Archean cherts derived from chemical, biogenic and clastic sedimentation in a shallow restricted...

Archean cherts derived from chemical, biogenicand clastic sedimentation in a shallow restricted basin:examples from the Gorge Creek Group in the Pilbara Block

K. SUGITANI*, K. YAMAMOTO , M. ADACHI , I . KAWABE and R. SUGISAKIà* Department of Natural Science and Informatics, Nagoya University, Nagoya 464±8601, Japan Department of Earth and Planetary Sciences, Nagoya University, Nagoya 464-02, Japanà Department of Teacher Education, Meijyo University, Nagoya 468±8502, Japan

ABSTRACT

The >3á0 Ga chert sequence of the Gorge Creek Group is exposed at Ord Ranges about

50 km east of Port Hedland in the Pilbara Block. The chert sequence examined in this

study is 15 m thick and consists of oxide-rich laminated chert, grey chert (silici®ed

clastic rock), carbonaceous black chert and carbonate-rich laminated chert. Although

the cherts have undergone postdepositional silica enrichment, such as cementation

and metasomatic silici®cation, primary precipitation of silica at the site of deposition

is indicated by abundant microstructures (mosaic and spherulitic structures). Other

primary to early diagenetic components were carbonates, sulphates (gypsum and

anhydrite) and organic matter. Although these mineral associations, on the whole,

correspond to those of modern marine evaporites, they are different from modern

equivalents with respect to abundant precipitation of amorphous silica and presumed

primary precipitation of iron-carbonate (siderite). This feature is a possible

manifestation of peculiar physicochemical conditions in the water mass from which

the chemical sediments were precipitated; compared with modern ocean waters, the

concentrations of Fe and Si were signi®cantly higher and the pH value might have

been lower. These conditions could be obtained by contributions of Fe- and

Si-enriched hydrothermal solutions and continental run-off to the site of deposition.

Grey cherts contain detrital quartz and altered Fe±Ti oxides and were formed in a

period of input of terrigenous detrital materials. They are characterized by higher

concentrations of TiO2, Al2O3, Cr, Ni, Zn, Rb and Zr compared with the other types of

chert and by very low (< 4) Al2O3/TiO2 values. These features are attributed to the

supply of terrigenous detrital materials that contain abundant Fe±Ti oxides (ilmenite

and titanomagnetite) and ®ne TiO2 particles. Such detrital materials might have been

formed by extensive chemical alteration of source rocks and residual enrichment of Ti

relative to Al.

INTRODUCTION

In the Pilbara Block, Western Australia, thegreenstone belt is composed of the Pilbara Super-group, which is subdivided into the Warrawoona,Gorge Creek, and Whim Creek Groups in ascend-ing stratigraphic order (Hickman, 1983). TheWarrawoona Group consists largely of ma®c±

ultrama®c volcanic rocks with subordinate chertand felsic volcanic units. During its formation,shallow-marine environments developed on thelow-relief ma®c±ultrama®c volcanic shield withfelsic volcanic-cones in places (Barley et al.,1979; DiMarco & Lowe, 1989a,b); silici®ed evap-oritic sediments were found from several strata(Lowe, 1983; Boulter & Glover, 1986; Buick &

Sedimentology (1998) 45, 1045±1062

Ó 1998 International Association of Sedimentologists 1045

Dunlop, 1990). The Gorge Creek Group, on theother hand, consists largely of sedimentary rockssuch as shale, sandstone, conglomerate, chert andiron formation, and is interpreted as an orogenicterrigenous assemblage (e.g. Lowe, 1994). Incontrast to the Warrawoona Group, petrologicaland geochemical characteristics of sedimentaryrocks of the Gorge Creek Group have scarcelybeen studied, despite their potential for revealingthe evolution of the greenstone belt in the PilbaraBlock.

We report here sedimentological, petrologicaland geochemical characteristics of chert layers inthe Gorge Creek Group. Although the chert layerswere silici®ed and have suffered partial subaerialalteration, they exhibit well-preserved primary toearly diagenetic textures. Previous studies onArchean cherts suggest that their mineral compo-sitions and sedimentary textures are of great helpwhen considering their origins and sedimentaryenvironments (e.g. Lowe & Knauth, 1977; Lowe,1983; Paris et al., 1985; Buick & Dunlop, 1990).Recently accumulated geochemical data fromvarious types of Archean cherts (e.g. Sugitani,1992; Manikyamba et al., 1993; Minami et al.,1995; Sugisaki et al., 1995; Sugitani et al., 1996)have also placed some constraints on theirorigins. Detailed petrological and sedimentologi-cal descriptions of Archean cherts, combinedwith geochemical characteristics, should provideimportant information about their origins andsedimentary environments that are closely con-nected with surface environments on the earlyearth. The chert layers discussed in this paperconsist of: (1) silici®ed chemical and biogenicsediments; and (2) silici®ed clastic sediments. Inthe former, various structures related to silicadiagenesis, postdepositional silica-enrichment,microbial activities, and precipitation and re-placement of sulphate minerals are well pre-served. From the latter, new petrological andgeochemical data of titaniferous cherts character-ized by very low Al2O3/TiO2 values (< 4) areobtained. The very low Al2O3/TiO2 values forArchean cherts from other areas of the PilbaraBlock, together with their origins and sedimenta-ry environments, were recently discussed bySugisaki et al. (1995) and Sugitani et al. (1996).

GEOLOGICAL SETTINGAND STRATIGRAPHY

The chert layers occur at the southern edge of achert-clastic complex, about 8 km in diameter,

called `Ord Ranges', �50 km east of Port Hedland,Western Australia (Fig. 1). The complex is amember of the Cleaverville Formation of theGorge Creek Group (Hickman, 1983) that isassumed to have been deposited before 2á95 Ga(Oversby, 1976). The 15 m thick outcrop exam-ined is subdivided into four units (Fig. 2) in termsof lithologic features: Unit 1 consists dominantlyof oxide-rich laminated chert; Unit 2 consistsexclusively of grey chert similar to clastic rocks inappearance; Unit 3 consists dominantly ofcarbonaceous black chert with minor oxide-richlaminated chert; Unit 4 consists mostly of car-bonate-rich laminated chert with lesser amounts

Fig. 1. Sampling location of cherts in the PilbaraBlock. The star shows the location of Ord Ranges in thePilbara Block, and the arrow shows the outcrop exam-ined at Ord Ranges. Simpli®ed geological map of thePilbara Block is after Hickman (1983). Geological mapof Ord Ranges is after the Australian 1 : 250 000 Geo-logical Series (Port Hedland-Bedout Island).

1046 K. Sugitani et al.

Ó 1998 International Association of Sedimentologists, Sedimentology, 45, 1045±1062

of oxide-rich laminated chert and carbonaceousthin layers that are equivalent to carbonaceousblack chert. In addition to these four mainlithologic types, light-grey, translucent and struc-tureless chert occurs commonly within the chertlayers in Units 1, 3 and 4. In this study, 53samples collected during the ®eld research in1989 and 1993 were examined.

METHODS

Major and minor elements of the samples wereanalysed by an automatic X-ray ¯uorescencespectrometer. For major elements, a fusion glassmade from powdered sample and ¯ux (Li2B4O7)in the proportion of 1 : 5 was used (Sugisaki et al.,1977). For minor elements, a pressed disc wasmade by mixing powdered sample and binder in

the proportion of 2:3. Ferrous iron was colouri-metrically determined using o-phenanthroline(Sugisaki, 1981). For inductively±coupled plasmaspectrometry (ICP-AES) analyses of rare earthelements (REEs), samples were decomposed pri-marily by HCl±HNO3±HF solution and residueswere decomposed by Na2CO3 fusion (Kawabe,1995). Total C and H were analysed by an ele-mental analyser (Carlo Erba-EA1108). Constituentminerals were examined under the microscopeand qualitatively analysed by X-ray diffraction(XRD) using Cu-Ka radiation. Some samples weretreated by dilute HF (10%) before XRD analyses;during this treatment quartz is dissolved, but ®ne-grained carbonates enclosed with quartz are lessattacked and the diffraction peaks of thesecarbonates can be separated from those ofquartz. The analytical results are listed inTables 1 and 2 (whole data set is available uponrequest).

DESCRIPTIONS OF QUARTZOCCURRENCE

Quartz is the dominant mineral in the chertsamples, and its occurrence is useful for esti-mating the metamorphic grade of the chert andthe secondary enrichment of silica. Therefore,quartz occurrences are described before thedescriptions of each unit. In most parts of thecherts, individual quartz microcrystals becomeextinct randomly, but locally they showspherulitic extinction or domain extinction (andvarious intermediate phases), as described forEarly Proterozoic cherts and iron-formations ofNorth America by Simonson (1985, 1987). Theseextinction ®gures occur within irregular voids,layers parallel to local bedding and interstices ofbrecciated portions (vein stockworks). Smallvoids consist of a single spherulitic mass(Fig. 3A), whereas larger voids and beddinglayers have spherulitic masses growing inwardwith a texture of `swapped rims' (Fig. 3B,C)(Voll, 1960). In the latter case, the centres ofvoids are often ®lled with megaquartz-mosaic(Fig. 3B). In vein stockworks, mosaics of mega-quartz typically coarsen inward (Fig. 3D) and acolloform texture isopachously coating the brec-cia is also observed. Brecciated fragments aremostly angular and have sharp outlines (Fig. 3D),but some fragments are peripherally discolouredor entirely blurred. In addition to these quartzmorphologies, euhedral-megaquartz occurs lo-cally in cherts.

Fig. 2. Stratigraphic column of the chert sequencefrom which samples were collected in 1993. Theboundaries between units are not always clear at theoutcrop.

Archean cherts from the Gorge Creek Group 1047


Table 1. Bulk chemical compositions of chert samples (average, minimum and maximum values).

Unit 1(n � 8)

Unit 2(n � 8)

Unit 3(n � 12)

Unit 4-a(n � 4)

Unit 4-b(n � 7)

average min. max. average min. max. average min. max. average min. max. average min. max.

SiO2 (wt%) 85.1 76.4 91.6 96.7 90.3 99.4 95.8 87.4 99.0 88.1 83.3 90.6 95.8 93.5 99.0TiO2 0.018 <0.005 0.28 0.28 0.18 0.56 0.009 <0.005 0.024 0.005 <0.005 0.006 0.014 <0.005 0.058Al2O3 0.53 0.08 1.82 0.57 0.15 0.91 0.26 0.05 1.16 0.11 0.06 0.18 0.21 <0.05 0.59Fe2O�3 13.58 7.57 22.89 1.68 0.12 7.25 2.79 0.43 9.61 10.42 6.46 16.69 2.23 0.58 4.09MnO 0.031 0.011 0.102 0.012 <0.005 0.042 0.006 <0.005 0.021 0.020 0.007 0.042 0.007 <0.005 0.015MgO 0.11 <0.05 0.20 <0.05 <0.05 <0.05 0.06 <0.05 0.17 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05CaO 0.07 0.01 0.12 0.02 0.01 0.04 0.04 <0.01 0.23 0.03 <0.01 0.05 0.04 <0.01 0.09Na2O 0.06 <0.05 0.08 0.07 0.04 0.14 0.08 <0.05 0.13 0.06 <0.05 0.09 0.06 <0.05 0.07K2O 0.01 <0.01 0.03 0.06 0.01 0.14 0.01 <0.01 0.03 0.02 <0.01 0.03 0.02 <0.01 0.03P2O5 0.035 0.019 0.077 0.010 0.006 0.019 0.025 <0.005 0.139 0.051 0.019 0.066 0.032 <0.005 0.058H2O (total) 1.16 0.32 2.36 0.32 0.23 0.62 0.34 0.05 1.40 0.43 0.32 0.68 0.20 0.07 0.32CO�2 0.05 0.04 0.18 0.01 <0.01 0.03 0.12 <0.04 0.51 1.11 0.77 1.39 0.47 0.04 0.95FeO 0.02 0.09 0.30 0.18 0.10 0.30 0.27 0.02 0.80 1.72 1.37 2.47 0.52 0.03 1.29Cr (p.p.m.) 134 4 354 506 345 788 57 tr. 266 18 6 34 41 5 65Ni 7 4 13 31 13 60 10 7 13 4 1 5 9 tr. 13Zn 16 9 27 22 12 30 7 3 15 14 9 18 8 6 12Rb tr. tr. tr. 4 tr. 10 tr. tr. tr. tr. tr. 1 tr. tr. tr.Sr 4 tr. 14 11 1 22 3 tr. 18 21 tr. 77 17 tr. 48Zr 8 tr. 32 18 10 35 2 tr. 11 tr. tr. 1 2 tr. 4

Fe2O�3 is total Fe as Fe2O3; CO*2 is total C as CO2; Unit 4-a and Unit 4-b show ferruginous samples (Fe2O�3; > 5%) and others, respectively; `tr.' for minor

elements is `not detected' and is conveniently assumed to be 0 p.p.m. for calculation of average values; for calculation of the average values, the values of<0.01%, <0.05% and <0.005% for major elements are conveniently assumed to be 0.01%, 0.05% and 0.005%, respectively.

