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Diagenetic braunite in sedimentary rocks of the proterozoic Manganese Group, Western Australia
Ore Geology Reviews, 5 (1990) 315-323 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
315
Diagenetic Braunite in Sedimentary Rocks of the Proterozoic Manganese Group, Western Australia J. OSTWALD and B.R. BOLTON Broken Hill Pty. Co. Ltd., Central Research Laboratories, P.O. Box I88, Wallsend, New South Wales 2287 (Australia) Department of Geology and Geophysics, The University of Adelaide, G.P.O. Box 498, Adelaide, South Australia, 5001 (Australia) (Revised and accepted July 6, 1989 )
Abstract Ostwald, J. and Bolton, B.R., 1990. Diagenetic braunite in sedimentary rocks of the Proterozoic Manganese Group, Western Australia. In: B.R. Bolton (Editor), Metallogenesis of Manganese, Vol. II. Ore Geol. Rev., 5: 315-323. Braunite occurs in shales in the Proterozoic Manganese Group, Western Australia as flattened concretions which post-date the shale sedimentation and pre-date lithification. The mineral is here classified as diagenetic in origin. Braunite may have grown as concretions in the still-moist sediment by reaction between Mn 2+, liberated from older manganese oxides by decay of trapped organic remains, and silica. Apatite within the nodules possibly results from organically-derivedphosphate. Polycrystalline braunite also occurs in the Manganese Group sediments, as a result of recrystallisation of an older generation of braunite of undefined genesis.
Introduction The mineral braunite, of general composition Mn 2+ Mn~ + SiO12, (Fleischer, 1987) was first described by Haidinger (1831). Though known for over 150 years, details of its crystal chemistry and genesis remain unresolved. The mineral exists in two polytypes, normal braunire (or braunite I) which corresponds to chemical composition 3MneO3"MnSi03 with about 10% SiO2 and a rarer form, braunite II, which appears to contain essential calcium and iron, much less SiO2 (about 4%) and has an ideal formula Ca(Mn12Fe2)3+Si024 (De Villiers, 1980). The existence of other elements, such as magnesium, barium and boron, in the mineral has been reported by Frenzel (1980). Because it is so characteristic of the meta-sedimentary 0169-1368/90/$03.50
manganese ores of India, irrespective of the grade of metamorphism (Roy, 1966), and of similar diagenetic to metamorphosed rocks in South Africa and South America the mineral is commonly considered to be a metamorphic reaction product. It is also a common manganese mineral in manganese-containing base-metal veins, in association with fluorite, barite, carbonate, which may be the ancient equivalent of present-day hydrothermal emanations at oceanic spreading centres (Roy, 1981). Both modes of origin are of elevated temperature and/or variable confining pressure. There is also a small number of reports which suggest that braunite may have developed at low temperature and pressure, in the supergene weathering zone of manganese deposits (Hew-
© 1990 Elsevier Science Publishers B.V.
316
Y. OSTWALD AND B.R. BOLTON
itt, 1972) or as a product of a manganese-containing sediment diagenesis (Roy, 1981). In most of these reported cases the evidence for braunite development under normal surface temperature and pressure is not very good. For this reason, the discovery of braunite showing a range of morphologies and textures in sedimentary rocks of the Proterozic Manganese Group, Western Australia (De la Hunty, 1963 ), allowing the postulated low-temperature origin of braunite to be examined, has added to the knowledge of the mineralogy of this mineral.
Regional geology The deposits examined here occur on, or in, sediments of Early- to Mid-Proterozoic age making up the eastern part of the Hamersley Basin (De la Hunty, 1963; Blockley, 1975; Goode, 1981; see Fig. 1). This sequence, known as the Mt. Bruce Supergroup, comprises clastic, chemical and volcanic material up to 12 km thick. The Mr. Bruce Supergroup comprises the basal Fortescue
Group (dominantly basic volcanics with minor clastics), the Middle Hamersley and Turee Creek Groups (banded iron formations, shales, dolomites, dolerites and acid volcanics) and, in the northeast, the Manganese Group (clastics, carbonates and volcanics). The Manganese Group rests unconformable on the underlying sequences and is probably the equivalent to the Wyloo Group of the southwestern part of the basin (Goode, 1981). The manganese deposits considered here occur either in the Hamersley Group, represented here by the basal Marra Mamba Iron Formation and the overlying Carawine Dolomite, or in the Manganese Group. Goode (1981) divides the Manganese Group into the following three lithologies. ( 1 )Chert breccia (known locally as the Pinjian Chert Breccia) - the basal part of the group. This formation is comprised dominantly of angular chert fragments and is developed most frequently, via an irregular contact, over the underlying Carawine Dolomite (Hamersely Group).
J. Ostwald graduated from Queensland University with a B.Sc. in Geology and Mineralogy, and later gained the degrees of B.A. and Ph.D. (London University) and M.Sc. (Macquarie University). Since joining the Broken Hill Proprietary Company Limited, Central Research Laboratories in 1966 he has been involved in mineralogical research into iron and manganese ores and sinters, base metal deposits, slags and steelmaking raw materials.
Barrie Bolton graduated in 1976 with B.Sc (Honours) Geology from the Flinders University, Adelaide, South Australia. His research has centred on the stratigraphy and sedimentology of manganese deposits in Australia and particularly those at Groote Eylandt. In recent years he has also initiated studies on manganese deposits in the U.S.S.R., Morocco and Gabon. He is co-leader of I.G.C.P. Project 226 (Correlation of Manganese Sedimentation to Palaeoenvironments) and is currently employed as a Senior Research Fellow at the Department of Geology and Geophysics, University of Adelaide, Adelaide, South Australia.
DIAGENETICBRAUNITEIN SEDIMENTARYROCKS
317
21000.
LEGEND LATE [ ~ PROTEROZOIC
Bangemall Group
MIDDLE [~Manganese PROTEROZOIC EARLY PROTEROZOIC
ARCHAEANEARLY
Group
I- ~ Fortescue & L I ~ ' ~ JHamersley Group
[~
Greenstone belts
i÷ t . IGranite batholiths
LOCATION
22o00,
120°
/~Wyn.dh Port Head la d.a~.,.~ n 21°--
~
u
am
J PLAN AREA 21°
{ HA~SI.EY
Carnarvon~
BAS?I
.Kalgoorlie i
Perth~~ ~ i 120°
23~00' 120°00'
121'~30
Fig. l. Location of the Hamersley Basin and the examined formations.
(2)Clastics and volcanics - formations that generally lie in well-defined depressions in the chert breccia. The sedimentary succession in each depression or basin is variable, but general lithologies are similar. These include litharenites with minor chert pebble conglomerates, hematitic sandstones, glauconitic sandstones, vitric tufts, hematitic welded tuffs and rare oolitic-hematitic rocks. (3)Carbonates - These occur only in the eastern part of the region and appear to be stratigraphically equivalent to (or partly above) the clastic facies. The carbonates are dominantly shaly dolomites with minor, basal stromatolitic, conglomeratic and sandy dolomites, chert, and hematitic and glauconi-
tic sandstones. Most of the Manganese Group rocks are essentially unaffected by deformation. The dominant structural feature in the area is the broad, north-south trending, Oakover Syncline (Fig. 1 ). Dips are usually less than 25 °, although locally, abrupt steepening is apparent in some depressions in the Pinjian Chert Breccia (Goode, 1981). The area has undergone only low grade regional metamorphism which, in places, reaches greenschist facies (Goode, 1981 ). Sediments of the Hamersley Basin, including the Manganese Group, were deposited mainly during a major transgressive/regressive cycle beginning at approximately 2200-2400 Ma ago
318
Y. OSTWALD AND B.R. BOLTON
(Goode, 1981). According to this model, the prolific iron formations of the sequence are interpreted by Goode (1981), to have been deposited at the time of maximum transgression. Major iron formation deposition in the Hamersley Basin essentially ended with the regression at the end of Turee Creek Group time. The widespread development of chert breccia in the basin indicates extensive sub-aerial exposure, particularly in the shallower parts of the basin at the beginning of both Wyloo and Manganese Group sedimentation. Manganese Group sediments appear to have been deposited during a minor transgression within the overall transgressive/regressive cycle (Goode, 1981). Sedimentation in the basin ceased ~ 1700-1800 Ma ago.
Sample locations The braunite samples investigated in this paper come from two localities, Milbeena Bore and Woodie Woodie, shown on the geological map (Fig. 1 ). In the area of Milbeena Bore braunite occurs in disseminated nodules within well-bedded, purple, chocolate to green coloured micaceous to sandy shale-siltstone (elsewhere referred to as the Noreena Shale; De la Hunty, 1966). Manganese deposits at Woodie Woodie (De La Hunty, 1966) are dominantly replacements or cavity fillings of Carawine Dolomite or enrichments in the overlying Pinjian Chert Breccia. The deposits are generally small, usually totalling no more than a few thousand tonnes. Braunite-containing samples were collected from small quarries in the northern part of the Woodie Woodie area. The manganese oxides occurred as irregular cavity-fillings (up to 1 m thick) at the contact between Carawine Dolomite and Pinjian Chert Breccia.
Braunite mineralogy Braunite occurs in the samples in two basic morphologies.
Fig. 2. P h o t o g r a p h s showing Milbeena Bore b r a u n i t e concretions weathered from shale in plan (a) a n d section (b). Smallest grid division is 1 mm.
(1)As flattened, lenticular, micro-concretions oriented parallel to the bedding plane of shales at Milbeena Bore. In the shale bedding planes the nodules appear as circular bodies generally less than 3 cm in diameter. The lenticular structure is visible in sections cutting through the shale bedding, where the maximum dimension of the concretions is 1 cm (Fig. 2). Grain size in these concretions is very fine, generally less than 50 ttm. The braunite concretions contain abundant micron-sized particles of quartz, clay minerals, apatite, etc. (2)As irregular bodies in a chert or carbonate
DIAGENETICBRAUNITEIN SEDIMENTARYROCKS
matrix, showing a microstructure of polycrystalline braunite (Fig. 3), commonly altered into secondary manganese oxide such as cryptomelane, chalcophanite, etc. (Woodie Woodie). The coarse grain size, often greater t h a n 0.1 m m suggests recrystallisation. Electron microprobe analyses from a number of areas of lenticular and polycrystalline braunite indicate t h a t both types of braunite belong to the normal braunite type, with almost 10% of SiO2, and are not braunite II, with 4.4% SiO2 as defined by De Villiers and Herbstein (1967; see Tables 1 and 2). The analysis also suggest: (1)Calcium, at 0.4-1.4% CaO, is a constant feature of all the analyses. (2)The lenticular braunite contains iron and magnesium which are absent from the polycrystalline braunite. The deviation of the analytical totals from 100% is probably due to the presence of different valencies of manganese present, which cannot be determined by electron microprobe analysis. According to De Villiers (1980) calcium is an
319 TABLE 1 Microanalyses of braunite from the Mibeena Bore shales (wt,%) Area 1 2
Mn20~
Fe2Os CaO
MgO
K20
83.2 84.7
3.2 0.8
0.4 0.7
0.6 0.4
0.3
0.1 0.1
nd
10.1
85.1
0.4
0.8
0.8
nd
10.1
97.2
0.6 0.7
0.1 nd
0.2 nd
9.8 9.7
99.1 98.2
9.8 9.7
97.3 96.4
86.3 87.3 87.2
1.1 0.6
7
86.3
0.7
0.9
0.8
0.8
0.3
0.1
0.1
nd
10.1
10.2
98.2 98.2
88.1
nd
0.5
nd
nd
9.9
98.5
88.6 89.7
nd nd
0.5 0.5
nd
nd
100.0
86.9 84.9 87.5
nd nd nd
0.5 0.6 0.4
nd nd nd nd
10.0 9.8
12 13 14
nd nd nd nd
9.9 9.9 10.1
97.3 95.4 98.0
15
86.3
nd
0.5
0.1
nd
10.2
97.1
16
87.4
nd
0.8
0.2
nd
9.8
98.2
17
86.1
86.4
0.4
Total
3 4 5 6 8 9 10 11
1.2
Si02
98.3
99.1
18
87.3
nd
0.1
0.4
nd nd
nd nd
9.7 9.9
97.7
19 20
87.6 88.2
nd nd
0.6 0.4
nd nd
nd nd
9.8 10.0
98.0 98.6
0.5
96.6
*oxide stoichiometry expressed as Mn20~. nd = not detected
Fig. 3, Scanning electron micrograph of polished section showing polycrystalline, recrystallised braunite, Woodie Woodie.
