Summary:
Boddington consists of greenstone diorite hosted mineralization and exploration activities continue to develop the known reserve.
The deposit style is still somewhat controversial. Features consistent with porphyry-style mineralization, classic orogenic shear zones, and intrusion-related gold–copper–bismuth mineralization, are all recognized, giving rise to a variety of genetic interpretations.
Boddington does not fit any classic Archean orogenic gold deposit model, having a general lack of quartz veins and iron carbonate alteration, a copper ± molybdenum ± bismuth association, zoned geochemical anomalism, and evidence of high-temperature, saline, ore-forming fluids. Detailed petrographic, geochemical and melt inclusion studies suggest a late stage ~2,612 Ma, monzogranite intrusion as one of the principal sources of the mineralizing fluids. However, there is also local evidence for older, perhaps proto-ore, porphyry-style copper ± gold in the dioritic intrusions and patchy, locally high-grade, orogenic-style gold mineralization associated with enclosing shear zones and brittle-style deformation, which was focused on the relatively competent dioritic intrusions (Turner et al., 2020).
The Boddington deposit lies within a 6 km strike length of the Wells Formation. For descriptive purposes the deposit is subdivided at approximately 12,200 N into two main centers of bedrock mineralization, referred to as Wandoo North (North Pit) and Wandoo South (South Pit).
Most of the primary mineralization at Boddington is hosted within intermediate to felsic intrusive, volcanic, and volcanosedimentary rocks, with approximate dimensions of 9,000–11,000 mE; 8,500–14,500 mN; and -675–324 mRL. The deepest mineralization intercept to date is at approximately 1,219 m. The volcanic rocks are dominated by dacites and andesites.
The Wells Formation is intruded by at least three magmatic suites:
• A suite of quartz-feldspar-phyric diorite, porphyritic diorite, and microdiorite intrusions (Diorite suite) which are spatially linked to the bulk of the Au-Cu mineralization. These were emplaced between 2,714 and 2,691 Ma (Roth, 1992; Allibone et al., 1998; McCuaig and Behn, 1998);
• A separate suite of granodiorite-quartz diorite-tonalite intrusions (Eastern suite), which intrude the Marradong and Wells Formations, is dated at ~2,675 Ma (Allibone et al., 1998);
• The “Late Granite” (Wourahming monzogranite) suite is the final intrusive event at ~2,612 Ma (Turner et al., 2020).
The N05 extended layback, at North Pit, is dominated by diorites, with lesser fragmental volcanic rocks. The diorites at North Pit are mainly porphyritic and generally more felsic compared to the predominantly aphyric diorites of South Pit. A suite of rhyodacitic porphyries are identified at North Pit, but is rarely observed at South Pit.
The South Pit is centered on a composite diorite stock, the Central Diorite, which has a known strike length of approximately 1,200 m and thicknesses varying from 300–600 m. The southern portion of the Central Diorite strikes north, and dips subvertically and steeply to the west, with an apparent southerly plunge. To the north, the strike of the diorite changes from north to northwest, following the orientation of a transecting dolerite dike. The dip changes from westerly, to subvertical, to steeply to the southwest.
The diorite is in contact with three volcanic units:
• Southern volcanic unit: sequence of porphyritic volcanic rocks in the south and west;
• Northern volcanic unit: sequence of tuffaceous volcanic rocks to the northwest;
• Eastern volcanic unit: characterized by aggregated clusters of plagioclase. Separated from the Central Diorite by the Eastern Shear Zone, a north-striking, steeply west-dipping brittle, ductile tectonic feature.
Thin units of fragmental volcaniclastic rocks consisting of angular to well-rounded diorite and andesite clasts ranging from fine ash to agglomerate sizes are common within and around the diorite stock. A series of fine-grained microdiorite dykes, ranging from a few centimeters to several meters wide, cross-cuts andesite, diorite, and fragmental lithologies.
A suite of Proterozoic dolerite dykes with three prominent orientations cross-cuts the entire mine sequence, but does not host any significant mineralization.
Structure and Alteration
The following structural/alteration events were identified at Boddington:
• Early (pre-deformation) albite and biotite–silica alteration associated with the dioritic intrusions was interpreted by Roth (1992) to signify potassic alteration (Turner et al., 2020);
• D1–D2 ductile shearing was accompanied by silica–sericite–pyrite ± arsenopyrite alteration. Lacks significant gold–copper mineralization. Formed north–south-striking sub-vertical to east-dipping broad ductile shear zones;
• D3 northeast-trending ductile shearing produced mylonite zones with silica–albite–biotite–pyrite alteration. Associated with development of massive quartz veins in D2 shear zones;
• D4 northwest-trending, brittle-ductile deformation. Late D4 clinozoisite–quartz–chlorite–sulfide veins commonly impart a fine fracture-fill network or mesh texture to the rocks and are generally associated with the bulk of the lowgrade gold–copper mineralization. In addition, late D4 actinolite–sulfide veins have a narrow selvage of phlogopite–clinozoisite or quartz–albite, and are associated with zones of higher grade gold and copper (Turner et al., 2020).
