Deposit Type
The Plutonic Gold Mine deposits are Archean Greenstone gold deposits. The gold mineralisation is predominantly structurally controlled occurring in a variety of stratigraphic settings, mainly associated with replacement-style lodes and stockwork veining within a wide variety of host rocks ranging from ultramafic and mafic volcanic rocks, metasediments, felsic intrusive, volcanoclastic units, and banded iron formations.

In the Hermes Gold Mine and BBJV Projects there are two broad mineralisation styles (Outhwaite, 2013) referred to here as the Peak Hill Type and the Bryah Type.

The Peak Hill Type gold deposits are hosted in rocks that are generally highly deformed (four or more fold events) and metamorphosed (up to amphibolite facies), generally represented by the Peak Hill Schist Formation. Mineralisation is early in the paragenesis, (syn- to post-D1 isoclinal folding) with folded mineralisation commonly observed. Previously mined examples include Peak Hill Main/Five-Ways, Mt Pleasant, Jubilee, Wilgeena and St Crispin. The Hermes deposits may be examples of mineralisation that belongs in this category.

The local controls on this deposit class are difficult to identify, because of the effects of strong, postmineralisation deformation, and probable remobilisation of Au.

Mineralisation
Plutonic
The Plutonic Gold Mine Mineral Resources mined and unmined lie with a surface area of approximately 10 km east-west ........

Plutonic Underground
Plutonic Underground is a mechanised access underground mine that has been in continuous operation since 1995. Historically the mine has produced at a rate of up to 1,400 ktpa ore, although in recent years' production has stabilised at approximately 800 ktpa. The current constraint on underground production is not the infrastructure: the constraint is gathering the data to move the stopes through the planning process and into production.

The Plutonic Underground has seven active mining zones. Seven of these mining zones are well-established having been active for several years (Established Areas). The eighth zone, Baltic Extension, is a new mining zone (New Areas).

The underground mining operation covers an area of about three kilometres north-south and two kilometres east-west and currently extends over a 500 m vertical extent from approximately RL 150 m to RL 650 m. The Baltic Extension is a new mining area adjacent to existing workings and is planned to extend the depth of operation by a further 500 m vertical including Inferred Mineral Resources (total depth below surface 1,150 m). As the depth of mining increases, the ground stress also increases and can create significant mining issues. The mining studies to date have not considered stress as part of the Baltic Extension, however mining to this depth will occur gradually and the issue is not imminent or expected to materially impact successful extraction of the Mineral Re ........

Crushing
Run of Mine (ROM) ore is trucked to the ROM pad from the underground mine. The ore is classified and stockpiled according to gold grade, arsenopyrite content, pyrrhotite content, and graphitic content so that blending can be undertaken to maintain an optimal feed to the processing plant. Oversize ore and tramp metal are sorted from stockpiles and broken on the ROM pad using a loader or excavator. Any oversize that cannot pass through the primary crusher grizzly is broken by a rock breaker mounted at the grizzly.

The PP1 crushing circuit has a nameplate capacity of 2.5 Mtpa and consists of three stages of crushing:
A 60 x 48 Jacques primary double-toggle jaw crusher,
A Symons 7’ SXHD secondary standard head cone crusher, and
Two Symons 7’ SXHD tertiary short-head cone crushers.

In addition, there are separate surge bins that are operated in closed circuit with two Nordberg 7.1 m x 2.4 m double deck vibrating banana screens. Crushed ore exits the product screen with a top size of 10 mm and is stored in the fine ore bin. The fine ore bin has a live capacity of 3,000t.

PP1 crushing circuit contains 2 x Thermo Scientific Ramsey 10-17 belt scales (CV07 and CV13) for measuring mass of circuit ore.

The now decommissioned PP2 oxide crushing circuit consists of a 48 x 42 Kemco double toggle jaw crusher with a nameplate capacity of 1.2 Mtpa, a product conveyor and a coarse ore stockpile with a live capacity of 2,200 tonnes. Crushed oxide ore was transferred to PP2 grinding mills using two variable speed belt feeders.

Grinding
Crushed ore is withdrawn from the Fine Ore Bin via two belt feeders (CV 14/15), which transfer ore onto the mill feed conveyor (CV04) that feeds into the primary grinding mill (ML01). Mill feed can also be fed via an emergency feed hopper (CV02) which is fed via the oxide coarse ore feed slots. Quicklime is discharged onto CV04 via a variable speed, manually controlled rotary valve from a 200t lime silo. Liquid lead nitrate (40% w/w) is discharged directly into CV04 head chute into the grinding circuit.

The grinding circuit comprises a Svenson 4.5m diameter by 5.63m long primary mill and two Svenson 4.2m diameter by 5.63m long secondary ball mills. The primary mill has a grate discharge and is rubber lined. Its speed is fixed at 14.6 rev/min (72 per cent of critical) and the installed power is 1,600kW (1,350kW drawn). 78mm diameter forged steel grinding media is used in the primary mill.

The secondary mills are rubber lined overflow mills run at 15.8 rev/min (75% of critical), also with 1,600 kW power (1,450 kW drawn). The grinding circuit throughput is currently operated at 165 tph with a primary mill and one ball mill configuration; this however can be increased to 230 tph by running the stand-by ball mill. 40 mm High Chromium steel grinding media is used in the secondary mills.

The primary mill discharge slurry is screened on a 6 mm aperture scalping screen and oversize is returned to the primary mill. Screen undersize reports to the ball mill discharge hopper. ML01 mill undersize and ML02/ML03 mill discharge is pumped to a hydrocyclone cluster consisting of 18 x 250 mm Cavex cyclones. Operating pressure is 130 to 150 kPa. Each cyclone contains 90 mm ceramic vortex finders and 75 mm ceramic spigots. Coarser cyclone underflow is returned to the operating secondary ball mill for further size reduction. Cyclone overflow (approximately 80% passing 75µm) discharges over a trash screen (1mm) with screen undersize reporting to the leaching circuit.

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