Wiluna District - Wiluna, Caledonian, Galaxy, Bulletin, Regent, Matilda, Lake Way, Williamson,Prior, Carrol
Western Australia, WA, Australia
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The Wiluna gold mines are located just to the SE of the town of Wiluna near the northern extremity of the Archaean Norseman-Wiluna greenstone belt of the Yilgarn Craton, Western Australia. Wiluna is 500 km north of Kalgoorlie which is in the same greenstone belt, and 50 km SW of Jundee. In addition to the main Wiluna mine, other workings and operations in the district within a 20 km radius of Wiluna include Bulletin, Regent, Galaxy, Matilda, Caledonian and Lake Way, the latter of which includes the Williamson, Prior and Carrol deposits.
(#Location: Wiluna - 26° 37' 59"S, 120° 14' 22"E; Matilda - 26° 45' 6"S, 120° 12' 54"E).
The Wiluna Mining Centre mines exploited a group of individual lode hosted gold deposits within a series of very low grade metamorphosed (prehnite-pumpellyite facies) Neoarchaean mafic and ultramafic lava flows (the "Mine Sequence"). This sequence includes basalts and high-magnesian basalts, with intercalated felsic intrusions, lamprophyre dykes, metasediments and dolerites that have been subjected to both brittle and ductile deformation. The lodes are controlled by the Wiluna strike-slip fold system - a north and north-east trending, west and east dipping set of dextral principal and subsidiary faults (with minor oblique reverse or oblique normal movement) and an overall maximum cumulative displacement of 1350 m. The mineralised lodes are located on dilational jogs, divergent bends and horsetail splays, with shoots plunging moderately to steeply south.
Fault hosted ore breccias, net veined breccias and brecciated veins are significant loci for gold and contain mainly quartz (chalcedony), carbonates, fuchsite, arsenopyrite, pyrite and/or stibnite.
The Bulletin deposit is 5 km SE of Wiluna and lies within the Happy Jack - Bulletin Fault Zone, a 50 m wide brittle-ductile NE trending structure. The resource is contained within 5 shoots/lenses, with 90% being in the main zone which has a strike length of 200 to 300 m, persists down plunge for 600 m and is 3 to 6 m wide in the upper sections increasing to 30 m in the high grade core at a depth of 300 m.
The Galaxy deposit contains gold mineralisation in quartz reefs and is hosted within volcanics and high magnesium basalts of the Wiluna Mine sequence. Mineralisation appears to be controlled by a macro-ptygmatic, 'Z-folded', quartz vein array, resulting in stacked, relatively flat lying mineralisation envelopes. Most of the Galaxy mineralisation is contained within parallel, NW- striking NE-dipping shoots. The entire sequence is cut by NE-trending, syn- to post-mineralisation dextral strike-slip faults and fracture zones where narrow dilation, resultant fluid flow and gold mineralisation occurs peripheral to quartz pods and veins delineating the major faults. Although carbonate alteration is widespread, sericite-pyrite is localised around mineralisation (Wiluna Mining, ASX Announcement, 17 Nov., 2021).
The Matilda deposits occur in association with multiple shear zones within the broader, NNW trending, semi-regional, Coles Find Shear Zone developed along the contact between a Neoarchaean felsic and mafic volcanic sequences to the east and west respectively. The host sequence of mafic volcanic and sedimentary rocks are interpreted to be a parallel folded and faulted repetition of the Wiluna sequence to the east. The host sequence forms a greenschist to lower amphibolite facies metamorphic domain with predominantly ductile deformation. It comprises a volcano-sedimentary sequence on the western limb of an interpreted major north-west trending synclinal structure, with the axis close to the Perseverance Fault. The sequence comprises basal banded iron formation in the west, overlain by komatiitic volcanics with limited basal peridotite members, grading upwards into high-magnesium basalt and basalt with interflow chert and graphitic sediments. Metabasalt predominates in the deposit area. Felsic volcanic rocks and sediments are interpreted to form the core of the syncline, with the main Wiluna-Galaxy-Lake Way centres on the eastern limb. The main mineralised interval at Matilda is distributed over a 2.5 km long interval that is 500 m wide in the north and up to 750 m wide to the south. Shear zones contain multiple, ENE plunging, en echelon lodes that are 2 to >10 m thick, carrying grades of 2 to 10 g/t Au. This mineralisation is predominantly contained within moderately north-plunging shoots, which may represent boudinaged older tabular lodes. Typically, lodes are continuous down-plunge, with lesser up-dip continuity.
The Lake Way mining centre is ~20 km SSE of Wiluna and encompasses the Williamson, Prior and Carrol deposits. It covers a strike length of ~6 km with multiple mineralised structures. The area is covered by shallow (0 to 4 m thick) sediments of the Lake Way playa. The host sequence comprises a westerly younging, steep to vertically dipping lateral variation of the Wiluna Mining Centre stratigraphy. This sequence is composed of, from footwall to hanging wall: Unit 3 basalt, the Wiluna Mine komatiite marker; Unit 2 tholeiite; Unit 1 high Mg basalt and Unit4 basalts/interflow sediments, overlain by a thick sequence of volcaniclastics. All of these units are intruded by differentiated dolerite and minor felsic porphyry or monzogranite.
