Larvotto Resources has finished a metallurgical auger drill program to determine just how much gold, tungsten and antimony was left behind in tailings from mining campaigns dating back to the eighties at its Hillgrove project in NSW. Encouragingly, old records show gold averaging 1.34 grams per tonne and plenty of tungsten was shipped into the 1.4m tonnes of tailings during historic antimony mining.
The company undertook the drilling to assess the viability of re-processing the historic tailings in the Hillgrove tailings storage facility (TSF) for one or more of the three key commodities.
If viable, the move would be a cost-effective pre-curser to the main game at Hillgrove which boasts a 1.4m ounce gold equivalent resource going 6.1 grams per tonne across the broader 254 sq km project. Any mineralisation contained in the tailings is not included in Larvotto’s current mineral resource.
Larvotto Resources managing director, Ron Heeks said: “Hillgrove has historically focussed on extracting antimony, so a considerable amount of gold was not extracted from the processed ore, as is highlighted by production records. Additionally, no tungsten was extracted and went straight to tails. Larvotto has the potential to treat these tailings independently or combined with future underground feed to the processing plant onsite.”
“Tailings” is a term used to describe the discarded post-processing residues from mineral treatment operations. They are often the subject of renewed economic interest as they can carry residual potentially economic grades of the commodities originally sought in higher commodity price cycles.
Minerals were sometimes sent to tailings because of poor prices or due to poor historic mining technology and recoveries.
Notably, tailings are usually retained in surface dumps or impoundments that are easily accessible and commonly amenable to free digging or slurry pumping, which means they not only represent potential resources that are essentially “already mined” but in most cases have also been crushed and ground to fine screen sizes, making them readily amenable to lower cost re-treating.
Larvotto’s broader Hillgrove project lies 23km east of Armidale, NSW and comprises four exploration leases and 48 granted mining leases across its 254 square kilometres of tenure.
Multiple high-grade drill targets have also been identified for near-term drilling by Larvotto beyond the current mineral resource, including Bakers Creek and the TSF.
Over 204 individual mineral occurrences have been identified to date in the greater region, with the mineralisation developed as strike extensive veins extending over 20km with potentially depth extensive steeply dipping fissures.
The area has been mined since the 1870’s and the Hillgrove operations have produced over 740,000 ounces of gold and about 40,000 tonnes of antimony, making it Australia’s largest antimony deposit and placing it among the world’s top ten antimony resources. In addition, over 2000 tonnes of scheelite concentrate, the principal ore mineral of tungsten, have been produced from the area.
Both antimony and tungsten are designated as strategic commodities by many countries including the US, EU, China, and Australia. Larvotto says tungsten is currently worth a whopping A$45,000 a tonne – almost double the current nickel price.
A tungsten recovery section was partly installed by the original processing plant but was not completed, ensuring that almost all tungsten at Hillgrove was sent to tails.
The TSF was commissioned in 1982, by New England Antimony Mines. It covers over 113 hectares, all within current mining leases. The TSF lies about 500m from the current processing plant that is now on care and maintenance and its 1.4 million tonnes of tailings at an average grade of 1.34g/t gold are confirmed from production records and physical surveying. The Hillgrove mine ceased production in 2002 and went into receivership.
In 2013, Hillgrove Mines carried out a diamond drilling program on the TSF to confirm historical processing grades and undertake recovery testwork. The in-house results also supported the documented grades and indicate that viable recoveries of gold and antimony could be achieved.
Management says the analytical results from the auger drilling will be available within weeks and are expected to further confirm the tailings grades, while metallurgical recovery testwork is expected to take several months.
If metallurgical testwork results prove positive, the potential exists for the TSF resource to add significant further gold ounces to Larvotto’s operational plans, along with antimony and tungsten credits- and the kicker is that the mining is already done.
If the estimated 1.34 grams per tonne gold holds up across the entire 1.4m tonnes of tailings – and that is yet to be proven - Hillgrove’s front end cashflow forecasts just might get a serious shot in the arm.
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By Doug Bright
