The origin of the iron-formation-rich Hamersley Group of Western Australia — deposition on a platform

Precambrian Research, 21 (1983) 273--297

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Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

THE ORIGIN OF THE IRON-FORMATION-RICH HAMERSLEY GROUP OF WESTERN AUSTRALIA -- DEPOSITION ON A PLATFORM

R.C. MORRIS and R.C. HORWITZ CSIRO, Institute o f Energy and Earth Resources, Division o f Mineralogy, Private Bag, P.O., Wem bley , Western Australia 6014 (Australia)

(Received May 24,198'?.; revision accepted December 16, 1982)

ABSTRACT

Morris, R.C. and Horwitz, R.C., 1983. The origin of the iron-formation-rich Hamersley Group of Western Australia -- deposition on a platform. Precambrian Res., 21: 273-297. With only minor exceptions, the 1.5 km thick sediments of the 2.5 Ga Hamersley Group are either chemical/biological (iron-formation, chert and carbonates) or pyroclastic/chemical ("shales"). Terrigenous clastics are sparse or absent. Palinspastic reconstructions indicate that the sediments were deposited on a submarine, essentially volcanogenic platform or bank (the Fortescue Group) built on an older Archaean, sialic, northwesttrending shelf protruding into, or marginal to, an ocean. A deep ocean basin is precluded by the geologic setting. Deposition in a barred basin is considered unlikely in the combined absence of terrigenous clastics, a defined shoreline or lateral facies changes. Upwelling of marine bottom currents resulted in precipitation of iron, silica and other components derived under anoxic conditions, largely from the pulsed output of a large oceanic rift or hot spot, possibly supplemented by normal continental drainage. The currents generally persisted during sedimentation of the Hamersley Group, temporarily interrupted or perhaps diverted by eustatic changes, growth of barrier reefs or the oscillating emergence and submergence of intervening volcanic chains. Ash emissions from the latter, combined with chemical precipitates, were largely responsible for the "shales" in the succession. INTRODUCTION T h e H a m e r s l e y I r o n P r o v i n c e o f Western Australia (Fig. 1) ( M a c L e o d , 1 9 6 6 ) c o n t a i n s e x t e n s i v e b a n d e d i r o n - f o r m a t i o n s ( B I F ) a n d large s u p e r g e n e iron ore bodies derived from them. These occur within the Hamersley Group which, w i t h t h e u n d e r l y i n g F o r t e s c u e G r o u p ( M a c L e o d e t al., 1 9 6 3 ) , f o r m s p a r t o f t h e Mt. B r u c e S u p e r g r o u p (de la H u n t y , 1 9 6 5 ; r e d e f i n e d , T r e n d a l l 1979). C o m p s t o n et al. (1981} h a v e suggested an age o f 2.5 G a f o r t h e H a m e r s l e y G r o u p , a n d t h e F o r t e s c u e G r o u p m a y be as old as 2.76 G a (R.T. Pidgeon, p e r s o n a l c o m m u n i c a t i o n , 1 9 8 2 ) . G r a v i t y d a t a (Fraser, 1 9 7 6 ) , i n d i c a t e t h a t , e x c e p t f o r a z o n e s o u t h w e s t o f t h e W y l o o D o m e (Fig. 1), t h e F o r t e s c u e G r o u p rests u n c o n f o r m a b l y o n t h e buried, s o u t h e r n e x t e n s i o n o f

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279 the Archaean Pilbara Block (Daniels and Horwitz, 1969). The southwestern gravity boundary to the substratum is reflected in the covering rocks by the boundary against the Ashburton (Wyloo) Trough (Doust, 1975). The term Hamersley Basin was first used by Trendall (1968) to refer to the depositional basin of the Hamersley Group, and was later extended to the depositional basin of the underlying Fortescue Group (Trendall, 1980, p. 113). T h e concept of a simple barred basin essentially limited to the present general area of outcrop rests largely on the isopach data for the Dales Gorge Member (Trendall and Blockley, 1970, pp. 279--283), and on the belief (p. 280) that the "general form for the basin follows closely the general distribution of depression during Fortescue Group t i m e . . . " . However, Horwitz and Smith (1978), and Horwitz (1981), indicated that the Fortescue Group thickened towards the southwestern margin of the Hamersley Iron Province (Figs. 2a,b), forming a shelf for the deposition of the Hamersley Group. This concept was presented at the Fifth Australian Geological Convention (Morris and Horwitz, 1981) as the basis of a model for the deposition of the Hamersley Group on an isolated platform and the present paper is an amplification of that thesis. THE HAMERSLEY PLATFORM

