RARE-EARTH ELEMENT MINERALIZATION WITHIN THE MT-WELD CARBONATITE LATERITE, WESTERN-AUSTRALIA

A thick weathering profile of a Proterozoic carbonatite intrusion possesses exceptionally high REE contents in the form of abundant secondary phosphates and aluminophosphates. REE experienced pronounced lateral and vertical mobility during weathering which was favoured by high fluid/rock ratios, long fluid residence times, abundant REE complexing agents in the groundwater solutions and the ready decomposition of primary igneous carbonatite minerals. The wide range of pH and alkalinity conditions, and carbonate anion concentrations in the groundwater solutions variably influenced the different stabilities of LREE and HREE complexes which caused fractionation, separation and deposition of LREE and HREE. Chemical-crystallographic controls and adsorption of REE onto non-crystalline substances led to the incorporation of LREE into plumbogummite-group minerals, rhabdophane and secondary monazite in the upper part of the weathering profile, whereas HREE and Y were preferentially incorporated into churchite at lower levels. lateral groundwater flow towards a central topographic low of the laterite and associated decreasing pH of the solutions favoured the mobilisation of large amounts of REE and the formation of a highly REE-enriched laterite centre. © 1990.