Mineralization
Two mineralization stages were recognized. The earliest phase consists of widespread silica–biotite alteration and complex quartz + albite + molybdenite ± muscovite ± clinozoisite ± chalcopyrite veins, all of which are variably deformed by ductile shear zones.
The second, major, alteration stage cross-cuts the first, and comprises:
• Quartz + albite + molybdenite ± muscovite ± biotite ± fluorite ± clinozoisite ± chalcopyrite veining;
• Clinozoisite + chalcopyrite + pyrrhotite + quartz + chlorite veins that host low-grade gold–copper mineralization;
• Actinolite + chalcopyrite + pyrrhotite ± quartz, carbonate + biotite veins that host high-grade mineralization.
Gold in the laterite zones occurs in association with iron and aluminum hydroxides. Gold in the saprolite is hosted in primary quartz veins, in clays immediately adjacent to mineralized quartz veins, and in secondary, shallow-dipping, goethitic horizons. Saprock mineralization reflects the mineralization distribution in the underlying bedrock.
Bedrock gold mineralization is hosted in veins, lenses and stockworks. Chalcopyrite and pyrrhotite the dominant sulfides, with lesser pyrite, sphalerite, cubanite, cobaltite, arsenopyrite, pentlandite, covellite, bismuthinite, digenite, marcasite and galena.
Quartz–albite–sulfide veins with coarse molybdenum, a dominant control for molybdenite distribution within the deposit, are found in both the Wandoo North and South areas but are dominant in the South Pit. Non-mineralized, thin felsic and intensely epidote-altered lithologies are seen in the Wandoo North area but are not reported from the South Pit.
Summary:
The milling plant includes a three-stage crushing facility (two primary crushers, six secondary crushers and four high-pressure grinding rolls), four ball mills, and hydrocyclone classification to generate a milled product with a P80 of 150 micrometers (µm) at a slurry density of around 35% solids.
Coarse Crushing (in mining area)
The mine haul trucks dump ore to two primary crushers (60/113 MK-II gyratory crusher). Crushed ore is transferred via an overland conveyor to a 230,000 t capacity (40,000 t live capacity) stockpile adjacent to the processing plant. Dozers operating on the coarse ore stockpile can increase the total storage capacity up to 400,000 t.
Fine Crushing and Screening (process plant)
Three apron feeders reclaim ore from beneath the coarse ore stockpile and delivers the ore to the secondary crusher feed conveyor and the six secondary crushers (MP1000 cone crusher, five original with a sixth crusher installed in late 2010), which are operated in closed circuit with four coarse screens (three original with a fourth coarse screen installed in 2010).
Oversize material returns to secondary crushing and the fine material reports to the tertiary crushing plant that consists of four HPGRs. The tertiary product is stored in a 20,000 t fine ore bin ahead of the ball milling circuit. Fine ore is reclaimed from the bin via eight reclaim belt feeders (two per four parallel milling trains) and delivered to the fine screens ahead of ball milling. Each line consists of two screening units.
Undersize material from the screens reports to one of four cyclone feed hoppers and the oversize returns to the HPGRs for additional crushing. Cyclone clusters classify the finely-crushed particles, with the finer cyclone overflow material (80% passing 150 µm) reporting to the flotation distribution box and the coarse cyclone underflow material to a split between the ball mills or flash flotation cell (for free gold recovery).
Tails from the flash flotation cells are recycled back to the ball mills for further grinding. The target final product grind size from the milling circuit is 80% passing 150 µm, although increased throughput rates in 2011 and 2012 resulted in grind size P80 coarsening to 170 to 190 µm at times. Improved process control and increased ball mill operating power draws have reduced the grind size back to around 150 to 160 µm at the elevated throughput rates.
The regrind plant consists of two Verti-mills (one duty and one standby) with product reporting, via cyclone clusters, to the cleaner flotation plant. The cleaner flotation facility has three sequential stages with final product being transferred to the concentrate thickener, then storage in two, 1,000 m3 tanks before being sent to the filtration plant. Concentrate is trucked to the port of Bunbury to be exported by sea.
