Summary:
The Lofdal property is underlain by Paleoproterozoic metamorphic rocks of the Huab Metamorphic Complex, which outcrop as an inlier of the Congo Craton surrounded by stratified rocks of the Damaran Orogen. The metamorphic basement was intruded at ca 750 Ma by alkaline silicate rocks and carbonatites of the Lofdal Carbonatite Complex. The complex comprises an early silicate intrusive assemblage of dominantly nepheline syenite, and a later carbonatite intrusive assemblage ranging from calcitic through dolomitic and ankeritic carbonatites.
The Lofdal Carbonatite Complex comprises a central intrusive core characterized by several plugs of nepheline syenite and carbonatite with associated diatreme breccias, surrounded by a wide area of dyke intrusion and associated hydrothermal alteration. The phonolite and carbonatite dykes have exploited pre-existing structures in the basement that were re-activated during Neoproterozoic tectonism.
Rare earth element mineralization in the Lofdal Carbonatite Complex is closely associated with the carbonatite dykes and related hydrothermal alteration. These occur within an area of more than 200 km2 . The lithogeochemical database demonstrates that many of the dykes are geochemically anomalous in REE (which includes yttrium as a heavy rare earth) with a significant number being of economic interest. Of particular significance is the frequent enrichment of heavy rare earths in the dykes and in structurally controlled hydrothermal alteration zones, which trend predominantly in NE - SW and NNE - SSW directions.
The rare earth elements (REE) are subdivided into heavy rare earth elements (HREE) and light rare earth elements (LREE). Lanthanum, cerium, praseodymium, neodymium, promethium, and samarium are the LREE. Yttrium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium are the HREE. Although yttrium is lighter than the light rare earth elements, it is included in the heavy rare earth group because of its chemical and physical associations with heavy rare earths in natural deposits.
The rare earth mineralization in the Lofdal Carbonatite Complex is variable and includes both LREE and HREE enriched varieties that appear to have been introduced in separate mineralizing events. Petrographic evidence suggests that the heavy rare earth-rich mineralization resulted from a dominantly hydrothermal event that occurred relatively late in the history of carbonatite emplacement. Mineralogically, the heavy rare earth-enriched mineralization is dominated by xenotime (Y-REE phosphate), which is commonly associated with zircon, rutile, apatite and/or thorite. The mineralized hydrothermal alteration systems are continuous both along strike and at depth and produce clear geological, geochemical and radiometric signatures that are easily recognized, particularly in drillhole core.
Two areas have been evaluated by the recent drilling, these being known as Area 4 and Area 2B.
Mineralization in Area 4
The Huab Metamorphic Complex in this area is dominated by quartzo-feldspathic gneiss and metasedimentary grey gneiss with lesser amphibolite and pegmatite. The gneisses strike approximately ENE-WSW and generally dip steeply southwards.
A large number of carbonatite veins and carbonatic alteration zones have been mapped in Area 4. Although strike directions are dominantly NE-SW and NNE-SSW following the dominant structural grain of the basement, other directions are locally seen. Outside the central alteration zone, carbonatite veins are thin, (<1 m wide) and do not exhibit significant alteration beyond their margins. However, within the alteration zone, they are more continuous and alteration is ubiquitous.
A more or less continuous zone of albite-carbonate alteration with significant grades of REE has been traced by mapping, trenching and drilling for more than 1,100 m along strike and regional geological mapping to the east and west indicates that it continues for several kilometres beyond Area 4. Within this zone, the intense alteration typically thickens and thins, and locally forms lensoid bodies that can extend on surface up to about 100 m long and 10 m wide.
The mineralization in Area 4 occupies a structurally-controlled, linear alteration zone. The Area 4 alteration zone is the largest and best mineralised and is clearly manifested and easily mappable in surface outcrops by variably intense albitisation and brown carbonization with locally abundant phlogopite. Grab samples from outcrops typically return highly anomalous values of HREE and also have a very high HREE/TREE ratio.
Mineralization in Area 2B
The Huab Metamorphic Complex in Area 2 is dominated by amphibolitic schist interbanded at outcrop scale with leucocratic quartzo-feldspathic paragneiss and muscovite schist and locally intruded by coarse grained granitic pegmatites. The rocks are complexly folded on a fine scale.
Phonolite and carbonatite dykes and related alteration zones of variable orientation and thickness are common in the area. Carbonatite dykes average a few cm in width but carbonatitic and albititic alteration zones can range up to more than 10 metres in width in outcrop.
Dykes and alteration zones in this area dominantly trend from NE-SW to NNE-SSW and generally are at a considerable angle to the structural grain of the basement, which in this area trends from E-W to ESE-WNW. The area is bounded to the north and south by major sinistral faults interpreted from remote sensing data (Fugro, 2010) and it may be that the dominantly NE-trends of dykes and alteration zones in Area 2B reflect fracture systems related to these linked faults.
The REE mineralization in the Area 2B zone is restricted to the zone of alteration and carbonatization. In drillhole core, the zone is seen to consist of a zone of intense brecciation and hydrothermal alteration. The best assay values are related to late veining and alteration that cuts most of the pre-existing alteration lithologies.
Shearing is very common in the alteration zones. In most sections, there is a prominent shear zone at or near the footwall, which is itself variably albitized and carbonatized. The shear zones range from centimetres to as much as five metres wide and the shear fabrics are cut by both albitite and carbonatite suggesting that they represent structures that pre-date the mineralization. There is a main footwall shear zone in most sections, which may be the controlling structure for much of the alteration and mineralization.
Mineralization in Areas 4 and 2B is structurally controlled and hydrothermal in origin. The host structures are first- and perhaps second-order basement structures that were apparently reactivated more than once during the mineralizing event. Repeated movement promoted the introduction of several generations of hydrothermal fluids, which resulted in a complex series of overprinting alteration events. The mineralization is dominantly present in xenotime and is interpreted to be related to the waning stages of hydrothermal alteration related to carbonatite intrusion. The highest-grade mineralization does not occupy a consistent position within the structural zones. It is interpreted to occupy structures within the zone that were still open during the last phases of hydrothermal alteration. The mineralised structures can be traced from hole to hole and are variably mineralised.