The g 3chemistry of c bonatites and related ro&s from carbonatitecomplexes, south Nyanza, Kenya

CHRISTOPHER BARBER

LITHOS Barber, C. 1974: The geochemistry of carbonatites and related rocks from two carbonatite complexes, south Nyanza, Kenya. Lithos 7, 53--63.

Carbonatites, metasomatised country rocks, and earbonatitic calcite and magnetite have been analysed from two earbonatite complexes, Homa and Wasaki, W. Kenya.

The carbonafites are all greatly Ca-earth enriched, contain abundant 'carbonatitic' trace elements (Sr, Ba, Nb and REE), and generally low concentrations of Cr, Co, Ni, Pb, Ga, Ge, Sn, Bi, Li and Mo. At both complexes early s~vite is rich in Sr, and impoverished in other trace elements relative to the alvikites. The late-intruded melacarbonatites contain the greatest concentrations of Ba, REE, Fe and Mn.

It is concluded that the accumulation of these elements in the later earbonatites is mainly due to fractionation of carbonates from c.arbonatite magma which was initially rich in 'carbonatitic' trace elements.

Christopher Barber, Department o/ Geology, University o/ Nottingham, Nottingham, Englamt.

Carbonatites and their associated silicate rocks, such as ijolites, fenites and nepheline-syenites, contain a characteristic set of trace and minor elements, often in exceptional concentrations (Heinrich 1966). The carbonatites in particular contain large concentrations of Ba, Sr, Nb and the rare-earth elements (REE). These elements substitute for major elements of similar ionic size and chemical properties in the common carbonatitic minerals (e.g. calcite, apatite and magnetite) or more rarely form rare-element minerals such as pyrochlore, bastnaesite and monazite (Heinrich 1966).

Detailed studies of the geochemistry of carbonatite complexes, especially those in East Africa, are rare. Those which have been made show considerable variation in the distribution of both major and trace elements (Van Warn- bake 1964, yon Eckermann 1966, Temple & Grogan 1965).

This investigation was undertaken to deter-

mine the geochemical evolution of varied carbonatitic rocks from two complexes in western Kenya. It complements field mapping and petrographic study of a number of com- plexes in western Kenya and eastern Uganda by other members of the East African Re- search Project at Bedford College (London~ and Leicmter University (1963-69).

Material was selected from collections of carbonatites and associated rocks from Homa Mountain and Wasaki, western Kenya. Both complexes contain a variety of carbonatite types, for example ~vite, alvikite, melacar- bonatite, carbonatitic breccia and carbonatites with pseudomorphs after melilite, whose age relationships have been worked out in detail (Flegg 1969, Le Bas i972, pers. comm. [ms in preparation]). Also petrographic study of the rocks has revealed mineralogical trends which coincide with their order of emplacement and which require geochemical amplification.

54 Christopher Barber

Geological setting of the carbonatite complexes Homa Mountain and Wasaki are situated in the Kavirondo Rift Valley in western Kenya. The Homa Mountain complex, on the shores of Lake Victoria, is composed of carbonatite and 'ijolite' intruded into Nyanzian andesitic lavas in Miocene to early Pliocene times (Sag- gerson 1952, McCall 1958, Clarke & Flegg 1966, Flegg 1971). Emplacement of ijolite was followed by intrusion of carbonatite and car- bonatitic breccia, generally in cone sheets. Plugs and dykes of phonolitic nephelinite and olivine-melilitite occur, aud fenitisation of the country rock is widespread.

The Wasaki centres, lying to the west of Homa Bay and 40 km cast of the Kisingiri volcano, intrude l~recambrian granitic rocks. They consist o.~ an ijolite mass at Usaki, sur- rounded by fenites; a carbonatite complex at Sokolo; and the phonolitic volcanic remnant of Nyamaji (Pulfrey 1950,. 1954, McCall 1958, Le Bas 1966, 1970).

Petrology of the carbonatites and related rocks Detailed petrographic ~tudy of the carbonatites from W. Kenya, notably Homa Mountain (Flegg 1969), showed three main types of car- bonatite; (a) sevite, a coarse-grained white carbonatite, (b) alvikite, a fine to medium- grained carbonatite usually coloured pale brown, a~,d (c) melacarbonatite, fine-grained and deep brown in colour. In addition, car- bonatites containing abundant calcite pseudo- morphs after melilite, and intrusive breccias with a matrix of calcite are found. The latter are distinctly different from the three main divisions described above, and are called melilite-pseudomorph carbonatites, and car- bonatitic breccias below.

