BELLEVUE GOLD PROJECT
High Grade Gold Discovery - 3.0 Moz @ 9.9 g/t gold Global Resource, including a Maiden Probable Ore Reserve of 5.3 Mt @ 6.1 g/t gold for 1.0 Moz.
The Bellevue Gold Project (Bellevue Gold 100%) A Forgotten Treasure
Bellevue Gold Ltd is developing a high-grade underground gold mine at the Bellevue Gold Project. The Company recently released its Stage 2 Feasibility Study, which outlines a highly profitable gold mine, with a 1.0 Mt per annum processing plant, and using a conventional gravity and leaching circuit and conventional underground mining techniques. The project is currently under development with the first gold pour planned for mid-2023. The project has an initial 8.1 year mine life based on a LOM of 1.56 Moz @ 6.0 g/t gold and includes a probable ore reserve of 1.04 Moz @ 6.1 g/t gold.
When operational the Bellevue Gold Mine is forecast to be one of the highest-grade, lowest cost mines in Australia with the first 5 years producing an average of more than 200,000 ounces per year at an all-in sustaining cost (AISC) of $992 per ounce. LOM over the 8.1 years the mine is forecast to produce an average of 183,000 ounces per annum at an AISC of A$1,014/oz. The high-grade nature of the mine at Bellevue is conducive to profitability and the mine is forecast to produce $1.8 billion of free cashflow (pre-tax) over the life of mine (assuming gold price of A$2,400/oz). The projected cashflow underpins an outstanding payback period of 1.4 years (post-tax).
The Company is fully funded into production with a $200 million underwritten and credit-approved loan secured to fund construction and development activities.
Figure 1 – Inspecting the decline at Bellevue, September 2020
The Bellevue System is metallurgically very simple with exceptional conventional gravity and CIL recoveries both from the historic mine and recent testwork completed by Bellevue Gold Ltd on the new discovery areas. Total conventional leach recoveries averaged 97% with a gravity component up to 58-85%.
Bellevue Gold is currently re-establishing access to the historic underground which is concurrently being dewatered and will provide a platform for future underground drilling and access to new development.
There is significant potential to expand the global resource with further exploration drilling from both surface and from underground and the company is committed to an aggressive exploration budget at the project.
The BGP sits within the Yakabindie domain of the Agnew-Wiluna Greenstone Belt of the Eastern Goldfields Super Terrane, on the Sir Samuel 1:250,000 map sheet. The Yakabindie domain consists of the layered Kathleen Valley gabbro (dated at 2.736Ga) overlain by the tholeiitic Mt Goode basalt sequence. A mixed sequence of metamorphosed ultra-mafics and felsic volcanics overly the Mt Goode basalts and in turn sit underneath the unconformable boundary of a late basin conglomeratic sequence (the Jones Creek conglomerates). A simplified stratigraphic column of the Bellevue area is shown in Figure 3. The domain is bounded to the east by the crustal scale Keith-Kilkenny Shear (Perseverance Fault) and bounded by the Ida Lineament to the West (Figure 3). The Bellevue Gold Deposit is proximal to the Keith-Kilkenny Shear and the regional scale Waroonga and Miranda shear zones.
The Bellevue area stratigraphy broadly correlates to that of the Kalgoorlie terrane, with a lower mafic sequence (Kambalda sequence), an upper metasedimentary sequence (Kalgoorlie sequence) which is unconformable to the mafics below, and a late basin sequence, which is separated from the Kalgoorlie sequence with an unconformity. The Mt Goode mafic sequence is part of the lower mafic sequence of the Eastern Goldfields super terrane and equivalent to the Kambalda sequence of the Kalgoorlie Terrane.
The belt has a complex deformation history with structural reactivation and switching kinematics.
D1: Progressive extension and basin formation. Ongoing extension leads to exhumation of granites and creation of unconformable sub-basins with recumbent folding proximal to sheared margins.
D2: Largely E-W compression, reactivates extensional structures and establishes thrust and fold architecture with NNW striking shear zones and fold axis
D3: Progressive NW-SE compression leading to the development of sinistral lateral escape structures, often nucleating on the D2 fold axis. Continued compression leading to the locking up of lateral escape structures and the establishment of a conjugate architecture of thrust faults which cross-cut earlier sinistral structures. These structures are coincident with mineralisation in the Bellevue area and are possibly coeval with the regional 2.645 –2.63Ga Gold Event.
D4: Progressive NE-SW compression leads to post-mineralisation SE and NW dipping planar faults and minor shears.
D5: Orogenic collapse, planar faults with steep slickenlines.
