The Global Mining Research report on SFX website has a good summary of the situation from the last test mining pit.
There is both an Upper Laterite related indurated layer from near surface that dips shallowly to the west through the Ore Zone that was always recognised as the biggest risk. Below is an example of at surface induration RHS, but note on LHS photo the bands or induration deeper in the test pit standing proud in the walls of the test pit (yellow lines). Probably the bottom of expected upper laterite indurated layer, though I expect if you took a deep test pit into the area of deposit under deeper overburden being mined now you would see similar layers of induration in the ore below the expected Upper Laterite related induration.
Photos of heavily indurated sand below LHS (note iron cementing colour), friable but still 'blocky' sand ore next, HG blocky sand ore, and how the friable blocky HM sand breaks down from water and hand rubbing in a pan to liberate the sand grains from weak induration RHS.
The key sentence from Global was "The age of the deposit is reflected in that the mineralisation is not completely free flowing and includes patchy zones of induration near the surface (first 10-12m). In terms of oversize material this is estimated in the resource at 12% of the material."... except it turns out that approx 3x the sand is blocky indurated and it doesn;t want to break up from the dozer push into the DMU. In short, there is unexpected and considerable blocky OS ore through out the T2 deposit underneath the expected near surface lateritic induration...
The market should be very focussed on the possible fixes for this situation related to 'cheaply' screening the blocky indurated ore from the DMU so the DMU can operate with ~75% undersize sand and moderate OS through to the wet plant spirals. If 30% of the deposit is lost to OS, and only 75% of expected undersize sand is recovered to the wet plant, there are huge ramifications for increased mining rate to meet or get close to DFS like product tonnage, costs, shorter mine life, possible exclusion of low-grade T1 ore that was key to the low strip and high mining rate of the Stage 2 expansion, etc. Another DMU will get nameplate sand to the WCP but the cost and implications are ugly...
Hopefully there is a cheaper in-plant fix that can be instigated that lifts the DMU up to nameplate U/size sand tonnes through to the wet plant, and recovers close to the nameplate sand and HMC per tonne of ore such that DFS product is achievable at only modest cost increase, leaveing the mining rate and mine plan little changed? One idea I had is to install an in-pit, pre-DMU, grizzly screen for say >50mm blocky OS (just a size guess, maybe >100mm, no idea?) that sends the >50mm blocky OS to a rotary drum scrubber to break down the blocky OS into more friable undersize (instead of driving dozers over it back and forth) then forwarded to the DMU and onwards as usual. Rejects the heavily indurated >12mm OS to waste tailings???
Maybe
@VinceS2 has the time to consider the situation and give us a more expert opinion, because my knowledge of plant and machine processing drops out quickly beyond filling a ute with fuel and starting the engine... Bottom line, without some fix, losing 25% of planned recoverable ore sand to OS while incurring considerably higher costs to mine is a high-risk recipe for disaster.
Fingers crossed, GLTAH
(20min delay)