The earliest ir'truded carbonatites are the sevites, usually found in plug-like intrusions. They are cut by thin 10 cm-5 m dykes and cone sheets of abundant alvikites and car- bonatitic breccias. 'I'h,.-. melacarbonatites occur in veins, dykes and cone sheets. They were generally intruded later than the sevites and alvikites and represeat the final phases of carbonatitic activity at both Homa and Wasaki.

Rare plugs and dykes of melilite-pseudomorph carbonatite are found at Homa.

The m~neralogy of the corresponding types of car ~ onatitic rocks at both Homa and Wasald is similar. The sevites are made up of coarse- grained calcite in platy or equant crystals, which may or may not show orientation. The accessory minerals apatite, biotite, feldspar and occasionally pyrochlore are rarely abun- dant (Flegg 1969).

The alvikites are generally much finer- grained rocks than the sOvites; they may be equigranular oi porphyritic with phenocrysts of calcite in rhombic or irregular crystals in a matrix of calcite. Biotite, euhedral pyrochlore, apatite, pyroxene, amphibole and large mag- netite grains are the more abundant accessory minerals. They are usually found in greater amounts in the non-porphyritic types.

The melacarbonatites contain a charac- teristic mineral assemblage including barite, fluorite, monazite, bastnaesite, dahllite and collophane. An abundance of magnetite, haematite, and goethite gives ri:~e to a typical deep brown colour in hand specimen. Calcite, often iron-stained, makes up th~ matrix.

The melilite-pseudomorph carbonatites con- sist of calcite pseudomorphs after melilite, in a matrix of fine-grained calcite and opaque oxides. The calcite is occasionally iron stained.

The carbonatitic breccias are made up of breccia fragments of 'ijolite', Nyanzian lava, carbonatite and fenite, set in a fine-grained matrix of calcite and, usually, haematite and goethite. The fragments of country rock, ijolite and fenite are often K-feldspathised in a manner similar to the country rock adjacent to the carbonatite intrusions. These feldspath- ised country rocks usually consist of orthoclase feldspar, with minor amounts of calcite, opaque oxides, apatite and rare pyrochlore.

Analytical methods Powdered rock and mineral samples were analysed for both major and trace elements using an A.R.L. 29000B direct reading spectrometer (Quantometer). Analytical lines were set on this instrument for the determination of trace elements as follows: Ba(4550A), Cr(4254.4A), Cu(3274.SA), Co(3453.5A), Li(6103.1A), Mo(3170.3A), Ni(3424.TA), Pb(2833.1A), Sr(4607.3A), Ga(2943.6A), Ge(3039.1A), V(4379.2A), Zn(2183.6A), and Zr(3392A), and major elements Fe~2739.6A), Ca(3158.9A), AI(2378.4A), Mg(2790.8A), Mn(2933A) an-;~ K(3653.9A).

The method of analysis is essentially that described by Tennant & Sewell (1969) for the spectrochemical determination of several elements using DC arc anode excitation and no internal standard. Sample powders are mixed with their own weight of carbon powder (Magicol Black 888) and half this weight of NaF buffer to minimise matrix effects.

Rocks and minerals with a high carbonate content were also diluted with 'sx~cpure' silica to avoid ejec- tion of sample from the electrode during arcing. Each sample was loaded into a flat ended graphite electrode and arced at 15 amps using a graphite counter electrode and an electrode gap of 3 mm. Synthetic standards for the major elements were made up using 'spe~ure' compounds, diluted with CaCO 8 in the case of the carbonate rich samples. Standards for the trace elements were obtained by the addition of 'specpure' trace compounds to a synthetic base of major element composition similar to that of the unknown samples.

Replicate determinations on one carbonatite and one silicate rock sample throughout the analysis gave for most trace elenlents a precision of better than 10% at the 95% confidence level, except for Nb which gave a precision of 20%. Major element precision was always better than _+ 30%.

