Location: Gwalia, Western Australia
TENEMENT: M37/25, M37/333
OWNER: St Barbara Ltd 100%
OPERATOR: St Barbara Ltd (ACN 009 165 066) Level 10, 432 St Kilda Road, MELBOURNE,VIC 3000
Brief History of the Gwalia Gold Mine:
The Gwalia Deeps underground Gold Mine is located 3km south of Leonora in the north-eastern goldfields of Western Australia. Gwalia Deeps is the name given to the down plunge extension of the Sons of Gwalia ore body from 1,075 metres below surface. The Gwalia mine has produced in excess of 4,580,000 ounces of gold via open pit and underground mining since its discovery in 1896.
•1897 to 1963 -Underground mining of the Sons of Gwalia gold deposit with shaft access to depths of 1.08 km
•1984 to 1999 –Mined as an open pit
•1998 to 2003 –Remnant underground mining of upper levels using hanging wall decline access
•2007 to present –Mine decline development recommenced and is now below 1.1 km.
Gold mineralisation occurs as a number of en echelon, moderately east dipping foliation parallel lodes within strongly potassic altered mafic rocks and extends over a strike length of approximately 500m and to a vertical depth of at least 2,000m. Five lodes have been identified and the lode names and geometries are summarised in Figure 1 below. Further detail on the geology and estimation techniques can be found in the St Barbara Limited Mineral Resource Report.
|FIGURE 1 PLAN VIEW OF GWALIA LODES|
The primary mining method since resumption of underground production in 2008 is a Longhole Open Stoping method with paste fill. Stoping configurations are predominantly double lift (40m vertical interval) however single lift stoping (20m vertical interval) is used in some areas. The Double Lift stoping method (as illustrated in Figure 2) involves concurrently mining the uphole (part DL3.1) and downhole (part DL3.2) sections of the stope. The upper and lower portions of the stope are drawn back and bogged at the same time. Parts 2 and 3 are then paste filled, followed by the delayed extraction of the DL3 part 3 wedge (part DL3.3).
|FIGURE 2 SECTIONS OF DL3 STOPES WITH SEQUENCE MARKED|
The delayed extraction of the part 3 wedge serves to reduce the hanging wall dip length in the primary stopes and thus preserve HW stability despite the 40m vertical lift. The installation of HW cables and the use of a concurrent strike retreat blasting sequence of the part 1 and 2 sections, also assist in controlling HW stability.
Gwalia is operated on a fly-in-fly-out (FIFO) basis with personnel residing in camp facilities in Leonora. The FIFO roster for most personnel is 8 days on site, with 6 days off site.
The Gwalia mine is one of the world’s deepest trucking mines. The Hoover decline is currently at the 1500 level which is about a 10km journey to the portal. Combined ore and waste production of around 1 Mtpa is trucked to surface using a fleet of six DAT 60 and three Atlas Copco MT 6020 trucks. The truck fleet is matched with a fleet of one Sandvik LH621 and three Caterpillar R2900 loaders operating on a combination of tele-remote and manual control. Normal drilling fleet includes two Jumbos and three production rigs.
The operating depth and relatively high diesel loading at Gwalia requires chilled air intake. Chilling is via a 4MW bulk air cooler using direct contact chilled water located on a raise-bored intake shaft at the edge of the open pit. Primary ventilation is supplied by two Howden WB Model B Size 3300 single inlet centrifugal fans.
All Gwalia ore is trucked to the Gwalia processing plant. The processing plant is located at St Barbara’s Leonora Operations and consists of a three stage crushing circuit, single stage milling circuit and hybrid CIL circuit with one designated leach tank and 7 adsorption tanks. Gold is recovered from activated carbon into concentrated solution via a split AARL type elution circuit. Electrowinning and smelting are conducted in an adjacent secure gold room. The tailings from the process are thickened and pumped to a paddock type tailings storage facility with multi-spigot distribution (Zimmerman 2013). Gold doré bars are transported by armoured trucks to the Perth Mint for refining.
The declared Reserve is based on the current Life of Mine plan for the Gwalia Deeps deposit. Figure 3 shows the Life of Mine design colour coded by the reserve classification.
|GWALIA ORE RESERVE SUMMARY - JUNE 2013|
Browne, C. & Love, R. 2013, ‘SBM Gwalia Deeps Resource Report’, Internal St Barbara Report.
de Vries, J, Zimmerman, J & Beswick, M. 2013 ‘Gwalia Mine’ Australasian Mining and Metallurgical Operating Practices 3rd edition.
Wilson, L. Beswick, M and de Vries, J., 2013., St Barbara Limited Ore Reserve Report - Gwalia June 2013, SBM Report. June, 2013