The Golden Grove zinc-copper-gold-silver-lead deposits occur within the Warriedar Fold Belt (“WFB”) which is located in the southern part of the Yalgoo-Singleton greenstone belt of the Murchison Province within the Archaean Yilgarn Craton of Western Australia.

The Golden Grove volcanic-hosted massive sulphide (VHMS) mineralisation occurs predominantly within the Golden Grove Formation (“GGF”) and the Scuddles Formation (“SCF”) of the Gossan Hill Group (“GHG”). The GHG lies along the northeast flank of the WFB forming the eastern limb of a regional syncline and has an average thickness of 3km and a strike length of around 28km. The stratigraphy is west facing and dips steeply to the southwest (75-80°); the regional syncline has been cut by extensive northeast trending, steeply dipping dextral faults and a major regional structure, the Mougooderra Fault, is located to the west of Golden Grove.

Zinc and copper mineralisation are largely hosted in the Golden Grove Formation which is divided into six members (GG1-6) based on lithology and alteration. The GGF comprises a footwall sequence of rhyolite flows and tuffs, a host sequence of volcaniclastic litharenites and crystal tuffs, and a hangingwall sequence of felsic lavas. Mineralisation forms a series of steeply dipping lodes with most of the mineralisation occurring in the GG4 and GG6 members of the GGF, with stratiform zinc concentrated principally at the GG6 horizon and copper mineralisation typically lower in the stratigraphy at GG4. Some zinc mineralisation occurs within GG5 and in the SC2 and SC3 members of the overlying SCF. Zinc mineralisation in the form of sphalerite also hosts galena, gold and silver. Average zinc grades of lenses are typically in the range 7-16% Zn, with some exceptionally high-grade drill intersections of up to 20-30% Zn. Copper mineralisation is mainly present as chalcopyrite with copper lens grades typically in the range 1.5-3.0% Cu. Gold is present as electrum (Au-Ag alloy) or native gold and silver is present as native silver and freibergite (Cu-Sb-Ag sulphide), as well as electrum. The highest gold and silver grades are present within the SC3 member and in some parts of the GG6 member in the range 1-4g/t Au and 50-100g/t Ag. Elsewhere, in GG4 and GG6, gold grades are generally in the range 0.4-1.3g/t Au and silver in association with zinc ranges from 20-90g/t Ag.

A local mine grid is used at Golden Grove which is rotated 52.4° to the west of the national MGA94 grid. Three main deposits have been defined which are from north to south - Scuddles, Gossan Hill and Gossan Valley. Gossan Hill is located approximately 4km south of Scuddles and Gossan Valley is a further 7km south of Gossan Hill (Figure 4). A dextral fault zone (Racehorse Fault) passes between the Gossan Hill and Scuddles deposits, displacing Scuddles approximately 1.2km to the east.

At Gossan Hill, mineralisation occurs within both the GG4 and GG6 members, separated by about 50m of relatively unaltered GG5 clastics. The copper and zinc mineralisation tend to form separate lenses although copper mineralisation can also occur on the footwall of the zinc lodes. Copper and gold resources also occur in the oxide zone above the primary copper and zinc zones respectively. The known mineralisation at Gossan Hill occurs over a strike length in excess of 1,500m, although individual lenses typically have a strike extent of 200-300m and are up to 20m wide. A total of 18 separate mineralised zones are currently defined at Gossan Hill to a maximum depth of around 2,000m below surface, with a number of lenses remaining open in the down-dip direction. There is an oxidized cap above the ABCD primary zinc mineralisation, containing zinc, gold and silver. Oxide and supergene copper, mostly malachite with minor azurite, chalcocite and native copper, overlies primary copper sulphides in the Tryall zone. The oxide zone extends to a depth of 80-100m.

The Scuddles area contains the complete GGF sequence including GG3 which is absent at Gossan Hill. Dacite and rhyodacite in SC1 and SC2 overlie the zinc and copper mineralisation. Mineralisation occurs largely within the GG6 member and consists of massive zinc and massive pyrite with either dominant zinc or copper sulphides. The massive sulphide mineralisation is underlain by a stringer sulphide zone. The mineralisation extends about 500m along strike, is up to 50m wide, and extends to over 1,000m depth. Copper mineralisation occurs adjacent to the zinc mineralisation, but there are some later cross-cutting copper zones. Mineralisation is primarily sphalerite, chalcopyrite and galena with pyrite and pyrrhotite. Silver is present within the sphalerite-rich sections of the deposit and gold occurs mainly in the centre of the sulphide mineralisation near the transition between massive sphalerite and massive pyrite.

The Gossan Valley deposit is located around 7km south of Gossan Hill and presently comprises five separate mineralised lodes, the main Gossan Valley lenses and four smaller areas, Grassi and Flying Hi, 1-2km to the south of Gossan Valley and Felix and Conteville, 0.5-1km to the north.

Zinc and copper mineralisation are hosted in the GGF, chiefly in the GG4 and GG5 members, unlike Gossan Hill and Scuddles where zinc mineralisation occurs predominantly in GG6. Sphalerite and chalcopyrite have locally replaced massive magnetite in GG4 and GG5. Stringer chalcopyrite also occurs in GG1 and occasionally in the GV4 Member of the Gossan Valley Formation. Small lenses of sphalerite and chalcopyrite are also developed in GG6. The same style of mineralisation has been intersected at Felix approximately 500m to the north of Gossan Valley with massive magnetite locally replaced by sphalerite and chalcopyrite.

Post-mineralisation dacite/rhyodacite, dolerite and rhyolite dykes cross cut, displace and stope out portions of the mineralisation. The GGF dacite dykes are interpreted as feeders for the overlying dacitic volcanics in the Scuddles Formation. The dolerite dykes were emplaced after the dacite and form sub-horizontal to moderately dipping bodies. The rhyolite dykes are generally narrow and post-date the dolerite.

Development of the Scuddles mine started in 1988 with a decline and shaft sink; production commenced two years later. The Gossan Hill decline was started in 1994 and production commenced in 1998. A number of open pits have been mined in the past for copper and gold ores, but no open pit mining is currently being undertaken.

Scuddles is accessed via a 630m shaft and a 1 in 7 gradient decline to a depth of 1,105m with deep drilling having identified ore grade mineralisation to depths of around 1,950m; Gossan Hill to the south is accessed by decline only, to a current depth of around 1,370m at Hougoumont and 1,437m at Xantho. The decline branches off to provide access to the two major current producing orebodies, Xantho and Hougoumont; deep drilling has identified ore grade mineralisation to depths of around 2,180m.

Gossan Hill is currently the primary source of ore, which is hauled to the surface via the decline, stockpiled at the Gossan Hill ROM, crushed and conveyed around 3km by surface conveyor to the processing plant adjacent to the Scuddles shaft; ore from Scuddles is crushed underground and hoisted to the surface by skips operating in the main shaft.

Production in the last two years came from a total of eight ore zones mined at Gossan Hill and Scuddles. The major ore producer over the last two years has been Hougoumont, with 30-40% of production coming from this ore zone. Xantho has been the next most significant ore source contributing around 20% in 2020. AB Zinc, Amity, Tryall and Scuddles each contributed around 10% of production tonnage in 2020.

In terms of ore type, Hougoumont and Xantho have been the main sources of zinc ore while Tryall, Scuddles and Hougoumont are the main sources of copper ore. Zinc ore (including mixed ZnCu ores) made up approximately 60% of the plant feed over the last two years with copper ore accounting for around 40%.

The underground mining method used at Golden Grove is conventional, mechanised bottom-up longitudinal long hole open stoping (LHOS) with either unconsolidated rock fill (mine waste) or cemented hydraulic fill (CHF). This method has been used at Golden Grove for many years and is well understood and successfully applied. Underhand, top-down variations of the method are planned in some areas in the future.

Transverse stoping is used in ore zones with significant width, typically 13m or more. Minimum mining width for longitudinal stopes is 3m before dilution is applied. Sublevel intervals throughout the operation are generally 30m.

Cemented Hydraulic Fill (CHF) is used, enabling the design of pillar-less stopes where geotechnically feasible, maximising recovery of the resource. Where practical the mine designs incorporate top level development to facilitate rock filling of stopes to reduce CHF requirements.

The mine production rate is planned to ramp up to 1.53Mtpa in 2021, from 1.44Mt in 2020 and 1.37Mtpa in 2019; production is forecast to average 1.6Mtpa from 2022. The development of the Gossan Valley project is also under consideration with a feasibility study due for completion in Q2 2021, which, if approved, could take total mine production to 2Mtpa.

A number of initiatives are in place to achieve a higher production rate including the introduction of paste fill at Gossan Hill. EMR Golden Grove Pty Ltd (now Golden Grove Operations Pty Ltd) is planning to hire a paste fill plant on a three- or five-year contract. The proposed plant was designed and assembled for a BHP contract and it is anticipated that some modifications will be necessary. The proposed commencement date for paste fill is Q2 2021 and drill holes and piping for paste reticulation are currently being established.