The Williamson and Williamson South, mineralisation is free milling and occurs as disseminated sulphides and free gold within a broad gold-anomalous envelope around a north striking, east dipping monzogranite. Higher grade sulphides and free gold are associated with shearing on the contacts of the granite and dolerites. Mineralisation within the monzogranite varies from broad, low grade, disseminated sulphides in the monzogranite itself, to high-grade veins formed within possibly conjugate fractures containing visible gold. Alteration ranges from weak carbonate chlorite distal to the main structure, to strong hematite-carbonate-silica-pyrite-sericite development in associated with high grade mineralisation.
At Carrol and Prior, 50 to 100 m thick dolerites occur within volcaniclastic sediments. Mineralisation is interpreted to be primarily controlled by the intersection of north-south mineralising structures with dolerite/volcanic contacts, although mineralisation is contained within both dolerite and volcanic host rocks. Within the dolerites, alteration is typically zoned and gold mineralisation is more discrete and higher grade, and gold is associated with quartz and quartz-pyrite veining (Wiluna Mining, ASX Announcement, 17 Nov., 2021).
These deposits are part of the greater Wiluna belt, a 45 km strike length of the Wiluna "Mine Sequence" which hosts the Galaxy, Wiluna, Bulletin, Golden Age, Regent and Williamson deposits, plus the Matilda deposits, 6.5 km to the SW of Regent, within a 10 km mineralised interval of the parallel Coles Find Shear Zone.
The Wiluna district lodes were discovered in 1901 and by 1990 had produced 68.5 tonnes of gold. The mine was closed in 2007 after producing >120 t of Au.
District resources in 1990 were: 37 tonnes of remaining contained Au in ore with a grade of 3.8 g/t Au.
Historic production to 1987 at the Wiluna mine itself was: 8.936 Mt @ 6.18 g/t Au for 55.254 t Au.
Wiluna tailings retreatment from 1983: 6 Mt @ 0.4 g/t Au
Reserves at the Wiluna Mine in 1987 were: 8.58 Mt @ 1.82 g/t Au for 15.623 t Au.
Probable reserve at Bulletin, 1998: 1.9 Mt @ 8.5 g/t Au + Inferred resource of: 0.6 Mt @ 7.3 g/t Au.
Total remaining measured + indicated + inferred resources in the district in 2015 were (Blackham Resources Ltd, December, 2015):
Matilda mine - 12.5 Mt @ 1.8 g/t Au;
Williamson mine - 6.3 Mt @ 1.7 g/t Au;
Regent - 3.9 Mt @ 2.2 g/t Au;
Galaxy - 0.6 Mt @ 2.9 g/t Au;
Golden Age (at Wiluna) - 0.6 Mt @ 6.7 g/t Au;
Bulletin South open pit - 2.6 Mt @ 3.4 g/t Au;
West Lode Calvert - 4.2 Mt @ 5.3 g/t Au;
Henry 5-Woodley-Bulletin Deeps - 2.9 Mt @ 5.6 g/t Au;
Burgundy-Calais - 1.6 Mt @ 6.0 g/t Au;
Happy Jack-Creek Shear - 2.9 Mt @ 5.4 g/t Au;
Other Wiluna Deposits - 2.8 Mt @ 4.1 g/t Au.
TOTAL Resource - 44 Mt @ 3.3 g/t Au containing 145 t of gold.
Total remaining measured + indicated + inferred resources in the district at 30 June 2021 were (Wiluna Mining, ASX Announcement, 17 Nov., 2021):
Wiluna Mining Centre - 36 Mt @ 3.9 g/t Au;
Matilda Mining Centre - 2.14 Mt @ 2.13 g/t Au;
Lake Way Mining Centre - 3.06 Mt @ 1.74 g/t Au;
Galaxy Mining Centre - 0.15 Mt @ 3.02 g/t Au;
TOTAL Resource - 41.3 Mt @ 3.65 g/t Au containing 150.73 t of gold.
Tailings - 33.2 Mt @ 0.57 g/t Au;
Stockpiles - 3.89 Mt @ 0.59 g/t Au;
TOTAL Global Resource - 78.4 Mt @ 2.19 g/t Au containing 172 t of gold.
For detail see the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 2021.
Record last updated: 18/11/2021
This description is a summary from published sources, the chief of which are listed below.
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Chanter S C, Ellu P, Erickson M E, Jones G F P, Mikucki E 1998 - Bulletin gold deposit: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne Mono 22 pp 105-110|
Hagemann S G, Groves D I, Ridley J R, Vearncombe J R 1992 - The Archean lode Gold deposits at Wiluna, Western Australia: high-level brittle-style mineralization in a strike-slip regime: in Econ. Geol. v87 pp 1022-1053|
Hagemann S G, Luders V 2003 - P-T-X conditions of hydrothermal fluids and precipitation mechanism of stibnite-gold mineralization at the Wiluna lode-gold deposits, Western Australia: conventional and infrared microthermometric constraints : in Mineralium Deposita v38 pp 936-952|
McGoldrick P 1990 - Wiluna gold deposit: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v1 pp 309-312|
Morgan P J, El-Raghy S 1990 - Matilda gold deposits, Wiluna: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v1 pp 313-318|
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