Fortescue Group The southern extension of the Pilbara Block, the basement to the Mt. Bruce Supergroup, had an uneven surface. Palaeogeographic reconstructions of boundaries and abutments (Fig. 3) indicate that the Fortescue Group mantled a ridge or palaeohigh which protruded from the east. Horwitz and Smith (1978, Fig. 6) have s h o w n h o w thickening of individual units to the southwest around this ridge indicates tilting of the whole substratum (or stronger subsidence in this direction) during sedimentation. Figure 3 shows h o w both the a b u t m e n t to the east and the thickening to the southwest persisted up to the upper unit of the Fortescue Group, the Jeerinah Formation. These features are emphasised by a dominant subaqueous facies in much of the upper part of the Fortescue Group to the southwest. This contrasts with the view that the dominance of this facies and maximum thickening, hence maximum subsidence, occurred at the Turner Syncline in the centre of a basin (Trendall, 1975b, 1980, p. 116}. No feature has y e t been found that indicates either a basinal shallowing or a proximity to a shore line in the far south and southwest for anything b u t the basal unit of the Fortescue Group. In the Mt. Brockman area and particularly in the north, east and southeast of the Hamersley Platform a shoreline facies is present. This is suggested by thinning of the members such as the Jeerinah Formation and the development of basal clastic units (Fig. 2, d o t symbol) which contain granitic material (Woodiana Sandstone Member), and the presence of stromatolitic carbonates such as those of the Tumbiana Formation in the middle sequence of the Fortescue Group.

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Thus, on the data available to us, we suggest that by the beginning of Hamersley Group times, the Fortescue Group was a submarine platform open to an ocean to the west and southwest. Considerable uncertainty arises in the delineation of the northern and eastern boundaries. The area may have abutted a continental margin (the Pilbara Block proper) or even covered the entire Archean substratum. A third alternative is that the area of deposition of the Hamersley Group was an isolated shelf, geographically analogous to the present Bahamas Platform. To support the concept of such a platform we need to examine in more detail the type of sedimentation characteristic of the Hamersley Group.

Hamersley Group Chemical sediments The Hamersley Group (MacLeod et al., 1963) shown in Fig. 4 has been described in many publications and in particular by Trendall and Blockley (1970). Apart from a thick sequence of intrusive and extrusive acid rocks and a sequence of thick basic sills in the Weeli Wolli Iron Formation, the Group contains five major and several minor BIF units and a major carbonate unit, all with Province-wide distributions. These, together with abundant cherty or ferruginous chert horizons, can be considered as essentially chemical sediments. The major BIF units show no evidence of lateral facies changes of the type suggested by James (1954) and the lateral continuity of the strata in general, and the BIF in particular, is a notable feature of the Hamersley Group. However, while remarkable correlations have been made in fine laminae over distances of hundreds of kilometres (Trendall and Blockley, 1970; Ewers and Morris, 1981), it seems likely that the exceptions may prove to be the rule. "Shales" Roughly one fifth of the 1.5 km of sediments included in the 2.5 km thick Hamersley Group are classified as shales. Unweathered samples in drill core or mines are usually very fine grained, ranging in various shades of green and grey and, with increasing carbon and sulphide, to black. Little is known of the original mineralogy, though in general, the sparse X-ray data, mainly from around the mining centres, show that the end product of weathering is commonly kaolinite. Stilpnomelane appears to be common in unweathered samples, particularly where the rocks are closely associated with BIF, but other components such as chlorite, ferroan talc and micas may overshadow this, depending on both the regional and stratigraphic position. Lamination is highly variable, some zones showing repetitive features not unlike varying, while at the other extreme the material is virtually devoid of structure. Pyroclastic features have been documented in S-macrobands (see Fig. 4) of the Dales Gorge Member (La Berge, 1966; Trendall, 1966), and Trendall and Blockley (1970, pp. 288--290) argued that virtually all the "shales" of

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the Hamersley Group resulted from a combination of air-fall volcanic ash and chemical precipitation. There is no evidence that the "shales" have a composition directly attributable to any known igneous rock type. However, Ewers and Morris (1981} suggested that if an igneous source could be cited, then the Zr/TiO2 ratios of shales from the Dales Gorge Member at Paraburdoo are consistent with an andesitic origin. Since the alumina and titania in these rocks show an invariable, sympathetic variation (Fig. 5), and since neither c o m p o n e n t could be reasonably introduced in solution with the normal BIF components, an extraneous source is required. In the absence of unequivocal evidence of a terrigenous clastic contribution to rocks in the area, a pyroclastic origin seems logical. Despite the excellent lateral continuity of strata in the Hamersley Group, there is no evidence that individual "shaly" members within the S-macrobands (Ewers and Morris, 1981), or the various equivalent aluminous horizons within such units as the Mt. McRae Shale, are likely to correlate throughout the platform area.

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Examples of localised deposition in these "shales" are not uncommon. For example, coarse pyroclastic debris, of up to 2 cm in the Dales Gorge Member at Wittenoom (La Berge, 1966), does n o t appear in equivalent horizons at Paraburdoo. A thick stilpnomelane band that marks the approximate mid-point of BIF 16 in gorges around Wittenoom is attenuated at Paraburdoo. Cases where complete S-macrobands are missing have been reported from the Paraburdoo Range (R. McKenzie, personal communication, 1982) though there is no certainty that this is n o t a tectonic feature. Perhaps more significantly, the succession of the internal subdivisions of S-macrobands at Wittenoom (Trendall and Blockley, 1970, p. 63) shows little similarity to that at Paraburdoo (W.E. Ewers, personal communication, 1980). The data in Table I indicate that the alumina from equivalent horizons is generally lower at Paraburdoo than at Wittenoom. TABLE I Alumina in the Dales Gorge Member a Average A1203 (%) BIF-maerobands S-macrobands Total Dales Gorge BIF-maerobands BIF-macrobands S-macrobands S-macrobands

0--16 at Paraburdoo 1--16 at Paraburdoo Member at Paraburdoo 12--16 Paraburdoo 12--16 Wittenoom 6+12--16 Paraburdoo 6+12--16 Wittenoom

0.09 2.04 0.36 0.13 0.20 1.42 2.53

aExtracted from Ewers and Morris, 1981 (Tables 1 and 4).

Though the data are very sparse considering the large area involved, the general pattern supports a northern area rather than a southern area as the main source of the pyroclastic material. SEQUENCE OF DEPOSITION

We have now laid the foundation for our depositional model of the Hamersley Group -- a wide shelf or platform built of volcanogenic rocks which, by the end of Fortescue Group times, was almost entirely marine (Jeerinah Formation), followed by a sequence of chemical sediments periodically supplemented with pyroclastic material. There is no evidence o f any terrigenous clastic contribution to the Hamersley Group except during the final stages of deposition. Marra Mamba Iron Formation

Clear exposures of the contact between the Jeerinah and the first HamersIcy Group unit, the Marra Mamba Iron Formation, are rare, but occasional