320
Y. OSTWALDAND B.R. BOLTON
TABLE 2
Frenzel (1980) lists values of CaO ranging from 1.2 to 4.3% in braunite. On the basis of the above it appears that the Manganese Group braunite contains calcium in its structure but, in the recrystallised example, at least, there is no indication that Ca has exchanged with Fe. Although the lenticular nodules give the appearance of bi-convex lenses or compressed spheres, their internal microstructure is not entirely concentric. Sections within the enclosing shale bedding do show vague concentric layering, mainly due to differing concentrations of manganese oxides and illite/chlorite, but sections of the nodules transverse to the shale bedding show very definite "relic" or "ghost" layering (Fig. 4), strongly suggesting that the shale bedding originally extended through the nodules. This phenomenon is restricted to the Milbeena Bore lenticular nodules. We interpret this as an indication that the braunite bodies grew within the shale while it was unconsolidated, probably replacing some of the siliceous minerals in the shale. Such a reaction would have SiO12).
Microanalyses of braunite from Woodie Woodie area (wt.%) Area
Mn20~
Fe2Oa
CaO
K20
Si02
Total
1 2 3 4 5 6 7 8 9 10
85.2 86.7 89.1 88.0 87.3 87.4 86.3 87.1 86.6 87.1
nd nd nd nd nd 0.1 0.1 nd 0.2 nd
1.0 1.0 1.1 1.1 1.1 1.2 0.9 1.4 0.9 0.8
nd nd nd nd 0.1 nd 0.2 0.1 nd nd
10.1 10.6 10.4 10.4 10.5 10.2 9.9 10.3 10.4 9.9
96.3 98.3 100.6 99.5 99.0 98.9 97.4 98.9 98.1 97.8
*oxide stoichiometry expressed as Mn203. All analyses from polycrystalline braunite in quartz and carbonate nd = not detected
essential feature of braunite II, which he classified as Ca(Mn12Fe2)3+SiO24. More recently, Abs-Wurmbach et al. (1983) identified coupled isomorphic exchanges between Mn a+, Fe and Mn 2+ and Ca in braunite, and discussed the calcium end-member, neltnerite (CaMn~ +
Fig. 4. Scanning electron micrograph of polished section showing traces of original shale layering within Milbeena Bore concentrations. White is braunite, dark is clay and quartz.
DIAGENETIC BRAUNITE IN SEDIMENTARYROCKS
produced essentially spherical (or spheroidal) concretions. Subsequent compaction and dewatering compressed the braunite concretions, producing the bi-convex morphology. In contrast to the above, braunite from Woodie Woodie, with its well-defined polycrystalline structure and triple-junctions, appears to be a recrystallisation of an earlier morphology (Stanton, 1972). Such recrystallisation will destroy any evidence for its primary mode-oforigin. Discussion
The Manganese Group braunite occurrences show no evidence, either mineralogical or textural, of a hydrothermal origin, as discussed by Roy (1981). Vein-type occurrence are lacking, as are the characteristic association of manganese oxides with fluorite, barite, calcite, manganese silicates, carbonates and sulphides. Neither do the occurrences show any evidence of metamorphism, as indicated by the host rock mineralogy (Table 3 ). Field studies on the area confirm this (Daniels, 1975). The manganese mineralogy of the Manganese Group is described by De La Hunty (1963) as pyrolusite, cryptomelane, braunite, and wad. Microscopic studies by one of the authors (Ostwald, unpubl, data) indicate that TABLE 3 H o s t - r o c k m i n e r a l o g y of b r a u n i t e occurrences
Illite Chlorite Kaolinite Quartz Hematite Cryptomelane Calcite Dolomite Apatite
Milbeena Bore
Woodie Woodie
P P L P L T P T
-
P = p r e s e n t , L = low, T -- trace.
L
P P -
321
micro-concretionary, concentrically-layered cryptomelane and goethite make up the bulk of the manganese, suggesting that the visible mineralisation is a form of arid climate desert varnish (Roy, 1981). The earlier manganese mineralisation can only be surmised, but it may have included manganese carbonates and oxides. Various interpretations of the stratigraphic occurrence of the Mangenese Group (Goode, 1981; Muhling and Brakel, 1984) stress its occurrence within sediments of a transgressive/regressive cycle. As mentioned earlier, the Manganese Group of the Hamersley basin succession, appears to have been deposited during a minor transgression within a larger transgressive/regressive cycle (Goode, 1981 ). As such, the deposition of manganese in such a sequence is consistent with similar marine transgressive/early regressive manganese deposits described elsewhere (Cannon and Force, 1983; Frakes and Bolton, 1984). This analysis points to the braunite being either a primary constituent of the sediment or a later diagenetic product. A primary origin for braunite has been postulated by Su Junhua (1983) on the basis of its occurrence in the Dounan manganese deposit, People's Republic of China, and also by Serdyuchenko (1980) from studies of Precambrian manganese deposits. The Dounan deposit appears to belong to the orthoquartzite-glauconite-clay association of Stanton (1972), though glauconite is not specifically described by Su J u n h u a ( 1983 ). Sediments ranging from sandstone to calcareous mudstone were deposited on the Yangzi platform during a Triassic transgression. Manganese occurs in the (deeper-water) calcareous mudstones (perhaps similar to the Groote Eylandt "marls" of Smith and Gebert, 1969 ) in the form of oolitic braunite, of grain size 0.31.7 mm; oolitic manganese carbonate, of grain size less than 2 m m and massive braunite/carbonate ore. Su J u n h u a (1983) believed that both braunite and manganese carbonate were primary sediments, the former developing under shallow water, moderately oxidising, con-
322
ditions the latter as a product of deeper water, more reducing conditions. Su J u n h u a considers the braunite ooliths were biogeochemical in origin, on the basis of their enclosed filamentous microrganisms (bacteria a n d / o r microalgae), and he developed a conceptual model of decaying shallow marine organics precipitating on suspended clay particles in coastal embayments, etc. Under such conditions Mn 2+ in solution (concentrated by water anoxia) reacted with the suspended clay particles to form braunite nuclei which gradually grew to oolitic size. Ooliths composed of zoned braunite and manganese carbonate were considered a response to varying levels of water anoxia. This biogeochemical model for braunite formation must be considered speculative, especially as there appears to be no reports in the literature of braunite formation by microrganisms under laboratory conditions. A sedimentary origin for braunite has also been advocated by Serdyuchenko (1980), on the basis of the occurrence of the mineral in stra-
Y. OSTWALDAND B.R. BOLTON
tiform bodies in Precambrian rocks. He considers Precambian braunite deposits as the product of coastal lagoons and evaporite flats (the braunite facies), as distinct from deeper-water, off-shore, marine calcium-manganese carbonate sediments (rhodochrosite facies). Such a facies sequence appears to be a variant of the better-known proximal manganese oxide-distal manganese carbonate model (Stanton, 1972). Serdyuchenko quotes the experimental work of Listova to the effect that braunite precipitates from seawater at pH of 9.5-9.9 and Eh of + 430mV to validate his model for braunite sedimentation. Unfortunately confirmatory studies on this topic have not been found by the authors. The remaining possibility for braunite origin is diagenesis. This mechanism was first suggested by De la Hunty (1966) in relation to the Manganese Group braunite occurrence and it appears to be the most likely mode of origin for the Milbeena Bore braunite. Textures as depicted in Fig. 4 indicated that the braunite is
Fig. 5. Scanning electron micrograph of polished section showing apatite grains (marked × ) within Milbeena Bore braunite concretions.
