Geochemistry and genesis of sulphide-anhydrite-bearing Archean carbonate rocks from Bahia (Brazil)

Chemical Geology, 29 (1980) 323--331

323

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

G E O C H E M I S T R Y AND GENESIS O F S U L P H I D E - - A N H Y D R I T E - B E A R I N G A R C H E A N C A R B O N A T E ROCKS FROM BAHIA (BRAZIL)

G.P. SIGHINOLFP, B.I. KRONBERG:'*, C. GORGONP and W.S. FYFE 2 1Mineralogy and Petrology Institute, University ofModena, Modena (Italy) 2 Geology Department, University of Western Ontario, London, Ont. N6A 5B7 (Canada)

(Received December 10, 1979; revised and accepted January 31, 1980) ABSTRACT Sighinolfi, G.P., Kronberg, B.I., Gorgoni, C. and Fyfe, W.S., 1980. Geochemistry and genesis of sulphide--anhydrite-bearing Archean carbonate rocks from Bahia (Brazil). Chem. Geol., 29: 323--331. Archean carbonate rocks hosting sulphide--sulphate (anhydrite) phases are found in association with high-grade metamorphic rocks in the Carafba Complex (central Bahia State, Brazil). The S-bearing rocks appear in drill-core samples in association with a metasedimentary (marble, calc-silicate, metapelite) sequence. 8 :'O values (+12%0 SMOW) are similar to those found in other Precambrian carbonates. IsC values range from --6.5 to --12°/00PBD. A model for anhydrite genesis is not obvious. Low Fe leyels, isotopic data and lithologic affinities with Archean carbonate sequences suggest the possibility of an evaporitic origin. Evidence of decarboxylation reactions and local accumulations of graphite suggest the possibility of sulphide formation by sulphate reduction. In any case, the extensive rSle of Archean life systems in the Carafba paleoenvironment is evident. INTRODUCTION Because only m i nor portions (~ 3%) of Archean crust are observable to-day o u r arguments f or Archean t ect oni c models are mainly inferential. R e c e n t reports o f gyps um - - c a r bonat e associations in Archean rocks in Australia, India, South Africa (reviewed in Lambert, et al., 1978) and o f a possible evaporitic sequence in Brazil in t he Caral'ba com pl ex (Leake et al., 1979) are significant in providing i n f o r m a t i o n on the Archean geosphere envelope. In the geochemical study presented here, the anhydrite-bearing rocks of the Carafba c o m p l e x in Brazil are c o m p a r e d t o similar carbonate rocks f o u n d in o t h e r Precambrian terrains. GEOLOGY AND MINERALOGY A geological description o f n o r t h e r n Bahia State (9°52'S) is given by Leake *To whom correspondence should be addressed. 0009-2541/80/0000--0000/$02.25 © 1980 Elsevier Scientific Publishing Company

324 TABLE I Mineralogical--petrographic notes of the C a r a ~ a carbonate--calcsilicate samples Sample

Mineralogical paragenesis

Sulfate-free samples: CA 1, CA 2, C A 3 CA 4, CA 5, CA 10 CA 11, CA 12

diopsidic pyroxene: 60--90 vol.%; fine-grained phologphitic mica: 5--20 wt.%; 2.1--5.5 wt.%; sulphide (pyrrhotite): 0.5--3 wt.%; relic brown hornblende: trace; sericitized plagioclase (samples CA 4 and CA 10); fas~aite: 20 wt.% (sample CA 10)

A nhy drite-bearing samples." CA 6, CA 7, CA 8 CA 9, SS 14

diopsidic pyroxene frequently chloritized: 60--85 wt.%; coarse-grained phlogopitic mica: 10--20 wt.%; anhydrite : 0.6--30 wt.%; calcite: 0--2 wt.%; apatite: trace (5 wt.% in the CA 9 sample) ........

et al. (1979). These rocks form part of the S~o Francisco craton (De Almeida et al., 1973, 1976). The anhydrite--carbonate rocks are associated with the predominantly mafic, meta-igneous Carm~ba complex (Barbosa, 1970) consisting of high-grade gneisses, migrnatites and pyroxene granulites. These rocks are considered to be of Archean age, and have been reworked during tectonic cycles (Wernik and de Almeida, 1979). The sedimentary rocks (marbles, calc-silicate rocks, metapelites) which come into contact with the Carm~ba gneiss--granulite complex, are cut by more recent (~2.0 Ga) granitic gneisses, probably of igneous derivation, and by K-rich permatic material. The metasedimentary sequence is dominated by calc-silicate rocks with varying mineral assemblages (Table I): (1) (2) (3) (4)

diopside (~ 90 wt. %) green spinel, apatite and sphene diopside, calcite, microcline diopside, anhydrite, microcline diopside, anhydrite, calcite, forsterite

