Diagenetic reactions in early precambrian graywackes of the Barberton Mountain land (South Africa)

Sedimentary Geology--Elsevier Publishing Company, Amsterdam--Printed in The Netherlands

DIAGENETIC REACTIONS IN EARLY PRECAMBRIAN GRAYWACKES OF THE BARBERTON MOUNTAIN LAND (SOUTH AFRICA) THOMAS O: REIMER

Geology Department, Pretoria University, Pretoria (South Afi'ika ) l (Received May 17, 1971)

ABSTRACT Reimer, T. O., 1972. Diagenetic reactions in Early Precambrian graywackes of the Barberton Mountain Land (South Africa). Sediment. Geol., 7 : 263-282. The Early Precambrian Fig Tree graywackes contain indications suggestive of the diagenetic origin of a large part of their matrix. It formed through the decomposition of clastic labile constituents an~l the rearrangement of the decay products during diagenesis. Calcium, derived from the destruction and partial albitisation of the plagioclases in the graywackes, and especially in the intercalated shales, was incorporated in dolomite, the magnesium of which was derived from original mafic minerals and rock fragments within the graywackes. These reactions caused a volume increase of the solid matter of the rock at a rate of 1.6-1.9 parts of matrix + dolomite to 1 part of labile constituents. This increase was made possible by a corresponding decrease of the original porosity of the graywackes. It indicates that the reactions described took place at a rather early stage after the deposition of the sediments. The shales alternating with the graywackes are greatly depleted in CaO and lost about 1.5~ of this element to the graywackes. At the same time the shales were depleted in Sr compared to the graywackes. These elements probably were transported from the shales into the graywackes by pore fluids rich in carbon dioxide. The absolute amount of dolomite in the graywackes is dependent on the average composition of the plagioclases and their amount in the size fraction above 0.03 ram. These reactions took place between 500-1000 m of overburden at temperatures possibly not greatly exceeding 165 °C. The occurrence of siderite together with pyrite suggests that the diagenetic processes took place at a pH of about 6.5-9.5 and an Eh of 0.2 to 0.4. It is suggested that dolomite as cement in appreciable amounts occurs only in graywackes which originally contained a considerable amount of material derived from mafic volcanic rocks.

INTRODUCTION

Lasius (1789), one of the earliest students of graywackes, described this rock as "a quartz breccia with mica flakes and fragments of siliceous shales (Kieselschiefer) or sandstones in a shaly matrix". This mostly high content of material with grain size below 0.03 ram, commonly referred to as matrix, has always been considered a characteristic feature of graywackes. It is therefore not surprising that a number of authors used a certain amount of matrix, usually between 10-20~0, to distinguish graywackes from other clastic sediments (e.g., Pettijohn, 1957). Tacitly this matrix was mostly understood to be ofdetrital origin

1 Present address: Johannesburg Consolidated Investment Co., P.O.Box 2, Randfontein (S. Aft.)

264

T . O . REIMER

although, in some descriptions, diagenetic to weakly metamorphic changes were suggested as a possible explanation of some of the matrix. Their quantitative importance, however, was not elaborated on. Dapples (1962), in an outline of the diagenetic changes which can take place in sandstones, stated: "The graywacke as seen in thin section is only indirectly representative of the particle-size distribution and the mineralogy of the sediment as deposited, and the properties of the original sediment could be very different from those of the final graywacke." 'The matrix problem was discussed in detail by Cummins (1963) who came to the conclusion that the matrix of graywackes formed mainly through reassemblage of chemical compounds set free during the partial to complete decomposition of labile constituents. Under the latter he understood fragments of feldspars, mafic minerals, and of igneous and mafic volcanic rocks. Their decay was understood to have taken place under diagenetic to mildly metamorphic conditions. Emery (1964) suggested that water, included in shales alternating with most graywackes, is driven out during compaction and carries fine-grained material upwards. This is deposited in the coarser clastic layers, thus accounting for their high matrix content. Cummins' theory was criticized by Dzulynski and Walton (1965) mainly on the basis of its being not substantiated by sufficient analyses. They follow Fischer (1933) in suggesting that the occurrence of graywackes preferably in Paleozoic and older formations might be due to the lack of a plant cover of any large extent over the ancient land masses. This would have given rise to weathering conditions, and consequently weathering products, differing from those of younger times. These authors present a diagram (Dzulynski and Walton 1965, fig.18, p.29) in which the amount of matrix is plotted against the labile constituents. The diagram is intended to disprove Cummins' theory and in the explanation the authors state: "The Coarsewall figures show a uniformly low matrix proportion (despite their high labile constituents)." This remark actually lends support to Cummins' theory. When it is assumed that the matrix is derived from the decomposition of the labile constituents, a negative correlation would actually have to be expected. Kuenen (1966) supported Cummins' ideas, stressing especially the importance of pressure solution for the decompositional reactions. He advocated the development of graywackes from flysch sandstones under diagenetic to slightly metamorphic conditions. From an investigation of Silurian turbidites, Rust (1965) concluded " . . . t h a t there is little evidence in the graywackes for a diagenetic matrix which is ...largely detrital in origin". Calcite can replace up to 70~, of these rocks. Graywackes with a presumably original detrital matrix were described by Audley-Charles (1967) from the Miocene of Timor. Unfortunately, the data supplied by him are not sufficient to completely evaluate the importance of his findings. Brenchley (1969) could show that even wellsorted shallow-water sandstones can attain the typical graywacke texture by diagenetic decomposition of

265

DIAGENETIC REACTIONS IN GRAYWACKES

their labile constituents, which in his case were mainly volcanic rock fragments. Recently, Dickinson (1970) introduced terms to distinguish between different types of matrix. He suggested "proto-matrix" for the original clastic matrix, "ortho-matrix" for recrystallized clastic matrix, "epi-matrix" for diagenetically formed matrix and "pseudo-matrix" for deformed clastic grains. Recent experiments by Whetten (1971) with medium to coarse grained river sand show that temperatures of about 200°C can produce a texture similar to that of graywacke when applied to the sediment over about 5 months. GEOLOGICAL SETTING

The Fig Tree Group represents the middle part of the Swaziland sequence ( - Supergroup) in the Barberton Mountain Land of the eastern part of South Africa (Fig.l). This sequence consists of a lower, mainly volcanic, Onverwacht

~

x x

younger cover

31"E x~ x A'11~ " Nelspruit x x~J~x x ~ Uiundi I X~.rl~l IW" a'Jx j l T l , ~ -

Vloodies Group LbSOUthorn Sto(z- ~ ~ _ , . ~ , ~ . . facies

oo. o,o ,,c,es

L.."

