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The Gold Ridge intrusion-related low sulphidation epithermal gold deposit is located ~40 km by road, and 23 km in a straight line, to the south-east of Honiara on Guadalcanal Island, in the southeastern Solomon Islands. The Gold Ridge project comprises four deposits, the Valehaichichi, Kupers, Namachamata and Dawsons deposits, which lie within a diamond shaped, hydrothermally altered, 2.5 x 1.4 km zone trending north-northeast.
(#Location; 9° 34' 35"S, 160° 8' 4"E).
Gold was first reported from the mouth of a stream draining Gold Ridge by Spanish explorers in 1568. Gold was again discovered in the Gold Ridge catchment in 1931, and was traced to soils and bedrock at Gold Ridge in 1936. Active prospecting commenced in 1931 and in 1939 sluicing of the Gold Ridge area commenced. Government surveys in the 1950s and 1960s failed to find any anomalies, until a geochemical survey in 1972 outlined arsenic anomalies over the four deposits. The land was tendered in 1974, with CRA Exploration Pty Limited being the successful bidder. Drilling indicated grades of 0.25 to 2.0 g/t Au which was too poor at the prevailing gold price (USD 35/oz or USD1.12/gram) and the title was surrendered. Subsequently a JV between Amoco Minerals and Cyprus, and then Arimco drilled over 35 000 m of core and RC drilling, but failed to outline a feasible resource. In 1994, the government issued another tender to Saracen Minerals, who soon after sold out to Ross Mining. Ross Mining then drilled another 29 000 m of RC and 300 m of core and put the project into production in August 1998. In May 2000 Delta Gold purchased Ross Mining, but in June of the same year had to suspend operations due to civil unrest. As a result of the abandonment, AIG, the American insurance group, paid out on a political risk policy held by Delta (by then part of Placer Dome), and in return assumed ownership of the mine. During September 2004, AIG put the project out to tender with the support of the Solomon Islands Government and Gold Ridge Community. Toronto-listed Australian Solomons Gold Ltd acquired the Project after an international bidding process that involved a number of mid-tier gold miners, and took control of the Project in May 2005, before being taken over by Allied Gold during 2009. The latter company subsequently announced an AUD150 million refurbishment and redevelopment program during March 2010. Additional exploration by Allied confirmed extensions to the gold mineralisation. Following the refurbishment, stockpiling commenced in March 2011 and commissioning was completed in June 2011. In September 2012, the Australian company St Barbara acquired Allied Gold and continued production. However, in April 2014, torrential rain and flooding caused by tropical cyclone Ita led to the suspension of operations at Gold Ridge. Force majeure notices were issued under certain mining and supply agreements when it became clear that production could not be resumed due to factors outside the operator’s control. In May 2015, St Barbara sold the project and its legal liability to Gold Ridge Community Investment Ltd (GCIL), a local landowner company, for AUD100. GCIL subsequently secured interest in redeveloping the mine from Australian-based, Chinese owned (parent Shanghai-based Xiangfu group) company AXF Resources who purchased a 90% interest, a share of which was on-sold to Hong Kong-listed Wanguo International Mining who will hold a 70% interest in the operating company, Gold Ridge Mining Limited. Refurbishment is to include construction of long term hydoelectric and interim diesel power stations; repairs to access roads and bridges to the coast; accomodation, plant and equipment on site, tailings storage as well as port facilities, at a cost of USD 825 million. In August 2020, Wanguo had planned to commence production in late 2021, but this schedule has been delayed due to the COVID-19 pandemic.
Guadalcanal lies within the Melanesian Arc of northeastern Papua New Guinea and the Solomon Islands. The arc extends from New Britain and New Ireland in the NW, through Bougainville Island and to the SE of Guadalcanal. Copper and gold mineralisation is related to four main associations within this arc, namely:
i). Porphyry copper and related gold mineralisation within Eocene to Oligocene volcanics associated with 30 to 22 Ma Late Oligocene intrusives;
ii). Porphyry copper-gold and epithermal gold associated with the Mid Miocene 17.6 to 11.2 Ma multiphase Yirri Intrusive Complex on Manus Island, to the north of New Britain;
iii). Porphyry and low sulphidation copper-gold associated with Latest Miocene to Pliocene intrusives. The deposits of this and the next grouping, have been the most important to date. This Latest Miocene to Pliocene grouping is represented by the Panguna porphyry copper-gold deposit on Bougainville Island, PNG and by the Gold Ridge low sulphidation gold deposit on Guadalcanal in the Solomon Islands.
iv). Porphyry to epithermal transition gold mineralisation along the Tabar-Feni linear Pliocene to Quaternary volcanic arc to the east of New Ireland, which is characterised by alkaline and under-saturated magmatism. The two key deposits in the Tabar-Feni chain are Ladolam on Lihir Island and Simberi on the northern most island of the Tabar group.
