Western Australia, WA, Australia
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The Red October gold mine is located within Lake Carey, ~195 km NE of Kalgoorlie and ~80 km south of Laverton in the Mount Margaret Mineral Field of Western Australia, and is 12 km South of the major Sunrise Dam gold mine (#Location: 29° 13' 2"S, 122° 24' 54"E).
The deposit was discovered in 1993, after geologists of Mt Burgess Gold Mining NL defined second and third order structures in the region from helicopter borne magnetic data. This work defined a NE trending structure similar in orientation to that at the Sunrise Dam deposit. This structure was tested with aircore drilling, with the best intersections of 2 m @ 1.78 g/t Au, 7 m @ 0.42 g/t Au, 3 m @ 0.37 g/t au and 2 m @ 0.3 g/t Au. Follow-up drilling on a 50 x 400 m pattern intersected 21.5 m @ 53.7 g/t Au from 28 m to the bottom of the hole (Graham, 2003).
Sons of Gwalia entered into a joint venture with Mt Burgess, carrying out RC and diamond drilling to define an open pittable reserve before purchasing Mt Burgess' remaining equity. Open pit mining by Sons of Gwalia from 1999 to 2001 produced 538.93 Kt @ 6.5 g/t Au that yielded 3.48 t Au. This was followed by an underground scoping study. Following the liquidation of Sons of Gwalia in 2004, Saracen Gold Mines acquired the deposit, commencing underground development in September 2011 and production in 2012.
The Red October deposit lies within the Laverton greenstone belt and the 10 to 20 km wide, north-south trending Laverton Tectonic Zone, which represents the strongly deformed eastern margin of the Kurnalpi terrane, adjacent to the boundary with the Burtville terrane to the east. Both terranes are part of the Archaean Yilgarn Craton.
The stratigraphy in the Laverton greenstone belt commences with the Laverton sequence, which is an ~2800 Ma mafic-ultramafic succession, the basal units of which are a suite of BIF and ultramafic successions, overlain by basalt that has been intruded by gabbro and dolerite dykes, followed by additional complex successions of BIF, basalt and sedimentary rocks. The Laverton sequence is unconformably overlain by the Kurnalpi Sequence, which comprises calc-alkalic andesitic lavas and andesitic volcaniclastic sedimentary rocks that grade up into quartz-bearing sandy turbiditic rocks and conglomerate with clasts of dacite and andesite (Standing, 2008). This sequence has been dated at 2715±5 to 2704±4 Ma (Kositcin et al., 2008). Regionally, the Kurnalpi sequence is conformably overlain by the Minerie Sequence, composed of pillow basalt, dolerite and high-magnesian basalt with minor sedimentary rocks (Kositcin et al., 2008). The youngest Archaean sequence comprises a suite of siliciclastic rocks deposited in late synorogenic basins, that in the Laverton area are predominantly turbidites, but also include carbonaceous shale, sandstone, conglomerate, chert, and magnetic shales, and the polymictic Wallaby Conglomerate dated at younger than 2673±5 Ma (Krapez and Barley, 2008; Kositcin et al., 2008; Standing, 2008). The latter is intruded by temporally and chemically distinct suites of felsic (2664±3 Ma) to mafic (2665±4 Ma) intrusive rocks (Standing, 2008).
The principal structural fabric of the Central Eastern Yilgarn region is the result of a protracted deformation history from ~2800 to ~2630 Ma made up of at least five main deformation events which resulted in an architecture that is characterised by closely spaced north-south faults bounding NE-striking greenstone segments. The D1 event was composed of a couple of extensional episodes between ~2800 and ~2698 Ma that produced the main north-south architecture. The subsequent D2 event involved a NE compression at ~2670 Ma, accompanied by dextral shearing. D3 extension was accompanied by granite doming and core complex formation from ~2665 to ~2655 Ma, followed by the two stage D4a and D4b events from ~2645 to ~2655 Ma, with NE to SE compression that produced upright folding of the late basins and greenstone sequence. D5, the last significant deformation event, was the result of NE compression that produced dextral strike slip along N to NE striking faults (McLellan et al., 2014). The same deformation events have been proposed at the Sunrise Dam Gold Mine (Blenkinsop et al., 2007; McLellan et al., 2007), although there has most likely been a localised rotation of the stress field from NE to NNE for the maximum principal stress to be at an optimal orientation for sinistral movement on the Red October Shear zone, the main structure at Red October (McLellan et al., 2014).
The ore deposit is overlain by a variable 10 to 40 m thick saprolite layer of smectitic clays and 10 to 50 m of lacustrine sediments, primarily composed of clays, which progress upwards from kaolinitic → smectitic → palygorskite-halloysite → pisolitic plastic clays. These are overlain by an 8 to 9 m thick continuous blanket of ferruginous alluvium composed of halite, kaolinite, halloysite, goethite, gibbsite, smectite and hematite, capped by <1 to 2 m of halite, gypsum and clay in the floor of Lake Carey. The weathering front in basalts is at a depth of 50 to 75 m and in ultramafic rocks at 20 to 75 m (Graham, 2003).
