RE: What’s the potential for an IOCG deposit at the Kalahari Suture Zone14 May 2022 07:50
A few questions on IOCGs. I can't give definitive answers, simply because the term IOCG covers a wide variety of deposits, and currently we don't have enough info (in public domain or KAV confidential). KAV need to stick in a few deep boreholes!
All IOCGs are formed by multiple episodes of hot mineralised aqueous fluids rising from intruding magma. The depth and composition of the source magma, the temperature of each mineralising episode, and depth of deposition will all affect the final mineral profile of the deposit. Iron oxide is the main mineral deposited – hæmatite if from relatively cool fluids, magnetite if hotter. Cu concentration is usually between 0.5 – 1%, Au often less than 0.5%. Other economic concentrations of minerals are often present. The mineralisation is usually zoned, representing the extent (or overlap) of different mineralisation episodes.
All IOCGs have distinctive co-incident high gravity and magnetic anomalies. This is due to the presence of bulk iron oxide, which is more magnetic and denser than the surrounding rock. This is why back in November, when the magnetite-rich core came up from the GRS co-incident anomalies, I guessed that we had an IOCG. I have also posted a presentation showing how these co-incident anomalies are related to IOCG mineralisation in the Gawler Craton, S. Australia.
So do we have an Olympic Dam? No. OD is derived from granitic magma hosted in brecciated granite, GRS from mafic, unknown hosting. OD is hæmatitic & lower temperature, GRS is magnetitic, so higher temperature. Magnetite is more magnetic, hence the GRS having a much higher magnetic reading. OD is one of the largest known IOCGs, but GRS is bigger – hence the higher gravity reading despite the greater depth of GRS. Half the economic value of OD is from uranium – U is mostly found associated with the IOCGs of the Gawler Craton, so not too likely in the GRS. OD also has Au in an unusual format – an odd alloy of gold, bismuth, antimony and tellurium. Most IOCGs have fine native gold and electrum disseminated between, or within, the other mineral grains.
What might we expect in the GRS? The nearest IOCGs are in the Lufilian Arc in Zambia. They are all quite small, set in a mafic environment, as is the GRS. They have 0.5 – 1% Cu, and very low levels of Au – down to 0.1%, so clearly assays are needed to detect that. However, these are in an incredibly complex geological setting that may not be comparable to the GRS. Unfortunately, little exploration seems to have been done in the last 20 years, but there is some mention of Co, V and light REEs. There is Cu in the GRS – there was both disseminated and blebby chalcopyrite in the Proterozoic core (as far as I can see from the photos).
Incidentally, in the Sharetalk article yesterday, Alexander Crossing mentioned 4% magnetite – I didn't know assay results had been released - I don't think that is correct, it is likely to be a lot more.
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