All mining operations underground are carried out by the mine contractor, Byrnecut Australia Pty Ltd (“BAPL”), except for box-hole boring carried out by Redpath Australia Pty Ltd (“Redpath”) and shaft services and CHF installations carried out by Murchison Mining Services Pty Ltd (“MMS”).

The mine contractor has a mining fleet comprising 13 x 65t haul trucks, seven 20t loaders, five development jumbos, three production drills, and miscellaneous service vehicles including graders and charging vehicles. The equipment is serviced and maintained in the contractor’s workshop at Gossan Hill; there are no underground workshops although some refuelling is done underground. The mine fleet also includes a boxhole rig provided by a separate contractor.

The Gossan Hill crushing circuit consists of a 60" x 48" 150kW primary jaw crusher located at the surface of the Gossan Hill mine. The ore is crushed to 100mm and conveyed via an overland conveyor to the Gossan Hill stockpile at the Scuddles processing plant. The ore size is reduced further to 40mm via a secondary crusher (Weir Trio TC66 Cone Crusher with 315kW drive) before discharge to the Gossan Hill stockpile.

Scuddles ore is crushed underground through a 60" x 48" 150kW primary jaw crusher and hoisted up the Scuddles shaft and discharged to the Scuddles stockpile. This ore stream does not currently report to any subsequent secondary crushing.

The grinding circuit is a semi-autogenous (SAG) mill, ball mill and recycle crusher configuration (SABC) comprising:
• a 6.7m diameter by 2.1m 1500kW SAG Mill
• a Weir Trio TC51 150 kW pebble (recycle) cone crusher
• two ball mills, a 3.8m diameter by 6.7m 1500kW mill, and a recently re-purposed smaller 2.74m diameter by 4.26m 400kW ball mill, which was previously utilised as a regrind mill.

Classification is achieved via a bank of six Weir Cavex 400 cyclones.

The SAG mill operates in closed circuit with the single-pass pebble (recycle) cone crusher. SAG mill grinding media comprises 105mm and 125mm diameter steel balls. The SAG mill is a single pinion fixed speed mill that rotates at 12.5 revolutions per minute (“rpm”), 76% of critical speed. Many SAG mills in this grinding configuration are variable speed; the Golden Grove SAG mill is a fixed speed design, resulting in the potential for SAG mill charge weight variations with variable feed characteristics. Manipulation of the stockpile feeders and a bypass gate on the pebble crusher are used to assist in maintaining the SAG charge weight with varying feed characteristics, such as ore size and ore hardness. Rapid increases in ore size limit the mill throughput and induce surging through to the flotation plant. Decreases in ore size are less deleterious, as the pebble crusher can be bypassed with the pebble stream (uncrushed) re-cycled back to the SAG mill.

Secondary crushing of the Gossan Hill ore, introduced in December 2019, has increased stability in the grinding and flotation sections.

The SAG discharge passes through a 12mm trommel; the trommel oversize reports to the pebble crusher to produce a <10mm product which is returned to the SAG mill and the trommel undersize is pumped to the primary cyclones, which classify the slurry, with the coarser particles reporting to the cyclone underflow, and the finer particles reporting to the cyclone overflow. The cyclone underflow is recirculated to the ball mills for further size reduction, and the cyclone overflow is pumped directly to flotation feed. The target flotation feed is a P80 (80% passing) of 106µm and is monitored manually on a shift basis.

A 2.74m 400kw ball mill has been repurposed from regrind duties to run in parallel with the primary ball mill to increase grinding circuit capacity.

A portion of the combined SAG and ball mill discharge is treated via a SB2500B–SE Falcon gravity concentrator unit. This unit recovers approximately 30-40% of the gold present in the ores prior to flotation. The gravity circuit contributes to an increase in overall gold recovery of about 10% compared with flotation only. The concentrate produced from the gravity circuit is recombined with either the Pb or Cu concentrates (added to the Pb or Cu concentrate thickeners) and marketed as a high precious metal concentrate.

The Golden Grove processing plant has operated continuously since initial commissioning in 1990. The plant currently treats ore from two mining areas, Gossan Hill and Scuddles, containing zinc, copper and lead sulphides and precious metals. The processing plant is designed to produce saleable metallic sulphide flotation concentrates, and has the capability, depending on the ore feed, to produce three principal concentrates. Whilst internal reporting refers to additional concentrate classifications for metallurgical accounting purposes, the saleable concentrate classifications are as follows:
• Copper concentrate - LPM (low precious metal) or HPM (high precious metal) content, depending on ore feed
• Zinc concentrate
• Lead HPM (high precious metal) concentrate.

The processing plant is mature, having been in operation since 1990. Incremental upgrades have been made to the flowsheet, primarily focused on the milling and flotation circuits to improve recovery ........