RE: i10 RNS3 May 2022 10:08
Let me try (emphasis on try) to clear up some potential confusion over identification of carbonates / carbonatites, sedimentary/igneous. Sedimentary geologists tend not to talk to igneous geologists - in fact you hear of 'soft rock' and 'hard rock' guys & girls, and because of this there is frequent mis-identification of rocks, and several different competing and/or overlapping identification & classification systems.
Confusion 1. Extrusive carbonatites, i.e volcanic ash & lava flows, get laid down in layers, often over other sedimentary strata, and can easily be mis-identified as being of sedimentary origin. This is compounded by strata of both sedimentary and extrusive igneous carbonates/carbonatites getting altered and re-mineralised - either by weathering from above, or percolating hot mineralised fluids from below.
Confusion 2. Intrusive carbonatites, which are caused by magma upwelling from the mantle, but not reaching the surface, can look exactly the same as sedimentary limestone that has been heated and squashed to form marble. In some cases the two can only be identified by sophisticated lab experiments, such as stimulated fluorescence, or by chemical assay for different ratios of elements, REEs for example.
Ditau situation. KAV are looking for INTRUSIVE carbonatites, which are closely associated with economic concentrations of REEs. These give rise to the signature cluster of circular anomalies we have all seen on KAV's diagrams. We know that these have been intruded into surrounding sedimentary rocks, that may well include sedimentary carbonates (limestone). The RNS states that rocks have been identified that may be EXTRUSIVE carbonatites, further work is necessary to confirm this. If positive, that's great, since the extrusive stuff must have come from an intrusion close by.
It also states that 150m of carbonates that appear to be stromatolitic have been found. If this is correct, but also requires further work, then that is a bit puzzling, since stromatolites are formed by blue-green algae fixing carbonate material at the bottom of shallow seas. If the observation turns out to be correct, this means they are definitely NOT carbonatite, which leaves me wondering what is it that has caused the anomaly, particularly since the RNS suggests they have hit the target. I guess we will all have to be patient for a bit longer.
I hope this makes sense, I agree it is very confusing, in fact I almost gave up half way through.
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