DIAGENETICBRAUNITEIN SEDIMENTARYROCKS
younger than the shale bedding, while the flattened nodule morphology is consistent with concretionary growth in a sedimentary medium (Berner, 1980). Further details of the braunite formation mechanism remain obscure. De la Hunty considered that the nodules may have originally been carbonate (early diagenetic) before being converted into braunite, though he admitted that unreplaced carbonate nodules were not present in the braunite-containing shales. A diagenetic origin for braunite in Mamatawan-type manganese deposits in South Africa has been suggested by Miyano and Beukes (1987). We suggest that the traces of apatite (Fig. 5 ), found in the nodules examined, may indicate that the nodules grew as concretions with the unconsolidated sediment about accumulations of decaying biological material. Under such circumstances the resulting volumes of reducing sediment pore water could first solubilise any primary manganese oxides in the sediment and then allow reaction with silica and concretionary development. An attempt to simulate such reactions in the laboratory, either purely inorganically or under the influence of specific microganisms would greatly enhance knowledge of low temperature braunite genesis. References Abs-Wurmbach, I., Peters, T., Langer, K. and Schreyer, W., 1983. Phase relation in the system Mn-Si-O: An experimental and petrological study. Neues. Jahrb. Mineral Abh., 146: 258-279. Berner, R.A., 1980. Early Diagenesis: A Theoretical Approach. Princeton Univ. Press, 241 pp. Blockley, G.J., 1975. Hamersley Basin - Mineralisation. In: C.L. Knight (Editor), Economic Geology of Australia and Papua-New Guinea. I. Metals. Aust. Inst. Min. Metall. Monogr., pp. 413-415. Cannon, W.F. and Force, E.R., 1983. Potential for highgrade shallow marine manganese deposits in North America. In: W.C. Shanks (Editor), Unconventional Mineral deposits. Soc. Min. Eng., pp. 175-189. Daniels, J.L., 1975. Bangemall Basin. In: Geology of Western Australia. West. Aust. Geol. Surv. Mem. 2: 147-159.
323 De la Hunty, L.E., 1963. The geology of the manganese deposits of Western Australia. West Aust. Geol. Surv., Bull. 116, 122 pp. De la Hunty, L.E., 1966. Manganese nodules in Middle Proterozoic shale in the Pilbara Goldfield, Western Australia. West. Aust. Geol. Surv., Annu. Rep., 1965: 65-68. De Villiers, J.P.R., 1980. The crystal structure of braunite II and its relation to bixbyite and braunite. Am. Mineral., 65: 756-765. De Villiers, P.R. and Herbstein, F.H., 1967. Distinction between two members of the braunite group. Am. Mineral., 52: 20-30. Fleischer, M., 1987. Glossary of Mineral Species. Mineralogical Record Publ., 227 pp. Frakes, L.A. and Bolton, B.R., 1984. Origin of manganese giants: Sea-level change and anoxic-oxic history. Geology, 12: 83-86. Frenzel, G., 1980. The manganese ore minerals. In: I.M. Varentsov and G. Grasselly (Editors), Geology and Geochemistry of Manganese, Vol 1. Schweizerbart'sche, Stuttgart, pp. 25-157. Goode, A., 1981. Proterozoie geology of Western Australia. In: D.R. Hunter (Editor), Precambrian of the Southern Hemisphere. Elsevier, Amsterdam, pp. 105-193. Haidinger, W., 1831. Mineralogical account of the ores of manganese. Trans. R. Soc. Edinburgh, 11:119-142. Hewitt, D.F., 1972. Manganite, hausmannite and braunite: features, modes of origin. Econ. Geol., 67: 83-102. Miyano, T. and Beukes, N.J., 1987. physicoehemieal environments for the formation of quartz-free manganese oxide ores from the early Proterozoic Hotazel formation, Kalahari manganese field, South Africa. Econ. Geol., 82: 706-718. Muhling, P.C. and Brakel, A.D., 1984. Geology of the Bangemall Group. Geol. Surv. West. Aust., Bull., 128, 266 pp. Roy, S., 1966. Syngenetic Manganese Formations of India. Jadavpur Univ. Press, Calcutta, 219 pp. Roy, S., 1981. Manganese Deposits. Pergamon, Oxford, 457 pp. Serdyuchenko, D.P., 1980. Preeambrian biogenic-sedimentary manganese deposits. In: I.M. Varentsov and G. Gresselly (Editors), Geology and Geochemistry of Manganese, Vol 2. Stuttgart, pp. 61-88. Smith, W.C. and Gebert, H.W., 1969. Manganese at Groote Eylandt, Australia. In: 9th Comm. Min. Met. Congr., Min. Pet. Geol. Sect., pp. 1-20. Stanton, R.L., 1972. Ore Petrology. McGraw-Hill, New York, N.Y., 713 pp. Su Junhua, 1983. Dounan Manganese Deposit - A braunite deposit of sedimentary origin. Bull. Inst. Min. Deposits. Chinese Acad. Geol. Sei., 4:33-47 (in Chinese).
315
Diagenetic Braunite in Sedimentary Rocks of the Proterozoic Manganese Group, Western Australia J. OSTWALD and B.R. BOLTON Broken Hill Pty. Co. Ltd., Central Research Laboratories, P.O. Box I88, Wallsend, New South Wales 2287 (Australia) Department of Geology and Geophysics, The University of Adelaide, G.P.O. Box 498, Adelaide, South Australia, 5001 (Australia) (Revised and accepted July 6, 1989 )
Abstract Ostwald, J. and Bolton, B.R., 1990. Diagenetic braunite in sedimentary rocks of the Proterozoic Manganese Group, Western Australia. In: B.R. Bolton (Editor), Metallogenesis of Manganese, Vol. II. Ore Geol. Rev., 5: 315-323. Braunite occurs in shales in the Proterozoic Manganese Group, Western Australia as flattened concretions which post-date the shale sedimentation and pre-date lithification. The mineral is here classified as diagenetic in origin. Braunite may have grown as concretions in the still-moist sediment by reaction between Mn 2+, liberated from older manganese oxides by decay of trapped organic remains, and silica. Apatite within the nodules possibly results from organically-derivedphosphate. Polycrystalline braunite also occurs in the Manganese Group sediments, as a result of recrystallisation of an older generation of braunite of undefined genesis.
Introduction The mineral braunite, of general composition Mn 2+ Mn~ + SiO12, (Fleischer, 1987) was first described by Haidinger (1831). Though known for over 150 years, details of its crystal chemistry and genesis remain unresolved. The mineral exists in two polytypes, normal braunire (or braunite I) which corresponds to chemical composition 3MneO3"MnSi03 with about 10% SiO2 and a rarer form, braunite II, which appears to contain essential calcium and iron, much less SiO2 (about 4%) and has an ideal formula Ca(Mn12Fe2)3+Si024 (De Villiers, 1980). The existence of other elements, such as magnesium, barium and boron, in the mineral has been reported by Frenzel (1980). Because it is so characteristic of the meta-sedimentary 0169-1368/90/$03.50
manganese ores of India, irrespective of the grade of metamorphism (Roy, 1966), and of similar diagenetic to metamorphosed rocks in South Africa and South America the mineral is commonly considered to be a metamorphic reaction product. It is also a common manganese mineral in manganese-containing base-metal veins, in association with fluorite, barite, carbonate, which may be the ancient equivalent of present-day hydrothermal emanations at oceanic spreading centres (Roy, 1981). Both modes of origin are of elevated temperature and/or variable confining pressure. There is also a small number of reports which suggest that braunite may have developed at low temperature and pressure, in the supergene weathering zone of manganese deposits (Hew-
© 1990 Elsevier Science Publishers B.V.
316
Y. OSTWALD AND B.R. BOLTON
itt, 1972) or as a product of a manganese-containing sediment diagenesis (Roy, 1981). In most of these reported cases the evidence for braunite development under normal surface temperature and pressure is not very good. For this reason, the discovery of braunite showing a range of morphologies and textures in sedimentary rocks of the Proterozic Manganese Group, Western Australia (De la Hunty, 1963 ), allowing the postulated low-temperature origin of braunite to be examined, has added to the knowledge of the mineralogy of this mineral.
Regional geology The deposits examined here occur on, or in, sediments of Early- to Mid-Proterozoic age making up the eastern part of the Hamersley Basin (De la Hunty, 1963; Blockley, 1975; Goode, 1981; see Fig. 1). This sequence, known as the Mt. Bruce Supergroup, comprises clastic, chemical and volcanic material up to 12 km thick. The Mr. Bruce Supergroup comprises the basal Fortescue
Group (dominantly basic volcanics with minor clastics), the Middle Hamersley and Turee Creek Groups (banded iron formations, shales, dolomites, dolerites and acid volcanics) and, in the northeast, the Manganese Group (clastics, carbonates and volcanics). The Manganese Group rests unconformable on the underlying sequences and is probably the equivalent to the Wyloo Group of the southwestern part of the basin (Goode, 1981). The manganese deposits considered here occur either in the Hamersley Group, represented here by the basal Marra Mamba Iron Formation and the overlying Carawine Dolomite, or in the Manganese Group. Goode (1981) divides the Manganese Group into the following three lithologies. ( 1 )Chert breccia (known locally as the Pinjian Chert Breccia) - the basal part of the group. This formation is comprised dominantly of angular chert fragments and is developed most frequently, via an irregular contact, over the underlying Carawine Dolomite (Hamersely Group).
J. Ostwald graduated from Queensland University with a B.Sc. in Geology and Mineralogy, and later gained the degrees of B.A. and Ph.D. (London University) and M.Sc. (Macquarie University). Since joining the Broken Hill Proprietary Company Limited, Central Research Laboratories in 1966 he has been involved in mineralogical research into iron and manganese ores and sinters, base metal deposits, slags and steelmaking raw materials.
Barrie Bolton graduated in 1976 with B.Sc (Honours) Geology from the Flinders University, Adelaide, South Australia. His research has centred on the stratigraphy and sedimentology of manganese deposits in Australia and particularly those at Groote Eylandt. In recent years he has also initiated studies on manganese deposits in the U.S.S.R., Morocco and Gabon. He is co-leader of I.G.C.P. Project 226 (Correlation of Manganese Sedimentation to Palaeoenvironments) and is currently employed as a Senior Research Fellow at the Department of Geology and Geophysics, University of Adelaide, Adelaide, South Australia.
DIAGENETICBRAUNITEIN SEDIMENTARYROCKS
317
21000.
LEGEND LATE [ ~ PROTEROZOIC
Bangemall Group
MIDDLE [~Manganese PROTEROZOIC EARLY PROTEROZOIC
ARCHAEANEARLY
Group
I- ~ Fortescue & L I ~ ' ~ JHamersley Group
[~
Greenstone belts
i÷ t . IGranite batholiths
LOCATION
22o00,
120°
/~Wyn.dh Port Head la d.a~.,.~ n 21°--
~
u
am
J PLAN AREA 21°
{ HA~SI.EY
Carnarvon~
BAS?I
.Kalgoorlie i
Perth~~ ~ i 120°
23~00' 120°00'
121'~30
Fig. l. Location of the Hamersley Basin and the examined formations.