Summary:
The milling plant includes a three stage crushing facility (two Primary crushers, six Secondary crushers and four high pressure grinding rolls), four ball mills, a flotation circuit and carbon in-leach circuit. The flotation circuit process recovers gold-copper concentrate before the material is then processed by a traditional carbon-in-leach circuit where the remaining gold is recovered to produce doré.
The processing plant has a nominal capacity to process approximately 40 million tonnes of ore per year with optimization projects underway to further increase this capacity.
The cyclone overflow from the mill circuit is treated in a flotation circuit that produces a copper–gold concentrate for export. Rougher and scavenger flotation concentrates are reground and cleaned to achieve an acceptable final concentrate grade. The concentrate is thickened and filtered before being trucked to port.
Cyclone overflow from the mill circuit is treated in a flotation circuit that produces a copper– gold concentrate for export. Rougher and scavenger flotation concentrates are reground and cleaned to achieve an acceptable final concentrate grade. Concentrate is thickened and filtered before being trucked to the port of Bunbury.
The cleaner scavenger tailings stream is thickened and leached under elevated cyanide levels. Scavenger tailings are thickened and leached in a conventional leach/adsorption circuit. Leached slurry from the cleaner scavenger tailings leach circuit is delivered to the scavenger tailings circuit for combined recovery of gold.
Leach residue is pumped to the residue disposal area, and residual CNwad is maintained below a targeted level by a Caro’s acid cyanide destruction plant. This facility can treat the following streams:
• Decant water returning to the plant so that cyanide levels do not inhibit flotation;
• Decant water recycling to the decant pond to maintain CNwad levels in the pond at an average of 30 ppm and a not-to-exceed level of 50 ppm;
• Residue slurry from the plant to protect the decant pond from excursions caused by shortterm variability in the copper head grade.
The carbon from the scavenger tailings adsorption circuit is treated by conventional split Anglo American Research Laboratory (AARL) method elution and reactivated in horizontal reactivation kilns. Gold recovery from the eluate is by electrowinning, cathode sludge filtration and drying, and smelting.
There is a flash flotation and gravity circuit installed in the process plant. These circuits have not been operated and remain decommissioned.
Gold and Copper Recovery to Concentrate
The flotation distribution box transfers cyclone overflow product from each of the four parallel grinding lines in to three parallel trains comprising, eight-unit flotation cells. Concentrates produced by cells #1 and #2 report to the coarse cleaner cells for final cleaning, and product from the other cells reports to regrind thickening.
The regrind plant consists of two Verti-mills (one duty and one standby) with product reporting, via cyclone clusters, to the cleaner flotation plant. The cleaner flotation facility has three sequential stages with final product being transferred to the concentrate thickener, then storage in two, 1,000 m3 tanks before being sent to the filtration plant. Concentrate is trucked to the port of Bunbury to be exported by sea.
The cleaner circuit has a scavenger circuit consisting of six cells. Cleaner scavenger tailings are leached in a dedicated circuit of nine leach tanks with the product combining, for further leaching, with the tails from the eight-unit flotation cells.
Gold Smelting and Bullion Production
Scavenger flotation tails report via a flotation tailings thickener to two five-unit leach tank trains. From the leach tanks, it transfers to two trains of seven carbon-in-leach (CIL) tanks and finally transferring to the residue disposal area (RDA). Activated carbon used in the leaching process to adsorb gold leached into solution reports to a two-train elution plant. Each elution plant has two elution columns, two heat exchangers and two elution heaters. The barren carbon product from the elution columns is transferred to the carbon reactivation plant before transfer back to the CIL train, and the gold solution transfers to the electrowinning circuit. The final product from the eight-unit electrowinning circuit reports to the gold furnace for smelting.
Recoveries & Grades:
| Commodity | Parameter | 2023 | 2022 | 2021 | 2020 | 2019 | 2018 | 2017 | 2016 | 2015 |
|
Gold
|
Recovery Rate, %
| 85.4 | 84.7 | 84.5 | 86.1 | 85.3 | 83.2 | 83.4 | 83.3 | 82.5 |
|
Gold
|
Head Grade, g/t
| 0.75 | 0.8 | 0.65 | 0.62 | 0.65 | 0.65 | 0.75 | 0.79 | 0.82 |
|
Copper
|
Recovery Rate, %
| 84.2 | 81.5 | 80.7 | 80.2 | 80.3 | 79.7 | 78.9 | 79.4 | 78.5 |
|
Copper
|
Head Grade, %
| 0.16 | 0.14 | 0.11 | 0.08 | 0.1 | 0.12 | 0.13 | 0.13 | 0.13 |