Assemblages (2)--(4) contain varying amounts of phlogopitic mica. The calc-silicate rocks are not exposed and the samples studied here are taken from drill cores. Pyroxene composition varies -- in low-alumina samples (diopsidites) pyroxene is almost pure diopside (A1203 ~ 2 wt.%), and it is more aluminous (~ 6 wt.%) in samples with more petitic character, Scapolite occurs locally. Fassaite has been observed in one sample containing sericitized plagioclase and brown hornblende. Dolomite is present in minor amounts with respect to calcite. Interbedded in the carbonate sequence are thin layers of metapelitic rocks containing quartz, sericitized plagioclase, sillimanite and K-feldspar (khondalite). Graphite layers (up to 1 m thickness) occur in association with the pelitic sequences; however, the stratigraphic relation of the graphite beds is confused by tectonic overprinting.

325 Anhydrite-bearing assemblages appear in layers (0.05--0.5 m thick) within the metasedimentary sequence. Anhydrite occurs as large crystals, and in finegrained aggregates filling cracks and microfractures. Textural evidence shows anhydrite growth associated with phlogopitic mica. Sulphide mostly as pyrrhotite appears as fine-grained masses in all sulphate-free rocks. CHEMISTRY

The variations in chemical composition (Table II) reflect the variations in principal mineral phases -- diopside6, micas, sulphur phases. (Carbonates are among the minor components.) The high levels of A1, K, Ti, Fe and Cr, nor-

TABLE II Analytical results for the Carafba carbonate--calc-silicate rocks and for other similar Precambrian rocks from Bahia Range

SiO=(wt.%) TiO~ AI=O s F%O3 *~ MnO MgO CaO Na=O K=O P=O s S < S c ( , g g -I) V Cr Mn Co Ni Cu Zn Rb Sr Zr Ba Cs

K/Rb

27.66--54.38 0.16-- 0.83 6.30--12.72 3.43--11.54 0.11-- 0.38 6.89--16.27 10.67--22.92 0.18-- 1.48 0.14-- 3.43 0.03-- 0.69 0.03-- 8.73 7--26 60--340 16--57 350--3200 20--820 12---92 20--60 55--240 5--175 31--348 49--248 144--2600 0.1--1.6 112--232

Caraiba samples

Jequi~ (south Salvador)

average

sulphatefree average

sulphatebearing average

carbonates (A) .4 (avg. 7 samples)

calcsilicate (B) .4 (avg. 16 samples)

44.27 0.46 9.31 5.62 0.19 10.97 17.99 0.58 1.41 0.23 2.61

48.11 0.50 9.47 6.68 0.23 11.01 17.26 0.74 1.09 0.16 1.31

38.12 0.40 9.06 3.92 0.12 10.90 19.15 0.32 1.92 0.32 4.70

9.50 0.10 0.20 1.74 0.25 18.35 30.52 0.01 0.10

51.60 0.27 6.57 3.88

16 152 33 1,594 390 39 36 120 81 *2 181 114 672 1.0 *s 160

17 167 33 1,751 290 30 34 119 55 143 110 748 165

10 59 35 806 ~820 92 43 126 108 241 120 595

12.84 23.65 0.31 0.36

9

42

19

16

9 47

153

• i total Fe; *= 12 samples; *a 9 samples. • 4 References: A = Sighinolfi (1974); B = Sighinolfi and Fujimori (1974).

104

326 T A B L E III Rare-earth data o n s o m e selected samples (~g g- 1)

La Ce Pr Nd Sm Eu Gd Yb

CA 2

CA 3

CA 4

CA 7

CA 10

123 270 9.3 75 17.4 4.8 22.2 16.2

16 40 3.1 12 3.5 1.1 5.7 4.3

62 92 5.5 22 3.8 1.2 5.0 2.8

66 90 4.5 16 3.2 0.9 3.8 3.1

28 64 5.0 22 5.0 1.0 6.2 5.0

mally associated with the non-carbonate fraction of carbonate sediments (Wolf et al., 1967; Thompson, 1972), distinguishes the carbonate--calc-silicate rocks of Carm~ba from the corresponding lower Precambrian rocks of the same block (Sighinolfi, 1974). The latter contain typically carbonate-associated elements and rather pure silica. As all the rocks in the Carm'ba area have been metasomatized and deformed, a model for the chemical history is difficult. The anhydrite-bearing samples (Table II) on average contain more S as well as K and Rb; however, K/Rb ratios are low with respect to those normally found in high-grade metamorphic rocks. The calc-silicate rocks are relatively high in Ba and P (up to 1.17 wt.% P2Os in some S-rich samples). It is not clear if these are associated with the detrital clay fraction or if they have been precipitated during diagenesis (Puchelt, 1967, 1972; Koritnig, 1978). !

2O

Fig. 1. C h o n d r i t e - n o r m a l i z e d rare-earth p a t t e r n s in s o m e c a r b o n a t e calc-silicate r o c k s o f Carafba.

327 The rare-earth elements (REE) (Table III, Fig.l) display considerable variations in concentration, reflecting the detrital clay contributions to the metasediments. The chondrite-normalized pattern is typical of sedimentary rocks (Haskin and Haskin, 1966). Unlike most Archean sediments (Wildeman and Condie, 1973; Nance and Taylor, 1976), t h e y display a slightly negative Eu anomaly found in all the basement rocks of the Bahia block (Collerson and Fryer, 1978; Figueiredo, 1980). CARBON-

AND OXYGEN-ISOTOPE

DATA

C- and O-isotope data (Table IV) are consistent for the eight samples analyzed. Unfortunately, the amounts of CO2 liberated using H3PO4 were rather low for accurate isotopic analyses. Both C and O in the Carm~ba carbonates are much lighter than in normal marine carbonates. The O-isotope data displays the d o c u m e n t e d "age effect" (Schidlowski et al., 1975; Veizer and Hoefs, 1976). The lowest 8~sO value is found in the sample with the highest S content. The values are within the range for those of Archean carbonates. The ~ 13C values are unusually light n o t only with respect to normal marine carbonates but also with respect to Archean carbonates. (Sample CA 12 with the heaviest ~ x3C value was in direct contact with a metapelitic gneiss). Metamorphism is not considered t o influence isotopic fractionation of C. Some T A B L E IV Carbon- and oxygen-isotope data in the carbonate--calc-silicaterocks of Cara~a and in other Precambrian carbonate rocks of Bahia Sample

S content

~ ~sC P D B

6 I'O S M O W

(wt.%)

(°1oo)

(°Ioo)

0.4 0.1

--8.1 --9.7

+13.0 +11.3 +11.6 +8.7 +12.7 +12.2 +12.1 +12.3 +11.7

Carafba (2.7 Ga) CA CA CA CA CA CA CA CA

1 2 3 5 9 10 11 12

Average

< 0.1

--9.0

4.1 2.7 1.1 < 0.1 0.5

--12.0 --9.5 --10.1 --10.4 --6.5 --9.3

Jequi$ (2.5 Ga) Range Average (7 samples)

- - 1 . 7 to

0.0

1.0

+ 1 2 . 5 to . 1 3 . 5 .13.1

Ubat~ (approx. 2.0 Ga) Range Average

- - 4 . 2 to + 0 . 3

--2.1

* 1 4 . 1 to * 2 1 . 5 *19.5

328

work (Veizer and Hoefs, 1976) has shown a limited "age effect"; but this should result in an isotopically heavier pattern. The C pattern in the Cara~a carbonates may indicate an organic origin, or post-depositional (diagenetic) equilibration with isotopically light C as demonstrated experimentally by Emlich et al. {1970) and Wendt (1971). Work by Veizer and Hoers (1976) indicates that variations in C- and O-isotopic ratios are facies controlled. For example, evaporitic carbonates are usually characterized by heavier ~ ~sO than their coeval counterparts. Carbonates deficient in ~3C are usually freshwater carbonates (Craig, 1953), carbonates associated with marine brines (Rye, 1966; Craig, 1969; Schoell and Stahl, 1972) or sulphur- and sulphide-rich limestones associated with evaporitic salt domes (Srebrodol'skiy, 1975; Freyer, 1978). When the ~3C depletion is due to exchange reactions, very light biologically derived CO: {Lindsay et al., 1951) is usually assumed to be the exchanging phase. The marked depletion in ~3C in S-rich carbonates would underscore the abundance of organic material in the Cara~a depositional basin. GENESIS AND PALEOENVIRONMENTAL SIGNIFICANCE OF THE CARAIBA CARBONATE ROCKS