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PETROLOGICAL FEATURESOF THE CHERTS

Unit 1: Oxide-rich laminated chert

This chert is characterized by the presence of aferruginous layer containing hematite andgoethite that occur as granules, ¯akes andpseudomorphs after rhombic or spindle-shapedminerals. The layers are generally porous and/ordiscontinuous. Spindle-shaped cavities are com-monly found and, in some portions, they displaymound-like aggregation or subvertically orientedand fan-shaped structures (Fig. 4A). This unitalso contains thin, wavy, and discontinuouslaminae enriched in carbonaceous matter.

Unit 2: Grey chert

The grey cherts are similar to clastic rocks inappearance and petrologically quite differentfrom the other types of chert. The grey chertscontain ®ne TiO2 particles, detrital quartz, Ti-rich sand grains and silicate minerals (sericite).Detrital quartz is angular to rounded and oftenshows textures of secondary overgrowths. Ti-rich sand grains are composed of ®ne TiO2

particles, microcrystalline quartz and iron-oxide/hydroxide. The grey cherts also containthin layers enriched in intraclastic grains ofvarious shapes. These grains are quartz±sericitemosaics rimmed by ®ne TiO2 particles, ormixtures of ®ne TiO2 particles and microcrys-

talline quartz. Some intraclastic grains containdetrital quartz.

Unit 3: Carbonaceous black chert

Unit 3 consists of alternating black and translu-cent layers less than a few cm in thickness. Blacklayers contain ®ne carbonaceous matter and showlamination (Fig. 4B). Clots of carbonaceous mat-ter containing ®ne silicate minerals are alsopresent. Translucent layers are composed exclu-sively of microcrystalline quartz with little im-purity.

Portions enriched in carbonaceous matter werealso observed in cherts from Units 1 and 4. Theycan be regarded as equivalent to the carbonaceousblack chert and therefore are described here. Acarbonaceous fragment from Unit 4 shows aspongy structure (Fig. 4C) with densely packedcarbonaceous microgranules less than 5 lm indiameter. Carbonaceous laminae composed ofmassive or thickly walled spherules or ellipsoidsfrom 10 to 40 lm in length are also found in Unit4 (Fig. 4D).

Unit 4: Carbonate-rich laminated chert

Unit 4 is characterized by abundant ®ne carbon-ate grains. Hematite and goethite are commonlypresent, but occur as granules, ¯akes or pseudo-morphs after rhombic and spindle-shaped miner-als. Pyrite and carbonaceous matters are presentonly as minor constituents. The cherts displayvarious types of lamination, interlaminar struc-ture and microstructure. The laminae are mostlythinner than 5 mm and are parallel but locallywavy, anastomosed or discontinuous. The thinoxide/hydroxide-rich layers often grade laterallyinto oxide/hydroxide-poor ones (Fig. 4E). Addi-tionally, soft-sediment deformation and intraclas-tic grains are observed. Both the perturbed andadjacent unperturbed portions are often cut bycommon quartz veins.

Microstructures such as mosaic and spheruliticstructures commonly occur in carbonate-richlaminae. Each unit of mosaic structure generallycorresponds to an individual quartz grain in thematrix (Fig. 4F). Components of mosaic units,other than quartz, are mostly carbonates withlesser amounts of hematite, goethite, pyrite andcarbonaceous matter. Within the perturbed port-ions of the sediments, aggregates of ®ne carbonategrains shaped like euhedral-quartz are occasion-ally present (Fig. 4F). They occur inside largeranhedral quartz crystals in the matrix. Spherulitic

Table 2. REE data for selected samples.

Sample No. 89408 89418

La (p.p.m.) 1.99 1.67Ce 4.02 4.22Pr 0.451 0.438Nd 1.43 1.28Sm 0.23 0.202Eu 0.119 0.138Gd 0.277 0.354Tb n.d. n.d.Dy 0.164 0.289Ho 0.034 0.066Er 0.088 0.180Tm n.d. n.d.Yb 0.084 0.155Lu 0.018 0.027(La/Nd)N 1.23 1.15(La/Yb)N 1.86 0.85

Note: n.d. � not determined.



structures (quartz microspheres), from 5 to 30 lmin diameter, are also present in quartz-®necarbonate matrix and rarely in carbonaceous

portions. Two adjacent spherules are often con-nected with each other, and some spherules arecored with unidenti®ed impurities. Spherules

Fig. 3. Photomicrographs of spherulitic extinction ®gures of microcrystalline quartz masses. Scale bars are 500 lm.Crossed polars. (A) Spherulitic extinction ®gures in irregular small voids, where a single spherule is observed. (B)Irregular void with plural spherules growing inward and showing swapped rim texture (arrow). The centre of void is®lled with megaquartz-mosaic. (C) Spherulitic ®gures in voids parallel to local bedding. They show sharp contactsand swapped rim texture (arrows). (D) Spherulitic ®gures in interstices of breccia (vein stockwork). In the upper rightcorner, the inset shows the sharp outline of a brecciated fragment (plane polarized light).