On a district scale, the Red October deposit is hosted within a NE striking segment of ultramafic-mafic sequence of rocks bounded by north-south faulting, and cut by a related swarm of steeply-dipping, locally mylonitic north-south faults. The stratigraphic sequence within this segment comprises NW-dipping footwall tholeiitic pillow basalts, separated by a thin, mineralised shale-rich unit, from a hangingwall dominated by ultramafic flows interbedded with high-Mg basalts. Both the tholeiitic basalts and komatiite flows have undergone prehnite-pumpellyite facies metamorphism (Saracen Minerals, 2014; McLellan et al., 2014).
The shale-rich unit occurs on a NE-trending contact within the sequence and is the main host to the known mineralised zones. It ranges from tens of cm to 5 m in thickness, and comprises altered carbonaceous and sulphidic black shale. It is variably deformed, containing ductile textures defined by pyrite mineralisation (McLellan et al., 2014).
The main NE to orientated, 80°NW dipping mineralised Red October shear zone, which hosts the Main lode, is the product of brittle-ductile deformation along the shale-rich unit and adjacent hanging wall and footwall rocks. It is located on the northwestern limb of a tight, north-plunging, isoclinal antiform, which appears to buttress a felsic intrusion to both the west and NW of the deposit. The rheologically weak black shale unit localised strain, whilst its reducing nature provided a chemical trap for gold (McLellan et al., 2014).
In addition to the Main lode of the Red October Shear zone, two other styles of mineralisation have been defined, namely the Smurf (and Smurfette) and Marlin footwall subsidiary structures. When all three styles are present, it is not uncommon for gold assays to be >500 g/t (McLellan et al., 2014).
The Smurf and Smurfette subsidiary structures within the footwall basalts are defined by ductile shearing and quartz-biotite-sericite alteration, striking ~NNW at 350° with a dip of 40°NE, steepening to 60° as they approach the shale-rich unit and Red October Shear zone. These are interpreted to be related to limited movement on the north-south faults and form a number of relatively small mineralised structures at a high angle to the main Red October and Marlin lodes (McLellan et al., 2014).
The Marlin lode, which is tens of cm to metres thick, occurs as brecciated quartz-pyrite-gold mineralised zones, enclosed within an alteration selvage of sericite and pyrite. These breccia zones are best developed where pillows and other volcanic features provide rheological heterogeneity. At depths of <200 m, the Marlin lode occurs along the contact with the Red October Shear zone, locally overprinting the shale and giving it a silica-flooded appearance. In these areas the mineralised gold bearing zones are the thickest in the deposit. Deeper in the system, the Marlin lode diverges from the shale contact, and has a flat 10°S plunging intersection lineation (McLellan et al., 2014).
A third subsidiary structure type, known as Krill structures, are also found within the footwall basalt, and only appear towards the southern limits of the deposit, where they dip at 60°NE. They occur as quartz-sericite-biotite shear zones, but exhibit weaker alteration and sulphide mineralisation than the Smurf and Smurfette structures (McLellan et al., 2014).
All of these structures are regarded as being fault-related tensile vein/fracture arrays, most likely formed during a sinistral movement on the main fault, probably during the D4b regional event. There is also evidence of reverse dextral movement on the subsidiary structures, most likely related to the later D5 event (McLellan et al., 2014).
In both fresh rock and saprolite, gold is associated with quartz veining and with sulphides and their weathered products. Within the transported overburden, goethite-overprinted smectitic clays, and ferruginous nodules and pisoliths host Au at depths of 30 to 40 m, and in ferruginous and titaniferous gravel at the base of the cover (Graham, 2003).
Published ore reserve and mineral resource estimates at 30 June, 2015, were (Saracen Mineral Holdings Limited, ASX Release, 2015):
Open pit resource
Indicated resource - 251 Kt @ 1.7 g/t Au = 0.42 t Au
Measured resource - 9 Kt @ 8.6 g/t Au;
Indicated resource - 152 Kt @ 16.8 g/t Au;
Inferred resource - 33 Kt @ 13.9 g/t Au;
TOTAL underground resource - 194 Kt @ 15.9 g/t Au = 3.08 t Au.
Probable reserve - 225 Kt @ 6 g/t Au = 1.35 t Au.
NOTE: It is not stated in the Saracen Mineral Holdings release, as to whether resources are inclusive or exclusive of reserves.
Saracen Mineral Holdings exploits a number of mines between Red October and Carosue Dam 120 km to the SSW, where the ore from all is treated. These include the Karari, Whirling Dervish and Deep South mines and the Porphyry and Safari Bore deposits, each of which has associated satellites, which together account for a cumulative resource of 79 Mt @ 1.6 g/t Au. Red October is a low tonnage high grade deposit, while most of the others are higher tonnage, but with grades of 1 to 2.5 (and a few of 4 to 6) g/t Au.
The most recent source geological information used to prepare this summary was dated: 2014.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
References to this deposit in the PGC Literature Collection:
Graham, J.L., 2003 - Red October gold deposit, Northeast Goldfields, Western Australia: in Butt, C.R.M., Cornelius, M., Scott, K.M. and Robertson, I.D.M., 2003 Regolith Expression of Australian Ore Systems - A compilation of geochemical case histories and conceptual models CRC LEME 3p.|
McLellan, J.G., Conn, J., Howe, D. and Gates, K., 2014 - Structural controls and strain partitioning in the Red October gold mine, Western Australia: in Ninth International Mining Geology Conference, Adelaide, SA, 18-20 August, 2014, The Australasian Institute of Mining and Metallurgy: Melbourne, Proceedings, pp. 315-322.|
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