The Woodlawn VMS deposit lies on the eastern limb of the asymmetric northnorthwest plunging Woodlawn Syncline. The regional structures trend approximately north-northwest with the local foliation dipping moderately to steeply to the west. The Woodlawn deposit is hosted by regionally metamorphosed (green schist facies) fine to coarse grained felsic to intermediate volcanic rocks, volcanogenic sedimentary rocks and minor carbonaceous shale, known as the Woodlawn Volcanics. The axial plane dips at about 60° to the west and is paralleled by a strong slaty cleavage or in places a schistosity through the mine sequence.

Within the Woodlawn mine area, the Woodlawn Volcanics comprise a lower sequence of coarse-grained silica and sericite altered volcaniclastic quartz-phyric rhyolitic rocks with siltstone, mudstones and minor basalt overlain by strongly silica and sericite altered rhyodacitic to intermediate volcaniclastic mudstones and sandstones that host the massive sulphide lenses. Overlying the mineralised sequence is a less altered massive, coherent, quartz–feldspar phyric rhyolite unit. The Woodlawn Volcanics have been intruded by calc-alkaline and tholeiitic dolerites (McKay & Hazeldene 1987) related to the Currawang Basalt. Dolerites postdate hydrothermal alteration and mineralisation at Woodlawn and are boudinaged (along with mineralisation) indicating that they were emplaced prior to deformation.

The Currawang deposit is conformable to and hosted by sub-vertical sheared spilitic pillowed and massive basalts with basaltic tuffs and fine-grained clastic rocks of the Currawang Basalt. The Cowley Hills deposit is also hosted in the Woodlawn Volcanics. The mineralisation consists of a single massive sulphide lens of average 9-15m true width, 80m strike length conformably hosted in altered hydrothermally altered felsic volcanogenic sediments within dolerite and spilitic volcanics of the Currawang Basalt.

The Woodlawn deposit occurs in Silurian felsic volcanic rocks, volcanogenic sediments and carbonaceous shales intruded by doleritic sills. It is considered to be part of suite of Volcanic-hosted Massive Sulphide (VMS) deposits formed along the eastern coast of Australia in the Paleozoic (240 to 540 Ma) (Gemmell et al 1998). Other significant VMS deposits in this suite include Mt Lyell, Rosebery, and Hellyer in Tasmania, Stockmans in eastern Victoria, Captains Flat in NSW and Mt Morgan and Thalanga in Queensland.

Three mineralised horizons hosting twelve known massive sulphide lenses occur within a 400 by 600m wide and 900m+ long northwest plunging envelope at Woodlawn. Each horizon has distinctive differences in mineralogy, geochemistry, metal ratios, alteration, isotope geochemistry and geophysical characteristics which reflect evolving ore-forming processes. Two major mineralisation types are recognised: a “copper-rich” assemblage and a “polymetallic” assemblage (see McKay & Hazeldene 1987; Glen et al. 1995). The “polymetallic” assemblage includes fine- to medium grained, banded to massive pyrite–sphalerite–galena–chalcopyrite with the gangue mineralogy including talc, quartz, chlorite, phlogopite, muscovite and barite. The copper-rich assemblage includes pyrite–chalcopyrite, lesser pyrrhotite as well as chlorite, quartz and calcite as massive sulphide and stock-work veins.

The deposit geometry has been modified by both ductile and brittle deformation. The ductile deformation is preserved as a prominent foliation with minor rodding concentrated along the margins of dolerite, in fine grained units and phyllosilicate altered zones. The dolerites and mineralised lenses display stretching to the northwest in the plane of foliation and are cut by lens parallel and sub-parallel anastomosing brittle faults.

Mining at the Woodlawn Project will proceed from two independent ore sources, fresh ore from underground and surface reclaimed tailings from the existing tailings dams.

The tailings will be hydraulically mined using a bottom up approach with a channel feeding back to a sump.

The underground is scheduled to produce at an annualised maximum rate of up to 864 ktpa using three different mining methods:
- Drift and half uppers;
- Underhand Longitudinal Open Stoping; and
- Underhand Transverse Open Stoping.

The underground production is supplemented by the retreatment of the tailings from the previous mining operations. Work undertaken on the processing of the underground ore and tailings has determined that a steady-state production rate was not required for the underground operation, so no nominal production rate was defined for the underground operation. The operation was scheduled with no steady-state production rate target. Access to the Woodlawn mine will be through a new boxcut and portal located to the west of the existing open pit. Ore is scheduled from a combination of new and remnant resource blocks. Ore will be truck hauled to surface then transferred overland to the processing plant located 1.2km to the east via a 2.1km long haul road.

Crushing - Underground Plant Feed
Underground ore will be stockpiled on the ROM pad before being fed into the ROM bin via front end loader (FEL) for processing in the crushing circuit. The ROM bin will include a 550mm static grizzly to prevent oversize feed entering the crushing circuit. Oversize product will be removed from the static grizzly by FEL and broken using mobile rock breaking equipment on the ROM pad.

