Summary:
The target deposit type at Songwe Hill is a high-level, REE-enriched carbonatite intrusive complex. Carbonatites are traditionally defined as intrusive and extrusive igneous rocks that contain in excess of 50 % modal carbonate minerals (Woolley and Kempe, 1989). Mitchell (2005) defines carbonatites as “containing greater than an arbitrary 30 % by volume of primary igneous carbonate regardless of silica content”. Carbonatites can be named according to their carbonate mineralogy (e.g. calcite carbonatite, dolomite carbonatite and ankerite carbonatite), and chemically they can be divided into the three main varieties: calcio-, magnesio- and ferro-carbonatite.
Songwe Hill is interpreted as a carbonatite intrusion–breccia complex expressed as a steepsided hill with a diameter of approximately 800 m. The general geology of the complex was described by Broom-Fendley et al. (2017). Information from surface mapping and drill core indicates that the complex consists of a multi-phase intrusion characterised by early intrusion of nepheline syenite (the Mauze nepheline syenite) and phonolite cut by diverse carbonatites and breccias exhibiting a range of alteration from potassic fenitisation to low-temperature hydrothermal/carbohydrothermal overprinting. The entire complex is cut by phonolite dykes, which appear to represent either a continuous event or multiple pulses during and after carbonatite emplacement.
The intrusive/breccia complex abuts the western slope of the large Mauze nepheline syenite intrusion, but the contacts on the northern sides of the vent are hidden beneath recent surficial deposits. It is possible that the carbonatite complex is in contact with Precambrian gneisses in this area because Chenga Hill, which is located less than 200 m west of the probable western margin of the Songwe Hill intrusion, includes fenitised gneisses and breccias. A nearby remnant of Precambrian gneiss north and northwest of the complex is also fenitised, although a screen of nepheline syenite intervenes between the gneiss and the intrusion. The fenitisation is interpreted to be the result of carbonatite intrusion, although it is also possible that the Mauze nepheline syenite had some role in the fenitisation process. The occurrence of carbonate-silicate rocks along the eastern margin of Songwe Hill was interpreted by Garson (1965) to be the product of metamorphism of the nepheline syenite by the Songwe Hill carbonatite.
The carbonatites are best exposed along the northwest facing slopes of Songwe Hill. Broom-Fendley et al. (2017) identified three recognisable carbonatite phases in the Songwe Hill complex: coarse- grained calcite carbonatite (C1), fine-grained calcite carbonatite (C2), and iron-enriched ferroan calcite carbonatite (C3).
All carbonatite lithologies at Songwe Hill are mineralised with REE but the tenor of the mineralisation is variable. Broom-Fendley et al. (2017) summarised analytical data from the 2011 and 2012 drilling campaigns indicating that, on average, the ferroan (C3) carbonatites and Mn-Fe veins are more intensely mineralised than the grey calcic carbonatites (C1 and C2).
Mineralisation in the Songwe Hill complex occurs in all three geological domains, but the highest grades and most consistent mineralisation are found in the carbonatites. The mixed lithologies locally carry a high enough grade over significant widths to be included in the Mineral Resource but are not consistently mineralised throughout their full extent. Fenites only locally contain economically interesting concentrations of REE, and seldom over economic widths.
Mineralogical studies carried out on Songwe Hill samples were reported in detail by The MSA Group (2015) and are summarised here, supplemented by recently published data. Mineralogical studies using a scanning electron microscope (SEM), electron microprobe (EMP) and laser ablation, inductively coupled plasma, mass spectrometry (LA-ICP-MS) showed that the REE mineral assemblage at Songwe Hill, regardless of lithology, is dominated by fluorocarbonates, principally synchysite with minor parisite, apatite and occasional florencite. The synchysite crystals are homogeneous, typically occurring as randomly oriented laths or tabular crystals and/or fibro-radial to plumose aggregates. Crystal size varies, but laths typically range in length from 10 µm to 60 µm, and crystal aggregates can reach up to 400 µm.
Synchysite is invariably associated with strontianite and/or baryte either as inclusions and/or intergrowths, and together they form distinctive vein-like aggregates or segregations. In addition to these two phases, synchysite is locally associated with calcite, fluorite, alkali (K) feldspar, pyrochlore and titanite. The mineral association of synchysite with strontianite and baryte in the Songwe Hill carbonatites and their textural relationships was described by Broom-Fendley et al. (2017) and interpreted on the basis of paragenesis with various states of apatite as representing hydrothermal redistribution of the REE during early hydrothermal activity at 250 °C to 300 °C.
Fluorapatite in the Songwe Hill carbonatites has a complex history and paragenesis. Broom-Fendley et al. (2017) showed that there are five recognisable stages of apatite crystallisation: two early stages representative of crystallisation from a carbonatite magma, and three stages that are texturally atypical of magmatic apatite and progressively enriched in the heavy rare earth oxides (HREO). The HREO enrichment factor of Songwe Hill apatite, defined here as the sum of all HREO from Eu2O3 to Lu2O3 and Y2O3/TREO, ranges from 40 % to 85 % compared to 2 % to 11 % in apatite from other carbonatites (Hornig-Kjarsgaard, 1998). It is rare for apatite in carbonatite deposits to display heavy rare earth enrichment.