Fig. 4. Photomicrographs of microstructures in the cherts. Scale bars for (A), (B), and (E), and for (C) and (D) are500 lm and 100 lm, respectively. Scale bar for (F) is 200 lm. Plane polarized light. (A) Subvertically oriented andfan-shaped structure composed of replaced spindle-shaped minerals in oxide-rich laminated chert from Unit 1. (B)Carbonaceous black chert from Unit 3 containing very ®ne carbonaceous materials (carbonaceous dusts). Carbona-ceous clot (arrow) is also found. (C) Carbonaceous chert fragment from Unit 4 displaying spongy texture. Dark partscontain abundant carbonaceous microgranules, possible relicts of microorganisms. (D) Thin carbonaceous layer inchert from Unit 4. Laminae are composed of carbonaceous spheroids and ellipsoids up to 40 lm in length (arrows).These spheroids and ellipsoids are thickly walled with carbonaceous matter. (E) Alternation of thin layers composedof carbonates (grey) and of hematite and goethite (black) in carbonate-rich laminated chert. The arrows indicatelateral transition from carbonate-rich portion to a hematite and goethite-rich one within laminae. (F) Perturbation oflayer showing mosaic structure in carbonate-rich laminated chert. The mosaic units closely packed in the unper-turbed portion (right) are of irregular shape, whereas those in the perturbed portion (left) are occasionally shaped likeeuhedral quartz (arrow).



walled with ®ne carbonate grains are also com-mon. The mosaic structure and the spheruliticone described here are quite similar to those inhematitic cherts of the Warrawoona Group in thePilbara Block (Sugitani, 1992) and Archean he-matitic cherts (jasper and jaspilite) in India

(Spencer & Percival, 1952; Majumder & Chakra-borty, 1977).

Three additional microstructures in Unit 4 are:(1) nodular interlaminar structure; (2) brecciatedlaminae showing internal development of euhe-dral megaquartz; and (3) spindle-shaped cavities



(Figs 5 and 6). The nodular interlaminar struc-tures composed of microcrystalline quartz aresimilar to those reported from North Pole Chert(Buick & Dunlop, 1990; Fig. 3(c)). The brecciatedlaminae are characterized by internal develop-ment of euhedral-megaquartz grains that contain®ne carbonate grains inside and occasionallydisplay zonation (Figs 5A, 6). Pore spaces be-tween euhedral-megaquartz grains are ®lled withmicrocrystalline quartz. Fine and spindle-shapedcavities are commonly found in carbonate-richlayers (Fig. 5B) and in the matrices of brecciatedlaminae.

GEOCHEMICAL CHARACTERISTICS

Most cherts from Ord Ranges consist dominantlyof SiO2 and subordinately of Fe2O3* (total Fe asFe2O3). Concentrations of SiO2 and Fe2O3* varyfrom 76á4% to 99á4% and 0á1% to 22á9%, respec-

tively (Table 1). Concentrations of Al2O3 andTiO2 range up to 1á8% and 0á6%, respectively.Concentrations of other constituents are generallylower than 0á1%, except for 0á17% MgO, 0á23%CaO and 0á14% P2O5 in a carbonaceous blackchert (No. 93423). Some samples from Unit 4 withhigher FeO concentrations (> 1á3%) tend to beenriched in CO2 (total C as CO2 > 0á7%).

Grey cherts (Unit 2 samples) are discriminatedclearly from the other samples in terms of chem-ical compositions; the concentrations of TiO2,Al2O3, Cr, Ni, Zn, Rb and Zr of the grey chertstend to be higher than those for the other cherts(Figs 7 and 8). Additionally, the values of Al2O3/TiO2 for the grey cherts are signi®cantly lowercompared with those of the other cherts (Fig. 9).

Concentrations of rare earth elements weredetermined for two samples from Unit 4 (Ta-ble 2). The REE-patterns normalized to NASC(North American Shale Composite) are character-ized by slight enrichment in heavy rare earth

Fig. 5. Photomicrographs of sedi-mentary structures and relict grainsof carbonate-rich laminated chertsfrom Unit 4. Plane polarized light.Scale bars for (A) and (B) are 500 lmand 200 lm, respectively. (A) Brec-ciated laminae characterized by in-ternal development of euhedralmegaquartz. (B) Spindle-shapedcavities in the matrix composed of®ne carbonate particles and micro-crystalline quartz. A good exampleis indicated by arrow.



elements (HREE) and a positive Eu-anomaly. Thepatterns are, on the whole, similar to those ofbanded iron-formation and banded chert from theGorge Creek Group previously reported by Derry& Jacobsen (1990) and Minami et al. (1995), butour samples show more LREE (light rare earthelement)-enrichment and a slightly positiveCe-anomaly (Fig. 10A). The values of (La/Nd)N

and (La/Yb)N in the analysed samples are 1á23and 1á86 for No.89048 and 1á15 and 0á85 forNo.89148, respectively, and are relatively higherthan those of banded iron-formation ((La/Nd)N � 0á8, (La/Yb)N � 0á5; Derry & Jacobsen,

1990) and banded chert ((La/Nd)N � 1á2, (La/Yb)N � 0á6; Minami et al., 1995) (Fig. 11). TheREE patterns of all these samples from the GorgeCreek Group are different from those of hematiticcherts in the Warrawoona Group (Sugitani, 1992;Minami et al., 1995); the latter shows more HREE-enriched patterns (Fig. 10B). (La/Nd)N and (La/Yb)N-values of the samples from the Gorge CreekGroup tend to be higher than those of hematiticcherts from the Warrawoona Group (Fig. 11).

INTERPRETATIONS

Microstructures related to diagenesisof silica, and origins of ®ne carbonates

Quartz crystals forming mosaic structures in thechert layers at Ord Ranges occur in either ananhedral or euhedral habit (Fig. 4F). The pres-ence of euhedral quartz, as identi®ed by bipyra-midal distribution of ®ne carbonates, indicates aclose relation between the mosaic structure andsilica diagenesis (and crystallization). The pre-cursors of mosaic units are considered to havebeen clots of silica gel formed by gradual dehy-dration. In the portions where perturbation ofsediments occurred, the clots of silica gel weredispersed in porous portions and were subse-quently crystallized to euhedral quartz, whereasin the unperturbed portions, the more closelypacked clots of silica gel were crystallized toirregular and anhedral quartz (Fig. 4F). Mosaicstructures have previously been reported fromArchean hematitic cherts in India by Spencer &Percival (1952) and Majumder & Chakraborty(1977), who also suggested that the structure

Fig. 7. Stratigraphic variations of TiO2, Al2O3, and Fe2O3* (total iron as Fe2O3). Stratigraphic column shown in theleft is referred to Fig. 2. Plots of 0á005% for TiO2, 0á05% for Al2O3 and 0á5% for Fe2O3* include data whoseconcentrations of TiO2, Al2O3 and Fe2O3* are lower than 0á005%, 0á05% and 0á5%, respectively.

Fig. 6. Photomicrograph of euhedral megaquartz inbrecciated laminae in carbonate-rich laminated chertfrom Unit 4. Scale bar is 500 lm. Plane polarized light.Euhedral megaquartz grains ¯oating inside the laminaedisplay zoning. Inclusions are mostly carbonates withminor amounts of hematite, goethite and pyrite.



was formed by gradual shrinkage (dehydrationand consolidation) or desiccation of a colloidalsilica gel.

The spherulitic structure (quartz microsphere),is also most likely formed by diagenesis orcrystallization of silica. The possibility of biogen-ic origin is excluded by the size distributions ofthe quartz spherules (15�25 lm) which are most-ly larger than bacterial cells and are similar tothose of inorganically synthesized quartz micro-spheres (Oehler, 1976). Oehler (1976) showedexperimentally that silica gel crystallizes in theform of chalcedonic spherules under hydrother-mal conditions (3 kb, 100±300°C, 25±5200 hrs)and concluded that the spherulitic structure inPrecambrian cherts is not biogenic in origin.Spherulitic amorphous silica was found also fromhydrothermal silica chimneys of the GalapagosSpreading centre by Herzig et al. (1988), whosuggested that suf®cient supersaturation for thedeposition of amorphous silica was achieved bycooling of initial hydrothermal water.

The presence of mosaic and spherulitic struc-tures indicates that silica was an important

component of the primary sediments and thatthe ambient water mass was possibly super-saturated with silica. These two types ofmicrostructure possibly re¯ect ¯uctuations ofphysicochemical conditions controlling diagene-sis and crystallization of silica. In either case, theformation of these microstructures is believed tohave occurred during an early stage of diagenesis.These structures are present in both unperturbedand adjacent perturbed portions of the sedimentthat are cut by later common quartz veins,indicating that they were formed before fracturingof sediments.

The microstructures are generally accompaniedby ®ne carbonate grains, and these carbonates areconsidered to be primary precipitates or productsof early diagenesis. XRD analyses for severalsamples treated by dilute HF show that sideriteis present as the carbonate mineral, which issupported by the fact that samples with high FeOconcentrations (> 1á3%) tend to be enriched inCO2 (> 0á7%; Table 1). Fine siderite grains andsiderite microspheres in Archean to Early Proter-ozoic cherts and banded iron formations are

Fig. 8. Stratigraphic variations in concentrations of Cr, Ni, Zn, Sr, Rb and Zr. Stratigraphic columns shown in theleft are referred to in Fig. 2. Plots of 0á5 p.p.m. indicate the samples whose concentrations of the elements are notdetected.



generally regarded as primary to early diageneticprecipitates, whereas rhombic (sparry) carbonates(dolomite, ankerite and siderite) are diagenetic toneomorphic products (e.g. Dimroth & Chauvel,1973; Carrigan & Cameron, 1991; Winter &Knauth, 1992).

Post-depositional silica-enrichment

Post-depositional silica enrichment of the chertlayers is deduced from the presence of spheruliticextinction ®gures of the microcrystalline quartzmass (Fig. 3) and the internal development ofeuhedral megaquartz crystals (Figs 5A and 6). Assummarized by Simonson (1985), the spheruliticextinction ®gures of silica can form in severalways such as competitive growth duringvoid-®lling by precipitation of ®brous crystals(Roedder, 1968), replacement of other materials(Milliken, 1979), and neomorphism of amorphousor opaline silica (White & Corwin, 1961). In thechert layers at Ord Ranges, most of the extinction®gures, if not all, were formed by rapid void-

®lling precipitation of ®brous quartz crystals. Areplacement origin for the extinction ®gures is notpossible, because the portions showing spheruli-tic extinction do not contain relict rhombic grainsand other impurities that abundantly occur inadjacent portions (Simonson, 1985). Their regularmutual boundaries and extinction behaviour(Fig. 3C) also suggest that they were not formedby replacement of other materials (Simonson,1985). Some spherulitic extinction ®gures mighthave been formed by neomorphism of opaline oramorphous silica, but the inward growth ofspherulitic ®gures with megaquartz-®lled centresin voids implies their growth into open spaces(Fig. 3B,D). Furthermore, reliable evidence forsilica precipitation in cavities is the internaldevelopment of euhedral-megaquartz withinbrecciated laminae in Unit 4 (Figs 5A and 6).Such euhedral-megaquartz in unmetamorphosedrocks typically represents growth into free spacessuch as microcavities and fracture openings(Knauth, 1994). The quartz grains often showzoning identi®ed by variation in amounts ofinclusions (Fig. 6); the amount of inclusions(mostly ®ne carbonate) decreases from core torim of individual euhedral quartz crystals. Spacesamong the euhedral-megaquartz crystals are ®lledwith microcrystalline quartz. These convergentlines of evidence indicate that the euhedral-megaquartz is not recrystallized microcrystallinequartz.

The two different styles of postdepositionalsilica precipitation in cavities described above(i.e. ®brous crystals and euhedral-megaquartz)may be related to different silica concentrationsin solutions from which the two silica phaseswere precipitated. The ®brous crystals suggest arapid growth of quartz from interstitial watershighly supersaturated with quartz (Simonson,1985; and references therein). In contrast, theenhedral- and zoned-megaquartz shows slow andintermittent growth of quartz from solutionswhose silica concentration was not high enoughto allow rapid growth of ®brous crystals.

An additional mechanism for postdepositionalsilica enrichment is `metasomatic silici®cation',which has been often emphasized for cherti®cat-ion of nonsiliceous Archean sediments and igne-ous rocks (Lowe & Knauth, 1977; Paris et al.,1985; DuchacÏ & Hanor, 1987; Hanor & DuchacÏ,1990). In many portions of the cherts from OrdRanges, rhombic carbonates and spindle-shapedminerals are replaced by microcrystalline quartz.Some fragments in vein stockworks are peripher-ally or entirely discoloured and blurred. These

Fig. 9. Stratigraphic variation of Al2O3/TiO2 value.Stratigraphic column in the left is referred to in Fig. 2.Plots with arrows show samples whose TiO2 concen-trations are below the detection limit (0á005%); theirAl2O3/TiO2 values are calculated on the basis of0á005% TiO2. Al2O3/TiO2 value of 4 indicated by thevertical line shows the assumed minimum value ofterrestrial igneous rocks (Sugitani et al., 1996).



features suggest that the cherts have sufferedsome metasomatic silici®cation. However, thetiming and mechanism of replacement and theirrelation to the precipitation of ®brous or euhedralquartz in cavities are obscure.

