Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences

Shawa

stripes

Occurrence number: 
181-00-007
Country: 
Zimbabwe
Location: 
Longitude: 31.72, Latitude: -19.2
Carbonatite: 
Yes

Located 13 km south of Dorowa (No. 181-00-006) Shawa is a 6 km-diameter, nearly circular plug of serpentinized dunite which is partly enclosed by arcuate bodies of ijolite and with a central ring of carbonatite. The complex has been intruded into Archaean granite gneisses, which are fenitized. The carbonatite ring and the ijolites produce two systems of concentric ridges with an area between of low relief. The dunite plug is mostly serpentinized but in the south areas of fresh dunite occur. This rock, which is sometimes banded with magnetite-rich layers, averages 96% modal olivine with a composition of Fo83. Magnetite is about 4% of the typical homogeneous dunite and a colourless clinopyroxene is accessory. A little brown and green spinel may also be present. The serpentinite is extensively replaced by calcite and quartz but fresher material, although generally with some replacement by the same minerals, contains 5-10% magnetite and serpentine pseudomorphs after olivine. Veins of fine-grained magnesite rock intersect the serpentinites. A gravity survey of Shawa (Lee and Reid, 1977) has demonstrated that the dunite/serpentinite body is about 500 m thick and is interpreted as a cumulate from an olivine ijolite magma. Ijolite forms an arcuate zone on the west of the complex and occurs sporadically elsewhere marginal to the serpentinite/dunite plug, but notably in the northeast. The western ijolite extends over approximately 4x1 km and is normally a coarse rock but a chilled zone up to 100 m wide occurs along the inner contact with serpentinite; there may be a similar outer zone against fenites but exposure is poor. The ijolites are typically heterogeneous with bands of urtitic composition and melteigitic patches. Pegmatitic ijolites are widespread and Johnson (1961) was able to map extensive areas of this rock type in which in the southern part of the ijolite intrusion a banded structure is developed of 15 cm-wide alternating coarser and finer grained bands dipping steeply outwards. The ijolites consist essentially of augite, with rims and smaller crystals of aegirine-augite and nepheline with accessory biotite, apatite and magnetite and sporadic cancrinite, titanite and orthoclase. Many of the pegmatites contain orthoclase, in some cases sufficient to categorise the rock as a malignite. At a few localities in the serpentinites small angular fragments of olivine urtite occur. Within quartz fenites on the eastern side of the complex occurs a 10 m-diameter plug of a rock called juvite by Johnson (1961). This rock comprises 59% orthoclase, 10% albite (An10), 31% aegirine-augite, 1% nepheline and a little titanite. Dolomitic carbonatite forms a central ring-dyke about 1.5 km in diameter the dyke varying between 30 and 300 m in width. The margins of the ring-dyke are not clear-cut but break into veins which penetrate the serpentinite for up to 15 m. The rock generally displays a vertical foliation and varies widely in grain size from fine to places where dolomite crystals are 10 cm in diameter. Two shafts into the serpentinite inside the ring-dyke encountered dolomitic carbonatite at 13 and 28 m, which may represent dykes or the roof of a plug. The carbonatite consists of dolomite, magnetite, abundant apatite with readily apparent fluid inclusions, and green phlogopite. Carbonatite also forms dykes up to 30 m in length and 1-2 m wide within the serpentinites. They mostly comprise an unfoliated dolomitic carbonatite of a similar mineralogy to the ring-dyke. A zone of syenitic fenites almost completely encircles the complex only being absent adjacent to the major ijolite intrusion in the southwest.The fenite grades outwards into quartz syenitic fenite and fenitized granite gneiss with an ill-defined outer margin. The fenites are pyroxene-bearing, aegirine being dominant in the outer zones and aegirine-augite in the syenitic fenites. An area of barite mineralization is located immediately inside the carbonatite ring-dyke in the northeast. Barite veins range from 0.1 to 2 cm in thickness and may occupy half the rock. Vermiculite is developed over an area of ijolite, syenitic fenite and a little serpentinite on the northeast of the complex. The vermiculite occurs as disseminations and veins up to 60 cm wide. An area of hydrated biotite veins lies within serpentinite northwest of the carbonatite ring-dyke, and vermiculite is distributed sporadically throughout the serpentinite. Some spectrographic trace element data for carbonatites are given by Johnson (1961). Analyses, including a broad suite of trace elements and Sr, Nd and Pb isotope data, for three carbonatites are given by Harmer et al. (1998).

Economic: 
Vermiculite was formerly won from open-cast workings - known as the Shawa Mine. Reserves of residual apatite-bearing rock of 25 million tonnes to a depth of 25 m at an average grade of 10.8% P2O5 have been proved (Fernandes, 1989).
Age: 
Rb-Sr determinations on biotite from ijolite gave an age of 209±16 Ma (Nicolaysen et al., 1962).
References: 

FERNANDES, T.R.C. 1989. Dorowa and Shawa: late Palaeozoic to Mesozoic carbonatite complexes in Zimbabwe. In. A.H.J. Notholt, R.P. Sheldon and D.F. Davidson (eds), Phosphate deposits of the world. 2. Phosphate rock resources. 171-5. Cambridge University Press, Cambridge.HARMER, R.E., LEE, C.A. and EGLINGTON, B.M. 1998. A deep source for carbonatite magmatism: evidence from the nephelinites and carbonatites of the Buhera district, SE Zimbabwe. Earth and Planetary Science Letters, 158: 131-42.JOHNSON, R.L. 1961. The geology of the Dorowa and Shawa carbonatite complexes, Southern Rhodesia. Transactions of the Geological Society of South Africa, 44: 101-45.JOHNSON, R.L. 1966. The Shawa and Dorowa Carbonatite complexes, Rhodesia. In O.F. Tuttle and J. Gittins (eds), Carbonatites. 205-24. John Wiley, New York.LEE, C.A. and REID, A.B. 1977. A gravity survey of the Shawa carbonatite complex, Rhodesia. Transactions of the Geological Society of South Africa, 80: 43-5.NICOLAYSEN, L.O., BURGER, A.J. and JOHNSON, R.L. 1962. The age of the Shawa carbonatite complex. Transactions of the Geological Society of South Africa, 65: 293-4.

Map: 
Fig. 3_346 Shawa (after Johnson, 1961, Plate 13).
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith