added. But now the payback will come. “If it maintains a 1% improvement in overall zinc mineral recovery, the system’s payback is one year,” Rozon said.

From the environmental and site abandonment viewpoint, the tailings from the concentrator do not generate acid. In fact, the limestone host rock acts as a natural acid buffer and actually creates basic discharge.