Origins of iron-oxide/hydroxides(hematite and goethite)

Iron-oxide/hydroxides (hematite and goethite) aresubordinate components in the chert layers, and

can be regarded as secondary minerals becausethey do not occur as very ®ne dusts, typical ofprimary precipitates in oxide-facies banded ironformations (Dimroth & Chauvel, 1973). Insteadthey occur as granules, ¯akes, irregular aggregatesor pseudomorphs. The secondary origin of hem-atite and goethite is also strongly suggested by thelateral discontinuity of hematite- and goethite-rich laminae as shown by Fig. 4(E). The chertsalso contain abundant rhombic grains of hematiteor goethite, which appear to be oxidized iron-bearing carbonate (ferroan dolomite, ankerite andsiderite; Lougheed, 1983).

Accordingly, some hematite and goethite inthe chert layers were formed by in-situ oxidationof iron-bearing carbonates. More complex pro-cesses, in addition, participated in the formationof hematite- and goethite-rich laminae. Theselaminae contain abundant spindle-shaped cavi-ties and similar shaped hematite grains, indicat-ing precipitation of hematite in cavities thatwere formed by dissolution of the spindle-shaped mineral grains. Primary constituent min-erals were dissolved by penetration of surfacewaters containing iron; simultaneously or subse-quently, hematite or goethite precipitated in thecavities to form the iron oxide/hydroxide-richlaminae.

Occurrence of sulphate mineralsand their dissolution and replacement

Relict grains, nodules and laminae of sulphateminerals are commonly found in the chert layers atOrd Ranges. Among these three occurrences,spindle-shaped relict grains are the most abund-ant. They are either replaced by silica, hematiteand goethite or remain as cavities. In the carbonate-

Fig. 10. NASC (North American ShaleComposite)-normalized REE patternsfor cherts and banded iron-formationfrom the Gorge Creek Group (A) and theWarrawoona Group (B). NASC data arequoted from Goldstein & Jacobsen(1988). (A) Two samples of carbonate-rich laminated chert (solid circles)studied here and banded iron-formation(triangle, Derry & Jacobsen, 1990) andbanded chert (square, Minami et al.,1995) from the Gorge Creek Group. (B)Red hematitic cherts from Marble Barand Pillow Hill of the WarrawoonaGroup. Data are quoted from Sugitani(1992) and Minami et al. (1995).

Fig. 11. Histograms of (La/Nd)N and (La/Yb)N valuesfor cherts and iron-formation from the Gorge CreekGroup and the Warrawoona Group. Solid boxes showthe samples from Ord Ranges, boxes with oblique linesshow other chert and iron-formation samples from theGorge Creek Group (the same data as Fig. 10A), andopen boxes show samples from the Warrawoona Group(the same data as Fig. 10B).



rich laminated cherts, thin layers containingabundant small (up to 50 lm in length) spindle-shaped relict grains are observed (Fig. 5B). Sim-ilar-shaped and larger (up to 200 lm in length)relict grains are present in the oxide-rich lami-nated cherts, where they display mound-likeaggregations or subvertically oriented and fan-shaped textures (Fig. 4A). Spindle-shapedpseudomorphic or relict grains have often beenreported from Archean cherts and iron formationsand are generally interpreted as gypsum replacedby silica or iron oxides (e.g. Lowe & Knauth, 1977;Lougheed, 1983), although Grotzinger & Kasting(1993) critically noted that `gypsum pseudo-morphs' in some Archean cherts were possiblyderived from other minerals such as magnesiteand aragonite.

The nodular structure composed of microcrys-talline quartz could be interpreted as replacedsulphate nodule on the basis of their morpholog-ical resemblance to Permian aphanitic anhydritenodules in dolomitic sediments (Tucker, 1991;Fig. 54). Buick & Dunlop (1990) also reported asimilar interlaminar structure from North PoleChert in the Pilbara Block and implied that thestructure is a possible equivalent to the chicken-wire structure (sulphate nodules) diageneticallyformed in modern evaporitic sediments.

The internal structure of brecciated laminae(Figs 5A and 6) is quite similar to Mississippiansilici®ed sulphate nodules (Milliken, 1979:Fig. 9(C)). In the Mississippian nodules, euhe-dral-megaquartz crystals with anhydrite inclu-sions are isolated inside cavities ®lled withsecondary calcite or dolomite, whereas theeuhedral-megaquartz crystals in the brecciatedlaminae contain carbonate inclusions and areisolated in the quartz matrix (Fig. 6). Despite sucha discrepancy in mineral compositions, the inter-nal growth of euhedral megaquartz indicates thatboth the layers and the nodules were oncecavities. The brecciated laminae in the chertstudied here are most likely interpreted to beprimarily composed of soluble minerals such asgypsum and/or anhydrite.

Microbial textures

In carbonaceous black chert from Unit 3, carbo-naceous matter generally occurs as very ®ne dustor clot (Fig. 4B) and no possible microbialtextures could be found. In carbonaceous frag-ments or thin layers of the carbonate-rich lami-nated cherts, on the other hand, some texturesthat possibly resulted from biogenic activities are

well preserved. Although individual bacterialcells could not be identi®ed, the abundant mic-rogranules in the spongy structure (Fig. 4C) maybe relicts of microorganisms. The spongy struc-ture, which has not previously been reportedfrom Archean cherts, appears unlikely to beformed by inorganic processes such as silicadiagenesis and agitation of sediments. The mostprobable explanation for this texture is a bacterialcolony. Carbonaceous laminae composed ofspheroids and ellipsoids, in addition, may repre-sent another type of bacterial colony (bacterialmat). The relatively large spheroids and ellip-soids (up to 40 lm) shown in Fig. 4D are similarto textures in Archean carbonaceous cherts fromSouth Africa reported by Walsh (1992), whointerpreted that the textures were bacterialsheaths. Many microfossils have been reportedfrom Archean cherts older than those of the GorgeCreek Group (e.g. Dunlop et al., 1978; Walteret al., 1980; Schopf & Packer, 1987). Accordingly,it can be concluded that the carbonaceous matterin the cherts at Ord Ranges has been derived frommicroorganisms.

