Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
Chilwa Province
Longitude: 35.8, Latitude: -15.53

The Tundulu carbonatite complex lies some 3 km southeast of Lake Chilwa and comprises a central area of carbonatite, agglomerate, nepheline syenite and potassic fenite around which extends a broad aureole of fenitized Precambrian basement rocks, containing Karoo dolerite dykes, cut by dykes and sheets of agglomerate. The central area forms the hills of Nathace, Tundulu and Kamilala and the fenites similarly produce a number of hills several of which are isolated by soils and, in the north of the area, by beach deposits associated with Lake Chilwa. Garson (1962) recognised three igneous centres, or ring-structures, at Tundulu the first of which produced a dome some 5 km in diameter, corresponding approximately to the present area of the fenite aureole; updoming at the centre was of about 500 m. The extensive area of carbonatite agglomerate and arcuate sovite dykes that form most of Tundulu Hill together with dykes of nephelinite and the main fenitization are associated with this event. Emplacement of the second ring was centred on Nathace Hill the upper parts of which consist of agglomerate and the lower of feldspathic breccia and arcuate sheets of apatite and bastnasite carbonatite the whole of which is enveloped in a ring-dyke of nepheline syenite up to 550 m wide. Along the outer contacts of the foyaite with fenites a zone of nephelinization up to 30 m wide was recognised by Garson (1962) but in areas in contact with carbonatite, which form a kilometre long zone on the western side of Tundulu Hill, feldspathoidal carbonate-silicate rocks were generated. Associated with the foyaite ring-dyke is a series of radial and arcuate dykes of phonolite and micro-foyaite including leucite- and analcime-bearing varieties. After a period of major faulting rocks of the third centre were emplaced which comprise dykes, generally less than 0.5 m thick, and small plugs (<15 m diameter) of melanephelinite, which are concentrated around Nathace Hill, and thin sheets and veins of beforsite and carbonate-rich lamprophyres, some of which contain melilite laths replaced by carbonate. Garson (1962) has described in considerable detail the petrography of all the rock types at Tundulu. The agglomerates of Tundulu Hill consist of angular fragments of feldspathic breccia in a calcite matrix. The feldspar is potassic and these rocks are essentially fragments of potassic fenite. In contrast, the agglomerate that builds the upper parts of Nathace Hill consists of fragments and blocks up to 0.5 m across of trachyte, feldspathised fenite, sovite, dolerite and tuffisite in a trachytic matrix, in places cut by quartz-barite veins that Garson (1962) considered to be a high-level vent material. The sovite sheets that are abundant on Tundulu Hill and at various places cutting fenites in an arc to the west and southwest of Nathace Hill contain aegirine and/or biotite, with ubiquitous apatite, rare to abundant pyrochlore, magnetite and, in some rocks, ilmenite, barite, fluorite, quartz, celestite and other accessories. Analyses of calcite, apatite and magnetite are given in Dawson et al. (1996). Garson (1962) distinguished dark weathering dykes up to 25 m thick along the summit of Tundulu Hill, and elsewhere, as ‘siderite sovite’, although the carbonate has not been analysed. These rocks generally contain rare earth element carbonates. The carbonatites of the second centre, concentrated on Nathace Hill, include apatite sovite which is associated with rocks containing up to 90% apatite that also include quartz, calcite, an Fe-bearing carbonate and accessories including bastnasite. The other major carbonatites on Nathace Hill form concentric arcuate sheets up to 30 m thick and are characteristically bastnasite-bearing with apatite-rich and -poor varieties distinguishable. These are ferruginous rocks with much carbonate pseudomorphed by iron and manganese oxides, calcite, ankerite, apatite, quartz, pyrochlore, abundant sheaves of bastnasite, anatase, florencite, strontianite, synchysite, rare monazite and veinlets of clear calcite, dolomite and barite. A study of the rare earth minerals in the carbonatites has been made by Ngwenya (1994) who gives analyses of synchysite, parisite and bastnasite. The nepheline syenite