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Australian gold goes underground: as surface boom declines, companies consolidate for longer term developments.

AUSTRALIAN GOLD GOES UNDERGROUND

The 1980s Western Australian gold boom was built on the back of near-surface low-grade deposits of limited life and has been sustained by the discovery of fresh surface orebodies as these have become worked out. Comparatively little effort has gone into proving up deeper, often refractory, reserves for the longer term.

Declining gold prices, a shortage of exploration funds and tax changes which will see gold treated equally with other minerals from 1991, are bringing this phase to an end. Gold production in Western Australia has probably peaked and a far larger proportion of the nation's gold will in the future come from underground operations.

A consolidation of interests seems inevitable as the more financially extended companies seek partners for the increased costs of going underground. This consolidation is most clearly seen along the Kalgoorlie-Boulder Golden Mile where a succession of ownership changes has brought management of all the contiguous properties under Kalgoorlie Consolidated Gold Mines.

WMC'S JUNCTION

WMC started gold mining in 1981 at a series of open pits close to its nickel operations; Victory, Defiance, Orion and Orchin, all of which are located on the south shore of Lake Lefroy, a large salt lake (See E&MJ November 1987 issue). The Victory-Defiance complex had progressed to underground mining and a number of other open pits have subsequently been opened up. These operations include the Revenge pit, which is situated in Lake Lefroy, and the Junction Gold Complex a few kilometers further south near the old Jan shaft where the St. Ives gold concentrator has been constructed.

Unlike most Western Australian gold miners, the bulk of WMC's gold reserves are now only suitable for underground mining. Underground reserves are 21 million mt at 4.5 g/mt gold of which approximately half are at Junction where open pitting has already ceased. Other open-pittable reserves are currently only 1.6 million mt at 4.1 g/mt.

WMC is busy developing the Junction underground mine which is expected to be producing 40,000 mt/mo ore by the end of 1990 and 80,000 mt/mo by the end of 1991. A shaft has been collared and should be complete by June 1992. Meanwhile access is via decline from which main levels are being developed at 60-m intervals.

Stoping will be by mechanical cut-and-fill on entries averaging 9-m wide by 5-m high. A Secoma Pluton 24 with CTEV telescopic feeds to ease bolting has been purchased as the main drill jumbo for this purpose as it will be able to drill a full face from a single position.

NORSEMAN KEEPS KICKING

Central Norseman Gold Corporation is named after the horse who is reputed to have kicked up gold with his hoof at a prospector's bivouac. Now almost 100 years old, Norseman is still very much alive and famous as one of the only two gold mines in Western Australia to have continued working all through the great gold-price slump.

Norseman has been predominantly an underground mine but prices have permitted the development of a number of open pits which are now moving underground. Today, the complex consists of four underground mines, three serviced by decline shafts. The ore is free-milling, coming from quartz shear zones found in the basalts and gabbros within the Norseman field.

In 1987 Central Norseman initiated a development program which included the sinking of three declines and purchased the first of two Pluton 17 jumbos. The small 900-m long Viking decline was driven shortly after the commencement of the OK decline. This is now complete and services an orebody dipping at 20 [degrees] - 50 [degrees] where room-and-pillar stoping is proceeding. The Pluton 17 is now working on the main decline, the OK which has been driven from surface to 10 level (1,000 ft) and is continuing further. Total development will be over 8,000 m. Availability of the Pluton 17 from 1987 to date has been 95%.

It was on a straight stretch of the OK decline in 1987 that the Australian decline drivage record was set, 183 m in 10 working days. The 5.5-m x 5.5-m OK decline is being driven in hard, 70,000 lb/[in.sup.2] compressive strength basalt at a grade of 1 in 9. The twin Hydrastar 300 drifters on the Pluton 17 drill 48 45-mm diameter holes and four 102-mm burn-cut holes per round. Drilling time for the small holes averages 3.5 min/hole and a full round 2 hr-45 min. Pulls average 5-6 m/day and drivage costs about $A1,500/m.