285 drill core intersections show an alternation of black shale with ferruginous cherts. The latter increase in iron content in the upper member of the Marra Mamba to form BIF. The BIF is generally markedly podded and perturbation features such as small-scale truncation structures are abundant. This suggests a somewhat shallower environment than for the later BIF; an environment probably normally below wave base, but affected by periodic violent storms. Intercalations of shaly carbonate zones up to 2 m thick, comparable to the overlying Wittenoom Dolomite, are present in the upper BIF member (Ewers and Morris, 1980). Some of these have been correlated in a broad crescent from Newman through Mt. Bruce to the Brockman Syncline and down to Kungarra Gorge (unpublished company data; Blockley, 1979). The correlation, mainly by gamma logging in ore zones, once thought tenuous to the south (Bourn and Jackson, 1979) is now believed to be consistent over the main platform area (R.A. Harmsworth, personal communication, 1983). Major sedimentary slumping in the southwest at Mr. de Courcey (Horwitz, 1978) indicates a steepening of the sea floor slope during deposition of the uppermost beds of the Marra Mamba of this region. Wittenoom Dolomite, Mt. Sylvia Formation and Mt. McRae Shale No agreement has been reached on the origin of the next unit, the Wittenoom Dolomite and its correlative of the northeastern area of the Province, the Carrawine Dolomite. Though dominantly dolomitic in character, a lateral facies change into limestone is known near Wittenoom, and it is possible that both units originated largely as limestones with cherty horizons. Many geobiologists would deny the existence of pelagic carbonate-precipitating organisms during this period. One of them (Walter, 1982} has suggested that stromatolites in the Carrawine Dolomite (Goode, 1981) were a possible source of carbonate debris for a deeper water detrital carbonate facies to the west, i.e., the Wittenoom Dolomite. However, Goode (1981} points out that shallow-water features are rare in the Carrawine Dolomite and suggests only local shoaling. Nevertheless, he has recognised a pelletal cherty horizon overlying the unit toward the extreme eastern boundary which he tentatively correlates with a shallow water equivalent of the Mt. Sylvia/Mt. McRae Formations. The upper part of the Wittenoom Dolomite is dominated by "shales" with minor ferruginous chert, excellently exposed in Hamersley Gorge, 30 km west of Wittenoom, where a thick, generally massive, carbonate-spotted unit shows minor, fine-scale, current bedding and is demonstrably tuffaceous. Shales, ferruginous cherts, and minor carbonates persist in the sequence until the upper Mt. McRae Shale. This sequence contains only one significant BIF unit, a 5--6 m thick, laterally continuous unit known as Bruno's band which marks the top of the Mr. Sylvia Formation. Restricted outcrops of finegrained arenaceous equivalents of the shales occur along the Ophthalmia

286

Range. These are among the few examples known in the Hamersley sediments which show current bedding with slump structures. An important feature of the depositional conditions during this period is the apparent cessation of significant iron precipitation and the prominence of carbonate with lesser chert. Shale units show considerable variations in thickness and carbonate zones within them are generally not persistent. The upper member of the Mt. McRae Shale consists of macrobands of BIF or ferruginous chert alternating with shaly horizons, continuing into the macroband alternation of the Dales Gorge Member, and from there into the overlying Whaleback Shale Member. The Dales Gorge Member o f the Brockman Iron Formation The Dales Gorge Member is one of the most studied BIF units in the world, partly because of the spectacular exposure of its thirty-three laterally persistent macrobands, but also for its economic significance, originally related to crocidolite asbestos and now to the presently mined, high-grade hematite deposits. Detailed descriptions of the unit were presented by Trendall and Blockley (1970), together with a model for the origin of the BIF, summarised later by Trendall {1980}. More recently Ewers and Morris {1981) proposed a somewhat different origin. The first hypothesis is based on the interpretation of paired laminae (A.F. Trendall's microbands} as a varved {annual} precipitate of iron-rich and silica-rich zones. Compaction during diagenesis of specific groups of these microbands resulted in vertical displacement of silica, evenly from some groups, and reprecipitation of this silica, again evenly, into others, to give groups of laminae flmesobands} with different concentrations of iron components. Extreme compaction resulted in high-iron mesobands now consisting almost entirely of magnetite and/or hematite. Ewers and Morris {1981) supported the "varve" concept, but argued against Trendall's diagenetic model, partly on the grounds of chemical infeasibility of the silica redistribution. They suggested that variation in supply of iron and silica, and in meteorological conditions, controlled deposition of the mesobands. The change in style from precipitation of oxide-type BIF macrobands to the chert--carbonate--silicate-type S-macrobands is mainly attributed (Ewers, 1980; Ewers and Morris, 1981} to an increase in pH of the seawater, triggered and maintained by episodic influx of pyroclastic material. Figure 5 contains data from these studies that show, despite the apparently sharp lithological boundaries of the macrobands, that the change in conditions from oxide to S-type BIF was heralded by a gradual increase in specific components attributable to volcanic activity, reaching a climax with the accretion of identifiable tuffaceous material (La Berge, 1966}. A considerable proportion of the ash would have been fine enough to react with seawater and to precipitate with iron and silica already in solution, to give

287 rise to the "shales" and silicate-rich sequences within the mixed chert, chert--carbonate and "shale" lithologies that make up the S-macrobands.

