Graphite in Precambrian rocks of Southern Africa: implications on the carbon content of metamorphic rocks

Precambrian Research, 26 (1984) 223--234

223

Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands

GRAPHITE IN PRECAMBRIAN ROCKS OF SOUTHERN AFRICA: IMPLICATIONS ON THE CARBON CONTENT OF METAMORPHIC ROCKS

THOMAS O. REIMER

c/o Dyckerhoff Engineering Gm bH, Postfach 224 7, D-6200 Wiesbaden (W. Germany) (Received July 22, 1983; revision accepted May 21, 1984)

ABSTRACT Reimer, T.O., 1984. Graphite in Precambrian rocks of southern Africa: implications on the carbon content of metamorphic rocks. Precambrian Res., 26: 223--234. Data on the occurrence of graphite and other highly reconstituted carbonaceous matter as potential traces of carbon originally derived from organic matter are compiled from Precambrian rocks of Namibia, Botswana, South Africa, and Zimbabwe. While carbonaceous matter occurs in sediments throughout Precambrian sequences of the area, graphite occurrences appear to be concentrated in high-grade metamorphic rocks of the Early Archaean Beitbridge Complex of the Limpopo metamorphic belt and the late Precambrian Damara Supergroup of Namibia. The original sediments were mostly carbonaceous shales and rarely bituminous carbonates. The data compiled show that metamorphic rocks can contain appreciable amounts of carbon that cannot be disregarded in any calculation of carbon cycles. Previous estimates, weighted in favour of gneisses, give an average content of 200 ppm Corg for metamorphic rocks, i.e., the same as for igneous rocks. The inclusion of sediments of lower metamorphic grades in the calculation raises the mean to 0.054% Corg in metamorphic rocks. This results in a total of 6.75 × 1021 g Corg in the metamorphic portion of the crust, or 33% of the total Corg contained in the crust. This is a proportion which suggests a need for a revision of the presently accepted balance calculations of the interconnected carbon and oxygen cycles.

INTRODUCTION

The distribution in space and time of carbon as the element of life has always been of great interest for the origin and development of life on Earth. Special attention has been paid to the quantitative distribution o f carbon and its isotopic composition in sediments. Amongst the more recent contributions to the subject of carbon isotopes have to be mentioned the papers of Schidlowski et al. {1975) on carbonate carbon and by Eichmann and Schidlowski (1975) on Corg--Ccarb pairs in the Precambrian sedimentary record. Veizer and Hoefs (1976) presented additional data on the isotopic

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224 composition of Precambrian Ccarb and compiled almost 2000 analyses from a large number of sources, also covering the Phanerozoic. The observation that since the early Precambrian the isotopic fractionation between Corg and Cearb has remained virtually constant around 250/0o was used by Eichmann and Schidlowski (1975) as support for the ratio between Corg and Ccarb in the total sedimentary reservoir having remained relatively constant with time at - 20:80. Using this ratio and the average recent mass fluxes between atmosphere, hydrosphere, biosphere, and lithosphere, these authors developed a flow chart for the steady-state condition of the combined carbon and oxygen cycles. This chart, however, does not take into account the considerably larger portions of the igneous and metamorphic rocks in the lithosphere which, despite their lower carbon concentrations, nevertheless present reservoirs of considerable absolute size. Little is known on the carbon content of these rocks and its isotopic composition. Consequently, the implication of these additional carbon reservoirs on the carbon--oxygen cycle is difficult to assess. It is the aim of this paper to present data from a limited area of metamorphic rocks of the continental crust, i.e., the Precambrian of southern Africa, which support the thesis that even in highly metamorphosed rocks carbon can be preserved in nonnegligible amounts. For this purpose data on graphite occurrences are compiled from Botswana, Namibia, South Africa, and Zimbabwe, based on reports by Martin (1965), Wilke (1969), Massey (1973), and Whiteside (1976), supplemented by other literature data and own observations. In the papers evaluated the term graphite is used in an economic--geological sense, designating a material that is commercially acceptable as graphite without being necessarily fully recrystallized to graphite in the mineralogical sense, i.e., a nearly pure carbon phase possessing the full 3-dimensional ordering characteristics of graphite. As the methods of identification are not given in the papers mentioned, it is possible that some of the material contains hydrogen-poor carbonaceous residues displaying some of the characteristics of graphite such as x-ray diffraction patterns indicative of the extending aromatic planes of carbon atoms in graphite, but without the characteristic reflections showing the orderly stacking of these planes. The "geological" use of the term graphite is retained here with the above restrictions in mind. It does, however, not distract from the probable derivation of the vast majority of this carbon from organic precursors in original sediments. It has to be remembered, however, that graphite can form not only from metamorphism of organic material in sediments. This is suggested by its occurrence in meteorites and pegmatites as well as an apparently primary constituent in some igneous rocks (Putzer, 1968). For certain economic graphite deposits such as those of Sri Lanka where masive graphite occurs in veins or cavity fillings in Precambrian marbles, schists, and gneisses, it has been suggested that its carbon was derived through thermal dissociation of the CO2 of the enclosing carbonates (cf. Putzer, 1969). Owing to the large amounts of energy required for this reaction (2CO2 + 135.4 kcal ~ 2CO

225 + O:, 2CO + 52.8 kcal ~ 2C + O2), this origin of graphite appears to be the exception rather than the rule (Hahn-Weinheimer, 1965). As shown by the wide spread of isotope ratios in graphite (Hahn-Weinheimer, 1965; Hoefs, 1969) the graphitization process is accompanied by a number of isotope homogenization and fractionation reactions yet to be investigated in detail. GRAPHITE OCCURRENCES

Within the sediments of the southern African Precambrian carbonaceous matter is a regular constituent (cf., Hall, 1938; Visser, 1964). The sediments of the Early Archaean Swaziland Supergroup may serve as an example in which Reimer et al. (1978) noted an average carbon content of 0.43% with individual values as high as 2.12%. The graphite occurrences of southern Africa can be subdivided into five major groups according to their host rocks. They are compiled in Table I and the more interesting ones are described below. The locality numbers used refer to Fig. 1. Archaean schist belts

Minor graphite showings are known from schist belts of the northern part of South Africa and eastern Botswana (loc. 1--4). In the Barberton Mountainland graphite occurs in the dormant gold mines Figaro (loc. 5) and French's Bob (loc. 6) in stringers of graphitic schists within up to 6 m wide zones of carbonaceous cherts and shales. They most probably belong to the lower part of the Fig Tree Group. Graphitization was caused by strong shearing, possibly assisted by hydrothermal processes. A sample from French's Bob showed a content of 1.28% C and 53 000 ppm S (Moore et al., 1974). Its carbon isotope ratio 8 13Corg was -- 26.9°/00 (K. Kvenvolden, personal communication, 1974). Another, slightly more sheared, sample gave a 8 13Corg-value of --20.560/0o (M. Schidlowski, personal communication, 1983). Archaean gneissic terrane

Graphite occurrences appear to be concentrated in the eastern central, intensely folded, portion of the Limpopo Belt north of the Soutpansberg (inset of Fig. 1). The area is underlain by the highly m e t a m o r p h o s e d r o c k s of the ~ 3.4 Ga-old G u m b u Group of the Beitbridge Complex. In this area the largest graphite deposit of South Africa, the n o w dormant G u m b u Mine (loc. 7), is situated. Graphite here occurs mostly as disseminated flakes up to 4 mm in size in light to dark gray schistose graphitic gneiss and garnet-graphite gneiss, apparently forming a steeply dipping lens together with folded marbles, meas.uring at least ~ 100 m in length and 7--20 m width on the surface and over 50 m in depth. The ore

226 TABLE I Graphite in Precambrian rocks of southern Africa a Archaean schist belts 1 Bushman Mine

2 3 4 5 6

Mosetse river Goedehoop 489 LS Stranger's Rest 431 LT Figaro goldmine French's Bob goldmine

2.5 m thick zones of graphitic schists and phyllites along contact between sheared dolomitic limestones and granite gneiss Cuttings from water well in schists In basic chlorite schists of schist belt remnants in gneisses Graphitic schists (see text)

Archaean gneissic terrane 7 8 9 10 11 12 13 14

Gumbu Mine Dawn 71 MT Khononga Kop Wendy 86 MT Bali 84 MT Madimbo Albasini 524 MS Krige 495 MS, Prachtig 538 MS, S'Gravenhage 496 MS, Kempshall 497 MS 15 Inkom 305 MR, Steamboat 306 MR

Schistose graphitic gneiss (see text) Graphitic gneiss (see text) 2 bands of garnet--hypersthene granulite with 10% graphite granitic gneiss, garnet--biotite gneiss (2--4% graphite)

Graphitic gneiss (5--10% graphite) Marble (1% graphite) Garnet--biotite--sillimanitegneiss (1% graphite)

Biotite--graphite gneiss (5% graphite) Hornblende granulite

Early Proterozoic sedimentary formations 16 Moshaneng 17 Twyfelaar 11 IT 18 Maandagshoek 254 KT 19 Sinoia area 20 Lynx Mine 20a Orange Free State Goldfield

Graphitic pyrite-rich shear zones in dolomites Carbonaceous shale (see text) Carbonaceous shale xenolith in gabbroic Zone of Bushveld Igneous Complex 200--1000 m thick zone of graphitic slates (see text) Graphitic bands in sillimanite gneisses Graphite sheaves in auriferous conglomerates (see text)

Middle to Late Proterozoic metamorphic complexes

21 22 23 24 25

Oup Gains Garies A u k a m Mine Nchanga/Drummond area 26 LeJonquet/N'Dongini 27

Geiaus 6

Schists Schists, sphalerite Quartzite, gneiss Kaolinized granite (see text) Granitic gneiss, carbonaceous marble with 40.2% "fixed carbon" (Hall, 1938) Quartz--graphite schists in calc-silicate rich dolomitic marbles Fluid hydrocarbons in hydrothermal quartz (see text)

227 Table I

(continued)

Late Proterozoic sediments 28 SE of Windhoek 29 Karibib area 30 Vanrhynsdorp area

Graphitic meta-carbonates and schists (see text) Graphitic marbles and schists (see text) Partly graphitized schists (see text)

aNumbers refer to Fig. 1.

."" 0 ~

~-.:~..: ~

Phanerozoic cover rocks 1<~'/5;:: I late Proterozoic sediments

17~

met . . . . ~ c

ear: r~ ~]

-~

~' ~"" .~-i~ ~'~ ,t

~"

complexes

lI,'~:'~

.~v ""

rotero=lc me ose me ts

Archaean gnei.... schist belts

30

ti" /

?l

0k

2~,~,~1~" ~ ~ ~""

~o'%~ ,~,&

~ ep'&°

~

~,~

~

m

~J"

~"

//~l

Oulban .

I00 200 300 400 500km

Capmt0~

Fig. 1. Geological sketch map of southern Africa showing graphite occurrences in Preeambrian rocks (numbers refer to localities mentioned in the text).

contained ~ 30% graphite. As accompanying minerals sericite, talc, opal, calcite, epidote, and iron oxides have been reported, suggesting that some secondary processes affected the deposit. Graphite nodules are locally developed. Some zones of "iron-bearing" graphite gneiss were observed. The graphitic gneisses at Steamboat and Inkom (loc. 15), Dawn (loc. 8), Wendy (loc. 10), Bali (loc. 11), and Khononga Kop (loc. 9)contain similar "iron-

228 bearing" zones of considerable thickness. It is possible that the haematite of these bands was derived from original pyrites. In Dawn (loc. 8) bands of graphitic gneiss and graphite-bearing g r a n i t e gneiss are interlayered with marble, calc-silicate rocks, and garnet--graphite gneiss over more than 800 m at a maximum width of 50 m. The richest bands within the graphitic gneisses contain up to 25% graphite flakes. In most of the other occurrences (cf. Table I) flaky graphite occurs in gneisses and schists of different composition within the country rock usually consisting of leucocratic granite-~gneiss and pegmatitic granite containing folded beds of marble and calc-silicate rocks or rarely quartzites. Pegmatitic granite intruding the graphitic rocks contains flakes of graphite incorporated from the latter. At Albasini (loc. 13) up to 1% graphite occurs within marbles. In a sample of marble from an unspecified locality in the area, Eichmann and Schidlowski (1975) determined the carbon isotope ratios of the carbonate and graphite carbon. The ~ 13C-values were -+ 0% 0 for carbonate and --16.2°/0o for graphite.

Early Proterozoic sedimentary formations Minor occurrences of partly graphitized carbonaceous matter are found in the Malmani Dolomite of the ~ 2.2 Ga-old Transvaal Supergroup in various parts of South Africa and Botswana (e.g., loc. 16). In the Pretoria Group of the same supergroup, especially in its lower portion, prominent zones of carbonaceous shales are known in which locally part of this carbon has been graphitized by intrusive diabase sheets and by contact metamorphism along the southern edge of the 1.95 Ga-old Bushveld Igneous Complex. At various points along this belt some small-scale mining has taken place, notably on Twyfelaar (loc. 17) where graphitic shales with 24--28% C attain a thickness of 3--4 m below a diabase sheet contemporaneous with the Bushveld Igneous Complex. In the Piriwiri "Series" of northern Zimbabwe graphitic slates are developed over a wide area northwest of Sinoia (loc. 19). The slates show only wavy foliation but no distinct bedding and contain thin layers of black cherty quartzite, possibly original chert bands (Stagman, 1961). Graphite is presently being mined in sillimanite gneisses of this series at the L y n x Mine near Karoi (loc. 20). The Piriwiri appears to predate the 1.95--2.65 Ga-old Lomagundi Group. Microscopic sheaves of graphite were noted by Schidlowski (1967) in conglomerates of the Early Proterozoic Witwatersrand Supergroup of the Orange Free State Goldfield (loc. 20a). The mineral is explained as having formed from carbonaceous matter during regional metamorphism.

Middle to Late Proterozoic metamorphic complexes At the western end of the Namaqua--Natal mobile belt stringers of

229 graphite are frequently encountered in various types of metamorphic rocks (loc. 21--23). At the Gams lead--zinc deposit (loc. 22) graphite flakes also occur in sphalerite ore (Stumpfl and Clifford, 1977). The rocks are part of the Bushmanland Group for which an age of ~ 1.3 Ga has been reported (KSppel, 1978). In the Aukam Mine (loc. 24) in the southern part of Namibia, up to 30 cm thick seams of massive graphite occur in a pipe-like b o d y of kaolinized granite--gneiss, which also contains some disseminated graphite. The age of the rock is uncertain, probably Mid-Proterozoic. At the eastern end of the belt graphite has been observed at two localities (loc. 25--26). Doubly terminated quartz crystals with fluid inclusions containing hydrocarbons were described by Kvenvolden and Roedder (1971) from calcitefilled veins in a pre-700 Ma~liabase d y k e cutting metasediments o f the Bushmanland Group at Geiaus in the Warmbad District of Southern Namibia (loc. 27). The authors mentioned similar material from Aukam (loc. 24) without giving further particulars o f the geological situation of the locality. A connection with the graphite deposit on that farm cannot be excluded. The molecular composition and distribution of the hydrocarbons suggest a biological derivation. The age of the enclosing calcite veins is n o t known. Mueller (in Kvenvolden and Roedder, 1971) described vaselinous oils and dark resinous flakes from quartz crystals in veins cutting granite at an unspecified locality in Namibia. He suggested an inorganic derivation of the hydrocarbons. Late Proterozoic sediments

Within the metamorphic rocks of the 760--830 Ma-old Damara Supergroup of Namibia, zones rich in carbonaceous matter and graphite have been reported from the Hakos Stage, the lower part of the eugeosynclinal Swakop Group. It contains in its mainly dolomitic basal portion over a wide area graphite-bearing phyllites, concentrated especially in structural synclinoria. In the area south of Windhoek (loc. 28) "the greater portion of the carbonate rocks may grade into a graphitic facies" (Gevers, 1963). Around the Riefontein inlier of pre-Damara rocks (loc. 28) southeast of Windhoek, the Hakos marble is overlain by a graphitic schist, followed by a banded iron formation (Martin, 1965). In the marbles of the upper Hakos Stage, especially surrounding the Ababis Inlier of pre-Damara rocks in the Karibib area (loc. 29), finely disseminated graphite is widespread and in the coarser marbles visible flakes of graphite are developed (Martin, 1965). Intercalations of graphite schists have been noted in the overlying Khomas Group, a m o n o t o n o u s succession of various types of schists interlayered with quartzites, amphibolites, and marbles. Scapolite-bearing schists and scapolite--biotite gneiss from Proterozoic metamorphites of the Hakos Mountains near Outjo in the northern part of Namibia on farm Okunguarri 94 contain 0.18--0.24% C and 0.4% C, respectively (Visser, 1964). The rocks appear to belong to the lower part of the Swakop Group.

230 In the phyllites of the late Precambrian Malmesbury Group, graphitic beds occur over a large area in the Vanrhynsdorp district (loc. 30). Rogers (1905) described carbonaceous phyllites with up to ~ 46% C from several 100 m thick sequences of bluish-gray folded phyllites with minor quartzitic intercalations. They overlie ~ 1000 m of metamorphosed stromatolitic dolomites (Reimer, 1978). The carbon mostly occurs as stringers on bedding planes or as laminae rarely up to 2 mm thick. In some laminae elliptical carbon specks measuring ~ 1--2 mm in the long axis and ~ 0.5--1 mm thick are found, being elongated parallel to the prevailing lineation of the enclosing rocks. The "richer" portions consist of phyllitic laminae alternating with up to 2 cm thick carbon stringers. From the Oranje River Settlement ~ 15 km northwest of Vanrhynsdorp up to 12.5 cm thick "seams of graphite" have been reported (Rogers, 1905). Under the microscope the carbon is found to consist of small granules of carbonaceous matter. X-ray diffraction showed some of this material to consist of graphite while the remainder represents highly reconstituted kerogenous matter. Whiteside (1976) mentioned contents of up to 20% graphite from phyllites in the area. The 5 ~3Corgin a sample of these phyllites was found to be rather " h e a v y " at --15.28% (M. Schidlowski, personal communication, 1983). In metamorphosed carbonates of the same area Eichmann and Schidlowski (1975) observed similar " h e a v y " isotope ratios. A sample of dolomitic limestone gave a ~ 13Corg of --10.9% and a 6 13Ccarb of +4.8%. A dolomite gave values of --11.6% and +1.0%, respectively. DISCUSSION The compilation above shows that even in highly metamorphosed rocks carbon can be preserved over wide areas as graphite. Two particularly rich provinces have been delineated, viz., the Early Archaean Beitbridge metamorphites in the northern part of South Africa and the Late Proterozoic Damara Supergroup of central Namibia. The rocks enclosing the graphitic beds are mostly gneisses and schists locally together with marbles and rarely with calc-silicate rocks. The original sediments thus were mainly carbonaceous, locally possibly sulfide-bearing shales, while bituminous carbonates were of minor importance. There is a notable scarcity of data on the carbon content of metamorphic rocks. Hahn-Weinheimer (1960) presented values of 10--360 ppm C for metamorphic rocks of the Mfinchberger Gneiss Massif of northern Bavaria. Hoefs (1965, 1973) gave an average value o f 200 ppm C for metamorphic rocks, the same as for granites. Ronov and Yaroshevsky (in Ronov, 1968) gave a figure of 0.17% C for the granitic shell o f the Earth which was reported to consist o f 42.1% igneous and 57.9% metamorphic rocks. Using Hoefs' average of 200 ppm C for the igneous rocks, a figure of 0.28% C is obtained for the metamorphic rocks. The latter, according to Ronov and Yaroshevsky (in Ronov, 1968}, consists

231 o f 64.9% gneisses, 15.5% "crystalline schists", 2.6% carbonates, and 17% arnphibolites. In view o f the predominance o f gneisses among the m e t a m o r p h i t e s this calculated average of 0.28% C appears t o be rather high. For the non-gneissic por t i on it would require a carbon c o n t e n t o f ~ 0.76% which is considerably above the average o f 0.49% C given by the same authors for the rocks o f the sedimentary shell. The data o f R o n o v and Migidsov (1971) showed a weighted average of 0.1% C for the sedimentderived m e t a m o r p h i c rocks of the Precambrian o f the Russian Platform. Shaw et al. (1967) gave a figure o f 0.02% as an average for the Archaean o f the Canadian shield while Wedepohl (1967) gave 320 ppm C as an average for the igneous and m e t a m o r p h i c rocks of the Earth's crust. In view o f the widely differing estimates of the carbon c o n t e n t of metamorphic rocks the need for a new calculation is indicated. The respective data are presented in Table II. The values f or gneisses and amphibolites (200 p p m) were taken f r om Hoefs (1965). As some of the amphibolites will represent m e t a m o r p h o s e d sediments with a potentially higher carbon c o n t e n t , instead of mafic volcanic rocks, the 200 ppm C will be a conservative estimate. F o r m e t a c a r b o n a t e the 0.01% C o f R o n o v and Migidsov (1971) for Proterozoic m e t a ~ a r b o n a t e s was taken. This value appears t o be rather low when c o m p a r e d to the 0.47% C for Canadian Precambrian metacarbonates given by Shaw et al. (1967). However, in view o f the absence o f TABLE