X -~. ~P X X "~

Onverwach, Group granites, gneJ~ses

V

i I

..---< / i'_ . ...""_ i )

X /'C.._"5"-X ~ x ~x/./~ x

Mbabane

Fig. 1. Locality map showing position of the Barberton Mountain Land and distribution of rocks of the Swaziland Formation.

Group (--.~ 14,000 m); the middle, Fig Tree Group (-~ 2,100 m); and an upper, mainly arenaceous', Moodies Group (--~ 3,600 m). The age of these rocks is more than 3,000 m.y. (Allsopp et al., 1968) and a detailed investigation of the U-Pb isotopes in the granites and gneisses surrounding the Mountain Land led Oosthuyzen (1970) to conclude that the sediments have a minimum age of about 3,300 m.y. Thus the Fig Tree Group may well contain what can be called the oldest well-preserved graywackes known at present. The degree of metamorphism in the rocks of the Swaziland sequence and especially in the graywackes is generally

266

T.O. REIMER

very low, and only close to the contact with the surrounding granites and gneisses locally higher grades of metamorphism are approached. Two different facies were recognized in the Fig Tree Group by the author, a conclusion which was substantiated by recent investigations (T. Heinrichs (G6ttingen), personal communication, 1971). The graywackes are restricted to the northern facies (Fig. 1) and evidence exists that they were deposited under deeper water in a turbidite basin. In contrast to this, the sediments of the southern facies were mostly deposited in shallow, partially still, water. Graywackes mainly occur in the lower, 700-1200-m thick, Sheba Formation of the northern facies and in the eastern outcrops o f the middle Belvue Road Formation. As Condie et al. (1970,) have shown, the graywackes of the Sheba Formation were mainly derived from volcanic rocks of the underlying Onverwacht Group, while those of the Belvue Road Formation were mainly derived from the ancient sialic crust, which appears to have been tonalitic in composition. As can be deduced from sedimentary structures in the Sheba Formation, the source area lay in central Swaziland, south and southwest of the Barberton Mountain Land. The transport distance of the sedimentary material was not much more than about 150 km. PETROGRAPHYOF THE GRAYWACKES In this paper the graywackes of the Sheba Formation will be discussed in detail, with emphasis being laid on those from the Stolzburg and Ulundi Synclines (Fig.l). The petrography of these rocks has been described elsewhere (Herget, 1966; Condie et al., 1970). The modal composition of fresh samples of typical medium-grained graywackes is presented in Table I. Clear quartz grains without undulatory extinction, derived from acid volcanic rocks are a regular constituent of the graywackes (Reimer, 1971). The

TABLE I M O D A L C O M P O S I T I O N OF G R A Y W A C K E S OF T H E F I G TREE G R O U P

Mineral

S h e b a Formation i

Belvue R o a d Formation z

Quartz 26.5 25.6 Chert 16.5 6.7 Plagioclase 1.8 3.4 K-feldspar 8.5 16.5 Rock fragments 3.7 3.3 Mica 1.4 1.3 Matrix 34.9 41.0 Dolomite 6.7 2.2 tAverage of 4 sub-averages of a total of 71 analyses, includingdata of Herget (1966); 2Averageof 8 graywackes.

267

DIAGENET1C REACTIONS IN GRAYWACKES

same source rocks furnished the small idiomorphic crystals of K-feldspar which were found in a large number of samples. Up to 50~ of the K-feldspars contain perthitic exsolution lamellae of albite to varying extent. Rock fragments were derived from volcanic, igneous, and sedimentary rocks. Detrital grains of chalcedony and quartzine ( = length-slow chalcedony) are a regular constituent, as are relatively large, monocrystalline, rounded grains of prochloritic to penninitic chlorite. They most probably represent original biotites which have been chtoritized already in their mainly igneous source rocks. Mafic minerals are rarely found. Only one hornblende was observed in addition to several detrital grains of chrysotile. Heavy minerals include zircon, tourmaline, apatite, chromite, and chalcopyrite. Pyrite is a notable constituent of diagenetic origin and forms idiomorphic crystals of up to 2 mm in size as well as in clusters of small irregular crystals with total diameters up to 2 cm. Under the microscope it becomes obvious that diagenetic changes have affected the original clastic particles of the sediments to varying degrees. Feldspars are frequently marginally to completely replaced by chlorites and to a minor extent by dolomite. Another type of alteration, affecting only the plagioclases, is that .their margins are frequently more acid than their cores. It can be shown that this is no primary zoning, as it is controlled only by the detrital outline of the grains, and not by crystallographic directions, etc. Therefore it is considered to be of post-erosional origin and attributed to a slight incipient albitisation during the diagenesis of the enclosing sediment. The quantitative aspects of this reaction will be elaborated on in a later section. Similar albitisation which led to irregular secondary zoning in plagioclase, was described by Huckenholz (1967), Ojakangas (1968), and by Dickinson et al. (1969). Quartz is only rarely affected by replacement while chert fragments, unaffected by chlorite, are frequently replaced by dolomite. The clastic grains are 9'0 99

95 90 80 70 60 5O ~0 30 2O

j°,-~ ,Jj, j! ~-j f - °

/

,

"

17/"~

,,d~: f

!/

10 5

- 1.0

2

0

1.0

2.0

3.0

,;.0

5.0

6.0

1

0.5

0.25

0.125

0,062

0.031

0.016

mm

Fig.2. Grain-size distribution o f graywackes o f the Sheba Formation.

268

T . O . REIMER

floating in a groundmass consisting of chlorite, sericite and probably some illitic clay minerals. Dolomite is irregularly distributed throughout the rock, in places forming thin veinlets. When grain-size analyses of the graywackes are compared with similar data from the literature (e.g., Cummins 1963) it becomes obvious that their degree of sorting at similar median grain sizes is smaller than that of other graywackes (Fig.2). The analyses are based on counts of 500 grains each, over a regular grid, and the results are presented as number-frequency percentages. No conversion into volume or weight percentages was attempted, in order to facilitate the comparison of the results by other workers in this field with their own data. The differences in sorting on average amount to 0.5 Trask units, by which the Fig Tree graywackes are less sorted than are other graywackes. ORIGIN OF MATRIX

In order to find the nature and extent of any correlation between matrix and labile constituents, initially these two variables were plotted against each other. A negative correlation was clearly revealed, which is not surprising, as within certain limits, a fine-grained sediment with large amounts of matrix, i.e., particles below 0.03 mm in size, does not contain large amounts of coarser material and thus of labiles as well. Such a diagram, however, has the disadvantage that it does not take into account the above influence of the grain size on the petrographical composition of a sample. To account for this, first the relationship between the median grain size of a sample and its matrix content had to be determined. It will be shown later that dolomite takes part in the diagenetic reactions as well and, therefore, it has to be considered together with the matrix. As considerable differences in the original petrographic composition of samples could obscure any trends that might exist, samples from different levels of two graywacke beds of similar petrographic composition were considered. They come from a drill core from the Sheba Mine, which is situated within graywackes of the lower Sheba Formation on the northern limb of the Ulundi syncline (Fig. l). The inset in Fig.3 shows that for each phi-unit which the median increases, matrix+ dolomite decreases by about 17~o. A similar relation was found by Mizutani (1957) in Permian graywackes of Japan. The percentages, unless stated otherwise, represent volume-percentages: the correlations were determined with the aid of the least-square method. In order to convert the analyses to a common base it was decided to assume a model graywacke with a median grain size of 0.125 mm or 3 phi. For every phi-unit or part thereof, by which the median of a sample is smaller than that of the model graywacke, a corresponding amount (l 7~ or multiples and parts thereof) is deducted from the amount of matrix + dolomite and added to the constituents with grain sizes above 0.03 mm. But as part of

269

DIAGENETIC REACTIONS IN GRAYWACKES

90

¸

\*,\ \ \

80

~,oo}

\

!::I ,01 .:./

\

¸

\

\

\

\

\oe

6O

3

t

5

6

7

vm

median [~1

\

\

2 50 + ~0 "2. ,!

30

20

10

I0

20 labt'le constituents [%]

30

Fig.3. Matrix + dolomite vs. labile constituents for two graywackebeds of the Sheba Formation. Open symbols = corrected values; solid symbols = uncorrected. Inset:matrix + dolomite vs. median grain size. this calculated increase would consist of feldspar and other labile constituents, a certain percentage o f it would have to be added to these on the abscissa in Fig. 3. Feldspars account for an average of 18% of the fraction above 0.03 mm in the graywackes. However, as revealed by grain size analyses of the different clastic constituents, feldspar is enriched in the smaller size fractions. The latter would have mainly been affected by the above increase and, therefore, a higher amount of the increased material, estimated as 30~o, is thought to be represented by labile constituents. The procedure is shown schematically in Fig.3. This correction considerably reduces the spread of analyses which is mainly attributable to differences in grain size. The corrected values represent the relation between matrix ÷ dolomite and labiles in a model graywacke with a median of 3 phi, with originally uniform petrographic composition and which has been affected by diagenesis at a non-uniform rate. According to this the "freshest" graywacke had a theoretical content of about 36% labile constituents which decreases to about 20~ in the "most al-

270

r . o . REIMER

tered" sample. It is interesting to note that the increase of matrix ÷ dolomite is higher than the corresponding increase in labiles. The rate for all samples plotted is 1.6 : 1. It will be higher, up to 1.9 : 1, if one or both extreme values with more than 25~ labiles are not considered in the least-square calculation. Two separate explanations are possible and the author favours a combination of the two. Firstly, a volume increase can take place during the decomposition of labiles and the rearrangement of the decay products to form new mineral phases. Secondly, some of the new minerals could have been introduced from the outside into the system. Although it cannot be excluded that the above calculation is arithmetically not sufficiently founded, the emerging volume increase can be easily explained. Firstly, as will be shown later, a considerable part of the calcium carbonate component of the dolomite was introduced into the graywackes and thus, at an average dolomite content of 6.7~, could have contributed to this volume increase. Secondly, the correlation between the nickel and magnesium contents of the graywackes (Condie et al., 1970) indicates an originally high content of mafic minerals, such as pyroxenes, derived from ultrabasic to basic volcanic rocks in the source area. These relatively unstable minerals with rather high specific gravities were transformed during diagenesis into chlorites with lower specific gravities. This would also have caused a volume increase. The theoretical volume increase described did not cause an expansion of the rock as it would appear. It was completely compensated for by a corresponding loss in the porosity of the original sediment. This is only possible if it is assumed that the reactions took place during a time when the sediments were not yet completely compacted. From the data presented above it can be deduced that the Fig Tree graywackes originally were very impure medium-grained sandstones containing large amounts of labile clastic constituents and about 20~o detrital or proto-matrix. The latter amount compares favourably with the 10~o proto-matrlx assumed by Kuenen (1966) for coarse turbidites. The typical graywacke texture then-would have been formed by diagenetic decomposition of up to 50~ of the original labile constituents and the rearrangement of their decay products to form mainly chlorites and some of the dolomite. In addition the introduction of carbonate into the rock played an important role and will be outlined in the following section. DOLOMITE

Introduction

Condie et al. (1970) mentioned three possibilities for the origin of the dolomite in the graywackes: (1) it formed through reorganization and recrystallization of carbonates introduced by clastic fragments of carbonate rocks; (2) during the oxidation of organic matter entrapped in the graywackes, carbon dioxide could

DIAGENETIC REACTIONS IN GRAYWACKES

271

have formed which precipitated calcium and magnesium from the pore fluids of the sediments; (3) the dolomite was introduced by and precipitated from carbonate-rich pore fluids. Of these the first possibility is rather unlikely, as no traces of any large amounts of calcareous fragments were observed in the graywackes. Furthermore, no large amounts of such rocks are known from the underlying Onverwacht Group which constituted over two thirds of the source area of the Sheba sediments. However, some dolomite could have been introduced by fragments of carbonated lavas. The second possibility cannot be completely discarded, although most of the diagenetic reactions within the graywackes took place under reducing conditions as witnessed by the frequent occurrence of diagenetic pyrite. Nevertheless, some oxygen could have been formed by bacterial reduction of the dissolved sulfate to sulfide. The third possibility, although not completely satisfactory, offers the best explanation. In the following paragraphs an attempt will be made to explain the derivation of the different components of the dolomite. Calcium

As mentioned earlier, a number of plagioclases have been observed which show signs of irregular albitisation. This reaction, together with the decay of the more calcic plagloclases, is believed to have set free certain amounts of calcium which are difficult to incorporate in the diagenetic minerals observed other than dolomite. Zeolites which can also incorporate calcium have not been observed in the graywackes. As the calcium content of the graywackes is in no way unusual compared to others, most probably no calcium has been removed from them. Further evidence for the decomposition of considerable amounts of plagioclase in the graywackes comes from the observation that K-feldspar is the predominant feldspar in the graywackes while in the underlying Onverwacht volcanics, which constituted most of the source area, plagioclase predominates (Viljoen and Viljoen, 1969). The K-feldspar in the graywackes was mainly derived from granitic to granodioritic rocks of the ancient sialic crust. Although to a certain extent the K-feldspar predominance could be a source effect and plagioclase itself is slightly less resistent against weathering than K-feldspar, it is highly unlikely that the latter was concentrated during the rather short transport to the extent that it now predominates almost five times over plagioclase. In order to gain an impression of the amounts of calcium which can be set free during partial albitisation of plagioclase, a graywacke with 5~,, andesine (40~o An) and 5~o oligoclase (20~o An) was assumed. These percentages are weight-percentages. This was transformed into oligoclase, albite, and decomposition products, as shown in Table II. During these reactions some 0.55~ CaO was released which, when incorporated into dolomite, accounted for about 1.65~o of this mineral, corresponding to about 1.5 volume percent. Thus the complete decay of 4~o plagioclase and the partial albitisation of the remainder can add at

272

J. O. REIMER

TABLE II DECAY AND PARTIAL ALBITISATION OF PLAGIOCLASE 1

Original

Andesine (5Yo)

Oligoclase (5~o)

Product

oligoclase (23~i)

CaO

%

-0.10

Na20

%

+ 0.06 i

decomposed (3%)

-0.30

0.17

oligoclase (2~o) albite (2~) decomposed (1~)

-0.10 -0.05

+0.06 ! -0.03 I

Subtotal

Total

CaO

Na20

CaO

Na20

C~,;)

(%)

(°;)

G,)

0.40

0.11 •

0.15

0.55

0.08

0.03

aPositive amounts have to be added; negative amounts are released

least 4.95 volume-percent to matrix and cement if calcium is fixed in dolomite. The ratio of this increase is about 1.24:1 and thus comparable to the rate calculated for the volume increase during matrix formation. Table II furthermore shows that the reaction is almost self-sufficient as far as sodium is concerned. Originally it was assumed that the sodium required for the albitisation was derived from the pore fluids of the graywackes. Although the amount of calcium released by the above reactions is in no way sufficient for the 6.7~o dolomite in the average graywacke, it can be expected that a negative correlation exists between the amount and average composition of the plagioclase on the one side and the amount of dolomite on the other. To cast light on a possible correlation between these factors, it was assumed that the average composition of the plagioclase population of all samples concerned was approximately identical. The albitisation then would modify the median composition, rendering it less calcic as the effects of the reaction increase. Thus with progressing albitisation, more calcium would be released which could then be incorporated in dolomite. To determine the median composition the An-content of all plagioclases in a section was determined according to the zonal method described by Tr6ger (1967, p.743). This is a simplified procedure, but as a large number of grains were counted and the same method applied to all samples, the possible error can be considered sufficiently uniform to allow a comparison of the data. The median composition, expressed in degrees of extinction angle, was determined graphically and plotted against the amount of dolomite in the sample. From the resulting Fig.4 it becomes obvious that a negative correlation does exist. The dolomite content is higher in samples which have a more "acid" plagioclase population and which probably have suffered more albitisation than those with a more "calcic" population. The differences in the median composition correspond to about a 5 ~ difference in the An-content. A similar method was applied by Ermanovics (1967) in provenance studies.

273

DIAGENETIC REACTIONS IN GRAYWACKES

4.

•

~.~6

4,

~+j~ •

3

..

•

.*.

• U t u n d i Syncline + Stolzburg ,, 2

,~

6 dolomite

8

10

t2

1~

16

[%]

Fig.4. Dolomite vs. median extinction angle of plagioclase populations. This albitisation was probably of minor importance as a source of calcium, especially in the light of the low absolute changes in the median composition of the plagioclase populations indicated in Fig.4. This reaction, furthermore, leaves the feldspars as such intact, only changing their composition. Therefore, one should expect a positive correlation between dolomite and plagioclase, were this reaction the main source of calcium. As shown in Fig.5, a - c this is clearly not the case as plagioclase in the size fraction above 0.03 mm is negatively correlated with dolomite. This is consistent with plagioclase decay as the more important intraformational source of the calcium of the dolomite. As this decay preferably affected the more calcic plagioclases, it could have changed the median composition of a plagioclase population as well. As mentioned already, more calcium is required for the formation of dolomite than could be furnished by the above reactions alone. Although some calcium was included in the sea water entrapped by the sediments during deposition, this amount most probably was too low to account for the majority of the calcium in the dolomite. Its derivation, therefore, would have to be established in order to add support to the idea of its introduction into the sediment, especially as the present calcium content of the graywackes is in no way unusual. The calcium could have been introduced into the sediments from the outside, but this is rather unlikely as the permeability o f the formation as a whole was comparatively low. This was due to the unsorted nature of the graywackes and to their regular alternation with impermeable shales. This suggests an intraformational source of the calcium. It has to be stressed here that the calculations so far only dealt with the graywackes. They are alternating with dark to gray-green shales which mostly are only slightly cleaved. Mineralogical data from them are yet scarce but some information of their chemical composition can be obtained from the analyses ot Condie et al. (1970) and from Rb-Sr isotope studies of Allsopp et al. (1968). An average of three typical shales gives 0.19~ CaO. This is extraordinarily low compared to the

274

T.

O.

REIMER

a

% 7" 6, 5. ,B•

h, 3" 2'

lb

:5

k%

dolomite

#

70 dolom#e

%

7" 6" 5,

0 o O0

do--

•

3' 2'

k

lb

1~

2b

%

dolomite

Fig.5. Dolomite vs. plagioclase (in % of fraction above 0.03 ram). a. Sheba Formation, Ulundi

Syncline; b. Sheba Formation, Stolzburg Syncline;c. Sheba Formation, east of Stolzburg Syncline. Plagioclase in % of fraction above 0.02 mm. (After Herget, 1966.) average of 1.00% CaO given by Nanz (1953) for 33 Precambrian shales. Shales from similar depositional environments such as those of the Sheba Formation, i.e., from graywacke formations, according to the same author contain an average of 1.37~o CaO. This calcium depletion o f the Sheba shales suggests that plagioclase, originally included in the shales, was destroyed, probably during diagenesis and the calcium released was removed from the shales by pore fluids which could have been rich in carbon dioxide. The almost complete decay of the plagioclases in the shales can be explained by the well-established chemical principle that the reactivity of a substance increases as the particle size decreases. Thus the fine-grained

275

DIAGENETIC REACTIONS IN GRAYWACKES

clastic plagioclases in the shales would have easily succumbed to the destructive effects of the pore fluids. The resulting solution was expelled from the shales during compaction and introduced into the more porous graywackes where it could further react. Strontium was released together with calcium from the plagioclases. This is especially indicated by a comparison of the Rb/Sr ratio of shales and graywackes. qhese reactions will be elaborated on elsewhere (Reimer, 1972). Sodium was released during the decay of plagioclase as well. Part of it could have migrated together with the calcium, but most of it was incorporated in diagenetically-formed albite within the shales. Its presence is indicated by X-ray data. Although some of the sodium will have been incorporated in chlorites, most of it is present in albite which at an average content of 1.03'~i N a 2 0 in the shales accounts for some 10"~,, by weight. The approximate amounts of calcium and strontium which have been removed from the shales can be estimated if the following three assumptions are made: (1) no calcium and strontium has left the sedimentary pile; (2) shales and graywackes originally had similar chemical compositions: and (3) only negligible amounts of these two elements have been introduced into the system. However, the system was not closed during the diagenetic reactions and therefore the calculation can only give a rough estimate of the quantitative importance of the transport of calcium and strontium from shales into graywackes. The Sheba Formation on the north limb of the Ulundi Syncline, where the samples for the chemical analyses were collected, consists of about 85°.0 graywackes and 15,~, shales. This allows the average composition of the whole formation to be determined. The relevant losses and gains can then be calculated as shown in Table III. Calcium and strontium as chemically similar elements are positively correlated in the Fig Tree sediments (Fig. 6) and therefore it should be T A B L E II1 LOSS A N D G A I N C A L C U L A T I O N F O R R E M O V A L OF c a o A N D s r F R O M SHALES OF T H E SHEBA FORMATION

Graywackes CaO ('!~,) Average content Average content if uniform composition of Sheba sediments is assumed before diagenesis (based on 857~, graywacke and 15~'~ishale) Total loss or gain during diagenesis

1.89 1.64

0 . 2 5 Al

Shales

Sr CaO (p.p.m.) (°,3 91 81

-10 nl

Sr (p.p.m.)

0.19 1.64

25 81

1.45 A

56 s

A, A1 indicates Ca migration from shales to graywackes; B, BI indicates Sr migration from shales to graywackes.

276

T. O. REIMER

0

0

3.0

Ca

2,O

%

average Sheba

•

• °°

graywack¢ ~

0

• •

o

o

,"emov~ f~or~ ~hale~ . e .

1,0

Sheba Formation:

graywacke •

shale

( - a v e r a g e Sheba shale X

70

®

2"o

Be/vue Road Formation: X

3"0

• X

io ~o do ~odo~doo

graywacke

~

. . ~00

o

. 600 . . . 80O tO00 '

S F Dpm Fig.6., Calcium vs. strontium for sediments of Fig Tree Group. (After Condie et al., 1970.)

expected that the amounts of these two elements transferred from the shales into the graywackes fall approximately within this trend. This clearly is the case and strongly supports the processes outlined above.