The rare-earth elements La, Ce, Nd and Y were determined usins; a Siemens Krystalloflex 4 X-ray fluorescence spectrometer. Samples were prepared for analysis by pelletising 3-4 gms of powder with a backing of borax (Jenkins & De Vries 1967). Stand- ards were produced by the addition of known weights of 'specpure' oxides to a natural carbonatite con- taining low values for these elements. Cerium, yttrium and neodymium values were obtained from Ce Lfll, YKal, and Nd Lal radiations using a tungsten target and scintillation counter. Lanthanum was determined from La Lal radiation, using a chromium target and a gas-flow counter. Replicate determinations on one sample gave a precision of better than 10%. All the data were corrected for absorption effects, using major element data obtained from spectrochemical analysis.

Results The results of the analyses are summarised in Tables 1 and 2. In Table I, ar i thmetic means and ranges of concentration for each element in each carbonati te group are given for both complexes. In Table 2 similar data are given for calcite and magnetite.

Discussion Strontium and barium

The concentrat ion of these elements varies considerably in the carbonatites. The s~vites contain the least Ba and the highest Sr con- centrations. Ba is most abundant in the mela-

Geochemistry o~ carbonatites 55

carbonatites, while Sr is least concentrated in these rocks at bo th comrlexes.

The large concentrat ions of Ba in the mela- carbonati tes is due to the abundant barite. In the s~vite~ and alvikites, however, both ele- ments are mainly concentrated in calcite, and to a lesser extent in apati te and pyrochlore. Sr is more abundant in the s~vitic calcite than in c,,lcite f rom alvikite, whilst the opposite is t rue for Ba (Table 2).

The trend of increasing concentrat ion of Ba and decreasing concentrat ion of Sr in the carbonatites coincides with the order of em- placement of the carbonati tes from s~vite to alvikite to melacarbonat i te , at both complexes.

The melil i te-pseudomorph carbonatites, con- taining abundant calcite and no detecte¢~ barite, have low concentrat ions of Sr and high concentrat ions of Ba relative to the s~vites and alvikites at Homa. This suggests the late generation of these rocks, which agrees with field evidence.

It is more difficult to compare the geo- chemistry of the carbonati t ic breccias with that of the carbonati tes proper due to the diverse mineralogy of the former rocks. The breccias contain less calcite than the car- bonatites, and consequently have less Sr. Ba is more abundant than Sr in these rocks, being present to a greater extent in orthoclase as well as calcite.

Manganese and iron

Mn and Fc also show a considerable range of concentratiov in the carbonatitic rocks. Al- though there is great variation in the Fe content of the melacarbonati tes, these rocks contain overall the greatest concentrat ions of both Fe and Mn. The s~vites show the lowes*, concenUations of both elements, and the alvikites contain intermediate amounts. Thus both elements increase in concen~:ration from the early to late-intruded rock~. They are present to a large extent ir.l o~raque oxide minerals such as magnetite, haemafite and goethite in the s~vites and alvikites. Calcite contains minor amounts of Fe and Mn, wi'~h higher concentrat ions in calcite f rom alvikite compared with ~ v i t i c calcite (TaL'.e 2).

The unusually high concentrations of Mn in the melacarbonat i tes could not l~z traced to any Mn mineral , and probably are largely associated with the abundant iron oxide miner-

Tab

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ma

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Ho

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Was

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Ho

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Was

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carb

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<

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ean

18

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188

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35

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110-

480

110-

255

115-

240

Zr M

ean

< 10

<

10

69*

53*

29*

31"

68

46

Ran

ge

< 10

-20

< 10

-50

< 10

-240

<

10-2

10

< 10

-90

< 10

-60

40-1

00

30-6

0

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ean

349

322

1,09

3 1,

124

2,74

1 1,

898

306*

2,

691

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ge

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374

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1,00

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1,53

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ean

568

957

2, 3

20

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4 6,

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55

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4

6 20

7

8 5

6 3

Key

: N

-

Num

ber

of a

naly

ses.

*

- M

ean

is a

ppro

xim

ate

due

to s

ome

anal

yse~

l~

in~

he!

ow t

~

d~

tio

n

lim

it v

, ab

ove

the

uppe

r li

mit

of

sens

itiv

ity.

-

-No

m

ean

calc

ulat

ed,

due

to t

he m

ajor

ity

of a

naly

ses

bein

g be

low

the

det

ecti

on l

imit

. n

d-

Ana

lysi

s no

t at

tem

pted

.

58 Christopher Barber

Table 2. Major an6 trace el,~nents in carbonatitic calcite and magnetite. Cr, Ga, Ge, Li, Hi, Sn, lqb and Me were unde~cted in calcite; Cr, Ga, Ge, Hi, Li and Sn were undetected in magnetite. Element concentrations are in ppm; oxide concentrations in wt.%.

Mineral: Calcite Magnetite

Locality: Homa Wasaki Homa Wasaki Homa Rock: Sovite Sevite Alvikite Alvikite Alvikite

FeaO a Mean - < 0.1 0.38* 1.3 nd Range < 0.1-0.3 < 0.1-0.9 0.5-2.4 nd

MnO Mean 0.2 0.2 0.26 0.4 Range 0.1-0.3 0.1-0.7 0.1-0.7

MgO Mean - 0.5* 0.38* 0.3 Range < 0.1-0.3 < 0.1-1.6 < 0.1-0.7

Tie 2 Mean < 0,1 < 0.1 < 0.1 < 0.1 Range < 0.1 < 0.1 < 0.1

Ba

IVlean 576 250 1,347 587 Range 460--810 400--2,560 260--1,050

0.87 0.4--1.9

0.22 0.1-0.5

2.13 1 .(]-5.0

1,049 190-2,250

Co Mean < 5 < 5 < 5 < 5 9 Range < 5 < 5 < 5 5-35

CII Mean < 5 < 5 Z 5 < 5 6 Range < 5 < 5 <. 5 5-15

Pb Mean < 10 < 10 < 10 52 < 10 Range < 10 < 10-10 < 10--10 5-130

Sr Mean 3,400 2,440 1,620 1,220 < 10 Range 2,200-4,000 890--2,480 1,040--1,480 < 10

V Mean < 10 15 - - 4,591 Range < 10-20 < 10-55 < 10-15 2,700-6,500

Zn Mean < 10 41" 132 1,176 Range < 10 < 10 < 10~-120 10-340 78(~1,800

Zr Mean < 10 < 10 - - 31 Range < 10 < 10-95 < 10-35 10--70

N 3 1 8 3 8

Key:

N - Number of analyses. * -Mean approximate due to some analyses being below the detection limit, or above the upper limit of analytical sensitivity. - - No mean calculated due to the majority of analyses being below detection limit. nd - Analysis not attempted.

als in these rocks. However, Mn is consistently more abundant in the melacarbonatites than Fe, which suggests that i t is associated with two or more mineral phases in the mela- carbonatites. Calcites from these rocks at Homa contain Mn-rich cores (Flegg 1969), which would account for the anomaly. The data here similarly suggest that the Wasaki melacarbonatites contain Mn-rich calcites.

Niobium and zirconium

Nb and Zr vary irregularly in the carbonatites with Nb always the more abundant. Both ele- ments are present in greatest amount in the alvikites, and are least concentrated in the s~vites.

The variability in the concentration of Nb within the various groups is related to the irregular occurrence of pyrochlore in the car- bonatites and carbonatitic breccias. Pyrochlore is more abundant in the alvikites than in other carbonatite groups, and is the only Nb- rich mineral found in the Homa carbonatites (Flegg 1969).

Zirconium rarely exceeds 100 ppm in the carbonatites and carbonatitic breccias. No Zr- rich minerals have been reported from these carbonatites, although zircon, baddeleyite and zirkelite have been reported from other car- bonatitic rocks (Heinrich 1966). Only small amounts of Zr have been reported from several other carbonatites from Africa (Higazy 1954, Barber 1971, Dawson & Hawthorne 1973).

Analyses of pyrochlore from carbonatite have shown Zr to be present in this mineral in amounts up to 4.6% ZrO 2 (van der Veen 1963, van Wambeke 1964). The variability of Zr in the carbonatites, like that of Nb, is. thus probably related to the distribution of pyro- chlore.

Titanium, vanadium, chromium, cobalt, copper and nickel

Cr, "Co, Cu and Ni are never present in the carbonatites in large concentrations (usually in amounts below 10 ppm). Neither Ni nor Cr were detected in magnetite (less than 10 ppm), and only small concentrations of Co and Cu were found i~ this mineral (Table 2).

The concentrations of V and Ti, however, greatly exceed those of Cr, Ni, Co and Cu. Titanium is generally present in grtmter con-

Geochemistry o/carbonatites 59

centration in the alvikites than in sctvite, melacarbonatite, or melil~te-pseudomorph car- bonatite. However, this element is consistently more concentrated in the carbonatitic breccias.

Vanadium, unlike titanium, is not concen- trated to any great extent in the breccias, but shows a general increase in concentration from s~vite to alvikite to ferruginous alvikite at both coraplexes. Both elements are notably concentrv~ted in carbonatitic magnetite (Table 2). Titan/um is also present in large amounts of pyrochlore, shown by electron probe analysis to contain from 3.99-6.53% TiO 2 (Barber 1971). The greater concentration of V in the melacarbonatites is possibly due to concentra- tion of this element in goethite. V-rich goethite has been noted from carbonatitic rocks from the Kaiserstuhl complex, W. Germany (van Wambeke 1964).

Lead and zinc Zn and Pb are both concentrated in the mela- carbonatites. Zn increaseis in concentration from ~vite to alvikite to melacarbonatite and is present in both calcite and magnetite, although magnetite contains the greater con- centrations. Pb is undetected in most sCvites and alvikites. Small amounts of this element were found in magnetite.

Lithium and molybdenum

Little variation in concentration of these ele- ments among the carbonatite groups is ap- parent. However, the Homa melacarbonatites do contain unusually high concentrations of Mo in three analysed samples, varying from 110 to 680 ppm. Heinrich (1966: 241) noted a similar tendency for Mo to concentrate in the later stages of carbonatitic activity, repre- sented in this case by the melacarbonatites.

Lanthanum, cerium, neodymium and yttrium The rare-earth elements La, Ce and Nd, and Y occur" in exceptionally high concentrations in some carbonatitic rocks (Vainsb~ein et al. 1961, van Wambeke 1964, Heinrich 1966, Kapustin 1966); the Homa and Wasaki car- bonatites are no exception. All the carbon- atites are particularly rich in the Ce-earth elements. An approximate normalisation of the data for La, Ce, Nd and Y to chondritic abundances has shown these rocks to be greatly

60 Christopher Barber

enriched in Ce-earths relative to chondrites. This enrichment in carbonatitic rocks has been demo~strated elsewhere (Schofield & Haskin 196,~, Haskin et al. 1966) where more detailed investigation of the di,~tribution of the rare- earth elements has been andertaken.

In ~the Homa and Wasaki carbonatites, a general increase in the concentration of La, Ce, Nd and Y from sevite to alvikite to mela- carbonatite coincides with the order of em- placement of the carbonatites. Similai" enr~cll- ment trends have been reported for the Kaiser- stuhl complex (van Wambeke 1964) and for carbonatites of the Russian platform (Kapustin 1966, Vainshtein et al. 1961). Both the breccias and the melilite-pseudomorph carbonatites are similarly enriched in Ce-earths. The latter also contain greater amounts of Ce and La than the sevites and alvikites, which suggests their late intrusion.

The REE in the sevites and alvikites are dispersed in calcite, apatite and pyrochlore. Electron microprobe analyses of these minerals (Barb~:r 1971) showed that pyrochlore generally contained the most Ce and La, whilst calcite contained the smallest amounts. S~vitic calcite also contained less Ce than calcite from alvikite. The high concentrations of REE in the melacarbonatites and some alvikites are related to the presence of monazite and bastnaesite in these rocks. These usually occur as minute bleb-like crystals, which could be easily overlooked in thin section.

Magnesium

The concentration of Mg is surprisingly low in all analysed carbonatites, which is in part due to the absence of dolomite in these rocks.

It has been shown from experimental work (Wyllie 1966) that the composition of car- bonate minerals precipitated from carbonatitic magmas is pressure dependent, and that ' . . . for many initial compositions, low pressures favour the formation of carbonatites rich in calcite, and increasing pressure tends to favour the formation of dolomite and ankerite'.

The absence of dolomite in these rocks is most probably a reflection of the intrusion of the carbonatitic fluids at high levels in the crust, followed by crystaUisation of carbonate minerals under low pressure.

Relatively small amounts of Mg were found in carbonatitic calcite and magnetite. Car-

bonatitic pyroxene and mica contain major amounts of Mg, although these minerals are never abundant in the analysed rocks, and are usually only accessory.

The distribution of trace and major elements in K-feldspathised country rock adjacent to carbonatite intrusions at Homa Mountain

Analyses of five feldspathised rocks from Homa Mountain are summarised in Table 3, and compared with the average analyses for the Homa sevite and slightly altered (fenitised) lava at Homa.

