The study is based on metallurgical testwork and the construction of a centrally located multipurpose gold processing facility, examining the likely optimum treatment route and plant locations. The study is based on the potential for the treating of various mineralisation from Alexander River and Big River, as well as potentially treating material from other historical mines on the Reefton Goldfield.
This work has been extended to allow for additional works, including a review of the potential for processing of mineralisation from the recently acquired Sams Creek Project, as well as further test work to be conducted on stibnite related gold and antimony recovery from the various Reefton projects.
Study works have been successful to date, are progressing steadily and will be ongoing over the coming months.
Reefton Metallurgical Testwork
Total combined gold recovery of gravity and flotation is in the range 91 to 93% as shown in Table 1 below.
Table 1: Combined Au Recovery
The flotation concentrate from a composite sample of Reefton 1-5 gave a cyanide leach extraction of 22.9% after ultra-fine grinding and 98.5% after pressure oxidation (POX).
Table 2: Downstream Processing Test Results on Reefton 1-5 Composite Flotation Concentrate
In conclusion, the Scoping Study flowsheet of gravity recovery followed by flotation has been verified as an appropriate process. Based on the samples tested, gravity and flotation gold recoveries of 90-93% can be expected. If the flotation concentrate is treated with pressure oxidation followed by cyanidation a total gold recovery would be around 90%.
Stibnite (antimony sulphide) has been seen in significant quantities in the Reefton goldfield.
Stibnite is generally associated with gold mineralisation and in various Au-Sb operations around the world, free gold is recovered by gravity, followed by the production of a stibnite flotation concentrate.
The Brunswick Processing Plant at the Mandalay Resources Costerfield Mine in Victoria has a flowsheet which is simple, conventional and a well-proven circuit, with more than 14 years of operation.
The surface crushing and screening facility processes underground ore feed down to a particle size range suitable for milling through a two-stage, closed circuit ball milling circuit. Centrifugal style gravity concentrators are used on the combined primary milling product and secondary mill discharge to recover a gold-rich gravity concentrate. This is upgraded further over a shaking table and sold as a separate gold concentrate product, which is transported to local refineries.
Secondary milled products are classified according to size and processed through a simple flotation circuit comprising of rougher, scavenger and single stage cleaning. The flotation concentrate is dewatered through thickeners and with filtration to produce a final antimony-gold concentrate product, which is bagged, packed into shipping containers and shipped to customers overseas.
The feed to the plant during 2020 and 2021 was between 11.0 and 12.1g/t Au and 3.5 to 4.5% Sb.
The gravity gold production varies but recoveries are typically around 40 to 50%. The 2021 end of year (EOY) reconciled plant recoveries were 94.6% Sb and 93% Au, with the gold not recovered by gravity reporting to the flotation concentrate.
Should Au-Sb resources be delineated at Reefton the metallurgical solution is anticipated to be a simple and straightforward process.
Processing Plant Details
Based on the testwork reported above, GRES reviewed the process design criteria and flowsheet that was presented in the phase 1 Scoping Study. Little change was required, and the key features of the process plant are as follows:
- A nominal processing capacity of 1.25 million tonnes per annum, using a design head grade of up to 10g/t Au to cater for surges of high-grade ore.
- Three stage crushing, With fine ore bin storage and emergency reclaim.
- Single stage ball mill, with a flash flotation cell treating cyclone underflow.
- Separate gravity concentrators to treat ball mill discharge and flash flotation concentrate to produce Doré bullion output of up to 80% of the gold in the feed, again to handle high grade surges.
- Gravity plus flotation of approximately 93%, with an overall recovery estimated at approximately 90% with POX.
- Concentrate dewatering utilising a thickener and a filter to produce a transportable concentrate.
- Appropriate tailings handling facilities depending on plant location and underground paste fill requirements.
- Steinert Ore Sorters to reduce waste from the mining cycle and increase mill feed head grade.
The positive and successful metallurgical test results are another significant milestone and another positive step forwards for Siren Gold in advancing and developing a major gold mining operation on the Company’s Reefton and Sams Creek Gold Projects.
These results indicate a standard flow sheet will yield strong recoveries with the potential to feed a central processing plant from Siren's existing high-grade resources.
Siren Gold will continue to progress it's Scoping Study with the support of GR Engineering with the view of developing a significant gold mining operation in Reefton, New Zealand.