Hope Brook: If at first you don’t succeed . . .

Beyond such factors as grade, mining method and metallurgy, successful mining requires efficient, constant throughput from mine to mill to tailings. In its previous incarnation, under former owners BP-Selco, the Hope Brook mine never really achieved this, in spite of BP’s best efforts.

But under its new owner, Royal Oak Mines (TSE), Hope Brook, on the southwest coast of Newfoundland, has overcome the four key bottlenecks that hampered throughput in the past.

For example, it now has an environmentally clean bill of health, thanks to the Inco-developed SO2 cyanide destruction process. The mine’s earlier undoing was, in part, due to an inability to destroy cyanide in the tailings. “The environmental problem is a non-event,” Mine Manager Kevin Weston told The Northern Miner on a recent site visit. “We’ve been able to control the effluent since startup (in mid-1992).”

Royal Oak has also resolved the mafic pebble puzzle that caused downtime in the semi-autogenous grinding (SAG) circuit. Before, low-grade rock from mafic dykes that intrude the orebody was virtually impossible to grind below two inches in the SAG mill. A pebble-reject system was eventually installed complete with a pebble-crusher. But the solution itself created a problem — how to re-circulate the pebbles into the crusher and then back to the SAG mill.

Pumps proved nearly worthless because the 2-inch-sized pebbles, along with tramp steel balls, quickly wore out the pumps. The Royal Oak solution: a flexible, vertical-lift conveyor.

The ore-handling system between portal and mill posed a third hurdle. In winter, a post-crusher, open-air stockpile created downtime when fines from the damp ore agglomerated and then froze at key transfer points. Royal Oak now stockpiles ore ahead of crushing, essentially “freeze-drying” run-of-mine muck in the winter, if the post-crusher stockpile starts to build up.

The fourth big question mark that hung over the operation — hauling the ore to surface — has been tackled. For ore haulage, this ramp-access mine depends on 50-tonne-capacity, trolley-driven electric trucks manufactured by Kiruna.

The trucks are fast — at full load, up a 12% grade, they hit speeds of 12 km per hour versus a diesel truck speed of about three kilometres. They run quietly and smoke-free. Unfortunately, the electronic circuitry that keeps them moving is vulnerable in a mine and in an outdoor environment as punishing as Hope Brook’s.

On a late-March visit by The Northern Miner, one of the three trucks was down for repairs. Availabilities are about 70% and Mine Superintendent Lance Flewelling says this is sufficient for producing to the next-deepest (4900) level.

For having solved the key bottlenecks, Hope Brook’s young (thirty-something), aggressive management team deserves full marks. They now can focus on hitting their production target for 1993 of 120,000 oz. of gold. In the six months up to the end of February, Hope Brook produced 48,000 oz. Weston estimated that the March figure should reach about 9,500 oz.

Mine site operating costs in 1992 were $US254 per oz. Depreciation and amortization added another $US10 per oz.

“We anticipate getting better grades on the 4900 level,” Weston said. Development muck on 4900 is running a healthy 0.15 oz. per ton. The top two levels, developed during the BP-Selco days, provide ore. But by this fall, all production will derive from deeper levels, primarily 4900. Mill throughput is 3,575 tonnes per day at a millhead grade of 0.10 oz. per ton (the reserve grade is 0.11 oz.). Monthly production reaches about 110,000 tonnes, though February proved a poor month with only 90,000 tons. This was caused by sloughing from adjacent backfilled stopes that were structurally less competent than predicted. The mining stopes were rib pillars left over from BP’s ownership.

“We have two stopes running good grade and no dilution,” Weston said. “That will sustain us until we get production from 4900 by mid-April.” Mill recoveries, on average about 87%, recently dipped to 80%. As well, February’s mined tonnages were below average. To compensate, this cost-conscious operation reduced staff by 28. As a result, Hope Brook’s March financial figures had improved. In addition, immediate metallurgical tests were run to discover the problem. At the time of our visit, tests were incomplete. Weston suspects slightly different mineralogy from certain stopes caused the reduction.

As described by Michael Hibbitts, chief geologist, the orebody is hosted by the Georges Brook Formation. The Lower to Middle Ordovician-age formation is composed of felsic and mafic sequences with intercalated sediments. The Hope Brook sequence is a zone of pervasive silica pyrophyllite sericite alteration which can be clearly traced up to 5,000 metres along strike. The ore zone, with a current reserve of roughly 8 million tons (mineable ore) grading 0.112 oz. gold per ton) is highly silicified with an east-west strike of 700 metres, dipping to the south at 80. The deposit is as much as 60 metres wide and averages about 25 metres. Those widths and the evenly distributed gold mineralization allow Hope Brook to employ the bulk-mining method known as blasthole open stoping (no backfill, just cable-bolting for hangingwall support). The orebody is open at depth. The gold is associated with copper and pyrite, but it is not encapsulated within the grains. Copper is found mainly as chalcopyrite with minor bornite and chalcocite. The zone has been intruded by a post-mineralization mafic dyke swarm forming a labyrinth within the ore. At depth, the ore zone is split by a narrow east/west-trending sill of Chetwynd granite.

“The exploration potential”, said Hibbitts, “is excellent.” Hope Brook-style grades and widths have been intersected 2,000 metres along strike to the west of the known orebody. Deposits elsewhere in Newfoundland can also be explored because the mill is on tidewater.