An Elphinstone 2800 LHD loads broken ore into a Cat 769C truck for transport to surface. The sinking crew consists of just three men, one for the jumbo, one for the LHD, and one for the truck.

Shrinkage stoping is employed in the area serviced by the OK decline with stopes developed directly onto the decline at different levels. The objective is a stope width of 1.1 m but in practice stoping width is closer to 1.8 m due to instability of the hanging wall.

Mine manager Alan King says dilution in the shrinkage stopes is higher than anticipated and that shrinkage has another disadvantage in that ore is retained in the stope until the stope is completed, and the labor intensiveness of the hand-held drills which have to be used. He is looking at the feasibility of long-hole open stoping but needs to prove sufficient tonnage in suitable ore blocks to justify the investment in additional narrow-vein mining equipment. Two Quasar 12s, one for development and one for long-hole work, and at least two small LHDs are being considered.

A second Pluton 17 was acquired in March 1989 for the third decline, the Scotia, 30 km south of Norseman. The decline is being driven from the bottom of the Scotia open pit through extremely complicated geology. Mechanized cut-and-fill techniques will be used from ramps driven off the decline. Short 30-40 m ventilation raises are being successfully put in using a variant of vertical crater retreat mining, three burn holes in the center and six holes near the apices of a surrounding hexagon.

GYPSUM PILLARS

At the North Royal Mine, Central Norseman has around 1 [km. sup.2] of remnant room-and-pillar workings on a dip of 20 [degrees] - 45 [degrees] Some of the pillars have grades of 50-60 g/mt and the problem has been how to recover them safely. Norseman is doing this in a unique manner: using gypsum pillars for interim support. The gypsum occurs naturally on a nearby salt lake bed and waste heat from the Norseman power plant used for calcination.

The gypsum is scooped up by loaders and trucked to the calcination plant. Calcined gypsum is transported in tankers and fed pneumatically through pipelines to the stopes. A variable-speed rotary valve controls the feed rate of calcined gypsum. The space between existing pillars is cordoned off with a rough barrier of netting and hessian to form the boundaries of the new pillar. Water is added at the gypsum pipeline nozzle and the slurry used to fill the intervening space where it quickly sets into hard plaster-of-Paris. Once set, the high-grade rock pillars are removed.

Tests have shown that the gypsum pillars have a compressive strength of up to 20 MPa (about 2,600 lb/[in.sup.2]). Pillar recovery is done in retreat fashion and ground conditions are very carefully monitored at all times.

Cost of the calcination plant and ancillaries was approximately $A2 million and placement cost of gypsum pillars is less than $A50/[m.sup.3], far lower than cemented fill or any of the other alternatives examined.

COOLGARDIE QUASARS

Another company that is moving underground to mine narrow vein deposits is Coolgardie Gold NL at its property on the original Fly Flat, Coolgardie, Western Australia. Fly Flat was discovered by prospectors William Ford and Arthur Bailey in 1892 and was the discovery mainly responsible for sparking the gold rush that populated the Eastern Goldfields.

Underground production at Coolgardie Gold's new William Ford decline is contracted to Barminco which has purchased two Secoma Quasar 12 rigs. The rigs produce 400 m of 2.2-m x 3.5-m level development per four-week period. These are the first two Quasars in use in Australia and the changeover to hydraulic drilling from previously used jackleg drilling has seen productivity approximately double.

The Bayley's Reward lease has been worked by several companies since its discovery and was eventually closed in 1964 by its last owner, Gold Mines of Kalgoorlie Ltd. The lease was considered to have potential as it was well worked at a time when only high-grade ore was taken and subsequent exploration has borne this out. Halo zones around the Bayley's lode have been found which grade up to 12 g/mt. In addition, ore beneath the Kings Cross pit has grades exceeding 10 g/mt from the development headings.