Figure 2 – Regional Geology of the Bellevue area with a simplified stratigraphic column below
Figure 3 – Simplified stratigraphic column
Figure 4 – Geology of the Bellevue area
The Bellevue gold deposit lies to the west of the Miranda shear, a dextral strike-slip shear with an eastern block-down component. It separates the Mt Goode sequence from the younger felsic volcaniclastics and ultra mafics and also appears to be a Riedel splay between the transpressional Keith-Kilkenny and the Ida Lineaments. The Project is located in an area of greenstone belt constriction, and associated structural complexities, with evidence of multi-scale shear zones and controlling faults coupled with a long and complex intrusion history. To the west, lie high-Ca granites and granitic gneiss. The major geological features in the Bellevue area are shown in Figure 4.
The surface geology of the Bellevue Project area is readily separated into two areas; subcrop to outcrop in the north and transported alluvium/colluvium in the south. The outcrop consists of Archean mafic lithologies in a range of low hills with thin residual soils that have a depth of weathering between 10 to 30 metres. Shallow Tertiary and Quaternary colluvium and alluvium can be found along the project area which sits immediately north of Lake Miranda to the south, as part of the paleo-drainage system. Lake Miranda is a playa lake system dominated by gypisferous dunes, lunettes and sandy, clayey, evaporitic lake floor deposits. Pleistocene red sand sheets and dunes form remnant deposits to a few metres thickness on the lower, western flanks of the hills. This sand/silt material has been reworked into the lake floor deposits with the evaporites.
The main Bellevue lode can be traced at surface along a strike length of over 1,500 metres. The hosting shear zone strikes north-south and dips from 45° to 85° west. Further gold mineralisation occurs in hanging wall and footwall structures both sub-parallel to the Bellevue Lode and in conjugate low angle mineralised shears. The Archean geology around the mine is dominated by Mt Goode sequence, a suite of tholeiitic mafic basalts and dolerites which range in grain size from fine to coarse. Pillow and flow-top breccias have often been recognized by geologists. Minor basalt with megacrystic plagioclase has been mapped locally, with minor sediments and felsic intrusives (felsic porphyry and granite dykes). The tholeiites strike north to north-east and dip steeply to the west and north west. Stratigraphy at Bellevue is overturned, and the direction of younging is to the south east.
Metamorphic grade of the area is upper greenschist to amphibolite grade however the strain distribution is heterogeneous with mylonites (greenschist-amphibolite facies metamorphism) developed within shear zones bounded by relatively undeformed tholeiites (greenschist facies metamorphism).
Locally, a broad antiformal architecture was established under E-W compression during the D2 compressional event, in conjunction with the reactivation of earlier extensional structures. Ongoing, largely NW-SE compression in D3 established NNW striking sinistral shear structures, dipping steeply to the NE, including the Bellevue West, Highway and Yakabindie Shear Zones which are the most prominent mappable features in the project area.
It is thought that ongoing NW-SE compression under D3 lead to the ‘locking-up’ of these sinistral shear zones and the establishment of a conjugate architecture of thrust faults, which cross-cut the earlier sinistral structures. These structures are coincident with mineralisation in the Bellevue area and are possibly coeval with the regional 2.645 –2.63Ga gold event. A later relaxation event has resulted in normal reactivation of the D3 shears. This event has important controls on mineralised shoot geometries. All structural features are cross-cut by later brittle faulting which segments mineralisation throughout the deposit area.
In the Bellevue mine area, Bellevue geologists have subdivided the Mt Goode sequence into separate ‘panels’ referred to as, from west to east: the Waroonga Corridor, the Western Corridor, the Bellevue Corridor and the Vanguard Corridor. These corridors are separated by the north – north west trending, steeply north east dipping, D3 Sinistral Shear Zones, the West Shear, Highway Shear and Canberra Shear (refer to Figure 5).
Figure 5 – simplified geology of the BGP showing surface projections of lode geometries, major lithological subdivisions and major structural features.
Figure 6 – Diagrammatic representation of the Bellevue Structural Framework looking north.
Gold mineralisation in the area is structurally controlled and is generally associated with N-NNW trending, West dipping shear zones (dipping about 60 degrees), of 1 to 20 metre thickness. The exception is the Viago Lode, which is a low angle shear zone at 500 metres below surface which gently plunging to the South as well as the Westralia and Vanguard lodes, which dip about 45 degrees to the north east.
Locally, gold mineralisation is associated with three main structural orientations, most probably belonging to the same deformation event, including:
50-85° west dipping north south striking high angle reverse mineralised shears containing a fabric indicative of ductile deformation, e.g. Bellevue, Southern Belle, Tribune and Deacon Lodes.
50-70° east dipping north north-west striking mineralised ductile shears e.g. Westralia and Vanguard reefs.
