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Summary:
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.