The Citronen Deposit is interpreted as belonging to the SEDEX deposit class, forming syn-depositionally with sedimentation.

SEDEX deposits are formed in submarine environments by the precipitation of sulphides from metal bearing fluids introduced onto the seafloor through underlying fractures which act as metal-bearing fluid conduits. Large amounts of sulphur are precipitated principally as pyrite and focused around vent areas or ‘mounds’ on the sea floor. Base metal (Zn + Pb) bearing sulphides at Citronen are predominantly located within laminate horizons surrounding these larger sulphide accumulations.

Mineralisation at the Citronen Fjord Zn-Pb Deposit comprises several distinct sulphide mounds containing massive and net-textured pyrite-rich mineralisation, interpreted to represent the focal point of fluid influx, flanked by pyritic laminated sulphides that are locally sphalerite and galena-rich. These laminated sulphides host the majority of economic grade mineralisation.

The deposit consists of multiple sulphide mounds forming in three lateral positions (“vents”), defined as the Discovery, Beach and Esrum ore bodies.

The mineralisation is hosted within two fine- grained sedimentary units, separated by a mass carbonate debris flow. The majority of mineralisation is stratiform, with semi-massive net-textured to massive sulphides accumulating in the core of the mound structure. Structurally controlled stock-work style mineralisation is present within the carbonate debris flows, with the most notable termed the XX ore body. The stratiform mineralisation has been identified from outcrop to a depth of approximately 500 metres, with the mineralisation open at depth. Level 1 is located predominantly within the Discovery ore body, Level 2 is evident discontinuously across all three ore bodies and Level 3 contains the largest volume of sulphides, with a lateral extent of over 3,000 metres between the Beach and Esrum ore bodies.

The mineralisation is pyrite dominated with variable amounts of sphalerite ((Zn/Fe)S) and lesser galena (PbS) present as sulphide species. Minor chalcopyrite (CuFeS2) has been documented and interpreted as having formed during remobilisation and enrichment of primary stratiform-hosted mineralisation. No economically significant Cu or silver (Ag) has been identified to be associated with the sulphide mineralisation.

Primary mineralisation is generally fine to medium grained, weakly to moderately laminated and bedded parallel with regionally deposited sediments. Gangue mineralogy is primarily silt and clay from mudstones deposited contemporaneously with sulphide mineralisation.

The ore body’s nature and orientation is well understood and the room and pillar mining method has been selected as the most appropriate for this style of deposit.

With the mining method being room and pillar, a recovery factor of 100% recovery was assumed as ore loss should be negligible provided that a tele-remote bogging system is used for the recovery of the ore drive strip material. The likelihood of ore loss would increase if a line of site remote bogging system was used.

All mining has been planned for conduction via development jumbo and, as such, minimal overbreak (i.e. unplanned dilution) outside of the planned dilution skin is expected. As a result, no tonnage factors were applied to this design.

Open pit mining operations have been scheduled to commence at the end of the life of the underground mine in Year 11 (3.3 Mtpa scenario).

The open pit mining rate to continue to maintain a total ore feed rate of 3.3 Mtpa to the process plant until the open pit is depleted. Mining will utilise 10 x 60t haul trucks and excavators until the point where the project is closed out, subject to the discovery of additional resources.

Crushing
Crushing equipment selection and circuit configuration is based upon UCS, Bond Crushing Work Index and Macon testing (Metso’s in-house testing method) of both the Discovery and Beach ore composites. A compact two stage crushing plant has been selected, with DMS testwork indicating acceptable metal recovery and mass rejection can be obtained with a primary crusher discharge P80 of 130 mm reduced to a P80 of 35 mm in the second stage.

The circuit equipment has been selected to achieve approximately 550 t/h average throughput levels at 18 h/d operation (6.5 d/wk, 365 d/a). The ore is delivered by haul truck and either direct dumped into the run of min (ROM) bin or onto the ROM pad and later transferred by front-end loader (FEL) to the ROM bin. The ROM bin is protected by an 800 x 800 mm grizzly, and a rock breaker is mounted adjacent to break up oversize.

The ore is fed by a vibratory grizzly feeder drawing ore from the ROM bin and feeding into the jaw crusher where it is crushed to a P80 of 130 mm. A primary jaw crusher size has been selected to minimise the probability of blockages.

A high frequency vibratory double-deck secondary screen precedes the secondary cone crusher with undersize reporting to the crushed ore product transfer point. Tramp steel is removed by two magnets located over the conveyor. A metal detector is located on the secondary crusher feed bin feed conveyor with an interlock hardwired to the conveyors.

The secondary screen oversize is conveyed to the secondary crusher feed bin where a vibrating feeder feeds the secondary crusher. The secondary crusher discharge is conveyed to the high frequency vibratory double deck screen. The screen undersize is conveyed over a weightometer to the DMS feed bin.

Dust suppression is by individual suction hoods at transfer points ducted to a central fan and bag- house.

Buffer storage of crushed ore and mill feed is minimised due to space constraints.

Grinding
Ball Milling
A 4.42 m diameter x 7.5 m long 2.2 MW grate discharge primary ball mill operates at an approximate average of 298 t/h treatment rate in closed circuit with a scat recycle screen with a bottom deck aperture of 1.2 mm.

The screen separation at nominally 1 mm is facilitated by high volume water sprays, and uses the entire water requirement for the grinding circuit to achieve a clean separation. The screen undersize flows to a common pump hopper that also receives DMS fines spiral concentrate and secondary grinding mill discharge.

The mill has sufficient power to achieve the design grind size of 850 µm P80. The mill is rubber lined, takes up to a 24% by volume steel charge and operates at 71% of critical speed. Balls of 90 mm to 120 mm size are added by kibble at the mill feed chute.


Vertical Stirred Milling (VTM)
Primary screen undersize, VTM discharge and DMS fines spiral concentrate are combined in a hopper and then pumped to a cyclone cluster at a density that enables a 53 µm cut point to provide a P80 of 45 µm to flotation. The cyclone cluster holds 6+2 400 mm diameter cyclones. Cyclone underflow feeds the VTM circuit and cyclone overflow is gravity fed to a trash screen. The VTM unit discharges overflow back to the cyclone feed hopper.

Two 2.2 MW VTM units have been selected with 24 mm x 24 mm recharge size high chrome cylpebs that are added by kibble at the mill feed chute. A proprietary blocker reagent (D200) may be added to the cyclone feed hopper.

Dense Media Separation (DMS)
DMS performance at a 2.96 to 3.10 separation SG provides recoveries in excess of 97% of lead and zinc to the sinks plus the -1.0 mm fines product bypass throughout the size ranges tested.

The DMS feed bin discharges at an approximate average of 375 t/h by belt feeders to a wet feed preparation screen that removes all -1.0 mm material, with the screen oversize feeding into the DMS cyclone feed hopper.

The DMS cyclone pump feeds -38+1 mm material combined with dense medium to the DMS cyclones. The DMS cyclone overflow is stripped of medium on a drain-and-rinse dual screening system with screen oversize conveyed to the external floats stockpile. The DMS cyclone underflow is stripped of medium on a drain rinse screen system, and the screen oversize is conveyed to the sinks (ball mill feed) bin.

The DMS feed fines are separated at the wet feed preparation screen. Sixteen percent of the ore feed weight reports as -1 mm fine ........