Coolgardie Gold NL was listed in 1984 and entered production in 1986 on ore from the Greenfileds open pit. 12,000 oz were produced in the year to June 1989 from three open pits: Greenfields, King Solomon, and Kings Cross. Kings Cross and King Solomon have now been mined out and production is now coming from Lindsays open pit and underground workings off the William Ford decline.

Measured reserves at Greenfields to the 100-m level are 615,000 mt at 2.6 g/mt gold and at Lindsays, 560,000 mt at 3.3 g/mt. Mineable underground reserves are 203,000 mt at 8.3 g/mt. Production in the year to June 1990 was approximately 34,000 oz and should rise to 41,000 oz/yr in 1991-1992 with higher grade underground ore and full capacity at the expanded 300,000-mt/yr CIL plant at Greenfields. The company is confident that sufficient extra underground reserves will be proved to sustain operations in the future for at least 10 years.

The 5.5-m x 5.0-m William Ford decline has been driven from the bottom of the Kings Cross pit to access two intersecting lodes, the Kings Cross Reef and Bayley's New Lode Reef. Strike length of the Bayley's Reef is approximately 1,500 m. The lodes are 1.2 m quartz veins dipping at 65 [degrees] - 85 [degrees] with hard, competent basalt found in both the hanging and footwalls.

Currently, stoping is being carried out down to 7 level but plans are to go to the 10 level, the bottom of the old workings. Four 2.2-m wide by 3.2-m high sub-levels are driven at 15-m intervals leaving 12 m between floor and back for the long-hole drilling. A crown pillar is left between main levels.

A Secoma Quasar 12 fitted with a Hydrastar 200 drifter is used for this development. Each face consists of 26 45-mm diameter holes and four 76-mm reamer holes. 3.1-m rods give approximately 2.7-m pull per round. The Quasar carries its own Anfo loader, works four headings concurrently, and about five rounds are shot every three shifts. Broken ore is trammed to the decline by Toro 150 LHDs and loaded into Cat 769C trucks for transport to surface.

Long-hole drilling between the sub-levels was formely done by a pneumatic rig but in May 1990 Barminco ordered a second Quasar for this work. The 57-mm long-holes are drilled up into the floor of the sub-level above to check for accuracy and minimize dilution. Where the vein is wider than one meter, holes are drilled on the contacts with the basalt with a single hole in the center of the lode in a five pattern, as the pattern on dice. Where the vein is narrower than one meter, holes are drilled at one meter intervals with a similar pattern.

Development ore runs about 6 g/mt gold and stopped ore about 9 g/mt. Barminco managing director, Peter Bartlett, says that with the Quasar being only 1.26 m wide dilution could be reduced by making development headings as narrow as 1.5 m or 1.6 m but the restriction is the width of the Toro 150 LHDs used for mucking out.

Bartlett is still evaluating the full benefits of the changeover to mechanized hydraulic drilling but they are substantial. He estimates productivity at 25-30 mt/manshift compared with an average of 15 mt/manshift for jackleg drilling in the Kalgoorlie area. One of the problems in Kalgoorlie is finding miners. With the Quasars being operated by one man each Barminco has been able to halve the number of drillers and drill more meters.

Another factor has been the reduction in consumables cost with hydraulic drilling. When developing in the basalt drill bits will last 200 m but in the abrasive quartz vein material seldom last longer than 15 m. Both figures are greater with pneumatic drilling. The Hydrastar 200 has completed more than 20,000 m without undue maintenance although there has been a change to stainless-steel parts in the head to reduce the corrosive effects of salt water.

PHOTO : The Central Norseman Gold Corp.'s operations were reputed to have begun as a result of a horse kicking up gold at a prospector's bivouac some 100 years ago.

PHOTO : Long-hole drilling pattern at Coolgardie Gold

PHOTO : The small 900-m long Viking decline was driven to access an orebody dipping at [20 degress - 50 degrees].
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Article Details
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Author:Suttill, Keith R.
Publication:E&MJ - Engineering & Mining Journal
Date:Sep 1, 1990
Words:2234
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