Mineralogy and geochemistry of grey chert

Abundant ®ne TiO2 particles, detrital quartz andTiO2-rich sand grains conspicuously characterizethe grey cherts, and the mineralogical featuresare explained by contributions of terrigenousdetrital materials. This is supported by geochem-ical characteristics of the grey cherts; as shownin Figs 7 and 8, they contain more Al2O3, TiO2,Cr, Ni, Zn, Rb and Zr than the other cherts.Among these elements, Al, Ti, Cr and Zr aregenerally considered to be immobile elementsincorporated in detrital phases of sediments. Thegrey cherts also contain thin layers that consistof intraclastic grains, some of which are broken,suggesting reworking and transportation of pre-viously deposited sediments.

Values of Al2O3/TiO2 in the grey cherts aregenerally lower than 4 and the lowest is about 1(Fig. 9). Although Al2O3/TiO2 values in sedi-ments are generally considered to representvalues in the source rocks of detrital materials(e.g. Sugisaki, 1978; Sugisaki et al., 1982;Yamamoto, 1983; Sugisaki, 1984; Garcia et al.,1994; Wintsch & Kvale, 1994), such low Al2O3/TiO2 values have not yet been reported forterrestrial igneous rocks (Sugitani et al., 1996).Hence, the values show Al-Ti fractionation insource areas, which will be discussed in detail inthe later section.



COMPOSITIONS OF PRIMARYSEDIMENTS AND SEDIMENTARYENVIRONMENT

We infer that the chert layers, except Unit 2, wereprimarily composed of chemically precipitatedsilica, carbonates and sulphates (gypsum and/oranhydrite), plus organic matter. The examinedcherts show some textures suggestive of postde-positional enrichment of silica, such as precipi-tation in cavities and metasomatic silici®cation.The abundance of primarily precipitated silicarelative to other components is obscured, but itsamount was signi®cantly large in the carbonate-rich laminated chert of Unit 4, because micro-structures related to very early silica diagenesisare abundantly observed in them. The dispersed®ne carbonate grains contained in the chertsconsist mostly of siderite and the original dom-inant carbonate phase could have been siderite.

Mineral associations in the chert layers, on thewhole, possibly correspond to those of modernevaporitic sediments (carbonate±sulphate facies,e.g. Tucker, 1991). Unit 1, 3 and 4 representchemical and biogenic sedimentation under eva-poritic environments, whereas Unit 2 showsclastic sedimentation. Grey cherts in Unit 2contain sand-size detrital quartz, altered Fe±Tioxides and intraclastic grains, indicating in¯owof terrigenous materials to the site of deposition.Evaporitic sediments (cherti®ed by silici®cation)have often been reported from volcanic±sedimen-tary sequences of the Warrawoona Group in thePilbara Block (Lowe, 1983; Boulter & Glover,1986; Buick & Dunlop, 1990). On the other hand,there has been no detailed report of evaporiticsediments from the Gorge Creek Group composedof thick sedimentary successions includingshales, sandstones, conglomerates, cherts andiron-formations. Although the stratigraphic posi-tion of the chert layers studied here in the GorgeCreek Group is still unknown, they potentiallyrepresent sediments deposited in a shallow-waterenvironment.

REE CHARACTERISTICS: IMPLICATIONSFOR SOURCES

The precursors of the chert layers are differentfrom modern equivalents with respect to theprimary precipitation of silica and Fe-carbonate(siderite), which presumably re¯ects uniqueArchean surface environments. In the Archean,hydrothermal alteration of oceanic crust was

more active than today (e.g. Fryer et al., 1979;Veizer et al., 1989), which is deduced from theEu-anomaly commonly found in chemicalsediments such as banded iron formation andferruginous cherts. The contribution of hydro-thermal components to Archean±Early Protero-zoic iron formations and cherts has been empha-sized by many authors (e.g. Dymek & Klein, 1988;Jacobsen & Pimentel-Klose, 1988; Beukes &Klein, 1990; Derry & Jacobsen, 1990; Sugitani,1992; Manikyamba et al., 1993), although a fewauthors suggest a continental origin (e.g. Miller &O'Nions, 1985; Garrels, 1987). The Eu-anomalyfound in the cherts from Ord Ranges (Fig. 10)suggests that the water mass involved in chertformation was enriched in hydrothermally de-rived Si and Fe.

More detailed information about the source forthe chert layers at Ord Ranges is obtained by theirREE patterns. REE characteristics in marinechemical sediments are generally explained by amixing between marine bottom-water (hydrother-mal), surface-water and riverine components(continental). As shown in Fig. 11, (La/Yb)Nvalues in the present samples tend to be higherthan those in iron formation and banded chert ofthe same age: this is attributed to a largercontribution of continental components to thechert layers at Ord Ranges compared with theiron-formation and banded chert studied by Derry& Jacobsen (1990) and Minami et al. (1995). Thisis consistent with our hypothesis that the chertlayers were deposited in a shallow and restrictedbasin. Furthermore, chemical sediments from theGorge Creek Group studied here and by Derry &Jacobsen (1990) and Minami et al. (1995) havemore LREE-enriched characteristics (higher (La/Nd)N and (La/Yb)N values) than the equivalents inthe older Warrawoona Group (Figs 10 and 11).This implies an increasing contribution of conti-nental components to the oceans during thesedimentation of the Gorge Creek Group.

SIGNIFICANCE OF TITANIFEROUSGREY CHERTS

Titanium-enrichment and very low Al2O3/TiO2

values in some Archean cherts were recentlyreported by Sugisaki et al. (1995) and Sugitaniet al. (1996), who examined chert layers of theGorge Creek Group occurring at Point Samson,north-northeast of Roebourne. Sugitani et al.(1996) concluded that the Ti-enrichment result-ed from input of Ti-enriched detrital materials



and suggested that the Ti-enriched detrital ma-terials were produced by extensive chemicalalteration of source rocks, i.e. Al dissolutionand residual accumulation of Ti. Althoughpetrological features of grey cherts from OrdRanges are different in some aspects from thoseof the Point Samson chert layers, the enrichmentof Ti relative to Al is believed to have beencaused by the similar process to that for thePoint Samson chert layers.