Remaining Hamersley Group units Apart from a current study on the geochemistry of the Joffre and Whaleback Shale Members of the Brockman Iron Formation (McConchie, 1983), details of the remaining units are sparse. Nevertheless sufficient evidence exists to suggest that with the exception of the massive intrusive/extrusive igneous phase represented by the Woongarra volcanics, these units, even though they possess individual characteristics, conform to the general pattern of BIF and "shale" alternations. With no evidence of sub-aerial deposition, it is reasonable to assume, in the absence of significant perturbation within the BIF, that an adequate water depth was maintained by isostatic adjustment during the entire period. MATERIAL

SUPPLY

Iron and silica We suggest, as m a n y have done before, that the oceans offer the most logical immediate source for iron and silica, and that UpweUing currents offer the best transporting mechanism (Cloud, 1973; Holland, 1973; Drever, 1974). However, it seems unlikely that the ocean deeps acted as a vast storage system (Ewers, 1980; Ewers and Morris, 1981). Our calculations to support this assume an ocean roughly equivalent to that of today (1.3 × 1021 1), and that the iron and silica of the Hamersley Group sediments, assuming an area of 105 km 2, was ~ 1 X 102° g Fe and 1.5 × 102° g 8iO2 (Trendall and Blockley, 1970, p. 275). With the large figures involved, a factor of 2 either way makes little difference to the general argument. We further assume a n anoxic atmosphere enabling the Fe to remain in solution in the ferrous state at about pH 7--8. Thus, dissolving these quantities of Fe and SiO2 into this ocean results in ~ 7 5 ppm Fe and 115 ppm SiO2. With a total world BIF production during the Precambrian of between 10 and 100 times this figure, we obtain ranges of 750--7500 ppm Fe and 1150--11500 ppm SiO2 required to fit an ocean storage model. Table II shows the range of iron suggested as likely concentrations by recent workers in this field, and most sources {e.g., Siever, 1957} consider ~ 1 2 0 ppm silica as a maximum. Even if seriously underestimated, these values do not support the concept of a vast reservoir periodically overturned to give major episodes of iron-formation of the t y p e indicated by Eichler (1976). A continuing input seems more logical. If river systems comparable in size to those of today existed in the Precambrian, t h e y could, under anoxic conditions, have easily maintained suf-

288 TABLE II Suggested iron content of the early Proterozoic ocean Reference

Fe (ppm)

Eugster and Chou (1973) Holland (1973) Mel'nik (1973) Drever (1974) Ewers (1980)