II

Carbon content of various units of the crust Mass

1024 g Oceanic crust

6.11

C %

Total carbon 1021 g

0.025

1.22

0.0543 0.02 ~

6.75 1.68

Continental crust

Metamorphic rocks Igneous rocks Subtot~

12.53 8.43 20.9

Sedimentary shell Sediments Volcanics

2.181 0.4'

Subtotal

2.58

Total crust

29.58

0.494 0.022

10.68 0.08

0.064

20.41

(i) Veizer, 1983. (2) Hoefs (1965).

(3) This paper. (4) Ronov and Yaroshevsky (in Ronov, 1968).

232 further data, the value of 0.01% C is retained for the calculation. Owing to the small a m o u n t of metacarbonates in the rock record, the uncertainties introduced by it are only limited. The carbon c o n t e n t of the schists is more difficult to assess, as only few analyses are available. Furthermore, the distinction between this rock type and sediments of low metamorphic grade, on the one hand, and gneisses, on the other, is rather indistinct. Assuming a gradual reduction of the carbon content during metamorphism, a figure halfway between the 0.49% C of the average sediment and the 200 ppm C of the gneisses appears to be a reasonable estimate for the carbon content of schists of sedimentary derivation. Among the schists those of volcanic derivation also have to be considered. In the sedimentary shell, volcanics account for ~ 17% by mass (Ronov and Yaroshevsky (in Ronov, 1968). During metamorphism the more mafic volcanics, i.e., the majority, are turned into amphibolites while the intermediate to felsic ones initially will form "schists". It is estimated that these types do not account for more than 5% of the "crystalline schists". With a carbon c o n t e n t of 200 ppm, taken from Hoefs (1965) for felsic volcanics the average for the "crystalline schists" will be ~ 0.24% C. This compares with 0.33% C given by Ronov and Yaroshevsky (in Ronov, 1968) for paragneisses, phyUites, and schists of the Proterozoic of the Russian platform. Considering the masses of the various types of metamorphic rocks, a figure of 0.054% C is obtained as an average for metamorphic rocks, a value that is considerably above the hitherto accepted 200 ppm C. In Table II the various components of the crust are stated with their average carbon contents. For the subdivission of the continental crust, Ronov and Yaroshevsky (in Ronov, 1968) gave a ratio of igneous to metamorphic rocks of 43:57, while Wedepohl (1969) gave a ratio of 39:61. An average of 40:60 was used for this calculation. Of the total given by Veizer (1983) for the sedimentary shell the mass of the included volcanics, 0.4 X 1024 g according to Ronov and Yaroshevsky (in Ronov, 1968), has been deduced. With these figures an average carbon content of 0.064% is found for the crust, resulting in a total carbon mass of 20.41 X 1021 g. Of this ~ 17.43 X 1021 g, i.e., the carbon of the sedimentary and metamorphic rocks, or ~ 85% of the total have been derived from organic precursor material. The metamorphic portion of this amounts to ~ 39% of the total Corg. These considerations show that the a m o u n t of carbon contained in metamorphic rocks is of a magnitude which does not permit it to be excluded from isotope balance calculations for the carbon--oxygen cycle. For such calculations to be meaningful the data base on the carbon content of metamorphic rocks and its isotopic composition has to be expanded considerably. ACKNOWLEDGEMENTS The paper represents a contribution to IGCP-Project 157. The presentation of the paper at the 1983 meeting of the European Union of

233

Geoscientists at Strasbourg was made possible through the assistance of Messrs. Dyckerhoff Engineering GmbH, Wiesbaden/Germany. The help of Prof. M. Schidlowski Mainz who supplied additional carbon isotope data, and of Dr. W. Herzberg (Bonn) and Mr. H. Gohla (Kropfmfihl) who assisted in the search of literature data is gratefully acknowledged.

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