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Summary:
The North Stanmore clay-hosted rare earth element and scandium deposit occurs in in situ regolith above the North Stanmore alkaline intrusion immediately north of Cue in the northwestern part of the Yilgarn Craton. Despite having a distinctive bullseye geophysical magnetic signature, the North Stanmore intrusion had not been identified previously on regional geological map of the Cue area.
The absence of tectonite fabrics in non-weathered lithologies from ~250m deep diamond cores drilled by Victory Metals indicate the North Stanmore intrusion is post-tectonic and therefore post-Archean in age. The North Stanmore intrusion is interpreted to be associated with an early Proterozoic plume track (Fiorentini et al., 2020)8 that resulted in alkaline magmatism (carbonatites, kimberlites and differentiated ultramafic to intermediate intrusions), extending in a broad belt from Mt Weld carbonatite near Laverton through Leonora to Cue.
The North Stanmore intrusion was emplaced into Archaean gneiss-granitoid-greenstone lithologies of the Meekatharra-Mount Magnet greenstone belt in the Murchison Province of the Yilgarn Craton. Other volcano-plutonic greenstone belts in the vicinity include the Warda Warra, Dalgaranga and Weld Range greenstone belts. The greenstone belts are dominated by metamorphosed mafic volcanic (tholeiitic and high Mg basalt and komaiites), subordinate felsic volcanics, felsic volcanoclastic rocks, pelitic and chemical sediments (BIFs and jaspilite). Thermo-tectonism resulted in development of large-scale fold structures that were subsequently disrupted by faulting.
Two suites of granitoids intrude the gneisses and greenstone belts, a partly foliated and thoroughly recrystallised suite, and a later post-tectonic suite. The early most voluminous suite, comprises granitoids that are recrystallised with foliated margins and massive cores, containing large enclaves of ortho- and para-gneiss. The second suite consists of relatively small, post tectonic intrusions.
Two large Archaean gabbroic intrusions occur south of Cue, Dalgaranga-Mount Farmer gabbroic complex in the southwest, and the layered Windimurra gabbroic complex in the southeast. They are differentiated intrusions ranging in composition from pyroxenite to leucogabbro and are tholeiitic in affinity, unlike the alkaline North Stanmore intrusion.
The North Stanmore intrusion is a large (approximately 16km by 5km) differentiated ultramafic to felsic intrusion. Lithologies identified in thin section include kaersutite and carbonate-bearing peridotite, orthopyroxenite, clinopyroxenite, leuco- and leuco-gabbro (ijolite), diorite, monzonite and syenite. Some regolith samples are strongly LREE enriched indicating that the intrusion also contains carbonatite. However, the uniformly high HREO/TREO of the regolith (of approximately 34% to 38%) indicates that carbonatite represents a subordinate phase in the intrusion.
The rare earth element mineralisation above the North Stanmore alkaline occurs in a relatively flat-laying saprolite-rich clay horizons beneath a veneer of unconsolidated colluvium that ranges in thickness from 0-36m.
Importantly, geochemical studies have shown that Nb/Ta ratio of regolith over carbonatites and other alkaline intrusions closely resembles the Nb/Ta ratio of their source plume generated lithologies.
The uniformity of Nb/Ta ratios (Nb/Ta 16±4) in the regolith above the North Stanmore intrusion indicates that the regolith developed in situ above the alkaline intrusion.
REES are complexed with carbonate ions [CO32] and bicarbonate ions [HCO³] or occur as REE3+ ions in soil as groundwater migrates downwards during tropical weathering. Based on Ce/Ce systematics 10.
Ce/Ce* ratios >1 reflect the gain of Ce at shallower regolith depths while Ce/Ce* ratios <1 reflect the loss of mobile Ce* from deeper Ce³* rich parts of weathering profiles. There appear to be four zones of REE enrichment at North Stanmore. These zones are 15m to 25m, 28m to 37m, 41m to 46m and a deeper zone between 53m and 77m. QEMSCAN mineralogical studies show that the REES are mainly hosted by <20µm particles.
Development and the preservation of such a thick oxidised regolith is quite unique and reflects the combination of ancient tectonic and palaeoclimatic conditions. Geochronological studies have shown that intense chemical weathering occurred in Yilgarn/Hamersley region between ~45-55 Ma. The period was called the Eocene Thermal Maxima.
At this time there was a very small temperature gradient between the equator and the poles and a hot humid climate existed globally supporting the growth of tropical forests, even at high latitudes.
The thermal maximum also coincided with a time when there was a 100% increase in intensity of silicate weathering.
Continental reconstructions based on paleomagnetic data have shown that during the Eocene Thermal Maxima, the Yilgarn Craton was located near the south pole. Thus, the very thick regolith above the North Stanmore intrusion is a very ancient feature that has been preserved because the Yilgarn Craton is now located in an arid climatic zone where erosion is limited.
REES mobilised from host minerals (xenotime and monazite) in the North Stanmore intrusion were likely released by microbes and organic acids.
The REES were transported in groundwaters and were precipitated at redox fronts, or they were concentrated in saprock. They are adsorbed onto kaolinite and halloysite, or formed secondary REE phosphates, rhabdophane (after monazite), churchite (after xenotime), and florencite. The different REE rich horizons in the North Stanmore regolith are geologically quite unique and reflect fluctuations in water table depths over geological time.
Scandium anomalism occurs throughout the regolith profile released through chemical weathering of pyroxene in the North Stanmore lithologies. Scandium (Sc3+) behaves like the transition metals (Ni2+, Co2+, Cu2+) and, due to its smaller ionic radius, adsorbs onto vacancy sites in clay lattices or attaches to the surface of Fe and Mn oxides.