The moment of truth is close for Great Southern Mining and Gold Fields Limited, with the joint venture partners near to drilling a 2.4-kilometre-wide porphyry anomaly in Queensland, unearthed by an induced polarisation (IP) geophysical survey that appears to scream “drill here”.
The duo believe they may be onto a “chargeability anomaly” laden with sulphides that are harbouring a significant gold-copper deposit.
Great Southern Mining is gearing up for what could be the most exciting drill campaign in its 14-year history as it looks to drill out its Leichhardt Creek prospect - part of the company’s flagship Edinburgh Park gold and copper project in North Queensland.
When the drill bit finally starts turning, it will be the first time deep holes have been plunged into what the company believes could be an intact porphyry system and/or a massive intrusion-related gold system.
If the rotary truth teller does its job, the company could be on the cusp of unearthing a sizeable discovery.
Great Southern jointly owns the project with mega-miner Gold Fields, which has the right to earn 75 per cent of the project by spending $15 million on exploration across six years.
The drilling program is designed to drill three deep diamond holes to test two large IP anomalies at Leichhardt Creek and see what treasure might be lurking below.
In the past two years, the joint venture has conducted a detailed helicopter-borne aeromagnetic survey of the area. However, the breakthrough came when an IP geophysical survey was run across the prospect.
The ground lit up like a Christmas tree, showing two distinct targets stretching over 2.4km with extensive large-scale “chargeability” and “resistivity” readings.
Early analysis of survey data across the western anomaly has shown a curved electrical signal that might point to a sulphide halo around a preserved porphyry gold-copper system.
Additionally, the strong signal from the eastern anomaly could be linked to intrusion-related gold, similar to the 3-million-ounce Mount Leyshon gold-silver mine about 120km west.
Porphyry systems are usually formed around a fault that allows the heady mix of metals to rise towards the surface like a bubble before cooling. During this process, the sulphides tend to float to the top and form a secondary layer above the main body.
When an IP survey is run across the area, the highly conductive sulphides show up as highly charged material, usually as a red or yellow blob on a data map.
Since the lower section of the deposit doesn’t tend to contain sulphides and is therefore resistant to chargeability, it tends to show up on a data map as resistivity in a contrasting colour, such as blue, when plotted.
Whether mineralisation is charged or not is not that important in the scheme of things, given an IP survey shows the separation of conductive and non-conductive material.
What is critical is the type of structure it shows, indicating that a large intrusive body has risen from the depths as molten rock and has then cooled, bringing lots of potential metals with it.
Notably, since both Great Southern’s targets are close to the surface but covered with oxide material and marginal quartz-pyrite stockwork veining, they have remained largely untouched by previous explorers.
Great Southern says the chargeability and resistivity areas could be mineralised. One geological theory is that both had sulphides, which were possibly washed away in the resistivity area when the ground was altered. In such circumstances, it is not uncommon to find that the mineralisation has been left behind.
It has been a long journey for Great Southern since the company first claimed the ground in 2011.
The work of exploring a tenement package of the sheer size of the Edinburgh project - stretching more than 1750 square kilometres - can be a daunting prospect, particularly for a junior miner with limited financial resources.
By 2020, Great Southern had done enough detailed desktop work to delineate 29 individual target areas prospective for epithermal-style gold-silver mineralisation, porphyry-hosted gold-copper deposits and intrusion-related gold systems.
Out of the multiple targets, five, including Leichhardt Creek, Mt Dillon, Molongle, Red Rocks and Sledgehammer, were earmarked for priority exploration.
In the same year, two exploration campaigns picked up a total of 2200 soil and 33 rock chip samples across Leichhardt Creek in the northern section of the project area, revealing a large geochemical soil anomaly, showing all the hallmarks of an intrusive-related gold system.
Detailed mapping picked up strong associated metal content, including gold, silver, stibnite, arsenic, copper, bismuth, molybdenum, zinc and lead, which are all key potential indicators of a multi-kilometre-scale hydrothermal system.
To do any justice to a target this size, Great Southern realised it had to do a major geophysical survey of the area to get a better feel for the deeper potential, which doesn’t come cheap.
Wind the clock forward to 2023, and the company opened its data room and invited potential partners with deep pockets to look at a joint venture.
After a hotly contested bidding war, Gold Fields Limited won the right to earn 75 per cent of the project in exchange for dipping its hand into its very deep pockets.
Gold Fields’ interest was first piqued after it reviewed the initial surface work done by Great Southern. Coupled with quartz-pyrite stockwork veining at surface and rock chips running as high as 10.5 grams per tonne (g/t) gold, the mining giant could see some enormous potential.
As the partners gear up for a potentially game-changing maiden drill program, all eyes are likely to be laser-focused on early visuals from the core when drilling eventually kicks off.
To pick up a drill target as well-preserved as Leichhardt Creek, which represents only 4 per cent of the landholding, it bodes well for future exploration.
Back in February, Great Southern announced it would look to start drilling Edinburgh Park in the June quarter.
With the June quarter about to draw to a close, all eyes will be on the company and when it is likely to get the rods turning in Queensland.
And if Great Southern and its giant JV partner are lucky enough to jag a monster deposit in the first few holes across Edinburgh, then the sky might be the limit for this David and Goliath pairing.
Is your ASX-listed company doing something interesting? Contact: matt.birney@businessnews.com.au
By James Pearson