Magnesium

This second cation of the dolomite is present in the graywackes of the Sheba Formation in comparatively high concentrations. While the average MgO content for other graywackes varies between 1.2-3.3~ (Pettijohn, 1963) it reaches 4.50~ in the Sheba graywackes (Condie et al., 1970). As can be deduced from a comparison of the modal and the chemical analyses less than one third of the total magnesium is bound in dolomite while the rest is present in chlorites of varying, but mainly Mg-rich composition. Visser (1956) and Herget (1966) interpreted this high Mg content as a primary feature caused by the occurrence of large amounts of ultrabasic rocks in the source area. This is witnessed by the presence of mafic minerals such as chrysotile in the graywackes and by the extremely high Ni and Cr concentrations in the Sheba shales and graywackes (Danchin, 1967; Condie et al., 1970). As the decomposition of mafic minerals derived from such source rocks would furnish more magnesium than is incorporated in the resulting chlorites, enough of this element is available for the formation of dolomite. Therefore the pore fluid need not have been rich in this element as it can easily be derived from the rock itself. C a r b o n dioxide

As mentioned already, no indications of an originally high carbonate content

DIAGENETIC REACTIONS IN GRAYWACKES

277

could be detected in the graywackes and therefore most of this component of the dolomite would have had to be introduced into the sediment by the pore fluid. Calcium could then have been transported as the bicarbonate which has the advantage of explaining the excess of carbonate ions required to bind the magnesium in the dolomite. The pore fluid then would have had the character of a concentrated calcium bicarbonate solution and its calcium content was mainly derived from the shales. When it came into contact with the magnesium released by the decay of the original mafic minerals a reaction according to the following schematic formula could have taken place: MgR 2 + Ca(HCO3) z --~ CaMg(Co3) 2 ÷ 2H + + 2RDirect precipitation of dolomite from solution is possible when the carbon-dioxide pressure is high, as demonstrated by Chilingar (1956). The comparatively high carbon-dioxide content of the pore fluid necessary to keep calcium in solution during transpor; probably was inherited from the original sea-water entrapped during sedimentation. This could be interpreted as an indication of a higher carbon-dioxide content of the Precambrian sea-water and consequently of the contemporaneous atmosphere. This would be in agreement with the views of a large number of scientists as compiled by Holland (1968). TIME AND SPACE RELATIONSHIP OF DIAGENETIC REACTIONS

The time at which these reactions have taken place can be roughly estimated. According to the definition of diagenesis of Von Engelhardt (1967) the pore fluids play a major role in the reactions: "The diagenesis stops at a depth where in all sedimentary rocks the intercommunicating pore spaces have been closed up by physical and chemical processes." In an example mentioned by Von Engelhardt this depth is reached in Devonian to Carboniferous sandstones of northern Germany at an overburden of about 3000 m. As the pore fluid of the Sheba shales is required to furnish the calcium in the dolomite of the graywackes, the reaction described came to an end approximately after all this pore fluid had been driven out. The majority of water included in shales is driven out before an overburden of about 500 m is reached and the porosity is thereby reduced from about 75~o to 30~o (Miiller, 1967). The remaining water is then only slowly removed. From this it appears justified to assume that these diagenetic reactions reached their greatest intensity between 500-1000 m of overburden and were virtually completed at about 3000 m. At similar depths Dickinson et al. (1969) found already an average of 50~ of the plagioclases in their graywackes albitized. This is in good accordance with the observations in the Sheba graywackes. The maximum overburden for the Sheba Formation was only about 5500 m according to the descriptions of Visser (1956). When a geothermal gradient of about 30°C/km is assumed this depth would correspond to a temperature of about

278

T. 0. REIMER

165'C which is still well within the field of diagenesis as delineated by Winkler (1967). At this depth the pore space of the graywackes was already filled by dolomite and epi-matrix and no more diagenetic reactions could take place. As suggested above the reactions themselves took place mainly between 500-1000 m of overburden and therefore at much lower temperatures. Even if a higher geothermal gradient is assumed for the Early Precambrian, it is highly unlikely that the graywackes were subjected to temperatures above 200°C. The state of preservation of the volcanics underlying the Sheba graywackes excludes the presence of a much higher geothermal gradient during the deposition of the rocks of the Swaziland Sequence in this area. However, locally the physico-chemical conditions of the zeolite facies could have been reached, especially in the lowermost parts of the Sheba Formation. In an oolite band underlying the graywackes in the Sheba Mine area on the northern limb of the Ulundi syncline (Fig.l) zeolites, unfortunately unspecified, were described by Koen (1947). Oxygen isotope data led Perry and Tan (1972) to conclude that a jaspilite horizon of the lower Fig Tree Group in the southern facies was subjected to temperatures of 320-360 ~C. As the maximum overburden in this area was probably only 4500 m or even less, this temperature is astonishingly high. As the rock itself and the surrounding sediments do not show any signs of having ever been subjected to such temperatures most probably the basis of this determination is not sufficiently founded or the isotope ratios have been changed during weathering. According to X-ray and chemical data, the Sheba shales contain pyrite together with siderite. This allows the approximate Eh and pH ranges of the diagenetic reactions to be determined. According to Garrels and Christ (1965) these two minerals can occur together only at a pH between 6.5 and 9.5 and an Eh between - 0.2 and 0.4. Consequently, the conditions at least during the final stages ofdiagenesis were reducing and mildly alkaline. Similar values were reported by Nicholls and Loring (1962) who estimated a pH of about 7.5 and an Eh of about 0.2 for the diagenetic reactions in certain British Carboniferous shales. FURTHER CONSIDERATIONS

Although diagenetic calcite is frequently observed in graywackes, dolomite appears to be comparatively rare (Rust, 1965). This might be due in part to the difficulty of distinguishing the two minerals from each other in thin section. However, if it is assumed, as advocated above, that only the calcium-carbonate component of the dolomite was introduced into the rock, then it appears highly probable that dolomite only forms in graywackes which originally had a high content of magnesium, and the material of which was mainly derived from a source area rich in basic to ultrabasic rocks. As this is frequently the case for Precambrian

279

DIAGENETIC REACTIONS IN GRAYWACKES

%I

°J°/

60

50

a

b

Z"