The Gold Ridge deposits lie around 15 km to the northeast of the axis of a line of exposed 'dioritic' intrusive bodies that extends from the south coast of Guadalcanal to the northwest tip of the island and hosts a range of porphyry style, skarn, vein and disseminated Cu-Au occurrences, including the Koloula gold rich copper porphyry prospects. No intrusive bodies appear to have been mapped in the vicinity of Gold Ridge. However, the abundance of andesite clasts in the Gold Ridge Volcanics has been interpreted to indicate proximity to a now buried and eroded andesitic volcanic centre and possibly any intrusive within that centre.
Mineralisation at Gold Ridge is hosted by the Miocene to Pliocene Gold Ridge Volcanics, a distinctive, shallow dipping volcaniclastic facies at least 500 m thick at the base of the more widely distributed Toni Formation. The Gold Ridge Volcanics are composed of a series of upwardly fining cycles of epiclastic lithologies. Grain sizes within individual cycles vary from boulder conglomerate to sandstone with thin laminated siltstone interbeds, and are further characterised by an abundance of large, relatively fresh andesite boulders. By contrast, the Toni Formation usually has a greater abundance of lithic clasts including volcanic and lithic arenites (clasts are rarely calcareous), but grade to micrite and marl. The Toni Formation also contains some lenses of massive coralgal limestone, bedded calcarenite, pyroclastics, and extrusive rocks.
The volcaniclastic sequence within the Gold Ridge Project area underwent pervasive argillic alteration characterised by illite and illite-kaolinite assemblages, which overprinted an earlier and more widespread propylitic event. Significant propylitic alteration persists below the deposit, overprinting argillic alteration to depths of >100 m.
The Valehaichichi deposit hosts the most intense and concentrated argillic alteration. Propylitic alteration survived at Kupers and at Dawsons, where argillic alteration is less intense. Propylitic assemblages were controlled by variations in the primary porosity of Kupers and Dawsons volcaniclastic lithologies. Shallowly dipping, clast supported lithologies were important in the lateral dispersion of gold mineralising fluids and alteration away from the subvertical, narrow fissures that provided the conduits. In the vicinity of the deposits, the Gold Ridge Volcanics have been gently folded with dips of 10 to 30°.
Steeply dipping, NNW to NNE trending faults have displaced the orebodies locally, although overall the Gold Ridge Volcanics sequences are not significantly disrupted by faulting. A combination of compressional and strike-slip deformation produced most of the moderate to shallow dipping secondary fractures and veins. Shallow dipping lithological controls and moderate to shallow dipping fractures and veins combine to impart a strong subhorizontal distribution, which is constrained to varying degrees by steep dipping boundaries.
Gold Ridge is a low-sulphidation gold deposit with veins, stockworks and breccias carrying gold, silver, electrum and pyrite with lesser amounts of sphalerite, galena and chalcopyrite. The mineralisation is closely related to alteration, the degree of veining and, to a lesser extent, lithology. Of these, the alteration relationships are the most significant, with high-intensity alteration generally correlating with strong mineralisation.
Most of the economic gold mineralisation is accompanied by argillic alteration with pervasive silica-pyrite, while the highest grades are most frequently observed in zones of i). strong silica-pyrite; ii). quartz, quartz-carbonate, and carbonate-sulphide veinlets (pyrite-sphalerite-galenachalcopyrite); and iii). intense illite-kaolinite-pyrite alteration that are often fractured and locally crushed or brecciated. Unaltered, or weakly propylitic, zones seldom carry economic mineralisation. Although the alteration assemblages are similar throughout Gold Ridge, the relative abundance and intensity varies from deposit to deposit. Valehaichichi is the most intensely altered, followed by Kupers and Dawsons.
A zone of strong illite-kaolinite and illite rich assemblages covers a continuous zone of more than 2.5 x 1 km that embraces all four deposits.
Two styles of occurrence of primary mineralisation have been described from the Gold Ridge deposits: i). vein and fault-hosted mineralisation comprising high-grade shoots associated with fine quartz veins and clay-carbonate-pyrite alteration, often localised in steep north to northeast-trending faults; and ii). abundant coarse-grained disseminated pyrite containing inclusions of electrum without accompanying veins, accounting for much of the bulk of the deposits assaying >1 g/t Au.
At Valehaichichi, the only deposit mined to 2006, four categories of mineralisation have been recognised, namely: i). a supergene zone, including the weathered and transitional zones, characterised by a sub-horizontal layer of saprolite ranging in thickness from 5 to 14 m; ii). Siliceous mineralisation, comprising strong pyrite-silica alteration, accounting for 30% of the >1 g/t Au ore; iii). occasional high-grade sulphide-quartz veinlets; and iv). pervasive micaceous clay-pyrite-carbonate, and less commonly clay-kaolinite-pyrite, which together comprise the bulk of the ore.
