Dolocrete blanket over Mt Weld carbonatite (Western Australia) - A geochemical and mineralogical window to REE super-resource

The geochemistry and mineralogy of REE and Nb were examined in lens-shaped dolocrete covering the central part of the (REE, Nb)-rich regolith derived from the Mount Weld carbonatite, Western Australia. Dolocrete was divided into three distinct units: (a) Upper White Dolocrete (UWD), comprising dolomite interwoven by palygorskite and traces of quartz, florencite-Ce, Ti-oxides, monazite-Ce; (b) Middle Pisolite Dolocrete (MPD) containing dolomite matrix with nodules of Nb-goethite, allogenic quartz, monazite-Ce, florencite-Ce, and cerianite; (c) Basal Breccia Dolocrete (BBD) comprising massive dolomite hosting monazite-Ce, florencite-Ce, and ironstone clasts containing Nb-goethite, Nb-ilmenite, Nb-rutile, quartz, baddeleyite, and zircon. The whole-rock analysis revealed an increase in (La/Yb) N with depth from UWD at 74, 247 in MPD, to 723 in BBD. The concentrations of REE in dolomite (Ca/Mg=1–1.2) increased from 66 ppm (median) in UWD, 281 ppm in MPD, to 347 ppm in BBD. Niobium-bearing ilmenite was altered to Nb-leucoxene (max. 18 wt.% Nb 2 O 5 ). Incorporation of Nb into goethite (0.1–1.4 wt.%) and the absence of pyrochlore suggest Nb migration during the dolocrete formation. Stable isotope values ( δ 18 O VPDB = −2.9‰ – −0.6‰, δ 13 C VPDB = −6.7‰ – −5.9‰) support a sedimentary origin of dolomite formed in saline groundwaters. A fluctuating water table in a semi-arid environment promoted dissolution and reprecipitation of REE-enriched dolomite with Mg most likely derived from carbonatite. These results reveal a complex evolution of REE and Nb during the weathering of metal-enriched regolith, underscoring the importance of dolocrete as a geochemical archive and a tool for exploration targeting ore deposits of critical minerals.