Shallowly east dipping (10-20o), shallowly (20o) southern plunging ductile shear zones eg: Viago, Vlad and Mavis Lodes
The mineralisation at the Bellevue Mine was characterised by bonanza grade mineralised shoots that contained the bulk of the mined metal within the overall lode system These shoots generally had an up-dip component of approximately 20-40 metres and a significant greater down plunge component of several hundred metres. Ore shoots occur within dilational zones of the shear structure, with gold mineralisation being present within massive or disseminated sulphides in quartz breccias. An example of these shoot geometries, repeatability and predictability are shown on the Tribune long section
The established shear lodes and veins have been deformed by a post-mineralisation, normal re-activation of the mineralised structures, resulting in the mineralised veins being folded and boudinaged into 10-20 metre wide shoots. Lineations and fold axes plunge between 10 to 20 degrees to the south, parallel to the orientation of the high-grade shoots. Semi-massive to massive sulphide minerals (predominantly pyrrhotite and chalcopyrite) have been remobilised under normal reactivation of the structures to the boudin interstices and fold hinges. It is common that gold concentrates in these lower strain domains. The Bellevue structural framework is represented in the Figure 6, showing the post mineralisation folding of the shear structures under normal re-activation, which governs the high-grade shoot geometries.
Mineralisation has been segmented by later brittle-ductile structures of which the most prominent within the mine area are:
An un-mineralised shallow easterly dipping strike slip array, e.g. Bellevue Fault which terminates the Bellevue Main Lode at the 13th level underground
A series of east-west striking normal faults e.g. ‘A’ and ‘C’ faults.
At the BGP, all lithologies have been metamorphosed to upper greenschist/lower amphibolite facies. Alteration is typically narrow, focused in and around shears. Observed alteration minerals include:
Chlorite – both early (shear foliation parallel) and late (shear foliation overprinting).
Amphibole – both early (shear foliation parallel) and late (shear foliation overprinting). Amphibole Intensity often has a negative association to gold mineralisation.
Biotite – an early phase, dark – foliation parallel (Figure 7) and a very late phase (light brown, matted, unoriented, overprints late chlorite). Biotite intensity has a positive association to gold mineralisation.
Albite/Silica – often observed in the Tribune and Bellevue lodes, but absent in the Viago lode.
Late alteration – a sericite-epidote-FeCO3 retrograde assemblage.
Of these, the early dark, foliation parallel biotite is the most effective vector to mineralisation, but often on a limited scale (20-50cm around mineralised intercepts and associated with shear intensity).The late biotite is often observed as ‘clots’ of 1-2 mm randomly oriented flakes overprinting the late coarse, matted chlorite-amphibole veins giving paragenetic timing. It exploits all existing fluid pathways giving a patchy to pervasive overprint of pre-existing vein networks and shear fabrics. Biotite alteration intensity is controlled by folding within shear zones, resulting in higher intensity biotite alteration corresponding to fold plunge orientations. It is observed that significant gold intercepts do not always have biotite alteration, however, the better intercepts nearly always have moderate to strong biotite alteration.
Pervasive biotite alteration does not correspond to higher grades which may be more a function of degree of shearing, namely stronger developed shears are pervasively altered but have sheared out mineralized veins.
Figure 7 – Dark biotite altered shear fabric folded around F7 fold axis with fabric picked out by remobilized pyrrhotite and chalcopyrite.
Mineralisation and Gold Characteristics
Pyrrhotite (po) and Chalcopyrite (cpy) mineralisation are observed in multiple settings:
Foliation parallel sulphides: discontinuous foliation parallel sulphides in zones of higher strain
Foliation cross cutting.
Fracture fill in veins: networks of po+/-cpy often hosted in sheared quartz veins.
Boudin interstices: Around the margins and in the interstices of boudinaged (often auriferous) quartz veins.
Basalt pillow rims.
Extension veins: late tension veins (Quartz, Quartz-Carbonate, Quartz-Epidote gangue) –post deformation.
Disseminated – rarely observed – up to 20% of the host rock and intergrown with po and cpy are remobilised late into the paragenesis and so are found across multiple settings.
At Bellevue, gold grades have a strong association to remobilised pyrrhotite (and lesser chalcopyrite) in and around the shear structures and associated veins, making DHEM an effective targeting tool. Remobilised massive to semi-massive sulphide exploited vein margins and internal fractures and often contain cataclastic breccia fragments of quartz. Pyrrhotite and chalcopyrite display evidence of ductile remobilisation by mechanical processes such as cataclastic flow and dislocation flow rather than diffusive mass, liquid state or magmatic transfer. Arsenopyrite is late, euhedral, with no obvious gold association, although it is also often observed in late fractures. Pyrite replacement of pyrrhotite is mainly observed in the top 100 metres of the Tribune and Bellevue lodes or proximal to workings and is interpreted to be a result of low temperature retrograde processes.
Visible gold occurs with quartz fragments and within late (open) fractures in veins. There are two types of mineralised veins observed within 1-20 metre-wide zones of high strain:
Type 1: Smokey grey, highly strained, often highly folded and boudinaged with po and cpy remobilised to boudin interstices. Visible gold is observed in late fractures.
Type 2: Opaque (sometimes partially recrystalised) quartz veins, frequently observed cross-cutting Type 1 veins. Po and cpy observed as fracture fill in opaque veins with visible gold is observed in late fractures.
*DMIRS (2021), MINEDEX: Production summary of Bellevue - Sir Samuel Group