The Mt Weld Rare Earths Project is covered by four mining tenements with long-term tenure that can be renewed for 21-year periods upon application. These tenements are M38/58, M38/59, M38/326 and M38/327. Mt Weld Mining Pty Ltd, a subsidiary of Lynas Rare Earths Limited, has 100% rights to all mining tenements outlined above.
- subscription is required.
Summary:
The Mt Weld Carbonatite (MWC) is a sub-vertical cylindrical igneous intrusion approximately 3.6 kilometres in diameter which has intruded strongly deformed Archaean volcanic and sedimentary rocks of the Laverton Tectonic Zone situated within the north-eastern section of the Yilgarn Craton. Wall rocks are generally unaltered approximately 500m away from the MWC boundary, with alteration progressively increasing towards the contact with the intrusion and associated dykes.
The carbonatite is generally massive with no evidence of large-scale shearing or faulting. This suggests that emplacement was after the last major regional deformation event. Local deformation has occurred from a dolerite dyke intrusion which runs from southeast to northwest through the centre of the MWC.
A complex regolith of variably oxidised carbonatite, residual mineral concentrations, palaeo-soils and locally transported alluvial sediments varying from a few metres to more than 50 metres has developed over the MWC. Prior to the sedimentation, a prolonged widespread lateritic weathering process over the Laverton geological region has concentrated rare earth element (REE) minerals in the carbonatite’s upper oxidised (saprolite) zone while more mobile carbonate minerals were removed by fluids during the weathering process. The saprolite zone has variable thickness from about 80m to 120m below the surface. The mineralised regolith profile contains fours zones of economic significance which form a concentric zonation of grades with light rare earths in the centre and heavy rare earths enrichment on the outside. The central zone is predominantly monazite with an overprint of cerianite and the outer zone comprises monazite with an overprint of xenotime, churchite, florencite and crandallite.
The four zones of economic significance are:
1) LI – Limonitic Zone. Limonitic and silica rich nodules. A yellow-brown nodular to concretionary variably cemented ironstone gravel to limonitic clay with an irregular thickness and distribution. It is generally gradational into Caprock (CR) above and variably gradational down into the CZ zone below. Geochemically and macroscopically it is difficult to distinguish from CZ lithology, with the main criterion being a subjective estimate of the amount of nodular or cemented iron oxides and silica present. The REE Content may be up to 30%.
2) CZ – Central Zone. Goethitic and haematitic silty zone. This zone constitutes the main REE mineralised lithology of the MWC. It comprises mainly friable, extremely fine-grained low density, goethitic silty clay with variably visible secondary monazite. The major components are typically 30-50% iron oxides, 15-30% lanthanide oxides, 5-10% phosphate, 1-5% alumina, 2-6% silica and 1-10% manganese oxide. The material is medium yellow-brown in colour grading into grey-brown or black below approximately 50m depth where minor manganese oxides stain the otherwise homogeneous lithological zone. Textures and features indicative of the origin of the REE mineralisation are not clearly diagnostic. Very fine to medium-grained crystals and aggregates of earthy to euhedral secondary monazite occur scattered through the siltstone matrix and clustered in irregular sub-horizontal lenses or layers. Most commonly the monazite-rich siltstone has a sedimentary aspect.
3) AP – Apatite Zone. Apatite rich material. The underlying Apatite Zone is the residual mineralised part of the carbonatite regolith. Apatite is not as heavily weathered as the lithological zones above with lower REE content, generally between 2% and 10%, however does exceed 10% below the central part of the deposit. The AP lithology varies from unconsolidated magnetite-apatite of pristine primary minerals to various states of oxidation, solution, reprecipitation and cementing with iron oxides, secondary carbonate, secondary apatite and crandallite. The diagnostic feature is the presence of at least some recognisable primary apatite. Over most of the carbonatite the upper contact is reasonably sharp (1-2m) with medium to heavily weathered supergene derivatives, which becomes less oxidised with depth. The basal contact of AP is an irregular karstic surface with residual transitional carbonatite. The thickness of the unit is variable due to this karstic surface, with the deepest part of the unit located centrally and decreasing in thickness and depth, outwards to the periphery of the deposit. Metallurgical test work has shown the main REO mineral is monazite, but possibly not the same variety as that found within the Ll and CZ stratigraphy above. Furthermore, the ratio of individual lanthanide elements is generally the same.
4) DAP – Deep Apatite Zone. A second, deeper apatite zone approximately 15 to 20 meters below the Apatite zone. The thickness and extent of the zone is variable. Mineralisation is similar to that of the Apatite Zone above.
Additional units which are significant geologically include the:
• MN – Manganese Zone. Secondary manganese oxidation is common along the boundary between residual apatite and the supergene regolith above. Localised areas of manganese mineralisation have been identified where the original lithology is indurated and cemented by MnO approximately greater than 10%.
• QA – Alluvium Zone. Transported Alluvium.
• BH – Hematitic Clay Zone. Clay with Lake sediment hematite clay.
• BP – Plastic Clay Zone. Lake sediment plastic clay.
• MC – Mineralised Clay Zone. Clay zone with mineralisation ranging from 0.5% up to 3%REO.
• BC – Calcrete Zone. Calcretised sediments.
• CR – Caprock Zone. Hematitic caprock; vuggy and fissured.
• DO – Dolerite dyke.
Below the regolith is the partially weathered saprock of the Mt Weld Carbonatite called the Transition (TR) zone followed by the unweathered Fresh (FR) zone.
The primary (i.e. unweathered) carbonatite rock is a combination of dolomite carbonatite with fluorite and ankerite, dolomite with phoscorite, characterised by magnetite, olivine, apatite and phlogopite, and medium grained sovite (calcite carbonatite mineral). Mineralisation within the Transition and Fresh Carbonatite units is consistent with the mineral zonation found within the enriched regolith profile, with higher grades in the central area decreasing outwards. Coarse grained carbonate rare earths such as parisite, synchysite and bastnaesite are also present within the fresh carbonatite.
The Mt Weld Carbonatite also hosts other ore deposits of niobium, tantalum and phosphates.
There are rare metal deposits surrounding the central REE mineralised core of the MWC; these are known as Gunyala (previously Anchor), Marloo (previously Crown), Mingkiri (previously Coors), Warla Wurdu (previously Swan) and Karlaya (previously Emu) and contain niobium, tantalum and phosphates. Mineralisation is hosted within the same weathered carbonatite regolith units with niobium and tantalum concentrated within the supergene enriched LI and CZ layers, and phosphates concentrated within the residual AP layer. These deposits also contain REO mineralisation in the range of 1% to 5% but are not included within this Mineral Resource Estimate.
Dimensions
The reported Mineral Resource extends approximately 2,000m east to west, 1,500m north to south and extends 200m below local surface topography which is relatively flat between 410 to 420m above mean sea level.