Testwork carried out by aspiring gold developer Middle Island Resources (ASX: MDI) on the company’s Sandstone gold project in Western Australia provided a ‘Eureka’ moment.
Middle Island’s focus at the Sandstone project is to extend and enhance its proposed gold production profile to recommission the on-site processing plant.
One major hurdle had been Sandstone’s Two Mile Hill tonalite deeps deposit, for which Middle Island has released an Exploration Target of 24 to 34 million tonnes at 1.1 to 1.4 grams per tonne gold for 0.9 to 1.5 million ounces of gold.
Two Mile Hill tonalite deeps sits four kilometres north of the 600,000 tonnes per annum Sandstone plant, and was previously considered incompatible with existing processing plant capacity.
Because of this perceived incompatibility, the company was considering Two Mile Hill tonalite deeps as a potentially separate project, even a possible farm-out deal.
The company reviewed its assessment of Two Mile Hill tonalite deeps after positive results from preliminary ore sorting testwork conducted on drill core demonstrated the deposit’s amenability to pre-concentration of gold mineralisation.
This ticks off several boxes for the recommissioning of the Sandstone processing plant, including reduced throughput and costs, and an increase to the feed grade to a level that is anticipated to be compatible with the plant’s capacity and capability.
This means the Two Mile Hill tonalite deeps deposit has potential to provide a substantially higher production profile over an extended period, without the requirement for a new larger processing plant, and Middle Island has adjusted its focus accordingly.
“We were running two separate projects within the Sandstone project,” Middle Island Resources managing director Rick Yeates told The Resources Roadhouse.
“The first was focused on recommissioning the mill, and the second was how we would develop the Two Mile Hill tonalite deeps deposit.
“We are very confident the initial ore sorting results can be substantiated by further test work.”
The process of ore sorting is not, as might be thought, grading of small bags of ore material by weight or colour.
It is rather, a simple mechanised pre-concentration process that facilitates ‘upgrading’ of ore and mineralised waste and has been used across the industry for sorting myriad of commodities, such as diamonds, uranium, tungsten for more than 25 years.
Like other technological advancements in the industry, ore sorting has also benefitted from improvements in technology, including the use of sensors and applying faster data processing speeds, which have contributed to the application of the process.
Sorting has been found to be effective for managing dilution from mining operations, upgrading low grade stockpiles, reducing haulage costs for satellite operations and, most importantly, improving processing costs and efficiencies.
Middle Island appointed Nexus Bonum Pty Ltd, a highly experienced consulting group in sorting technology and its application to mining projects, to advise on all aspects of the sorting program.
“The ore sorting technology we have employed is a good example of how new or enhanced technologies can transform the economics of mining and processing operations,” Yeates said.
“Middle Island is proud to be an early mover in utilising this significant technological opportunity.”
The ore sorting study followed an encouraging mineralogical assessment on ore sourced from the Two Mile Hill tonalite deeps deposit in October 2017. This work identified that greater than 99 per cent of the gold is hosted within the quartz veins.
The test work aimed to determine if a mill feed upgrade was achievable and the optimum pre-concentration route to do so.
Two ore sorting processes were run.
The first by Tomra, was undertaken on 78 kilograms of -30mm/+10mm material following screening to remove fines.
A primary sort using X-ray transmission (XRT) was followed by a Colour scavenge on the product and waste fractions generated.
A -10mm screen upgrade to 6.38g/t gold recovered 19.4 per cent of the gold in just 7.3 per cent of the mass in this test with the Tomra results showing 64.5 per cent of the gold was recovered in 24 per cent of the mass by XRT sorting following screening.
Colour sorting of the XRT product did not result in any further upgrade. However, Colour sorting of the XRT waste fraction recovered a further 12.3 per cent of the gold to yield a total screening (fines) plus sorting gold recovery of 96.3 per cent in 41 per cent of the mass.
Middle Island interpreted the data to indicate that a mined grade of 2.38g/t gold could be upgraded to a plant feed grade of 5.61g/t gold by screening and XRT/Colour sorting.
A second sorting trial using a Steinert sorter was carried out to assess if Laser sorting would be more effective than Colour sorting.
The Steinert sort was conducted on the recombined Tomra products (73kg), following sub-sampling for gold analyses.
The 73kg recombined composite sample was run through a Laser at different settings to produce two concentrates.
The Laser waste fraction was then scavenged twice by the XRT sensor to produce high grade (HG) and medium grade (MG) fractions.
The Steinert results indicated 12.2 per cent of the gold was recovered in 14.2 per cent of the mass by Laser (concentrates combined).
Scavenging of the Laser waste fraction with XRT recovered a further 75.6 per cent of the gold (HG plus MG) to yield a total screening plus sorting gold recovery of 95 per cent in 81 per cent of the mass.
The MG material is below the marginal cut-off for processing and would not be recovered.
Overall, the Steinert test resulted in a 93.9 per cent recovery of gold in 54 per cent of the mass.
Middle Island saw the results as an indication that reviewing ore sorting is a valid means of upgrading mill feed, thereby lowering process operating costs – and consequently the mining cut-off grade – for a possible underground mining operation, is readily justified.
It also signalled the potential that the substantial scale of the Two Mile Hill tonalite deposit could ultimately prove compatible with the milling capacity of the existing Sandstone processing plant.
“The results from the ore sorting characterisation testwork are obviously, at this stage, only preliminary, however, these initial results are outstanding,” Yeates said.
“Even Nexus Bonum principal, Geoffrey Laing, described the indicative performance as one of the best he has seen.
“The opportunity to generate a 185 to 257 per cent increase in sorter product grade, whilst retaining more than 93 per cent of the gold, is an outstanding result.
“The expected equivalent decrease in unit haulage and mill operating costs will also feed back into the economic mining cut-off grade, thereby potentially increasing the material available for mining and processing.
“Assuming more definitive testwork on larger composite samples replicates or improves on these results, ore sorting has the potential to make a significant positive impact on the economics of the Two Mile Hill tonalite deeps deposit.
Middle Island has lodged a Program of Work (POW) application to complete a large diameter (PQ) diamond core hole to provide material for more definitive ore sorting trials.
On the back of the initial ore sorting testwork results, it also lodged another POW to drill out the upper levels of the tonalite deeps deposit (to approximately 450m depth) to a notional Indicated Resource status.
Middle Island Resources Limited (ASX: MDI)
…The Short Story
2 Richardson Street
West Perth WA 6005
Ph: +61 8 9322 1430
Peter Thomas, Rick Yeates, Beau Nicholls