WA1 Econs 101, page-220

That Equivest geo's interp of the depth to basement and physical colluvial concertation of laterite in topographical lows around the Luni carb tells me some of geology isn;t as straightforward to others as it seems. Given the demonstrable, probably critical role weathering supergene enrichment plays in creating economic Nb deposit in the West Arunta (WA) it's fitting I elaborate my understand gin on that process here, under the Econs 101 thread. Some will be all or partly over it already, but I'll walk through it from scratch for anyone just catching up.

Starting with the company's illustration where carbonatites sit in WA, heavily eroded to a relatively flat surface long before the latest laterite profile was developed. Carbonatitesw of the WA were probably intruded approx 1.8B years ago, +/- many 100's M years. Lot of erosion over many erosional periods since then has developed a relatively flat 'basement' of Proterozoic rocks and later infill basin sediments now also metamorphosed into hard rocks. Overlying this 'basement' is shallow sands, soil and clay of very recent times. Any carbonatites in WA not overlain by younger sedimentary rock units will have the same profile and relatively flat surface as Luni.



The all important weathering to laterite event happened less than 50M years ago across most of West Aust, Sth Aust and the NT. Some of the laterite is almost completely preserved, like around W Arunta, other areas have been partly to completely stripped by subsequent erosion events. This map roughly shows Mt Weld and Luni for location purposes. There is unlike to be much transported carb-laterite colluvium around W-Arunta because there hasn't been any material incision or topographic relief since the laterite weathering event sometime between 25-50Mya. It's sat flat, and relatively undisturbed far as i can tell, with only the shallow sands and seds forming over the top.

Below is the passive seismic section that Equivest geo was referring to. Where they called the 'seismic reflection layer' depth to "basement", the reflected layer is of course a laterite clay weathering layer as drilling identified. The term 'basement' is common in oil-gas exploration because it actually refers to the crystalline igneous or metamorphic rocks at the bottom of sedimentary basins, or under transported cover. Interpreting the seismic boundary, coincident with Nb enrichment is the base error that probably caused the interp they made. Again, the reflection layer is not basement but an in-situ weathering feature that also prodcuces supergene Nb enrichment (as it does elsewhere in weathered carbs).


Below is the classic laterite profile chart to place Luni and Nb enrichment within the weathering sequence. Nb at Luni is depleted in 8-25m of heavily weathered clays (plasmic zone) above the Pedoplasmation Front, and highly enriched int he lower saprolite below it. On balance, as weathering decreases down through the lower saprolite to fresh rock Nb grades fall with lower weathering enrichment. Grade is king and so the SG HG zone is critical to making bank.


Thing with the laterite weathering profile is that it's not always flat. WA1 says that's the HG UZ is laterally continuous and broadly flat, but it's early days and infill drilling may prove otherwise. The HG UZ Pedoplasmation Front may prove flat, associated with either some level of water table or slow migration of extreme weathering, but generally there are variations in weathering depth of both the multiple layers in laterite profile, but also in deeper penetration of weathering down faults, fractures, breccias or other weaknesses that encourage water to migrate down further, faster than elsewhere in the carb. Not relevant to carbs, but this section demonstrates weathering profile being dragged down a fault zone as described.


To the observation that Nb enrichment is or may be higher when the weathering profile is deeper, there are a few explanations. Firstly, if there is a thicker pile of depleted clays above, then there is more Nb to be concentrated below into the supergene zone (ie high grades). Secondly, water will locally travel from higher to lower weathered zones, especially where deeper zones are faulted allowing more water transmissivity. Thus Nb will migrate short distances via hydromorphic movement and principate out deeper and perhaps not over the better primary Nb primary grades.

However, there is a horse and cart thing happening, because often the best Nb grades are found in late carb intrusions/breccia/fault derived hydrothermal alteration (ie see company's top illustration of multiple carb intrusions and hydrothermal veining). Weathering is often deepest over the highest REO-Nb grades etc, because the primary carb was most enriched in REO-Nb in the same place it was most brecciated and fractured. Virtuous cycle of highest grades being most enriched by the most intense and deepest weathering profile... We see this at Mt Weld, Ngualla, Cummins and any other carb hosted deposits because that's the way geology works.

Below are infill drilling sections ~40m apart with 40m spaced drilling along the Cummin's REO enriched deposit in the centre of the larger carb complex, showing the wreathing depth is generally maximum where the controlling primary REO enriching structure runs along centre of the deposit. Blue-Green is weakly weathered rock below supergene enriched saprolite in red. Ignore the detail other than the point weathering depth can vary across the carbonatite on a macro scale, but also within the 'supergene deposit' on a smaller scale.



Turns out that the SG enrichment along the deposit trend didn;t translate into HG REO structure in the primary rock after REE drilled it last year. The primary carb rock is enriched in patchy breccia lodes and structures that don;'t hang together well. The SG deposit was extremely flattered by weathering enrichment of chemical removal of mobile elements (ie Ca, Mg) and concentration of relatively immobile REO and Nb. This shape and SG situation at Cummins reminds me of Luni in some respect, where the HG may be developing into a long, thin, possibly structurally controlled SG development. The coincidence that the higher grades of Nb at Luni line up in WNW strike same as much of the structures seen in magnetics at greater scale right across the Aileron area seems more than coincidental... for someone who doesn't believe in coincidence. (This discussion is obviously releavnt to any carb exploration right across the WA).

Already too long, Luni deposit interp is for another post. Penance done, GLTAH