(2)Clastics and volcanics - formations that generally lie in well-defined depressions in the chert breccia. The sedimentary succession in each depression or basin is variable, but general lithologies are similar. These include litharenites with minor chert pebble conglomerates, hematitic sandstones, glauconitic sandstones, vitric tufts, hematitic welded tuffs and rare oolitic-hematitic rocks. (3)Carbonates - These occur only in the eastern part of the region and appear to be stratigraphically equivalent to (or partly above) the clastic facies. The carbonates are dominantly shaly dolomites with minor, basal stromatolitic, conglomeratic and sandy dolomites, chert, and hematitic and glauconi-
tic sandstones. Most of the Manganese Group rocks are essentially unaffected by deformation. The dominant structural feature in the area is the broad, north-south trending, Oakover Syncline (Fig. 1 ). Dips are usually less than 25 °, although locally, abrupt steepening is apparent in some depressions in the Pinjian Chert Breccia (Goode, 1981). The area has undergone only low grade regional metamorphism which, in places, reaches greenschist facies (Goode, 1981 ). Sediments of the Hamersley Basin, including the Manganese Group, were deposited mainly during a major transgressive/regressive cycle beginning at approximately 2200-2400 Ma ago
318
Y. OSTWALD AND B.R. BOLTON
(Goode, 1981). According to this model, the prolific iron formations of the sequence are interpreted by Goode (1981), to have been deposited at the time of maximum transgression. Major iron formation deposition in the Hamersley Basin essentially ended with the regression at the end of Turee Creek Group time. The widespread development of chert breccia in the basin indicates extensive sub-aerial exposure, particularly in the shallower parts of the basin at the beginning of both Wyloo and Manganese Group sedimentation. Manganese Group sediments appear to have been deposited during a minor transgression within the overall transgressive/regressive cycle (Goode, 1981). Sedimentation in the basin ceased ~ 1700-1800 Ma ago.
Sample locations The braunite samples investigated in this paper come from two localities, Milbeena Bore and Woodie Woodie, shown on the geological map (Fig. 1 ). In the area of Milbeena Bore braunite occurs in disseminated nodules within well-bedded, purple, chocolate to green coloured micaceous to sandy shale-siltstone (elsewhere referred to as the Noreena Shale; De la Hunty, 1966). Manganese deposits at Woodie Woodie (De La Hunty, 1966) are dominantly replacements or cavity fillings of Carawine Dolomite or enrichments in the overlying Pinjian Chert Breccia. The deposits are generally small, usually totalling no more than a few thousand tonnes. Braunite-containing samples were collected from small quarries in the northern part of the Woodie Woodie area. The manganese oxides occurred as irregular cavity-fillings (up to 1 m thick) at the contact between Carawine Dolomite and Pinjian Chert Breccia.
Braunite mineralogy Braunite occurs in the samples in two basic morphologies.
Fig. 2. P h o t o g r a p h s showing Milbeena Bore b r a u n i t e concretions weathered from shale in plan (a) a n d section (b). Smallest grid division is 1 mm.
(1)As flattened, lenticular, micro-concretions oriented parallel to the bedding plane of shales at Milbeena Bore. In the shale bedding planes the nodules appear as circular bodies generally less than 3 cm in diameter. The lenticular structure is visible in sections cutting through the shale bedding, where the maximum dimension of the concretions is 1 cm (Fig. 2). Grain size in these concretions is very fine, generally less than 50 ttm. The braunite concretions contain abundant micron-sized particles of quartz, clay minerals, apatite, etc. (2)As irregular bodies in a chert or carbonate
DIAGENETICBRAUNITEIN SEDIMENTARYROCKS
matrix, showing a microstructure of polycrystalline braunite (Fig. 3), commonly altered into secondary manganese oxide such as cryptomelane, chalcophanite, etc. (Woodie Woodie). The coarse grain size, often greater t h a n 0.1 m m suggests recrystallisation. Electron microprobe analyses from a number of areas of lenticular and polycrystalline braunite indicate t h a t both types of braunite belong to the normal braunite type, with almost 10% of SiO2, and are not braunite II, with 4.4% SiO2 as defined by De Villiers and Herbstein (1967; see Tables 1 and 2). The analysis also suggest: (1)Calcium, at 0.4-1.4% CaO, is a constant feature of all the analyses. (2)The lenticular braunite contains iron and magnesium which are absent from the polycrystalline braunite. The deviation of the analytical totals from 100% is probably due to the presence of different valencies of manganese present, which cannot be determined by electron microprobe analysis. According to De Villiers (1980) calcium is an
319 TABLE 1 Microanalyses of braunite from the Mibeena Bore shales (wt,%) Area 1 2
Mn20~
Fe2Os CaO
MgO
K20
83.2 84.7
3.2 0.8
0.4 0.7
0.6 0.4
0.3
0.1 0.1
nd
10.1
85.1
0.4
0.8
0.8
nd
10.1
97.2
0.6 0.7
0.1 nd
0.2 nd
9.8 9.7
99.1 98.2
9.8 9.7
97.3 96.4
86.3 87.3 87.2
1.1 0.6
7
86.3
0.7
0.9
0.8
0.8
0.3
0.1
0.1
nd
10.1
10.2
98.2 98.2
88.1
nd
0.5
nd
nd
9.9
98.5
88.6 89.7
nd nd
0.5 0.5
nd
nd
100.0
86.9 84.9 87.5
nd nd nd
0.5 0.6 0.4
nd nd nd nd
10.0 9.8
12 13 14
nd nd nd nd
9.9 9.9 10.1
97.3 95.4 98.0
15
86.3
nd
0.5
0.1
nd
10.2
97.1
16
87.4
nd
0.8
0.2
nd
9.8
98.2
17
86.1
86.4
0.4
Total
3 4 5 6 8 9 10 11
1.2
Si02
98.3
99.1
18
87.3
nd
0.1
0.4
nd nd
nd nd
9.7 9.9
97.7
19 20
87.6 88.2
nd nd
0.6 0.4
nd nd
nd nd
9.8 10.0
98.0 98.6
0.5
96.6
*oxide stoichiometry expressed as Mn20~. nd = not detected
Fig. 3, Scanning electron micrograph of polished section showing polycrystalline, recrystallised braunite, Woodie Woodie.
