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The supergene phosphate ore deposit that overlies the Matongo intrusive carbonatite body is located 70 km northeast of Bujumbura in northeastern Burundi (#Location: 3° 3' 20"S, 29° 36' 34"E).

The Matongo carbonatite is associated with the 25 x 8 km, NNW elongated, Neoproterozoic Upper Ruvubu alkaline plutonic complex that is exposed along the western branch of the East Africa Rift. The alkaline complex was intruded into Mesoproterozoic (Kibaran/Burundian) meta-sedimentary rocks (phyllites and quartzites with interbedded meta-volcanic rocks and dolerite intrusions) of the Akanyaru Supergroup and granites, all of which are part of the Karagwe-Ankole Belt. The Ruvubu complex comprises two main bodies of silicate rocks, i). an inner magmatic unit of feldspathoidal syenite, forming the core of the complex dated at 699±13 Ma (Rb-Sr whole rock; Tack et al., 1984) and ii). quartz saturated rocks that vary from olivine-bearing gabbro and diorite to quartz-bearing syenite and granite of the outer magmatic unit, forming two wings to the NW and SE of the inner unit, dated at 707±17 Ma (Rb-Sr whole rock; Tack et al., 1984). The carbonatite has been dated at 690±32 Ma (Pb/Pb; Demaiffe, 2008). Zircon megacrysts intruding the carbonatite in the weathered zone have yielded U-Pb LA-ICP-MS dates of 705.5±4.5 Ma (Midende et al., 2014).

The Matongo carbonatite, which is concealed, covers a NNE elongated area of 2750 x 250 m, and occurs below a depth of 40 to 80 m. It is located within a broader 10 x 1 km, similarly NNE trending zone of kaolinised outer magmatic unit quartz syenites, sandwiched between an enclave of metasedimentary countryrock to the east, and the eastern margin of the main inner magmatic unit feldspathoidal syenite (Songore, 1991).

The carbonatite is composed of fine and coarse-grained calcio-carbonatite and ferro-carbonatite. The dominant facies is fine-grained sövite, mainly composed of saccharoidal calcite, containing discrete zones of vanadiferous aegirine, and small grains, or small prismatic crystals up to 5 mm across, of apatite. The aegirine and apatite may represent local cumulates in which pyrochlore and K feldspar are also present. The carbonatite is partially capped by a zone of brecciated phosphate ore within th eoverlying quartz syenite (Decrée et al., 2016).

The ore is hosted within quartz-bearing syenite above the carbonatite which has been subjected to two main alteration styles: i). kaolinisation to almost pure kaolin, which generally only contains <1% P2O5, and ii). phosphate-enriched rock, which is weathered but has only been weakly kaolinised (Songore, 1991).

The phosphate ore occurs as two horizontal lenses that correspond to weathered/eluvial horizons in the weathered syenite overlying the concealed carbonatite. A typical sample from the weathered phosphate ore contains ~30% fluorapatite and 17% caxonite (an iron-phosphate mineral), the rest is composed of clay, feldspar and limonite (Kurtanjek and Tandy 1989). The ore lenses generally vary from 10 to 55 m, but may locally be up to 102 m, in thickness. The oxidised ore also contains preserved millimetre-sized aggregates with remnant textures related to the pre-weathering concentration of fluorapatite related to the carbonatite system. This primary ore is also enriched in light rare earth elements (LREE), which is especially apparent in the final generation of magmatic fluorapatite that contains up to 1.32 wt.% LREE
2O3 (Decrée et al., 2016).

Following a metasomatic event (fenitisation), which led to the formation of the K feldspar and albite, the magmatic fluorapatite-rich rocks were partly brecciated. Oxygen and carbon isotope compositions of the calcite forming the breccia matrix (δ
18O = 22.1‰ and(δ13C = −1.5‰) are consistent with the involvement of a fluid resulting from the mixing of magmatic-derived and metamorphic fluids, the latter originating from the country rocks (Decrée et al., 2016).

A subsequent post-magmatic event, resulted in the dissolution of the carbonates hosting fluorapatite, leading to further and intense brecciation of the fluorapatite-rich rocks. Secondary carbonate-fluorapatite, which is less enriched in LREE with 0.07 to 0.24 wt.% LREE
2O3, local associated monazite, and coeval siderite, comprise the breccia matrix. Siderite has δ18O values between 25.4 and 27.7‰ and very low δ13C values (from -12.4 to -9.2‰), consistent with the contribution of organic-derived carbon from groundwater. These latter signatures are taken to reflect supergene alteration (Decrée et al., 2016).

After these two events, the remaining voids were filled with a LREE-poor fibrous fluorapatite (0.01 wt.% LREE
2O3), producing hardened phosphorite under supergene conditions (Decrée et al., 2016).

The supergene processes imposed on the earlier magmatic fluorapatite accumulation results in the ore containing 0.72 to 38.01 wt.% P
2O5, with enriched in LREE (LaN/YbN from 47.1 to 83.5; and ΔREE between 165 and 5486 g/t, Nb of up to 656 ppm and V of up to 1232 ppm (Decrée et al., 2016).

A detailed feasibility study, based on testing by the United Nations, British Sulphur Corporation and various other agencies and institutions resulted in resources of (Kurtanjek and Tandy 1989):
    17.3 Mt @ 11.0% P
2O5 (cutoff 5% P2O5 and 1.5 m mining thickness), within a broader resource of
    40 Mt @ 5.6% P

The most recent source geological information used to prepare this summary was dated: 2016.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Decree, S., Boulvais, P., Tack, L., Andre, L. and Baele, J.-M.,  2016 - Fluorapatite in carbonatite-related phosphate deposits: the case of the Matongo carbonatite (Burundi): in    Mineralium Deposita   v.51, pp. 453-466

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