The feldspathised rocks contain orthoclase rich in Ba (greater than 1000 ppm) and Ga. They also carry accessory pyrochlore, mag- netite and apatite, emphasising their affinities with the carbonatites, and explaining the high concentrations of Nb, Zr, Ti and REE (in pyrochlore), Fe, Mn, Ti, V and Zn (in mag- netite) and Sr and REE (in apatite).

The abundance of these 'carbonatitic' trace elements in the feldspar-rich rocks strongly suggests that they were introduced into the country rock by feldspathising solutions emanating from the crystallising carbonatite magma, and indicates that the magma was enriched in these trace elements before intru- sion of the carbonatite. The concentrations of Fe~ Ti and Zr, higher than the sevites, sug- gest that these elements are to a large extent inherited from the original country rocks. On the other hand, the concentrations of Ba and Nb are higher than in either ~vite or un- altered country rock, and indicate that those elements not entering the crystaUising phases in the carboiiatitcs in large amounts were transported by the feldspathising solutions to the surrounding country rocks.

Summary and conclusions The Homa and Wasaki carbonatites, like other carbonatitic rocks, contain high concentra- tions of St, Ba, REE and Nb. They are also greatly enriched in Ce-earths relative to chon- drites, which along with the peculiar major element composition, contrast with the chem- ical features of other more abundant :igneous rocks.

Geochemistry ot carbonatites 61

Table 3. Comparison of the major and txace element distribation in s~vite, feldspathised country rock, and slightly fenitised country rc ~k at Homa Mountain. Oxide concentrations are in wt.%, and trace element concentrations in ppm.

Sovite Feldspathised rock

Mean ',.Range) Mean (Range) Country rock

F%O s 0.1" (< 0.1-2.1) 13.0 (7.2g~0.2) nd MnO 0.3 (0.2--0.6) 0.2 (0.1-4L3) nd TiO a <0.1 (< 0.1 ) 0.9 (0.4-1!. 2) nd CaO nd nd 5.3 (0.9-10.0) nd Ba 1,170 (600-2,800) 4,132 (1,080--8,000) 300 Be nd nd 38 (30-70) 8 Co < 10 (< 5-10) 17 0~32) 12 Cu < I0 (< 5--60) 36 (4-70) 5 Ga < 10 (< 10) 75 (30-175) 25 Li 20 (15-25) 10" (<10-72) 10 Nb < 50 (< 50) 434 (170-670) 50 Pb < 10 (< 10) 44 (< 10-105) 12 Sr 2,185 (1,610-2,550) 774 (270-1,650) 90 V 24 (10-40) 244 (130--430) 10 Zn 18 (10-30) 216 (15-435) 190 Zr < 10 (< 10-20) 892 (180-2,350) 520 La 349 (333-374) 192 (110-285) nd Ce 568 (475-700) 148 ( 11 0-225) nd Y 10" (< 10-31) 26 (9-44) nd

* - Signifies an approximate mean due to some values being below the limit of delection. nd - Analysis not attempted.

The large concentrations of Nb and REE also distinguish carbonatites from sedimentary limestones (Heinrich 1966). However, the ir- regular variation in the concentration of Nb, and, the generally low concentrations in the sctvites, suggest that Nb is not so useful. The large concentration of RE~, coupled with the marked Ce-earth enrichment relative to chow drites, seems to be a more suitable criterion for the distinction between these rock types.

The carbonatites have Be, Ga, Li, Ge, Sn, Cr, Ni, Bi, Co and Cu contents generally below 20-30 ppm, and in this respect resemble carbonatites from S.W. Uganda and the Congo (Higazy 1954), and S.E. Uganda (Barber 1971). This may in part be due to the paucity of lattice positions in which these elements may diadochically replace major' elements (e.g. Ga replacing AI, Li replacing K). The low con- centrations of Cr, Ni, Co and Cu in magnetite, however, would suggest that the carbonatitic magma cont~ned low concentrations of these metals.

The carbonatite types from both complexes are geochemicaily similar. Differences do exist in the level of concentration of elements in

the same type of carbonatite from the two complexes; the s~vites from Wasaki are mar- ginally richer in Nb and poorer in Sr than the corresponding rocks from Homa, but the greatest differences are found in the mela- carbonatites, where Fe, Mn, REE, Zn, Pb and Ba are all present in greater amounts at Homa.