A nhydrite pro blem The overprinting of tectonic and high-grade metamorphic episodes have strongly disturbed the original chemical and mineralogical features and it is difficult to draw any unequivocal conclusions. Among the questions to be answered are: Was the anhydrite in the Carm'ba carbonates a primary (sedimentary) phase or was its formation induced by later events? The possibility of primary anhydrite is corroborated by the lithologic similarities of the Carm~ba sequence to unmetamorphosed or slightly metamorphosed sulphate--carbonate sequences found in other Archean terrains (Viljoen and Viljoen, 1969; Heinrichs and Reimer, 1977), in which gypsum and/or anhydrite are interpreted as indicators of evaporitic conditions (Lambert et al., 1978). In the case of primary anhydrite, the sulphides in the calc-silicate rocks could be generated by sulphate reduction either during early diagenesis {e.g., anaerobic bacteria) or during metamorphism. In the latter case, reducing conditions would be sustained by the graphite laid down in an earlier environment. Anhydrite as a secondary phase is more problematical. If the anhydrite [up to 30% {Table I)] in these rocks were produced by sulphide oxidation, we would expect considerable amounts of original pyrite; but it is noted that the Fe contents are low. There are some anomalous accumulations of trace metals as for Co and Mn, but Ni is low (Stanton, 1972). These carbonates are associated with Cu mineralization (one phlogopitic mica contains 5 wt. % Cu (G.P. Sighinolfi, unpublished data, 1974), and thus a source of Cu remains a problem. Textural evidence is inconclusive. Anhydrite sometimes appears unaffected by metamorphism, but there is evidence for extensive anhydrite crystallization along cracks and fractures. The abundance of anhydrite--mica inter-

329 growths is indicative of its participation in metasomatism. The case for secondary anhydrite generation by mobilizing fluid phases is difficult to postulate, because metamorphic--igneous fluids are normally reducing. It should be noted that CaSO4 mineralization (with complex carbonates, sulphides and ore metals) can result from seawater convection processes. Thus, T6masson and Kristmannsd6ttir (1972) have described the almost total removal of sulphate from the thermal brines of the Reykjanes system in Iceland. Basalts may contain several per cent anhydrite. In the McIntyre mine of Ontario (Langford and Hancox, 1936), vein anhydrite is present and is associated with oxidative alteration of the volcanic hosts with hematite formation. In this mine, there is a clear association of carbonate--quartz--sulphide (including chalcopyrite)--Au-bearing veins with the anhydrite veins. All are associated with various types of Archean volcanics, cherts and graphitic sediments. There is little doubt in this case that the sulphate is derived from convecting seawater and, in fact, pockets of saline water are reported in this mine. Intense deformation, metamorphism and metamorphic differentiation of parts of the McIntyre assemblage would produce a granulite-facies rock of great complexity. This type of anhydrite formation could be common in Archean volcanics.

Cara(ba paleoenvironment Several features of the Carafba carbonates are different from those of Archean marine carbonates in the same crustal block and in other Precambrian terrains: (1) The heterogeneous composition of the Carafba rocks attributed to original contributions of non-carbonate (pelitic) material. (2) Ba and P levels are high, and may have been accumulated in biogenic debris (Pettijohn, 1963; Lange, 1974). (3) The original S content of these rocks was substantial. These differences must, at least in part, reflect variations in the original d e p ~ sitional basins. The association of high-grade metamorphic Archean terrains with marble-orthoquartzite--K-pelite strips has been interpreted (Windley, 1978) as indicative of shallow-water continental margin/platform conditions, although probably poorly developed in Archean time. If these conditions were typical of the unrestricted Archean carbonate facies, the Carafba carbonates may represent a restricted environmental variation. The existence of separate Archean seas with varying degrees of oxidation is suggested by Lambert et al. (1978). The abundance of biological debris could explain the presence of thick graphite layers. The importance of biological activity within the depositional basin is corroborated by the C-isotope data, which is compatible with reactions between carbonates and biologically derived COs. It thus seems reasonable to conclude that living organisms played an extensive rSle in the Carm'ba paleoenvironment.