320
Y. OSTWALDAND B.R. BOLTON
TABLE 2
Frenzel (1980) lists values of CaO ranging from 1.2 to 4.3% in braunite. On the basis of the above it appears that the Manganese Group braunite contains calcium in its structure but, in the recrystallised example, at least, there is no indication that Ca has exchanged with Fe. Although the lenticular nodules give the appearance of bi-convex lenses or compressed spheres, their internal microstructure is not entirely concentric. Sections within the enclosing shale bedding do show vague concentric layering, mainly due to differing concentrations of manganese oxides and illite/chlorite, but sections of the nodules transverse to the shale bedding show very definite "relic" or "ghost" layering (Fig. 4), strongly suggesting that the shale bedding originally extended through the nodules. This phenomenon is restricted to the Milbeena Bore lenticular nodules. We interpret this as an indication that the braunite bodies grew within the shale while it was unconsolidated, probably replacing some of the siliceous minerals in the shale. Such a reaction would have SiO12).
Microanalyses of braunite from Woodie Woodie area (wt.%) Area
Mn20~
Fe2Oa
CaO
K20
Si02
Total
1 2 3 4 5 6 7 8 9 10
85.2 86.7 89.1 88.0 87.3 87.4 86.3 87.1 86.6 87.1
nd nd nd nd nd 0.1 0.1 nd 0.2 nd
1.0 1.0 1.1 1.1 1.1 1.2 0.9 1.4 0.9 0.8
nd nd nd nd 0.1 nd 0.2 0.1 nd nd
10.1 10.6 10.4 10.4 10.5 10.2 9.9 10.3 10.4 9.9
96.3 98.3 100.6 99.5 99.0 98.9 97.4 98.9 98.1 97.8
*oxide stoichiometry expressed as Mn203. All analyses from polycrystalline braunite in quartz and carbonate nd = not detected
essential feature of braunite II, which he classified as Ca(Mn12Fe2)3+SiO24. More recently, Abs-Wurmbach et al. (1983) identified coupled isomorphic exchanges between Mn a+, Fe and Mn 2+ and Ca in braunite, and discussed the calcium end-member, neltnerite (CaMn~ +
Fig. 4. Scanning electron micrograph of polished section showing traces of original shale layering within Milbeena Bore concentrations. White is braunite, dark is clay and quartz.
DIAGENETIC BRAUNITE IN SEDIMENTARYROCKS
produced essentially spherical (or spheroidal) concretions. Subsequent compaction and dewatering compressed the braunite concretions, producing the bi-convex morphology. In contrast to the above, braunite from Woodie Woodie, with its well-defined polycrystalline structure and triple-junctions, appears to be a recrystallisation of an earlier morphology (Stanton, 1972). Such recrystallisation will destroy any evidence for its primary mode-oforigin. Discussion
The Manganese Group braunite occurrences show no evidence, either mineralogical or textural, of a hydrothermal origin, as discussed by Roy (1981). Vein-type occurrence are lacking, as are the characteristic association of manganese oxides with fluorite, barite, calcite, manganese silicates, carbonates and sulphides. Neither do the occurrences show any evidence of metamorphism, as indicated by the host rock mineralogy (Table 3 ). Field studies on the area confirm this (Daniels, 1975). The manganese mineralogy of the Manganese Group is described by De La Hunty (1963) as pyrolusite, cryptomelane, braunite, and wad. Microscopic studies by one of the authors (Ostwald, unpubl, data) indicate that TABLE 3 H o s t - r o c k m i n e r a l o g y of b r a u n i t e occurrences
Illite Chlorite Kaolinite Quartz Hematite Cryptomelane Calcite Dolomite Apatite
Milbeena Bore
Woodie Woodie
P P L P L T P T
-
P = p r e s e n t , L = low, T -- trace.
L
P P -
321
micro-concretionary, concentrically-layered cryptomelane and goethite make up the bulk of the manganese, suggesting that the visible mineralisation is a form of arid climate desert varnish (Roy, 1981). The earlier manganese mineralisation can only be surmised, but it may have included manganese carbonates and oxides. Various interpretations of the stratigraphic occurrence of the Mangenese Group (Goode, 1981; Muhling and Brakel, 1984) stress its occurrence within sediments of a transgressive/regressive cycle. As mentioned earlier, the Manganese Group of the Hamersley basin succession, appears to have been deposited during a minor transgression within a larger transgressive/regressive cycle (Goode, 1981 ). As such, the deposition of manganese in such a sequence is consistent with similar marine transgressive/early regressive manganese deposits described elsewhere (Cannon and Force, 1983; Frakes and Bolton, 1984). This analysis points to the braunite being either a primary constituent of the sediment or a later diagenetic product. A primary origin for braunite has been postulated by Su Junhua (1983) on the basis of its occurrence in the Dounan manganese deposit, People's Republic of China, and also by Serdyuchenko (1980) from studies of Precambrian manganese deposits. The Dounan deposit appears to belong to the orthoquartzite-glauconite-clay association of Stanton (1972), though glauconite is not specifically described by Su J u n h u a ( 1983 ). Sediments ranging from sandstone to calcareous mudstone were deposited on the Yangzi platform during a Triassic transgression. Manganese occurs in the (deeper-water) calcareous mudstones (perhaps similar to the Groote Eylandt "marls" of Smith and Gebert, 1969 ) in the form of oolitic braunite, of grain size 0.31.7 mm; oolitic manganese carbonate, of grain size less than 2 m m and massive braunite/carbonate ore. Su J u n h u a (1983) believed that both braunite and manganese carbonate were primary sediments, the former developing under shallow water, moderately oxidising, con-
322
ditions the latter as a product of deeper water, more reducing conditions. Su J u n h u a considers the braunite ooliths were biogeochemical in origin, on the basis of their enclosed filamentous microrganisms (bacteria a n d / o r microalgae), and he developed a conceptual model of decaying shallow marine organics precipitating on suspended clay particles in coastal embayments, etc. Under such conditions Mn 2+ in solution (concentrated by water anoxia) reacted with the suspended clay particles to form braunite nuclei which gradually grew to oolitic size. Ooliths composed of zoned braunite and manganese carbonate were considered a response to varying levels of water anoxia. This biogeochemical model for braunite formation must be considered speculative, especially as there appears to be no reports in the literature of braunite formation by microrganisms under laboratory conditions. A sedimentary origin for braunite has also been advocated by Serdyuchenko (1980), on the basis of the occurrence of the mineral in stra-
Y. OSTWALDAND B.R. BOLTON
tiform bodies in Precambrian rocks. He considers Precambian braunite deposits as the product of coastal lagoons and evaporite flats (the braunite facies), as distinct from deeper-water, off-shore, marine calcium-manganese carbonate sediments (rhodochrosite facies). Such a facies sequence appears to be a variant of the better-known proximal manganese oxide-distal manganese carbonate model (Stanton, 1972). Serdyuchenko quotes the experimental work of Listova to the effect that braunite precipitates from seawater at pH of 9.5-9.9 and Eh of + 430mV to validate his model for braunite sedimentation. Unfortunately confirmatory studies on this topic have not been found by the authors. The remaining possibility for braunite origin is diagenesis. This mechanism was first suggested by De la Hunty (1966) in relation to the Manganese Group braunite occurrence and it appears to be the most likely mode of origin for the Milbeena Bore braunite. Textures as depicted in Fig. 4 indicated that the braunite is
Fig. 5. Scanning electron micrograph of polished section showing apatite grains (marked × ) within Milbeena Bore braunite concretions.
