The 1 Billion year old Ngualla Carbonatite is a 4km x 3.5km pipe like intrusive complex of oval outline emplaced into Precambrian felsic volcanic, quartzites and gneissic rocks. A more resistant fenite alteration zone forms a ring of hills around the concentrically zoned carbonatite complex.
The Ngualla Carbonatite Complex comprises three distinct carbonatite phases and at least one contemporaneous phase of ultramafic magma. The predominant components of the complex are an annular calcite carbonatite and a later central body of ferroan dolomitic carbonatite. A third carbonatite phase in the form of cross-cutting dykes of dolomitic carbonatite occurs in the transition zone between these two carbonatite phases.
The outer calcite carbonatite is relatively phosphate rich, containing apatite, whereas the central ferroan dolomitic carbonatite is relatively barite–enriched with no phosphate minerals present and is the host to primary rare earth mineralisation.
An irregular karstic profile is developed over the central parts of the carbonatite (the Bastnaesite Zone and the hills to the north) and has resulted in the residual enrichment of rare earths in the regolith above the carbonatite through the complete removal of other carbonate minerals during the weathering process. Rare earth mineralisation also occurs from surface in the South
-west Alluvial Zone within unconsolidated ferruginous gravels up to 30m thick that have been eroded from the Bastnaesite Zone and surrounding area.
Oblique aerial view of Ngualla Carbonatite, drill holes and Mineral Resource block model coloured by REO grade.
The weathered Bastnaesite Zone comprises in-situ weathered ferroan dolomitic carbonatite, colluvium and ferricrete above the irregular karstic surface. This portion of the greater Ngualla rare earth deposit is identified as having the most favourable mineralogy for low cost metallurgical extraction as well as having the highest rare earth grade and is the focus of this Bankable Feasibility Study. Rare earths, hosted almost exclusively by the flourocarbonate mineral bastnaesite, are enriched to 3 to 8% REO from surface to depths of up to 140m within the weathered Bastnaesite Zone. The primary ferroan dolomitic carbonatite beneath the irregular karstic weathering surface is also mineralised at typical grades of 1.5 to 2.5% REO.
Schematic geological cross section across the central Ngualla Carbonatite illustrating styles of rare earth mineralisation.