50

eo

Z,O

20"

20"

20

10 dolomite

20

3O%

10

20 dolomite

30

~0 %

Fig.7. Dolomite vs. matrix for graywackes of Sheba Formation. a. Ulundi Syncline : b. Stolzburg Syncline.

formations, dolomite as cement should preferably be found in sediments of that age. That Precambrian graywackes do have a high magnesium content is shown by the compilation of Condie et al. (1970). The average for Precambrian graywackes is 3.72% MgO, while post-Cambrian graywackes on average contain only some 1.60%. It is frequently observed that diagenetic carbonate and matrix are negatively correlated with each other (Brenchley, 1969). This does apply to the graywackes of the Sheba Formation as well and is in accordance with the observation that carbonate cement is preferably developed in clastic sediments with a low content of detrital or proto-matrix (Siever, 1959). Fig.7 illustrates this relation for the Sheba graywackes. Porosity and permeability have an important influence on whether matrix or carbonate cement form as the predominant phase. It this respect it is interesting to note that, while dolomite can be completely absent in the Sheba graywackes, matrix rarely accounts for less than 20~ of the rocks (Fig.7). If it is assumed that the early formation of diagenetic carbonate inhibits the development of appreciable amounts of diagenetic matrix, then the 20% would be the maximum amount ofdetrital or proto-matrix in the graywackes. The same amount was mentioned earlier for the diagenetically unaltered graywackes. CONCLUSIONS

Observations are presented suggestive of the diagenetic origin of most of the matrix of the Sheba graywackes of the Fig Tree Group. It formed through the

280

T.o. REIMER

re-assemblage of chemical compounds derived from decayed labile clastic constituents such as feldspars, mafic minerals, and fragments of igneous and mafic volcanic rocks. The resulting minerals are chlorites of varying composition, sericite, and dolomite. When matrix 4- dolomite is plotted against the labile constituents and the analyses corrected in the manner described, in order to compensate the influence of the grain size on the petrographic composition of a sample, a volume increase at the rate of 1.6-1.9 parts of matrix 4- dolomite for 1 part of labiles becomes apparent. This increase is accounted for mainly by the introduction of most of the components of the dolomite from the pore fluids into the graywackes. Some of it was caused by the decay of mafic minerals with higher specific gravity and the reassemblage of the decomposition products to form chlorites with lower specific gravities. This volume increase is completely compensated for by a corresponding loss in the original porosity. Dolomite is the second constituent of diagenetic origin in the graywackes. The derivation of its constituents is investigated and it is concluded that part of its calcium was derived from the decomposition and partial albitisation ofplagioclase within the graywackes. The amount of dolomite in a sample is shown to be negatively correlated with the average An-content of its plagioclase population. Furthermore, the amount of dolomite varies inversely with the relative amount of plagioclase in the size fraction above 0.03 mm. Most of the calcium of the dolomite was derived frorh the pore fluids of the sediments. The latter obtained the calcium from the destruction of the plagioclases in the shales alternating with the graywackes. The calcium was transported as the bicarbonate. The removal of calcium together with strontium from the shales is furthermore indicated by peculiarities in the Rb/Sr ratios of both, graywackes and shales. The calcium content of the average shale was reduced from about 1.64% to 0.197o CaO and the strontium content from about 81 p.p.m, to 25 p.p.m. At the same time the calcium content of the graywackes increased from about 1.6470 to 1.89% CaO and the Sr content from 81 to 91 p.p.m. Magnesium was mainly derived from the original clastic constituents, especially from mafic minerals and rock fragments. Carbon dioxide was almost exclusively introduced into the sediment by the pore fluids which consequently had the character of concentrated calcium-bicarbonate solutions. The diagenetic reactions reached their maximum intensity under an overburden of about 500-1000 m and were virtually completed at about 3000 m. As the maximum overburden for the Sheba graywackes was about 5500 m, the temperature was below about 165 c'C and thus still within the field of diagenesis. The pH of the diagenetic solutions was between 6.5 and 9.5 and the Eh between -0.2 and -0.4. Finally it is suggested that, while calcite is the mineral cement usually found in graywackes, dolomite preferably formed in those which originally had a high

DIAGENETIC REACTIONS IN GRAYWACKES

281

content of magnesium, bound in mafic detrital minerals. Such graywackes should be mainly found in Precambrian formations. The maximum amount of detrital matrix in the graywackes was about 20~o and they originally were impure turbidite sandstones which attained their characteristic texture during the diagenesis of the whole formation. ACKNOWLEDGEMENTS

