The Mt Gibson Gold Project (MGGP) is located approximately 280 kilometres northeast of Perth and less than 10 kilometres from the main arterial Great Northern Highway, in the Murchison region of Western Australia. It is 150 kilometres SSE of Yalgoo (Golden Grove VHMS base metal deposits) and 240 kilometres SSW of Cue and the Big Bell and other lode gold mines.
Gold production commenced at the MGGP in 1986 on a modest laterite resource. By 1999, when the mine was placed on care and maintenance, the MGGP had mined several shallow laterite pits, 14 open pits with a maximum depth of approximately 100 metres, mining oxide, transitional and primary ore zones and an underground mine.
The CIL plant (decommissioned and removed) processed 12.5 million tonnes of ore at an average grade of 1.99g/t gold for production of 799,600 ounces. A further 68,868 ounces of gold was recovered from a 4 million tonne heap leach operation, taking total historic gold production at the MGGP to 868,468 ounces. Historical gold recoveries through the CIL plant of >90% were indicative of the ores treated at that time being free milling.
The MGGP tenements are located at the southern extremity of the Retaliation Greenstone Belt, in the SW portion of the Yalgoo-Singleton Greenstone Belt in the Murchison Province of the Yilgarn Craton. The tenements are mostly covered by a veneer of alluvial quartz sands and laterite gravels, with sporadic greenstone subcrop and outcrop, increasingly exposed in the north of the project area. The mineralised laterite gravels are situated slightly down-slope from the lode deposits on the Gibson trend. Regionally, the greenstone belt has been metamorphosed to middle amphibolite facies and hosts a number of Au-Cu deposits and prospects, including Golden Grove, 90km to the northwest of Mount Gibson.
The deposit has been defined by drilling over an 8km strike length and as deep as 950m down-dip where it is still mineralised and open down-dip. The mineralised shoots are present in drilling as broad zones up to 50m wide and are continuous down plunge. It is thought the shoots are developed in dilation zones along the main structures. A large laterite and oxide weathering zone is developed over the primary geology and this is mineralised in the near surface, up-dip position of the main shoots of primary mineralisation. A thin veneer of transported sand and colluvium soil covers the deposit and is typically less than 6m thick, the transition/fresh rock boundary is about 40 to 60m below surface.
Mineralisation – The Gibson Trend
The lode style mineralisation at Mount Gibson is predominantly hosted by three main trends of which the majority of the known and mined mineralisation is hosted by the Gibson Trend.
It is believed to have originally been a gold-copper-zinc rich Volcanogenic Hosted Massive Sulphide (VHMS) deposit that has been overprinted by a later hydrothermal gold mineralising event. This mineralised shear zone has an arcuate north-south to north-easterly strike (trending more north-easterly in the north) and extends for more than seven kilometres from the southern granite contact to beyond the Hornet ore body.
The so-called “Mine Sequence” is around 400 metres wide and consists of a parcel of sheared, metamorphosed and chlorite-biotite-muscovite altered mafic volcanics. Numerous felsic porphyries intrude the Mine Sequence. Mineralisation is hosted within multiple sets of elongate lodes with strong strike continuity, which anastomose and pinch-swell along strike and to depth. The main lode systems include the historical Hornet, Enterprise, Orion and S2 open pits.
Unique MGGP Opportunity
The combined area of the tenure covers approximately 139 square kilometres and in excess of 15 kilometres of strike on the gold bearing Retaliation Greenstone Belt, in the SW portion of the Yalgoo-Singleton Greenstone Belt.
The MGGP has been the subject of approximately 660,000 metres of exploration and operations drilling, of which less than 5% is deeper than 150 metres below surface. Further, as noted above, the deepest open pit at the MGGP finished only approximately 100 metres below surface while the average depth of mining is between 60 – 80 metres below surface. The gold price in 1999 when the mining operations ceased was in the order of A$450 per ounce (compared to current price of circa A$2,400 per ounce).
Mineral Resource Estimate
Capricorn has completed a JORC 2012 compliant Mineral Resource Estimate for the Mt Gibson Gold Project. A summary of the MRE is provided below:
- Mineral Resources are estimated using a gold price of A$2000/ounce.
- Mineral Resources are estimated using a cut-off grade above 0.4g/t Au.
- The above data has been rounded to the nearest 100,000 tonnes, 0.1 g/t gold grade and 1,000 ounces.
Errors of summation may occur due to rounding.
The Mineral Resource is reported using cut off grades of 0.4g/t for all material types. Key points from the resource below:
- The Mineral Resource is all classified in the Inferred category until database validation drilling is completed;
- The density of drilling in the resource ranges from 25m x 25m to 50m x 25m;
- The Mineral Resource estimate extends over a length of 8 kilometres to an average depth of 140 metres and maximum depth of 220 metres below surface;
- Historical mining has depleted all laterite ore, with 12% of the resource by ounces being oxide, 9% being transitional and 80% being fresh rock; and
- Ounces per vertical metre peak at over 17,000 between 70m and 120m from surface.