Middle Island Resources (ASX: MDI) recently completed a Stage 1 infill diamond drilling program at the company’s wholly-owned Sandstone gold project in Western Australia.
Middle Island Resources had targeted the program of infill diamond drilling in the upper half of the tonalite deeps deposit at Two Mile Hill.
The Two Mile Hill tonalite deeps deposit comprises a comprehensively gold-mineralised tonalite (granite) plug or stock, which at surface measures around 250 metres in length, 80m to 90m in width and extends to at least 700m depth.
Two Mile Hill is located four kilometres north of Middle Island’s 600,000 tonnes per annum Sandstone gold processing plant via an existing haul road.
Although there has been insufficient exploration carried out to estimate a Mineral Resource to date at the Two Mile Hill tonalite deeps deposit, Middle Island has determined an Exploration Target of 24 million tonnes to 34 million tonnes at 1.1 grams per tonne gold to 1.4g/t gold for 0.9 million to 1.5 million ounces of gold situated between 140m and 700m vertical depth, below which it remains open.
The recent program consisted seven drill holes (MSDD262-MSDD268) comprising 988m of RC pre-collar and 1,121.2m of NQ2 diamond tails, for a total of 2,109.2m.
The aim of the drilling was to infill existing diamond drilling within the upper half of the tonalite deeps deposit, extending from 140m depth (base of the open pit Mineral Resource) to approximately 420m depth.
The Stage 1 program was carried out with the following objectives in mind:
To provide enough drill density to permit the upper half of the Exploration Target to be re-classified to at least a JORC 2012 Inferred Resource;
To provide clarity on quartz vein densities and gold distribution within the tonalite to facilitate an improved structural understanding and optimise future drill targeting; and
To optimise the number of intersections of banded iron formation (BIF), marginal to the tonalite, to identify new positions of high-grade, replacement-style mineralisation within the stacked BIF units.
The tonalite results are generally consistent with previous diamond drilling with results returning:
165 metres at 1.11g/t gold from 85m, including 7m at 4.73g/t gold from 123m, 5m at 6g/t gold from 142m and 10m at 3.34g/t gold from 202m depth; and
150m at 1.03g/t gold from 84m, including 34m at 1.85g/t gold from 96m depth.
The drilling also encountered a new zone of high-grade mineralisation within the banded iron formation (BIF), returning:
5m at 21.9g/t gold from 339m, including 2m at 54g/t gold from 340m depth.
Middle Island explained that the drilling indicated the BIF-hosted gold mineralisation peripheral to the tonalite was behaving in a similar vein to elsewhere at Two Mile Hill, by returning higher grades associated with massive to semi-massive pyrite replacement of magnetite.
One hole, MSDD262, demonstrated the host comprises moderately brecciated BIF with the company noting this to be the first occasion that gold mineralisation has been identified within brecciated elements of the stacked BIF units.
Although the MSDD262 intercept represents a new mineralised BIF position, Middle Island said results across the entire program indicated additional multiple similar, high-grade intercepts were encountered peripheral to the tonalite at Two Mile Hill.
The drilling also returned intercepts from footwall and hanging-wall basalts consistent with previous drilling.
Shallow gold intercepts within basalts along the north-eastern tonalite contact returned:
93m at 2.57g/t gold from surface, including 35m at 6.27g/t gold from 58m depth; and
9m at 5.23g/t gold from 85m.
Middle Island believes that, based on these and other shallower results within the basalt, the resource model for the upper half of the Two Mile Hill tonalite deeps Exploration Target may well justify re-estimation.
“As anticipated from infill diamond drilling, the tonalite intercepts returned from the Two Mile Hill deeps gold deposit serve to confirm the earlier results,” Middle Island Resources managing director Rick Yeates said.
“It is also extremely pleasing to identify a further significant new zone of pyrite replacement-style gold mineralisation within the BIF adjacent to the tonalite contact, particularly as it is somewhat divorced from the main tonalite body.
“It is also the first significant intersection identified within brecciated elements of the BIF package and one, of only a few, recorded in the more sparsely drilled Middle BIF unit.
“Utilising these results, resource modelling of the Exploration Target will provide a formal JORC 2012 Mineral Resource for the upper half of the Two Mile Hill tonalite deeps Exploration Target, provide better resolution on the distribution of gold mineralisation within the tonalite which, together with the recently completed ore sorting campaign and geotechnical findings, will facilitate a more robust assessment of the underground mining potential.”
Earlier this year, Middle Island achieved further positive results from Stage II ore sorting trials conducted on drill core from the Two Mile Hill tonalite deeps deposit.
Middle Island’s Stage 1 ore sorting trials were commissioned after the company realised more than 96 per cent of the gold at Two Mile Hill is hosted by quartz veins within the tonalite.
The initial ore sorting campaign in 2017 was based on composite samples of quarter NQ diamond core.
The trials indicated that sorting could deliver a 185 per cent to 257 per cent increase in feed grade, with gold recoveries more than 93 per cent.
The earlier trials focussed on underground mining via more selective, open stoping, therefore utilising composites comprising broader intervals of diamond core with a higher head grade.
The Phase II trials focussed on sub-level caving, therefore utilising all primary core, representative of the entire deposit, to generate composites.
A series of four primary (fresh) composites, and a single transitional (partially oxidised) composite comprising intervals of half HQ and half PQ diamond core derived from MSDD261, were selected for crushing prior to ore sorting.
The results from the Stage II ore sorting campaign on the Two Mile Hill tonalite deeps deposit demonstrated that the ore can be upgraded substantially via a combination of Colour and X-ray sensors.
The results gave Middle Island further confidence that its use of the technology could enable it to deliver a great deal of upside for the project.
The results confirmed upgrades of between 155 per cent and 213 per cent, with sorting gold recoveries in the range of 67 to 93 per cent.
Ore sorting product yields fell within the range of 39 to 51 per cent, indicating that the sorting process rejects more than half the sorter feed material as waste.
Middle Island intimated a further campaign of bulk ore sorting would be undertaken to focus on enhancing results through the application of a Laser sensor, in conjunction with X-ray and/or Colour.
“Ore sorting trials continue to demonstrate a significant enhancement in gold grade, along with a commensurate reduction in feed mass that improves the potential viability of underground mining at the Two Mile Hill tonalite deeps deposit,” Yeates said.
“Considerable news flow associated with the Two Mile Hill deposit can be anticipated in the current quarter, including diamond drilling results, a Mineral Resource upgrade and an updated underground mining concept study.
“We look forward to keeping shareholders updated on progress with the substantial Two Mile Hill tonalite deeps deposit at the Sandstone gold project during the remainder of 2018.”
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