The help of Prof. H. Martin (G6ttingen) in the initial phases of the investigation is gratefully acknowledged. The author is indebted to Dr. C. van Moort (University of Tasmania) and Mr. T. Heinrichs (G6ttingen) for helpful discussions and criticism which considerably improved the manuscript. Prof. H. Fiichtbauer (University of Bochum) gave constructive criticism which was of great help in the preparation of the final draft. Financial assistance was received through a postgraduate research bursary awarded by the Department of Cultural Affairs of the South African Government in cooperation with the Deutscher Akademischer Austauschdienst (D.A.A.D.), Bad Godesberg (Germany). The South African Council for Scientific and Industrial Research (C.S.I.R.) and the Rembrandt Tobacco Corporation gave financial assistance during the final stages of the project. REFERENCES Allsopp, H. L., Ulrych, T. J. and Nicolaysen, L. O., 1968. Dating some significant events in the histoD of the Swaziland System by the Rb-Sr isochron method. Can. J. Earth Sci., 5:605-619. Audley-Charles, M. C., 1967. Graywacke with a primary matrix from the Viqueque Formation, Upper Miocene-Pliocene, Timor. J. Sediment. Petrol., 37:5-11. Brenchley, P. J., 1969. Origin of matrix in Ordovician graywackes, Berwyn Hills, North Wales. J. Sediment. Petrol., 39:1297-1304. Chilingar, G. V., 1956. Relationship between Ca/Mg ratio and geologic age. Bull. Am. Ass. Pet. Geol., 40:2256-2266. Condie, K. C., Macke, J. E. and Reimer, T. O., 1970. Petrology and geochemistry of Early Precambrian graywackes from the Fig.Tree Group. Bull. Geol. Soc. Am., 81:2759-2776. Cummins, W. A., 1963. The graywacke problem. Liverp. Manch. Geol. J., 3:51-70. Danchin, R. V., 1967. Chromium and nickel in the Fig Tree shales from South Africa. Science, 158: 261-262. Dapples, E. C., 1962. Stages of diagenesis in the development of sandstones. Bull. Geol. Soc. Am., 73 : 913-934. Dickinson, W. R., Ojakangas, R. W. and Stewart, R. J., 1969. Burial metamorphism of the Late Mesozoic Great Valley Sequence, Cache Creek, California. Bull. Geol. Soc. Am., 80:519-526. Dickinson, W. R., 1970. Interpreting detrital modes of graywacke and arkose. J. Sediment. Petrol., 40: 695-707. Dzulynski, S. and Walton, E. K., 1965. Sedimentary Features o f Flysch and Graywacke. Elsevier, Amsterdam, 274 pp. Emery, K. O., 1964. Turbidites-Precambrian to Present. Stud. Oceanog., Tokyo, 1964:486-495. Ermanovics, I. F., 1967. Statistical application of plagioclase extinction in provenance studies. J. Sediment. Petrol., 37:683~87. Fischer, G., 1933. Die Petrographie der Grauwacken. Jahrb. Preuss. Geol., 54:320-343.

282

T . O . REIMER

Holland, H. D., 1968: The abundance of CO2 in the earth's atmosphere through geological time. In: H. Ahrens (Editor), The Or(~in and Distribution o f the Elements. Pergamon, London, pp. 949 954. Herget, G., 1966. Archaische Sedimente und Eruptiva im Barberton Bergland (Transvaal, Sfidafrika). Neues Jahrb. Mineral., Abh., 105:161 182. Huckenholz, H. G., 1967. Petrographie und Mineralfacies einer konglomeratischen Tanner Grauwacke aus den] Unterharz. Contrib. Mineral. Petrol., 14:65 71. Koen, G. M., 1947. Die Geologic van die Sheba Gebied in die Barbertonse Distrikt. Thesis, University of Pretoria, 49 pp., unpubiished. Kuenen, Ph. H., 1966. Matrix of turbidites : experimental approach. Sedimentology, 7 : 267 297. Lasius, G., 1789. Beobaehtungen in dem Harzgebirge. Hellwig, Hannover, 285 pp. Mizutani, S., 1957. Permian sandstones in the Mugi area, Gifu Prefecture, Japan. J. Egrth Sci., Nagoya Univ., 5:135-151. Mfiller. G., 1967. Diagenesis of argillaceous sediments. In: E. Larsen and G. V. Chilingar (Editors), Diagenesis in Sediments. Elsevier, Amsterdam, pp. 127-178. Nanz, R. H., 1953. Chemical composition of Precambrian slates with notes on the evolution of lutites. J. Geol., 62:51-64. Nicholls, G. D. and Loring, D. H., 1962. The geochemistry of some British Carboniferous sediments. Geochim. Cosmochim. Acta, 26:181 223. Ojakangas, R. W., 1968. Cretaceous sedimentation, Sacramento Valley, California. Bull. Geol. Soc. Am., 79:973-1008. Oosthuyzen, E. J., 1970. The geochronology of a suite of rocks from the granitic terrain surrounding the Barberton Mountain Land. Thesis, Witwatersrand Univ., Johannesburg, 112 pp., unpublished. Perry, E. C. and Tan, F. C., 1972. Oxygen and carbon isotope determinations in Early Precambrian metasediments of the southern African Shield, in preparation. Pettijohn, F. J., 1957. Sedimentary Rocks. Harper, New York, N.Y., 2nd ed., 718 pp. Pettijohn, F. J., 1963. Chemical composition of sandstones, excluding carbonate and volcanic sands. U.S. Geol. Surv., ProJl Pap., 440-S: 1 19. Reimer, T. O., 1971. Volcanic quartz as indicator mineral in graywackes. Sedimentology, 17:125 128. Re(mer, T. O., 1972. Strontium depletion in Early Precambrian sediments. Neues Jahrb. Mineral., Monatsh., in press. Rust, B. R., 1965. The sedimentology and diagenesis of Silurian turbidites in southeast Wigtownshire, Scotland. Scot. J. Geol., 1:231 246. Siever, R., 1959. Petrology and geochemistry of silica cementation in some Pennsylvanian sediments. In : H. Ireland (Editor), Silica in Sediments--Soc. Econ. Palaeontol. Mineral., SlY. Publ., 7 : 66 79. Tr6ger, W. E., 1967. Optische Bestimmung der gesteinsbildenden Minerale. Schweizerbart, Stuttgart, 822 pp. Viljoen, M. J. and Viljoen, R. P., 1969. An introduction to the geology of the Barberton granite-greenstone terrain. Geol. Soc, S. Afr., Sp. Publ., 2:9-28. Visser, D. J., 1956. The geology of the Barberton area. S. Aft. Geol. Surv., Sp. Publ., 15 :251 pp. Von Engelhardt, W., 1967. Interstitial solutions and diagenesis in sediments, In: E. Larsen and G. V. Chilingar (Editors), Diagenesis in Sediments. Elsevier, Amsterdam, pp.503-522. Whetten, J. T., 1971. Diagenetic origin of matrix minerals: laboratory and field evidence. Int. Sedimentol. Congr., 8th, Heidelberg, Abstr., p. 109. Winkler, H. G.~ 1967. Petrogenesis o/Metamorphie Rocks. Springer, Berlin, 2nd ed., 220 pp.