Detrital zircon grains are often found in Ti-richlaminae of the Point Samson chert layers, but notin the grey cherts from Ord Ranges. However,covariation between Zr and TiO2 (Figs 7 and 8)suggests the presence of ®ne-grained zircon un-identi®ed under the microscope. Ti-bearing min-eral phases in the Point Samson chert layers areacicular to granular rutile (anatase) and pellet-likeaggregates of Fe±Ti oxides other than ®ne TiO2

particles, whereas those in the grey cherts aremostly ®ne TiO2 particles and Ti-rich sand grains.Ti-rich sand grains are possibly altered Fe±Tioxides such as ilmenite and titano-magnetite thatare the most resistant minerals against chemicalweathering in addition to quartz and zircon(Nesbitt & Wilson, 1992). Morad (1988) andMorad & AlDahan (1986) reported that theseminerals in sedimentary rocks are diageneticallyaltered and authigenic Ti-oxides (anatase andrutile) occur; Fe±Ti oxides (titano-magnetite andilmenite) in sedimentary rocks are successivelyenriched in Ti and depleted in Fe, and ®nallyform pure TiO2.

There has been no previous study of titan-iferous cherts in the Archean, but the accumula-tion of data on Archean cherts should reveal morewidely distributed titaniferous sediments in thePilbara Block and in other Archean cratons.Sugitani et al. (1996) noted that Ti-enriched andAl-depleted detrital materials can be formed bytwo potential mechanisms, i.e. extensive chemi-cal weathering and hydrothermal alteration ofsource rocks. During extensive chemical weath-ering or hydrothermal alteration, most rock-form-ing minerals could be decomposed and quartz,Fe±Ti oxides and zircon would be residuallyaccumulated. Furthermore, if this process hadoperated under acidic condition (pH < 3á9 �soluble point of kaolinite and gibbsite; see alsoBrookins, 1988), Al±Ti fractionation (i.e. dissolu-tion of Al and residual enrichment of Ti) couldhave effectively occurred (Sugisaki et al., 1995;Sugitani et al., 1996).

SIGNIFICANCE OF SIDERITE-SULPHATEASSEMBLAGE

Although ®ne siderite grains contained in the OrdRanges chert layers are primary or early diageneticproducts, the bulk composition of carbonatesprecipitated during evaporation of the ambientwater mass is a problem to be resolved. It issuggested that the precursory presence of calcite(aragonite) and/or dolomite cannot entirely beexcluded. The water mass in the depositionalbasin is assumed to have contained substantialCa2+ and Mg2+, because the cherts contain pseu-domorphs after Ca-sulphates (gypsum and/oranhydrite) and show geochemical and petrologi-cal characteristics indicating the contribution ofcontinental (terrigenous) components (Ca2+ andMg2+). In Phanerozoic evaporitic basins, calcite(aragonite) and/or dolomite generally occur butsiderite does not. However, we emphasize herethat siderite is still important as a candidate for thedominant carbonate phase precipitated duringevaporation. Under particular conditions even inthe presence of Ca2+ and Mg2+, siderite could bepreferentially precipitated and the precipitation ofcalcite (aragonite) and dolomite could be inhibit-ed. Such conditions are inferred from the resultsof previous studies on Archean atmosphere andweathering. In the following discussion, we pres-ent a hypothetical mechanism that could explainthe precipitation of silica, siderite and sulphatesin an Archean evaporitic basin.

On the basis of the presumed high CO2 partialpressure in the Archean (e.g. Owen et al., 1979;Kuhn & Kasting, 1983), some theoretical studiesrecently suggest a slightly lower pH value (6á5�7)for ancient ocean waters than at present (e.g.Krupp et al., 1994), although some other studiesimply a higher pH value (e.g. Kempe & Degens,1985). It is, in addition, implied that the high CO2

partial pressure and acid materials (H2S and HCl)issued from volcanoes generated acid rain andsurface waters (Krupp et al., 1994; Sugitani et al.,1996). If this is the case, the pH value of watermasses in Archean shallow and restricted basinscould be shifted lower than that of open oceans.Under acidic conditions, Ca-carbonates are lessstable compared with Fe-carbonate and Ca-sul-phates (e.g. Brookins, 1988); CaSO4 á 2H2O andFeCO3 are stable at pH 3á9±6á5 and pH 5±7,respectively, whereas CaCO3 is stable atpH > 6á5. Thus, given the water mass had a pHvalue between 5 and 6á5, the precipitation of



Fe-carbonate and Ca-sulphates prior to Ca-car-bonates could have occured. On the other hand,primary or early diagenetic dolomite-formation isrestricted for the following reasons: as suggestedby Baker & Kastner (1981), high dissolved sul-phate concentrations in the water mass, which isdeduced from the abundance of sulphate miner-als in the cherts at Ord Ranges, could haveinhibited primary dolomite precipitation; alsothe former absence of primary calcite (aragonite)discussed above would not allow such dolomitegeneration as replacement of Ca2+ in calcite bydissolved Mg2+.

This model for interpreting mineral associationsof the chert layers at Ord Ranges is simpli®ed andincludes some uncertainties. The stability ®eldsfor CaSO4 á 2H2O, FeCO3 and CaCO3 on pH±Eh

diagrams could be modi®ed by coexisting ions,concentrations of dissolved O2 and CO2 andtemperature. The experimental results of gyp-sum-synthesis by Cody (1979), in addition, showthat lenticular (spindle-shaped) gypsum can notbe precipitated under acidic conditions. However,it is still worthwhile to assume that siderite andgypsum (and anhydrite) were originally precipi-tated from evaporating water mass in an Archeanshallow restricted basin.

ACKNOWLEDGMENTS

We thank the members of the Department ofNatural Science Informatics and the Departmentof Earth and Planetary Sciences, Nagoya Univer-sity. We especially wish to express our gratitudeto Dr K. Suzuki, who advised us on X-ray¯uorescence analyses for major elements and toDr K. Maekawa in Kobe University, who helpedus with X-ray ¯uorescence analyses for minorelements. We are also indebted to Mr S. Yogo andMr T. Nagaoka in Nagoya University, who pre-pared thin sections for samples.

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Manuscript received 27 August 1997; revision accepted28 January 1998.



Archean cherts derived from chemical, biogenic and clastic sedimentation in a shallow restricted...

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