From the sections shown in Wallis and Brady, (2006), the bulk of the drilling was to depths of around 100 m below surface, with a few on each section to 100 to 150 m depth. One hole drilled east from the Namachamata deposit went to 450 m below the surface where it encountered a broad zone (>50 m) towards the bottom of the hole of >1 g/t Au, including around 10 m of >5 g/t Au. Only gold grades (and no lithologies) are shown and it is presumed this is one of the feeders to the ore zone and not the top of an underlying, telescoped porphyry Cu-Au system.
Remaining resources quoted by Australian Solomons Gold Ltd (2006) were:
Total measured + indicated - 25.789 Mt @ 1.86 g/t Au;
plus inferred - 7.813 Mt @ 1.71 g/t Au.
TOTAL - 33.602 Mt @ 1.82 g/t Au, containing 61.3 tonnes of gold.
Resources quoted by Barton et al., 2007 for Australian Solomons Gold Ltd., were:
Probable Ore Reserves at 0.8 g/t Au cut-off:
Valehaichichi - 3.55 Mt @ 1.65 g/t Au in situ, 1.23 g/t Au recovered;
Namachamata - 1.35 Mt @ 2.21 g/t Au in situ, 1.57 g/t Au recovered;
Kupers - 6.57 Mt @ 1.83 g/t Au in situ, 1.51 g/t Au recovered;
Dawsons - 8.09 Mt @ 1.84 g/t Au in situ, 1.60 g/t Au recovered;
TOTAL - 19.56 Mt @ 1.82 g/t Au in situ, 1.50 g/t Au recovered, containing 35.6 tonnes of in situ gold.
Mineral Resources at 0.6 g/t Au cut-off:
Measured + Indicated - 8.90 Mt @ 1.28 g/t Au;
Inferred - 3.36 Mt @ 1.24 g/t Au;
Measured + Indicated - 2.12 Mt @ 1.84 g/t Au;
Inferred - 0.33 Mt @ 1.63 g/t Au;
Measured + Indicated - 12.28 Mt @ 1.46 g/t Au;
Inferred - 3.50 Mt @ 1.50 g/t Au;
Measured + Indicated - 15.87 Mt @ 1.43 g/t Au;
Inferred - 4.45 Mt @ 1.52 g/t Au;
Measured + Indicated - 39.17 Mt @ 1.42 g/t Au;
Inferred - 11.63 Mt @ 1.44 g/t Au.
Measured + Indicated + Inferred - 50.80 Mt @ 1.42 g/t Au, containing ~72 tonnes of gold.
JORC compliant Ore reserves and Mineral Resources at 31 March 2018 with a 0.6 g/t Au cut-off ( Hong Kong Stock Exchange release of 13 August 2020 by Wanguo International Mining):
Probable Ore Reserves - ~31.2 Mt @ 1.43 g/t Au, containing 44.6 tonnes of gold.
Measured Mineral Resource - 24.1 Mt @ 1.35 g/t Au;
Indicated Mineral Resource - 20.4 Mt @ 1.34 g/t Au;
Inferred Mineral Resource - 31.3 Mt @ 1.43 g/t Au;
TOTAL Mineral Resource - 75.8 Mt @ 1.43 g/t Au, containing ~109 tonnes of gold.
Production from the mine during the 22 month period from August 1998 and June 2000 is reported to be 4.4 Mt of ore @ 1.92 g/t Au for ~8.4 tonnes of contained gold, all from the Valehaichichi pit (Barton et al., 2007). During the Allied Gold and St Barbara operating period from 2011 to 2014 (~35 months), the Gold Ridge Mine produced around ~5.93 t of gold (Wanguo International Mining, 2020).
Wanguo International Mining (2020) consider the Gold Ridge Project to have exploration upside. In particular, the existing drilling data shows that the high-grade mineralisation has only been delineated by drilling to depths ranging from 150 to 300 m, with some deeper holes on each section, and remains open in all directions. Based on the Golder's Resource model that was prepared for the company, the orebody has additional deeper resources of 10.89 Mt at the Charivunga deposit with an average grade of 2.62 g/t Au, which might be exploited by underground mining.
This summary was prepared principally from details in: "Wallis C S and Brady B, 2006 - Technical Report on the Gold Ridge Project, Solomon Islands, Prepared for Australian Solomons Gold Ltd." with lesser information taken from:
"Barton, A., McLean, E., Yeo, W.J.A., Benson, G., Lee, M. and Gowan, M., 2007 - Technical Report of the Feasibility Study to Australian Solomons Gold Limited, Gold Ridge Project, Guadalcanal, Solomon Islands; prepared by Ausenco International Pty Ltd., 162p", and
"Wanguo International Mining Group Limited (Incorporated in the Cayman Islands with limited liability) - Voluntary Announcement Information on Gold Ridge Project; Release to the Hong Kong Stock Exchange 13 August 2020."
The most recent source geological information used to prepare this summary was dated: 2006.
This description is a summary from published sources, the chief of which are listed below.
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