DIAGENETICBRAUNITEIN SEDIMENTARYROCKS
younger than the shale bedding, while the flattened nodule morphology is consistent with concretionary growth in a sedimentary medium (Berner, 1980). Further details of the braunite formation mechanism remain obscure. De la Hunty considered that the nodules may have originally been carbonate (early diagenetic) before being converted into braunite, though he admitted that unreplaced carbonate nodules were not present in the braunite-containing shales. A diagenetic origin for braunite in Mamatawan-type manganese deposits in South Africa has been suggested by Miyano and Beukes (1987). We suggest that the traces of apatite (Fig. 5 ), found in the nodules examined, may indicate that the nodules grew as concretions with the unconsolidated sediment about accumulations of decaying biological material. Under such circumstances the resulting volumes of reducing sediment pore water could first solubilise any primary manganese oxides in the sediment and then allow reaction with silica and concretionary development. An attempt to simulate such reactions in the laboratory, either purely inorganically or under the influence of specific microganisms would greatly enhance knowledge of low temperature braunite genesis. References Abs-Wurmbach, I., Peters, T., Langer, K. and Schreyer, W., 1983. Phase relation in the system Mn-Si-O: An experimental and petrological study. Neues. Jahrb. Mineral Abh., 146: 258-279. Berner, R.A., 1980. Early Diagenesis: A Theoretical Approach. Princeton Univ. Press, 241 pp. Blockley, G.J., 1975. Hamersley Basin - Mineralisation. In: C.L. Knight (Editor), Economic Geology of Australia and Papua-New Guinea. I. Metals. Aust. Inst. Min. Metall. Monogr., pp. 413-415. Cannon, W.F. and Force, E.R., 1983. Potential for highgrade shallow marine manganese deposits in North America. In: W.C. Shanks (Editor), Unconventional Mineral deposits. Soc. Min. Eng., pp. 175-189. Daniels, J.L., 1975. Bangemall Basin. In: Geology of Western Australia. West. Aust. Geol. Surv. Mem. 2: 147-159.
323 De la Hunty, L.E., 1963. The geology of the manganese deposits of Western Australia. West Aust. Geol. Surv., Bull. 116, 122 pp. De la Hunty, L.E., 1966. Manganese nodules in Middle Proterozoic shale in the Pilbara Goldfield, Western Australia. West. Aust. Geol. Surv., Annu. Rep., 1965: 65-68. De Villiers, J.P.R., 1980. The crystal structure of braunite II and its relation to bixbyite and braunite. Am. Mineral., 65: 756-765. De Villiers, P.R. and Herbstein, F.H., 1967. Distinction between two members of the braunite group. Am. Mineral., 52: 20-30. Fleischer, M., 1987. Glossary of Mineral Species. Mineralogical Record Publ., 227 pp. Frakes, L.A. and Bolton, B.R., 1984. Origin of manganese giants: Sea-level change and anoxic-oxic history. Geology, 12: 83-86. Frenzel, G., 1980. The manganese ore minerals. In: I.M. Varentsov and G. Grasselly (Editors), Geology and Geochemistry of Manganese, Vol 1. Schweizerbart'sche, Stuttgart, pp. 25-157. Goode, A., 1981. Proterozoie geology of Western Australia. In: D.R. Hunter (Editor), Precambrian of the Southern Hemisphere. Elsevier, Amsterdam, pp. 105-193. Haidinger, W., 1831. Mineralogical account of the ores of manganese. Trans. R. Soc. Edinburgh, 11:119-142. Hewitt, D.F., 1972. Manganite, hausmannite and braunite: features, modes of origin. Econ. Geol., 67: 83-102. Miyano, T. and Beukes, N.J., 1987. physicoehemieal environments for the formation of quartz-free manganese oxide ores from the early Proterozoic Hotazel formation, Kalahari manganese field, South Africa. Econ. Geol., 82: 706-718. Muhling, P.C. and Brakel, A.D., 1984. Geology of the Bangemall Group. Geol. Surv. West. Aust., Bull., 128, 266 pp. Roy, S., 1966. Syngenetic Manganese Formations of India. Jadavpur Univ. Press, Calcutta, 219 pp. Roy, S., 1981. Manganese Deposits. Pergamon, Oxford, 457 pp. Serdyuchenko, D.P., 1980. Preeambrian biogenic-sedimentary manganese deposits. In: I.M. Varentsov and G. Gresselly (Editors), Geology and Geochemistry of Manganese, Vol 2. Stuttgart, pp. 61-88. Smith, W.C. and Gebert, H.W., 1969. Manganese at Groote Eylandt, Australia. In: 9th Comm. Min. Met. Congr., Min. Pet. Geol. Sect., pp. 1-20. Stanton, R.L., 1972. Ore Petrology. McGraw-Hill, New York, N.Y., 713 pp. Su Junhua, 1983. Dounan Manganese Deposit - A braunite deposit of sedimentary origin. Bull. Inst. Min. Deposits. Chinese Acad. Geol. Sei., 4:33-47 (in Chinese).