< 1--5 3--30 100--400 5--10 - 20

ficient iron and silica in the oceans for all contemporary BIF sedimentation (e.g., Holland, 1973). Supply of this kind only would imply that the oceans reached a generally steady-state condition and, therefore, precipitation on average should have maintained a constant ratio of iron to silica, consistent with the requirements of the TrendaU--Blockley model. However, the massive carbonate deposit of the Wittenoom--Carrawine Dolomite and the abundant low-iron cherts presumably derived from the same ocean do n o t contain significant iron. Even the 1 m "averages" for the Dales Gorge Member (Fig. 5) show major Fe:Si variations which increase further as the sample size decreases (Ewers and Morris, 1981). The Ewers--Morris model stresses a variable input of material in which mesobands represent periods of constant supply for as little as 2 or 3 years to many tens of years, with abrupt changes in conditions occurring essentially synchronously across the area. If such changes were determined mainly by a fluctuating input of material then a volcanic source offers a more compatible mechanism. However, volcanism, though supported by many workers (e.g., Trendall and Blockley, 1970; Goodwin, 1973; Mel'nik, 1973; Gross, 1980), was considered unlikely by Holland (1973) for the Hamersley Group. While we agree with his rejection of local volcanism as the iron source, we consider a distal origin distinctly possible in view of recent evidence of the vast potential of oceanic ridge systems (Spooner, 1974; Wolery and Sleep, 1976; Corliss et al., 1979; MacDonald et al., 1980; Spiess et al., 1980). The data show that even with today's relatively low igneous activity, iron and silica release is adequate to service at least one major BIF unit at a time provided some mechanism could confine deposition to a restricted area. Concentrated 'hot spot' activity rather than the dispersed ridge systems of t o d a y (Fyfe, 1978), combined with greater igneous activity (Kroner, 1981), if substantiated for this period, would simplify this problem. Therefore, in our model for the Hamersley Group we envisage a significant, b u t intermittent, contribution of iron from a relatively distant source. Silica from the same source (not necessarily synchronously), was possibly added to a regular precipitate from a saturated ocean (Siever, 1957), to give rise to the banding so characteristic of these sediments. Various mechanisms

289 of precipitation have been recently reviewed by Mel'nik (1973), Drever (1974), Eichler (1976), Ewers (1980), Ewers and Morris (1981) and in more detail by Ewers (1983) and will not be discussed here. We do, however, suggest that the general absence of base metals within the BIF resulted from precipitation at source of these components, probably as sulphides, and that whatever little oxygen might have been available was scavenged by iron and sulphur, leaving the excess iron available to be transported in solution to the precipitation site. The now widely held opinion espoused by Lepp and Golditch (1964), Cloud (1973, 1980, 1983) and Mel'nik (1973), suggests that the lack of massive iron concentration in the late Proterozoic and Phanerozoic was a result of increased biogenic production of oxygen to levels which satisfied the extant oxygen sinks and prevented further massive migration of ferrous iron in solution. Iron deposits as well as cherts are not uncommon in these later periods but, though locally extensive, did not reach the levels of the lower Proterozoic; most of the significant Phanerozoic deposits consist of one or the other component, rather than both. The hypothesis that the rise in the level of atmospheric oxygen prevented further major development of BIF by restricting significant transport of iron, is an attractive one; but why was there a parallel decline in silica? A current, strongly held view of geobiologists is that silica-secreting organisms, of the type now held largely responsible for the very low silica content of m o d e m oceans, did not exist until the Phanerozoic (Heinen and Oehler, 1979). Spherical structures of La Berge (1973) or more complex objects such as those of Klemm (1979) found in BIF, are considered by them to be products of silica precipitation or of the extraction methods (e.g., Oehler, 1976; Engel et al., 1968; M.R. Walter, personal communication, 1980). In m o d e m organisms such as diatoms, silica uptake is apparently linked to aerobic respiration (Lewin, 1955, and Heinen, 1967; quoted in Heinen and Oehler, 1979), which requires the presence of free oxygen, either gaseous or dissolved in water. It can be reasonably assumed that before the appearance of such organisms there was a "substantial prior history of biochemical experimentation with siliceous materials" (Heinen and Oehler, 1979, p. 441), possibly triggered by increasing atmospheric oxygen. Thus, some biochemical precipitation of silica may have occurred during this period. While this may not have depleted the oceans to the small amounts of today, it could have depressed the general silica content of the oceans sufficiently below the saturation level to limit the efficiency of whatever inorganic processes (e.g., evaporation) had previously operated. Massive local input of silica, however, with or without iron, would allow such processes to operate to form proximal deposits such as during the late Proterozoic--early Palaeozoic period. Therefore, if atmospheric oxygen was directly responsible for the lower level of iron deposits after the lower Proterozoic, it may well have been indirectly responsible for the parallel decline in major deposits of silica.