A variable speed vibrating grizzly feeder will reclaim ore from the ROM bin and control feed to the primary crusher. The grizzly feeder will have a 75mm aperture for scalping feed to the primary crusher. Undersize product will report to the crusher discharge conveyor (CV-01) along with the primary crusher crushed product. The primary crusher will be a jaw type crusher with a 70mm closed side setting and will produce a P80 product of 75mm. Product from the primary crusher and grizzly feeder will be conveyed to the product screen via CV-01 and CV-02. The CV-01 and 02 transfer station will include an inline self-cleaning tramp magnet. Tramp metal will discharge into a skip bin.

The product will be screened on a double deck vibrating screen with a top deck aperture of 28mm and bottom deck aperture of 14mm. Undersize product will be conveyed to the fine ore bin. Oversize product from both decks will be conveyed to the secondary crusher feed bin.

Product will be reclaimed from the secondary crusher feed bin with a variable speed pan feeder to control feed to the secondary crusher. The secondary crusher will be a cone type with a closed side setting of 22mm and will produce a P80 of 23mm.

The secondary crusher will operate in closed circuit with the product screen. Product from the secondary crusher will be discharged onto CV-01 and conveyed to the product screen for re-screening.

Final crushed product (minus 14mm) will have a P80 of 10.5mm and will be conveyed to the fine ore bin prior to the primary ball mill. The fine ore bin will provide 1,500t or ore storage or 12 hrs of nominal milling time when full. Ore will be reclaimed from the bin using a reclaim conveyor. The bin will include an opening to allow access with mobile equipment for maintenance. An emergency feed system has not been included. Any coarse product spillage will be re-introduced to the circuit via the ROM bin.

The primary milling circuit will consist of a ball mill closed with hydrocyclones. The treatment rate will be set for 125t/h, based on a 93.5% utilisation rate when treating underground feed. Ore will be reclaimed by the fine ore bin reclaim conveyor using a variable speed drive and weightometer for feed control to the ball mill. The ball mill will be closed with hydrocylone classifiers and will produce cyclone overflow P80 of 75µm. The cyclone overflow will report over a trash screen prior to being pumped to the gangue pre-float circuit.

Grinding - Underground Plant Feed
The primary milling circuit will consist of a ball mill closed with hydrocyclones. The treatment rate will be set for 125t/h, based on a 93.5% utilisation rate when treating underground feed. Ore will be reclaimed by the fine ore bin reclaim conveyor using a variable speed drive and weightometer for feed control to the ball mill. The ball mill will be closed with hydrocylone classifiers and will produce cyclone overflow P80 of 75µm. The cyclone overflow will report over a trash screen prior to being pumped to the gangue pre-float circuit.

The copper flotation circuit will be the same configuration as used for the tailings retreatment circuit with the addition of a regrind mill for the copper rougher concentrate. The copper rougher concentrate size will be reduced to a P80 of 30 µm in an SMD-355 operating in open circuit with cyclones used for density control.

The copper scavenger tail will be pumped back to the primary IsaMill for secondary grinding to a P80 of 30 µm.

The lead rougher concentrate size will be reduced to a P80 of 15µm in the lead regrind mill in open circuit (one SMD of 2.67m3 capacity and fitted with a 355kW motorin).

The zinc flotation circuit will consist of the regrind mills; one SMD, 355kW, 3.22m3 and a second 355kW regrind mill operating in parallel (or series).

Grinding – TSF Reclaim
Reclaimed tailings will be thickened in an 18m diameter high rate feed preparation thickener to an underflow density of between 60% and 65% solids (w/w). Thickener overflow will gravitate to the process water dam for use in the hydraulic mining process and process plant. Thickener underflow will be pumped to the mill feed surge tank by one of two thickener underflow pumps in a duty/standby configuration. The mill feed storage tank will provide a 2,200m3 storage capacity prior to the milling circuit. Thickened slurry from the mill feed surge tank will be diluted to approximately 50% solids within the mill feed sump and then pumped by centrifugal slurry pump to the horizontal type grinding mill. The grinding mill will be an IsaMill 10000 unit that will have a volume of 10m3 and will be equipped with a 3.0MW drive. Mill feed slurry will be ground from an average feed size F80 of 90µm to a product size P80 of 30µm using ceramic grinding media. The grinding mill will be configured in open circuit as the IsaMill has an internal product separator that effectively classifies the product stream prior to its discharge. Lime will be added to the mill feed to maintain a pH of 6.8.

The plant will treat either reclaimed tailing at a rate of 190t/h (1.5Mtpa equivalent) or the underground material at a rate of 125t/h (1.0Mtpa equivalent).

Treatment of underground material uses the equipment and basic flowsheet used for reprocessing of the reclaimed tailing. The process route incorporates two stage crushing, ball milling to a primary grind size P80 of 75µm, gangue pre-float, sequential copper, lead and zinc flotation with regrinding of rougher concentrate in each circuit, and secondary grinding of the copper rougher tailing to a P80 of 30µm.

The flotation circuit employed will utilise a gangue pre-float, copper, lead and zinc differential flotation sequence. Feed will be either cyclone overflow from the primary ball mill (when processing underground ore) or IsaMill product when processing tailings ore.

Pre-float cleaner concentrate will be discarded to final tails to remove some of the gangue and talc ahead of the differential flotati ........