THE INSIDE STORY: A new approach opened the potential of the Two Mile Hill tonalite deeps deposit within Middle Island Resources’ (ASX: MDI) 100 per cent-owned Sandstone gold project.
Middle Island Resources has been tracking the extension and enhancement of the proposed gold production profile at the Sandstone project with the aim of recommissioning its on-site 600,000 tonnes per annum processing plant.
The Two Mile Hill tonalite deeps deposit is located just four kilometres north of the company’s 100 per cent-owned Sandstone gold processing plant and presently comprises an Exploration Target of 24 million tonnes to 34 million tonnes at 1.1 grams per tonne to 1.4 grams per tonne gold for 0.9 million to 1.5 million ounces of gold.
Middle Island Resources has determined the deposit’s amenability to pre-concentration of gold mineralisation via an ore sorting study.
The initial study findings increase the mill feed grade, at the same time reducing the haulage and milling costs, and, importantly, the quantum of mill feed to a level compatible with the existing processing plant’s capacity.
Middle Island Resources initiated the ore sorting study in October last year after receiving encouraging results from a mineralogical assessment of material sourced from the Two Mile Hill deeps deposit.
The aims of the initial testwork, which involved the assaying of a series of hand-sorted composites of quartz vein material and un-veined tonalite, were designed to determine the relative disposition of gold within the deposit.
The mineralogical assessment suggested that over 99 per cent of the gold is hosted within the quartz veins.
Buoyed by these results, Middle Island embarked on an initial ore sorting trial, which demonstrated that ore sorting could deliver a 185 per cent to 257 per cent increase in grade.
“We have identified that 99.6 per cent of the gold occurs in the quartz veins, it is commonly coarse, and certainly free gold,” Middle Island Resources managing director Rick Yeates told The Resources Roadhouse.
“Some 60 per cent of the gold is recoverable in a gravity circuit and, at a relatively coarse grind size of 106 microns, we are getting 93 to 97 per cent leach recoveries after gravity concentration.
“The leach kinetics are also good, with about 90 per cent leach recovery in the first two hours, with low reagent consumptions and no deleterious elements.
“The recognition that 99.6 per cent of the gold occurs in the quartz veins led us to the concept of ore sorting and we have been investigating this opportunity for the past eight to nine months.
“The indicative trials we completed early this year identified that XRT and Colour sensors provide the best results, with gold recoveries over 93 per cent delivered into some 36 per cent of the mass.
“This results in a significant increase in mill feed grade and, very importantly, a significant reduction in the quantum of mill feed, potentially compatible with our existing processing plant.”
Riding the success of the initial ore sorting program, Middle Island drilled a large diameter diamond core hole, MSDD261, from surface to a depth of 284.5m to provide sufficient material for a second, more definitive ore sorting campaign.
50-gram fire assays of 1m quarter core intervals from MSDD261 returned a best intercept of 100 metres at 2.02 grams per tonne gold from 55m downhole depth, including 7m at 18.1g/t gold from 121m depth.
This intercept in MSDD261 is entirely consistent with previous diamond drilling at Two Mile Hill, variously including intersections of:
508.3m at 1.38g/t gold;
372.7m at 1.52g/t gold;
230.4m at 1.62g/t gold;
353.3m at 1.04g/t gold;
141m at 2.3g/t gold; and
156.3m at 1.14g/t gold.
“One of the characteristics of the deposit, particularly important for its amenability to ore sorting, is the highly consistent nature of the mineralisation,” Yeates said.
“They are certainly very substantial intercepts and demonstrate that the deposit is persistently mineralised.”
Drilling carried out to date at the Two Mile Hill deeps deposit has confirmed the presence of a substantial and ubiquitously mineralised system which, at surface, measures 250m in strike, up to 90m in width and is mineralised to at least 700m depth and remains open beneath this level.
Middle Island believes this latest, 100m at 2.02g/t gold intercept, combined with encouraging results derived from previous diamond drilling, metallurgical testwork, mineralogical studies and ore sorting trials, presents the potential for underground mining beneath the proposed open-pit cutback at Two Mile Hill.
“Sub-level caving is certainly the preferred mining option being assessed at this stage,” Yeates said.
“There is no question that ore sorting will be key to the project economics.
“The fall-back position for us is a more selective, open stoping, mining approach, and we have identified multiple higher-grade, sub-horizontal zones within the known 700 metre vertical extent of mineralised tonalite, on which we could focus to achieve that outcome.”
At the time of writing, more definitive ore sorting trials on bulk composites from diamond hole MSDD261 were complete, however the results of 2kg bottle roll assays of the various product fractions generated by the main trials were still pending.
These more definitive ore sorting trials include iterations around crush size and grade range, designed to refine the original ore sorting results.
The company has also recently completed Stage 1 resource definition diamond drilling, designed to bring the upper half of the Two Mile Hill tonalite deeps Exploration Target (from 140m depth, which is the base of the quantified mineralisation for the open pit, and approximately 420m depth) into at least an Inferred Resource category.
Logged vein densities and alteration intensities observed in diamond core are entirely consistent with previous diamond drilling, and multiple instances of course, visible gold have been identified.
Middle Island is confident of a successful outcome for the current, more definitive, ore sorting trials.
So much so, it is already planning to progress Stage II infill drilling, designed to bring the upper half of the deposit into the Indicated Resource category, prior to a more comprehensive economic assessment of its underground mining options.
Each drilling stage is also designed to maximise the number of possible intersections on the upper and middle banded iron formations (BIFs) that are intruded by the Two Mile Hill tonalite plug, to target additional high-grade gold mineralisation within the BIF units, peripheral to the tonalite contact.
While several such BIF intercepts have been recorded around the tonalite contacts to date, only one of these has been quantified as a Mineral Resource.
“The tonalite intrudes three BIF units,” Yeates explained.
“Immediately adjacent to the tonalite we see selective massive to semi-massive pyrite replacement of magnetite within the BIFs, with which the very high-grade intercepts are associated.
“These mineralised lenses are relatively small, so in the grand scheme of things they are not hugely material, but they are very nice sweeteners.
“We have only quantified one of these lenses to date, as a dense drilling pattern is required and can be expensive, but we have identified multiple ore positions that are marginal to the tonalite on the upper two of the three BIF units – we haven’t drilled the lower BIF at all.”
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