290

Pyroclastics Table I shows how little alumina is present in the oxide-facies BIF of the Dales Gorge Member. Trendall and Blockley (1970, p. 275) suggest a total for the entire Hamersley Group sediments of 0.6 × 1019 g A1. If this was largely introduced as an andesitic ash, a total input of ~ 4 × 10 ~ km ~ of ejecta would be required. With the assumed time span of 8--24 × 10 ~ y for the 1.5 km of Hamersley Group sediments (Trendall and Blockley, 1970), ~ 1 km ~ would be required on average every 200--600 years. However, since the shales are concentrated during much shorter intervals we estimate a 'worst' case requirement of some 4 km ~ of ash each century during the peak of activity. The 1980 Mt. St. Helens (1 km~), 1883 Krakatoa (18 km 3) and 1815 Tambora (80 km ~) eruptions show that supply could be met b y even a small volcanic chain. It is unlikely that all the pyroclastics would have settled on the platform. A wider distribution into the path of incoming currents could have helped maintain the pH increase suggested for the precipitation of the carbonate and silicate phases o f the sediments during the volcanic episodes (Ewers and Morris, 1981). The distribution of ash from each eruption should follow a

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291 more complex pattern than suggested by the Mt. St. Helens picture (Fig. 6), since ocean currents would also be involved. This is supported by isopach maps prepared for each of the 33 macrobands of the Dales Gorge Member (Horwitz et al., 1983). These show a significant difference between the generally simple pattern of the BIF-macrobands and a more complex nodal pattern of the S-macrobands. BASIN, SHELF OR PLATFORM? The depositional environment of the 2.5 km thick Hamersley Group must satisfy certain special conditions. These include: (1) a source of vast quantities of iron and silica which did not introduce terrigenous clastics; (2) water deep enough to prevent formation of pelletal BIF, but shallow enough, at times, to allow extensive carbonates to form; (3) an apparent absence of shoreline and transgressive facies despite rapid vertical facies changes; and (4) remarkable lateral stratigraphic continuity across at least 60 000 km 2. A deep ocean environment is precluded on the obvious grounds of the geological situation (Fig. 1). An intracratonic basin (Trendall and Blockley, 1970} containing 1.5 km of sediments deposited over 8--24 X 106 y and a further 1 km of volcanics, without either water or wind borne terrigenous clastics, would need unique conditions. Shoreline facies are present in both the underlying and overlying rocks, but despite intensive exploration and mapping for over two decades none have been recognised in the Hamersley Group. Total erosion of at least 1600 km of basin margin would be a coincidence of the highest order. Similarly, interfingering and lateral facies changes suggested for BIF environments elsewhere cannot be demonstrated in this area. The best clustering of modern intracratonic basins with some similarity to the physical requirements is the European, Middle Eastern area shown in Fig. 7. This area offers a wide range of topographic and climatic situations, but none of these basins fulfils the chief requirement that terrigenous clastics be absent. A sabkha situation seems unlikely for the Hamersley Group in view of the apparent requirement for relatively deep water for the BIF. Certainly it would be difficult to maintain the excellent lateral continuity of the BIF over such an extensive area if there was a constant problem of local ponding. We can find no appropriate oceanic or shelf situation for the Hamersley Group in the classification of the BIF sedimentary environments of Gross (1980). Thus, we turn to the Bahamas as a geographic analogue of our model. Drilling has indicated over 4.5 km of shallow-water carbonate sediments with no terrigenous clastics (Blatt et al., 1980}, going back to at least the Cretaceous. This is some 5 to 10 times the period suggested for deposition of the Hamersley Group. Unlike the Bahamas, our main platform area would generally be submerged below wave base. Hence no shoreline would normally exist, though local shoaling and occasional emergence would be expected.

292

Fig. 7. Modern intracratonic basins of the European--Middle Eastern area. Approximate areas of the Hamersley and Bahamas Platforms are shown for comparison. The Hamersley Group is free of terrigenous clastics, but each of these modern basins contains clastics from rivers or sand storms.