When correlated with field evidence, the data from both complexes show an overall pattern of accumulation of Fe, Mn and REE, and to a lesser extent Zn, Pb, and V, from the early rOvitic carbonatites to the late mela- carbonatites. Nb, Ti and Zr are predominantly concentrated in the middle carbonatite stage, in the alvikites, whilst Sr is most abundant in the sCvites. The carbonatitic breccias at Homa contain intermediate amounts of Ba, Mn, Fe, Nb, and Zr, which suggest an association with the alvikites. The large concentrations of Ba, REE and low Sr values in the melilite- pseudomorph carbonatites, relative to the sCvites and alvikites, emphasise the late intru- sion of these rocks.

The similar distribution of trace and major elements in the carbonatites at both complexes points to a single dominant process affecting

62 Chris topher Barber

the crystallisation of the carbonatitic fluids, resulting in the concentration of Ba, R E E Mn, Fe, Nb, Ti, Pb, Zn and V, and depletion of Sr in the later liquid fractions.

Two processes which could account for the accumulation of these elements are:

(a) Incorporation of Ba, REE, Nb, Fe, Zn and Mn by reaction of carbonatite magma with country rock (Nyanzian lava at Homa, granodiorite at Wasaki), giving large concen- trations of these elements in the later car- bonatites, and

(b) fractional crystallisation of minerals from a carbonatite magma which initially contained large concentrations of 'carbonatitic' trace ele- ments.

Although th. considerable alteration of the country rock around the intrusive centres of Homa and Wasaki is well documented (e.g. fenitisation and K-feldspathisation) it is the author's opinion that only small amounts of trace elements (Sr, Ba, Nb, Zr, REE) in the car ~ onatites are obtained by reaction with, or a .milation of country rock. The analyses of feldspathised country rock at Homa show that these trace elements are introduced into the country rock from crystallising carbonatite.

The loss of trace elements to the surround- ing country rocks from the carbonatitic magma suggests that the parental liquid to the series sOvite-alvikite-melacarbonatite was ini- tially rich in these trace elements. Fractional crystallisation of such a magma would thus seem to be a more attractive proposition for the origin of the carbonatite series.

Calcite is by far the most abundant mineral in the carbonatites, and would thus exert the most influence on elemental trends in the carbonatites, if fractionation were allowed to proceed. The trace element data for calcite (Table 2) suggests that calcite removed Sr from the magma preferentially to Fe, Mn, Ba, REE and Zn, the latter elements increasing in concentration in later liquids. These trends continued into the latest phase of intrusion in the melacarbonatites, where abundant opaque oxides, barite and REE minerals formed.

The elements Nb, Ti and Zr, which are not found/in calcite, increased in concentration in the liquid until present in sufficient amounts for pyrochlore to crystallise. Depletion of the

liquid in these elements was most severe in the alvikites at both complexes, as shown by the low concentrations of Nb, Ti and Zr in the later melacarbonatites.

Other trace elements which were not re- moved from the magma by crystaltisation of calcite, or in accessory pyrochlore, apatite and magnetite, would be expected to be present in increasingly larger amounts in the later car- bonatitic rocks. These elements, Pb, Mo and V, increase in concentration from sOvite to alvikite, and attain the highest concentrations in the melacarbonatites.

The distribution of these elements between the carbonatite types at Homa and Wasaki can thus be explained as being the result of frac- tionation of mainly carbonates (calcite) from a carbonatite magma, which was initially en- riched in 'carbonatitic' trace elements REE, Nb, Sr and Ba. Early crystal fractionation most probably occurred at a lower level in the crust than that now occupied by the car- bonatite intrusions, as the proportion of ex- posed s0vitic carbonatite is too small to have given rise to the abundant alvikites. This requires the presence of intruded carbonatite below the present topographical level of the Homa and Wasaki centres. The proportion of exposed melacarbonatite is in keeping with an origin by fractionation from the alvikite liquids.

Acknowledgements. - The author is grateful to Profes- sor B. C. King (Bedford College, London) and Drs. M. J. Le Bas, D. S. Sunderland, M. C. (3. Clarke, J. A. Dixon and A. M. Flegg (Leicester University) for providing samples and petrographic data. The X-ray fluorescence analyses were carried out at the Department of Geology, Leeds University, by kind permission of Dr. (3. Hornung, whose help with the X-ray methods was appreciated. Drs. M. J. Le Bas and R. B. EUiott read the initial manuscript and offered helpful criticism.

The work was supported by a grant from the Natural Environment Research CounciJ, whose help is gratefully acknowledged.

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Accepted for publication November 1973 Printed January 1974