Summary:
Monte Muambe is located in the central part of the Karoo Moatize-Minjova coal basin, which corresponds to the eastern part of the Zambezi Graben. The Monte Muambe carbonatite intrusion is hosted by Upper Karoo Sandstones of the Cádzi Formation. The age of the intrusion is presently unknown.
The Monte Muambe carbonatite intrusion is hosted by Upper Karoo Sandstones of the Cádzi Formation. It is usually assumed that the intrusion is Cretaceous and forms part of the Chilwa Alkaline Province.
Isolated trachyte intrusions form small hills located 8 to 11 km to the north, and 22 km to the northwest of Monte Muambe. They are considered by GTK Consortium (GTK Consortium, 2006) to be contemporaneous of the Monte Muambe carbonatite. Other Post-Karoo alkaline and carbonatite intrusions in the region include the Salambidue syenite, about 50 km to the north-northeast of Monte Muambe (mostly in Malawi), as well as Cone Negose, located 315 km to the west-northwest of Monte Muambe along the northern margin of the Zambezi Graben.
While the Monte Muambe structure resembles a ring-dyke, or a volcanic edifice, the outer ridge consists of sub-horizontal indurated Upper Karoo sandstones and is the product of differential erosion.
The basin formed by the inner part of the structure consists chiefly of fenites, various types of carbonatites, breccias, as well as pyroclastics. The diameter of the carbonatite intrusion at surface level is about 3.3 km. Carbonatites tend to outcrop in the form of small hills rising above the floor of the basin.
Fenites are often deeply weathered at near-surface levels and rarely outcrop, though float can be encountered on slopes. Fenites form a circular zone lining the contact between carbonatites and host sandstones, but the detailed relationships between fenites and carbonatites are a lot more complex, involving faulting during and after the emplacement of the intrusion, as well as the incorporation of xenoliths of various size (centimetre to decimetre size). Drilling in various parts of the intrusion shows that fenite outcrops often cover carbonatites. This, as well as the presence of pyroclastics, suggests that the present erosion level may corresponds to the roof of the carbonatite intrusion, immediately under the base of the volcanic edifice.
Monte Muambe REE mineralisation
Not all carbonatites carry REE mineralisation at Monte Muambe. Both low grade (0.5 to 1% TREO) and high grade (>1% TREO) mineralisation, as defined further below in this section, are encountered in specific REE-enriched parts of the carbonatite intrusion. Outcropping REE mineralisation is relatively easily identified using the soil sampling survey results, as the soil geochemistry largely reflects the bedrock geochemistry due to the limited thickness of the soil cover (typically 50 cm to 2 m). This does not apply to blind mineralisation such as that of Target 6.
The position of REE mineralisation reflects both primary (magmatic) and secondary (hydrothermal and supergene) processes. Areas with 1 to -2% TREO in soil (yellow), and with >2% TREO in soil (red), roughly correspond to areas with low grade and high grade mineralisation, respectively, in the bedrock.
At target 1, the interpreted mineralised is broadly lenticular in shape, strikes approximately 310° with a variable dip ranging from 25 to 35°, dipping towards the northeast. The mineralisation has a strike length approximately 760 m, outcrops on surface, and narrows with depth, pinching out approximately 135 m vertically below surface, The average horizontal width is 42 m.
At target 4, the interpreted mineralised is flat lying, broadly lenticular in shape, and striking between 295 and 320°. The dip that ranges from 05 to 30° to the northeast. The mineralisation has a strike length of approximately 260 m, is 230 m across strike with an average vertical thickness of 46 m. The mineralisation outcrops on surface and extends 96 m vertically below surface.
REE mineralisation geometry – Target 1
The core part of Target 1 consists of a 500 m long HGM zone, 40 to 80 m thick, and dipping towards the NE. The dip angle varies from 35° (central part) to 50° (NW and SE parts).
• The hanging wall of the HGM zone, where it does not outcrop, often corresponds to or is close to the foot wall of overlying fenites.
• HGM occurs in both Ca-carbonatite and Mg-carbonatite and across the boundary between these two lithologies.
• LGM (Nb2O5 > 1,200 ppm and TREO <1%) occurs mostly in Ca-carbonatite and forms a 20 to 60 m thick zone at the footwall of the HGM zone. LGM is also more rarely present at the hanging wall of the HGM, especially in the SE part of Target 1.
REE mineralisation geometry – Target 4
REE mineralisation at Target 4 is associated to a sub-vertical carbonatite pipe hosted in fenite and having a diameter of 130 to 170 m at surface level. The pipe consists mostly of Ca-carbonatite, with a ring-shaped Mg-carbonatite dike close to or at the contact between the pipe and the host fenite and pinching out on the NW side. HGM forms a zone which is largely associated to the carbonatite pipe, but which does not follow exactly its boundaries.
Fluorite mineralization
Monte Muambe was originally considered as a fluorspar (or fluorite) project, and fluorspar exploration has taken place from the 1960s to the 2010s.
Exploration for fluorspar at Monte Muambe culminated with the publication by Globe Metals of an Inferred Mineral Resource (JORC, 2004) of 1.63 Mt of fluorite mineralisation with a grade of 19% fluorite with a cut-off of 10% CaF2 in 2012.
Fluorspar occurs in two types of geological contexts at Monte Muambe):
• As an accessory mineral in carbonatites.
• As late hydrothermal veins cutting across carbonatites and fenites.
Fluorspar in hydrothermal veins has a botryoidal habit, which is relatively unusual for this mineral.
No hydrothermal fluorspar mineralisation has been observed during resource drilling at Target 1 and Target 4, although disseminated fluorspar is common. XRD analysis show an average fluorspar content of 15 wt% in the high-grade mineralisation. This is consistent with a 6.52 wt% average fluorspar content in HGM, which corresponds (assuming all the fluorspar contained in fluorite) to 13.4 wt% fluorspar.