Geology

The 905Ma Cummins Range diatreme forms part of small alkaline intrusive complex located close to the junction of the Halls Creek and King Leopold Orogens.  The diatreme has plan dimensions of approximately 1.5km x 1.2km and is interpreted to be a vertical, pipe‐like body based on modelling of magnetic data.  The alkaline complex intrudes the ~1870Ma Olympio Formation sediments and unassigned granite with no observed structural disruption or contact metamorphism along its margins.

Outcrop in the Project area is limited and the majority of geological information is based on drill data.  Thin, aeolian sandy cover is the norm with intermittent east-west sand dunes, little defined drainage and a thriving camel population.  The geomorphology limits the effectiveness of soil or stream sampling as a geochemical tool.

The Cummins Range diatreme is composed of pyroxenite intruded by a central plug of carbonatite.  The pyroxenite is highly variable in texture and mineralogy, and is locally pegmatoidal.

The diatreme is broadly zoned, starting with an outer relatively unaltered pyroxenite zone comprising an assemblage of diopside, biotite, magnetite, apatite and carbonate.  Associated accessory minerals which are more prevalent in local pegmatoidal phases include: baddeleyite, sphene, ilmenite, monazite, zircon, zirconolite, pyrrhotite, pyrite and chalcopyrite.  The second, inner zone is a mica rich pyroxenite zone, generally comprising of an assemblage of pale green pyroxene and coarse biotite‐phlogopite, magnetite, carbonate and amphibole, associated with the previously mentioned accessory minerals, plus pyrochlore, barite, bastnasite, aeschynite, baddelyite and thorianite.  Finally there is a central carbonatite zone about 500m x 400m, comprising >90% carbonate, mainly dolomite, though with some calcite and magnesite.  Here the accessory minerals include amphibole, biotite, magnetite, chlorite, apatite, zircon, monazite, bastnasite, aeschynite, fluorite, pyrochlore, allanite, pyrite and pyrrhotite.

The carbonate alteration is first manifest as an increase in interstitial carbonate, and minor replacement of pyroxene by amphibole.  As alteration increases, pyroxene is progressively veined and replaced by amphibole and chlorite, and the microveins of carbonate become increasingly denser and interstitial carbonate becomes the dominant mineral.

The principal rare earth minerals at Cummins Range are apatite and monazite which are considered to be primary minerals derived directly from weathering of the carbonatite.  Apatite may occur as grains up to 7mm in diameter whilst monazite is typically no more than 2mm, and is usually smaller.

Secondary apatite and monazite are developed to varying degrees in the weathered mineralisation, often as rims on primary apatite and monazite grains.  They typically have a much finer grain size and may grow in vughs of weathering origin.  These secondary mineral textures invariably occur in subordinate proportions to primary apatite and monazite and are interpreted to have formed in a near-surface lateritic weathering environment.

The process of deep oxidation, leaching and dissolution of the carbonates has resulted in concentration of the resistant accessories, including the rare earth oxide‐bearing phases, apatite and monazite; and, the niobium‐ and uranium‐bearing phase, pyrochlore.  Primary and transition zone REO‐uranium-phosphate mineralisation are present in the carbonatite but is not significant relative to that in the oxide zone.

The main rare earth mineral in primary (unweathered) carbonatite is apatite, with monazite and rare earth carbonate occurring in subordinate quantities.  Monazite and apatite have a grain size below 1mm and the rare earth carbonate has a grain size < 50 microns.