Sihayo Gold says transitional and fresh material from a zone beneath its proposed Sihayo-1 pit at its namesake project in North Sumatra in Indonesia could be economically mined following a recent uplift in metallurgical recoveries.
The company recently announced its high-pH caustic pre-leaching process could recover over 80 per cent of the site’s gold and now plans on testing samples from a seven-hole, 2216m campaign to confirm its theory.
The company says the area assessed is located beneath and to the south of a planned open pit mine. The gold developer says the zone has to-date received minimal drill work due to its composition of transitional and fresh ore – mineralisation types which were once believed to be too costly to mine.
Samples from the campaign have since been sent to a laboratory for standard assay analysis.
Sihayo has also dispatched composite samples with observed mineralisation to an ALS testing facility in Perth and plans to use data from the evaluation to confirm its new metallurgical model.
The upturn in recoveries follows a 72-piece metallurgical testing program in July.
The work suggested the introduction of caustic pre-leaching could drive total recoveries at the project from about 71 per cent to more than 80 per cent – a move that, in addition to boosting mineable areas, has the power to significantly bolster cashflow.
Figures from an updated feasibility study indicate the project could yield a pre-tax life of mine cashflow of US$156 million or an annual average of US$24 million from an initial mine life of approximately 6.5 years.
The company estimates the increased gold recovery could push its life of mine pre-tax cash flows north of 75 per cent.
Perth-based Sihayo plans on establishing a beneficiation plant at the project and says the oxide and transitional ore account for 62.5 per cent of the facility’s anticipated feed.
The balance of the feed is courtesy of the project’s oxide material.
The project’s fresh and transition material consists of pyrite crystals laced with gold-infused arsenian pyrite that under direct conventional cyanide leaching promotes meagre recoveries of about 20 per cent.
Sihayo believes its new metallurgical program could break down the arsenian pyrite rims and discharge the gold for subsequent cyanidation, ultimately driving recoveries to over 80 per cent.
Sihayo Gold Executive Chairman, Colin Moorhead said:“Low metallurgical recoveries from more refractory ore types at Sihayo have until now been a significant issue for the Sihayo Starter Project. Material improvements arising from high pH leaching has the potential to not only significantly increase project revenues, but also to unlock and extend fresh mineralisation below the current pit design.”
The company has been completing metallurgical test work on fresh and transitional ore from the Sihayo deposit for over a year and a half to determine the effects high pH pre-leach chemical treatments have on the ground’s ore.
Programs have so far focussed on the use of a 12-hour pre-leach with 40 kg/t of an acidic formula known as “caustic” followed by a conventional 24 hour cycle of cyanide leaching.
The company argues its metallurgical process could deliver an average oxide gold grade of about 1.57 grams per tonne with a projected 83.4 per cent recovery rate.
Treating the transition material is projected to yield grades of 2.27 g/t gold with a recovery rate of between 80-85 per cent.
Running the site’s oxide material through the program is anticipated to deliver a grade of 2.22 g/t gold with an anticipated recovery rate of also between 80-85 per cent.
The figures collectively account for an average grade of 2 g/t with an 80-85 per cent recovery.
Notably, Sihayo estimates the additional processing cost of its transitional and fresh ore with high pH leaching could be about US$4.50 per tonne.
The company recently tied down US$1.5 million loan facility from its largest shareholder Provident Minerals and plans to plough the cash into subsequent metallurgical test work.
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By Matt Birney