330 ACKNOWLEDGEMENTS

The authors are grateful to Dr. F. Lindenmayer and Z.G. Lindenmayer (Rio Doce Geologia e Minera~o, Salvador, Bahia) for assistance in the field and to Professor J. Torquato (Fed. Univ. de Cear~, Fortaleza) for carbon-isotope analyses. The authors also acknowledge Faye Murray for her assistance in interpreting data. REFERENCES Barbosa, O., 1970. Geologia economica de parte da regia'o do Medio S~o Francisco, Nordeste do Brasil. Dep. Nac. Min., Div. Fom. Prod. Min. (Rio de Janeiro), Bol., 140 pp. Collerson, K.D. and Fryer, B.J., 1978. The role of fluids in the formation and subsequent development of early continental crust. Contrib. Mineral. Petrol., 67: 151--167. Craig, H., 1953. The geochemistry of the stable isotopes. Geochim. Cosmochim. Acta, 3: 53--92. Craig, H., 1969. Geochemistry and origin of the Red Sea brines. In: E.T. Degens and D.A. Ross (Editors), Hot Brines and Recent Heavy Metal Deposits in the Red Sea. SpringerNew York, N.Y., pp. 208--242. De Almeida, F.F.M., Amaral, G., Cordani, V.G. and Kawashita, K., 1973. The Precambrian evolution of the South American Craton margin south of the Amazonas River. In: A.E. Nairn and F.G. Stehli (Editors), The Ocean Basin and Margins. Plenum, New York, N.Y., pp. 1411--1446. De Almeida, F.F.M., Hasui, Y. and Brito-Neves, B.B., 1976. The Upper Precambrian of South America. Bol. Inst. Geocience Univ. S~o Paolo, 7, 45--80. Emlich, K., Ehhalt, D.H. and Vogel, I.C., 1970. Carbon isotope fractionation during the precipitation of calcium-carbonate. Earth Planet. Sci. Lett., 8: 363--371. Feely, H.W. and Kulp, J.L., 1957. The origin of Gulf Coast salt-dome sulfur deposits. Am. Assoc. Pet. Geol., Bull., 41: 1802--1853. Figueiredo, M.C.H., 1980. Geochemistry of high grade metamorphic terrains in northeastern Bahia (Brazil). Ph.D. Thesis. University of Western Ontario, London, Ont. Freyer, H.D., 1978. Degradation products of organic matter in evaporites containing trapped atmospheric gases. Chem. Geol., 23: 293--307. Haskin, M.A. and Haskin, L.A., 1966. Rare earths in European shales: a redetermination. Science, 154: 507--509. Heinrichs, T.K. and Reimer, T.O., 1977. A sedimentary barite deposit from the Archean Fig Tree Group of the Barberton Mountain Land (South Africa). Econ. Geol., 72: 1426--1441. Koritnig, G.S., 1978. Phosphorus. In: K.H. Wedepohl (Editor), Handbook of Geochemistry, II. Springer, Berlin, Ch. t5. Lambert, I.B., Donnelly, T.H., Dunlop, J.S.R. and Groves, D.I., 1978. Stable isotopic compositions of early Archean sulphate deposits of probable evaporitic and volcanogenic origins. Nature (London), 276: 808--810. Lange, J., 1974. Geochemische Untersuchungen an pelagischen Sedimenten des Atlantischen und Pazifischen Ozean (DSDP, Legs I--VII). Thesis, University of GSttingen, GSttingen. Langford, C.B. and Hancox, F.G., 1936. Hypogene anhydrite from McIntyre mine, Porcupine district, Ontario. Econ. Geol., 31: 600--609. Leake, B.E., Farrow, C.M. and Townend, R., 1979. A pre-2,000 Myr old granulite facies metamorphosed evaporite from Caraiba, Brazil. Nature (London), 277: 49--50. Lindsay, J.G., Bourns, A.M. and Thode, H.G., 1951. C 13 isotope effect in decarboxylation of normal malonic acid. Can. J. Chem., 29: 192--196. Nance, W.B. and Taylor, S.R., 1976. Rare earth elements patterns and crustal evolution in Australian post Archean sedimentary rocks. Geochim. Cosmochim. Acta, 40: 1539-1551.

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