Murrawombie is a sulphide copper gold deposit located on ML1280 in central New South Wales (NSW), Australia. The deposit geology is described as a Besshi style volcanic associated massive sulphide occurrence. It contains economic grades of copper with minor gold and silver.

Regionally mineralisation is hosted within early to mid-Ordovician turbidite sediments, forming part of the Girilambone Group. Mineralisation is hosted within greenschist facies, deformed pelitic to psammitic sediments, and sparse zones of coarser sandstones.

The Murrawombie Deposit is considered to represent a stratiform volcanogenic massive sulphide system (Besshi style). Mineralisation varies from massive sulphide bands (pyrite +/ chalcopyrite) to erratic stringer pyrite/chalcopyrite. Mineralisation pinch and swells which is in part a result of bounding graphitic fault zones deforming the mineralised lenses.

The Murrawombie Deposit will be mined by a combination of underground and open pit methods. Underground mining will extract the majority of the ore. The open pit mining will be an expansion by a small push back of the south eastern wall of the existing pit to recover the remnant shallow resource. The pit expansion will be mined as the last stage of production from the deposit, to avoid complication of simultaneously working above an operating underground mine. No crown pillar will be left to separate open pit and underground workings.

The deposit has multiple lodes of mineralisation that are separated by waste. The lodes are numbered 101 through to 105 and modelled as separate geology domains.

At shallow depth, base of pit down to 4990mRL, only the 101 lode is sufficiently strongly mineralised to support an Ore Reserve. Below this depth the 101 lode narrows and the 102 lode becomes the dominate mineralisation of interest. Underground mining of the 102 lode supports the majority of the Ore Reserve estimate for the deposit, Isolated areas of stoping have also been identified on the 101 and 105 lodes and these contribute minor tonnage to the Ore Reserve.

The mining method applied to the 101 lode is up hole bench stoping with rock pillars. Stopes are mined the full width of the ore body and generally 20m between sub levels. Benches are mined in a bottom up sequence using dry rock fill for support of the hanging wall. Attempts to mine by top down sequence with only rock pillars for support was unsuccessfully trialed in 2016 and the extraction was converted to bottom up mining. A significant portion of the 101 lode above 4990mRL has been depleted by mining at 30 June 2017 reporting date.

The Ore Reserve estimate for the 102 lode is based on the use of two mining methods;
• Sub level open stoping with rock fill will be used to extract the higher grade south mining domain.
• Longitudinal retreat sub level cave will be used to extract the lower grade north mining domain.

The same 20 metre sub level interval is designed for both mining domains to allow flexibility in changing between methods if necessary to respond to changes in the resource models following from improved understanding of the deposit geology (dense grade control drilling, drive sampling and mapping). The need for flexibility in design has become apparent in 2016 - 2017 following from detailed geology mapping of the mineralisation in the 102 lode.

Mining of an approximate 70 to 50m wide pushback of the east wall of the open pit will expose 1.6 million tonne of ore in the wall and at the base of the open pit. The relatively narrow push back can be mined using a combination of new ramp in the upper few benches, then connecting to the old ramp located on the western or footwall side of the deposit. This allows effective mining of the narrow push back without need for a new ramp to full depth of the pit, reducing waste to ore strip ratios.

Waste and ore mining will be by conventional excavator and truck following light blasting. Trucks of 100 to 120 tonne capacity size and suitably matched excavators will be used.

Murrawombie copper ore is treated at the Tritton ore processing plant by flotation of sulphide minerals to produce a copper concentrate product.

The crushing circuit comprises an ore bin with a fixed 800 mm aperture grizzly, feeding ore via a vibratory feeder to a Kemco S7N single toggle jaw crusher with a feed opening of 1.22 x 1.02 m.

Crushed ore of P80 100mm is conveyed to a 6,000 tonne stockpile where it is reclaimed via three vibrating feeders.

Grinding comprises an ANI Ruwolt fixed speed high aspect 6.7m dia X 2.13, 1500 kW SAG Mill, discharging via a trommel screen to the secondary grinding circuit. Lime is added at the SAG to target pH 10.5 for pyrite suppression.

Secondary grinding comprises a Marcy 3.81 m dia x 5.18 m 1250 kW ball mill operating in closed circuit with 500 mm dia Linatex hydrocyclones. Cyclone overflow discharges to two parallel Metso Vertimills (VTM800STD) installed as part of the TEP and shown in the foreground of the lower photo.

The grinding circuit produces a cyclone overflow product of P80 75 µm at 30 to 35% w/w solids.

The tertiary cyclone overflow feeds two 50 m³ Wemco tank cells (installed as part of the TEP) in series for primary rougher flotation.

Regrind is achieved by a 2.0 m dia x 3.4 m 150 kW ball mill in closed circuit with Cavex hydrocyclones. The underflow reports to regrind and overflow to scavenger cleaner, rougher cleaner and final conc.

The Tritton flotation circuit produces a copper concentrate typically with 25% Cu, <1 g/t gold and 30 g/t Ag from a mill feed of 2.1% Cu, 0.15 g/t Au and 6- 7 g/t Ag.

Copper is recovered by cementation from the residual acidic copper solutions circulating in the Girilambone heap leach pads.