THE DEPOSITIONAL MODEL

It is obviously difficult to present a valid geographical map of the area in Hamersley times but, by way of illustration, let us take an antipodean view of an analogous situation (Fig. 8). A volcanogenic platform (shown in black) is envisaged on the continental shelf of the "Pilbara Block", generally isolated from any significant land mass, but possibly connected at various times, particularly to the "north and east". A major primitive ridge system of hot spot existed somewhere to the "west", producing pulsed outputs of a variety of components, including iron, into an anoxic ocean saturated in silica. Part of this iron may have locally precipitated as sulphide, which also scavenged any base metal contribution. Currents carried the excess components intermittently to the submerged platform, upweUed and deposited their load by some chemical mechanism. Volcanic chains, probably mostly to the "north", added sufficient ash at erratic intervals to temporarily disrupt the normal precipitation mechanisms, but not sufficient to overwhelm them. Thus, both ash and chemical precipitation acted together to give rise to the composite units called "shales". At times oscillating emergence and submergence of such chains might have affected incoming currents.

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The initial chemical sedimentation of the Hamersley Group resulted in the ferruginous cherts of the lower Marra Mamba but, b y upper Marra Mamba times, true BIF deposition was widely established in relatively deep water, though often disturbed by violent storm activity. Toward the later part of this period chemical precipitation of iron and silica alternated with that of carbonate and subordinate silica, swinging abruptly from one to the other, until the carbonate/chert pattern of the Wittenoom Dolomite became dominant, though sporadic volcanic eruptions continued. The abrupt changes indicate either a temporary cessation or a diversion elsewhere of the iron-bearing currents, since we know from evidence of pelletal iron-formations that such rocks could form in shallow water. Short-lived stromatolitic carbonate barrier reefs, terminated by pyroclastic activity, offer one solution. Eustatic changes and adjustments in the underlying volcanogenic pile could also have played some role. Following deposition of the carbonates, sedimentation was dominated by pyroclastic input, with varied b u t relatively minor quiescent periods of ferruginous cherts, and carbonates, until the Brockman Iron Formation was deposited. Further alternation of pyroclastic and BIF continued until the emplacement of 0.5--0.75 km of volcanic and intrusive acid rocks of the Woongarra Volcanics. This was followed by the Boolgeeda Iron Formation, indicating that the platform maintained its isostatic balance throughout the period. The top of the Boolgeeda in which chemical sediments alternate with finegrained terrigenous clastics, finally giving way to the dominantly clastic sequence of the Turee Creek Group, marks the end of the major BIF phase of the area and a radical change in the depositional environment. APOLOGIA

While we have adopted a simple approach to the model, avoiding as far as possible the fine detail inherent in such a complex situation, the model is in no sense simplistic. To a large extent the concept of a platform is dependent on the absence of significant terrigenous clastics, a defined shoreline or lateral facies changes, and a somewhat broader viewpoint could have been presented by using the term shelf, implying a continuous gradient from an outer shelf margin to a hypothetical shoreline. If the Hamersley Group sediments conformed to Walther's Law that "unbroken vertical sequences are a reflection of lateral facies" (Eriksson et al., 1976), then some evidence of a transgressive situation should be apparent. There is no evidence of this in the alternation of macrobands in the Dales Gorge Member nor in the broader situation of the major BIF units and their intercalations. If Walther's Law implies " n o r m a l " situations then the Hamersley environment represents an abnormal situation even for BIF, and a deeply submerged platform close to, b u t largely isolated from, a major land mass seems more appropriate than a shelf.

295 ACKNOWLEDGEMENTS We are g r a t e f u l f o r c o n t r i b u t i o n s in discussion f r o m o u r colleagues in t h e C S I R O , t h e G.S.W.A. a n d t h e m i n i n g i n d u s t r y , a n d in p a r t i c u l a r we t h a n k W.E. Ewers. F o r i n f o r m a t i o n i n c l u d e d in a m e n d m e n t s t o Fig. 3, we a c k n o w l edge D.F. Blight a n d D.B. S e y m o u r o f t h e G.S.W.A., T h e G e o l o g i c a l S o c i e t y o f F i n l a n d a n d Elsevier Science Publishers B.V. h a v e given p e r m i s s i o n t o p u b l i s h t h e a m e n d e d Fig. 2a a n d 3, r e s p e c t i v e l y . T h e m a n u s c r i p t was imp r o v e d b y suggestions f r o m A.J. G a s k i n , W.E. E w e r s a n d M.J. Gole.

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