of the Nathace Hill ring-dyke is variably medium- to coarse grained, sometimes porphyritic, in places banded and consists of K-feldspar, euhedral nepheline, aegirine-augite, titanite, biotite and magnetite with apatite, calcite and melanite in some variants. Two small dykes described by Garson (1962) vary from urtite through ijolite to melteigite and consist of nepheline, aegirine-augite, biotite and titanite. The carbonate-silicate rocks, which Garson (1962) ascribes to metasomatism by the nepheline syenite ring-dyke acting on sovite and sovite agglomerates, are extensively developed on the west side of Tundulu Hill, with a smaller area to the west of the ring-dyke. The Tundulu Hill rocks are of two types: very fine-grained orthoclase-nepheline-cancrinite rocks with scattered tiny grains of biotite, titanomagnetite, aegirine and calcite and very coarse biotite-magnetite rocks with reddish feldspar in a sparse matrix of calcite; locally aegirine is abundant and there are patches of apatite. The numerous silicate dykes and rather fewer plugs at Tundulu include phonolites, microfoyaite, solvsbergite and nephelinite. Analcime, natrolite and pseudoleucite phonolites have been distinguished. The dominant phonolites are alkali feldspar-nepheline-aegirine rocks with a little biotite and brown amphibole. Abundant analcime occurs in the groundmass of some dykes and others are striking rocks characterised by radiating spherical patches of natrolite up to 3.5 cm in diameter. The pseudoleucite phonolites are relatively rare, form dykes up to about 0.5 m wide, have pseudoleucites reaching 2 cm in diameter and consist of sanidine, nepheline, much altered to cancrinite, and a little natrolite. Garson (1962) also describes leucite analcimitite which forms two thin dykes (<25 cm thick) and consists of analcime containing leucite or pseudoleucite. Nephelinite associated with the first centre at Tundulu forms rare thin dykes and blocks up to 0.5 m across in a vent cutting fenites south of Nathace Hill. Some varieties contain phenocrysts of augite zoned to aegirine-augite, nepheline and titanite in a groundmass of pyroxene, nepheline, biotite and accessories. Aphyric varieties are similar but some contain barkevikitic amphibole and melanite. The melanephelinites of the third centre contain phenocrysts of augite with thin rims of aegirine, biotite and nepheline in a groundmass of the same minerals, variable amounts of calcite and cancrinite and accessories. Two plugs of melanephelinite are distinguished by the presence of partly serpentinized grains of olivine. Solvsbergite forms the youngest dykes at Tundulu and consists of orthoclase, a little quartz, aegirine and arfvedsonite. It is noteworthy that a swarm of solvsbergite dykes is associated with the Mlanje intrusion some 30 km to the south. Fenitization extends for up to 2 km from the central igneous complex and affects a broad range of Precambrian granulites, gneisses, granite and syenite porphyry and Karoo dolerite dykes. The outer parts of the aureole have typical sparse veinlets of aegirine but closer to the centre quartz-free syenitic fenites of alkali feldspar, aegirine and sodic amphibole are developed. Garson (1962) also describes and mapped a zone of nephelinisation up to 30 m wide superimposed on the fenites around much of the nepheline syenite ring-dyke. Analyses of all rock types are presented by Garson (1962).

The complex has been prospected for pyrochlore, rare earth minerals, apatite, barite and Fe an Mn ores (Garson, 1962). Over 800,000 tons of rock yielding >20% P205has been proved on Nathace Hill.
K-Ar dating of biotite from sovite gave 133(7 Ma (Snelling, 1965b).
DAWSON, J.B., STEELE, I.M., SMITH, J.V. and RIVERS, M.L. 1996. Minor and trace element chemistry of carbonates, apatites and magnetites in some African carbonatites. Mineralogical Magazine, 60: 415-25.GARSON, M.S. 1962. The Tundulu carbonatite ring-complex in southern Nyasaland. Memoir, Geological Survey of Nyasaland. 2: 1-248.NGWENYA, B.T. 1994. Hydrothermal rare earth mineralisation in carbonatites of the Tundulu complex, Malawi: processes at the fluid/rock interface. Geochimica et Cosmochimica Acta, 58: 2061-72.SNELLING, N.J. 1965b. Age determinations on three African carbonatites. Nature, London, 205: 491.
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith