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1 1 The Pilgangoora Greenstone Belt 2Introduction 3 This project was initially conceived as a quantitative study of the effects of 4metamorphism on bio-markers and -signatures, with particular emphasis on isotopes of 5carbon and nitrogen (map of sampling locations, Figure 1). In this respect, and in some 6others, the project was a failure. The factors that hampered completion of this objective 7were: (i) the lack of suitable assemblages allowing quantitative geothermobarometry at 8all grades; (ii) incomplete equilibration and small grain-sizes in low-grade rocks, 9hindering petrological and microprobe analysis; (iii) concentrations of organic matter at 10or below background (contamination) levels, particularly in higher-grade rocks, thwarting

13 15 11 Corg and  Norg isotope mass-spectrometry, macronutrient (CNPS) microprobe analysis

+ 12(Figure 2) and NH4 Fourier-Transform Infrared Spectroscopy (FTIR, Figure 3); and (iv)

13 13the large non-metamorphic variability of kerogen  Corg analyses in individual units. 14 However, because of several unexpected findings in Early Archaean rocks in 15Australia, Greenland and South Africa, focus shifted to geological evidence for the nature 16of ancient sedimentary environments, and biogeochemical processes therein. 17 This introduction serves to give a general overview of Archaean, Pilbara and

18Pilgangoora Belt geology, in increasing level of detail. Focus rests on particular 19geological aspects of deep-water carbonates of the Coucal Formation (further described 20in Chapter 5), and shallow-water carbonates of the Strelley Pool Chert Formation.

2 3 Pilgangoora Greenstone Belt

211. Generalized Archaean Geology 22 Judging from the tenacity and diversity of controversies, our understanding of the 23Earth’s crust during Early Archaean times can fairly be described as insecure. A good 24starting point is to consider how Archaean rocks differ from contemporaneous ones 25(‘inter-era differences’), and what this tells us about the early geosphere. Also worth 26considering are differences between Archaean terranes (‘intra-era differences’). 27 All continents bear Archaean supracrustal exposures (Chapter 1, Figure 2). These 28Archaean rocks represent about 20∙106 km2, or ~ 13%, of approximately 150∙106 km2 of 29continental material exposed today (Burke, 1997). Tradition has seen these divided into 30‘high-grade’ and ‘granitoid-greenstone’ –sequences. 31 High-grade Archaean terrains have commonly experienced middle-amphibolite to 32granulite facies metamorphism, with mineral assemblages equilibrated at ~ 10 ± 1 kbar 33corresponding to depths of 30 - 40 km (Tarney and Windley, 1977; Martin, 1994). 34Together with their considerable lateral extent, this has made them viable candidates for 35pre- and sub-greenstone basement (Kröner, 1981). 36 Most high-grade Archaean terrains have seen complex polyphase deformation, 37frequently under sub-horizontal stress regimes. In these and other respects they have 38often been compared to the batholithic root zones of the main arcs of Mesozoic-Cenozoic 39Cordilleran fold belts (Kröner, 1981). They are geochemically distinct from gneiss- 40migmatite complexes in granitoid-greenstone sequences, not least in being distinctly 41more potassic (Bridgwater and Collerson, 1976). As they have little to tell us about 42ancient depositional environments and life, they shall not be considered further here. 43 441.1. Granitoid - Greenstone Terrains 45 Greenstone belts are deformed volcanic-sedimentary sequences that are typically 46metamorphosed to greenschist facies, and are commonly intruded by slightly younger 47‘granitoid’ bodies. The resulting association is termed a Granitoid - Greenstone Terrain 48(‘GGT’). Globally, such exposed Archaean shields generally comprise ~ 50% 49trondjemite-tonalite-granodiorite bodies (‘TTG’), ~ 20% granitoid plutons, and ~ 10% 50tholeiite flows (Condie, 1993), though with much variability. 4 Pilgangoora Greenstone Belt

51 Volcanic rocks, especially tholeiites, dominate the volcanic-sedimentary 52sequences, with sediments imparting only minor contributions to most greenstone 53successions. Komatiites, though frequently absent and not restricted to the Precambrian 54(Echeverria, 1980; Aitken and Echeverria, 1984), are characteristic. The minerals 55chlorite, epidote and/or actinolite are responsible for the distinctive green appearance of 56greenstone metabasites. 57 In order to account for shallow (~ 3 – 6 km) crustal depths inferred through 58geophysical studies, older estimates of stratigraphic thicknesses on the order of ~ 10 – 20 59km have come to be revised to ~ 5 km, usually by positing tectonic duplication (see 60Chapter 1, Figure 6). 61 The tectonic style of the granitoids typically takes the form of 30 - 150 km 62diameter circular to ovoid domal granite-gneiss batholiths1, with dominant metamorphic 63foliation and both volcanic and sedimentary bedding all dipping steeply away from the 64dome centre, giving rise to the characteristic symmetric, synformal greenstone belt cross- 65sections. 66 In comparison to structurally similar Proterozoic orogenic belts, such as the 1.7 - 671.8 Ga Snow Lake-Flin Flon belt in southeast Monitoba (Bell et al., 1975; Moore, 1977), 68the ~ 2.0 Ga Birimian in West Africa (Burke and Dewey, 1972), and the ~ 1.3 Ga 69Grenville Province in eastern Canada (Sethuram and Moore, 1973), GGTs exhibit smaller 70length:width ratios, shorter wavelength folding, and a noteworthy lack of blueschist- 71facies assemblages. 72 731.2. Granitoids 74 On the basis of contact relationships with surrounding volcanosedimentary 75sequences, GGT-granitoids are divided into three groups (Martin, 1994; Sylvester, 1994): 76gneissic complexes and batholiths [sic], syntectonic plutons, and late discordant post- 77tectonic granite plutons. 78 Gneissic complexes and batholiths, in view of compositional domination by 79tonalite, trondjemite and granodiorite, are often referred to as ‘TTG’s’. These complexes 80tend to contain large infolded remnants of supracrustal rocks, commonly including

51 As in greenstone-belt studies elsewhere, the term ‘batholith’ is employed colloquially, and should not be 6taken to imply intrusive relationships with all surrounding rocks. 7 Pilgangoora Greenstone Belt

81plentiful inclusions of surrounding greenstone belt material. Contacts with greenstone 82belts proper are usually intrusive, and heavily deformed. Geochemically, they resemble I- 83type granites, with strong HREE-depletions and a lack of significant Eu anomalies. 84 Syntectonic plutons display a variety of fabrics, from massive to strongly foliated, 85concordant to discordant. A gradational fabric, massive in the core and increasingly 86foliated towards the margins, can be considered characteristic. Geophysical studies have 87constrained their vertical extent to less than ~ 15 km. Well-developed concordant 88foliation around the margins of many Archaean syntectonic plutons is suggestive of 89forceful diapiric injection. Like TTG’s, syntectonic plutons also geochemically resemble 90I-type granites. It is not uncommon for greenstone felsic volcanics to show strong 91geochemical similarities with contemporaneous granitoids, possibly indicating 92cogenicity. 93 Post-tectonic granites, lastly, are often the only bona fide granites found in 94Archaean terrains, and may be anorogenic. They exhibit massive interiors and discordant 95contacts. Most are of the peraluminous or calc-alkaline variety, resulting from A- or S- 96type melting of sedimentary or igneous rocks in lower crust. 97 There is some indication that the duration of orogenic cycles has changed over the 98Archaean, with Barberton and Pilbara plutonism continuing for ~0.5 Gy after major 99volcanism, while entire cycles of volcanism, sedimentation, deformation and plutonism 100occured in less than 50 My in the Late Archaean, frequently followed by ‘late’ discordant 101granitoids in the following 50 My. 102 103Archaean Basement 104 Seismic studies of Granitoid-Greenstone Terranes have consistently revealed 105crustal thicknesses in the range of 30 to 40 km – much thicker than the 106volcanosedimentary belts. The nature of the ‘basement’ underlying Archaean supracrustal 107sequences has been the subject of intense and prolonged debate, with hypotheses 108spanning the compositional spectrum from komatiitic through basaltic to sialic. It is an 109important question, informing our understanding of the onset of continental 110differentiation and plate tectonics (Blackburn et al., 1985). 8 Pilgangoora Greenstone Belt

111 The compositional dichotomy of GGTs has led some to draw analogies with 112continental vis-à-vis oceanic crust, whereby either (1) ancient tonalitic gneisses represent 113ancient continental-type basement onto which greenstone volcanics were erupted; or (2) 114greenstones were effectively interpreted as proto-ophiolites, representing primitive 115oceanic-type crust into which tonalities, granodiorites and granites intruded (Kröner, 1161981). 117 Those who would have a sialic Archaean basement look to the abundant outcrop 118of ancient granitoid plutons and/or orthogneiss equivalents (Figure 2, see also orthogneiss 119enclaves at Isua in Chapter 4, Figure 1). It has long been noted that many of these 120Archaean plutons, however, exhibit clear intrusive relationships with pre-existing 121volcanic-sedimentary sequences, raising the possibility that they are genetic derivatives 122thereof (Percival and Card, 1983), and inducing some to posit basement(s) of mafic or 123even ultramafic affinities (Glikson and Jahn, 1985; Jensen, 1985). 124 Basal remelting and recycling of a continually basalt-thickened crust has also 125been considered (Kröner, 1981). Glikson (1995), on the basis of REE and LIL element 126data, has suggested that early Archaean GGT’s were produced through a process of 127‘sima-to-sial transformation’. In this model, mantle melting produced mafic-ultramafic 128volcanic units in the first stage, with minor felsic units produced through fractionation. 129Partial melting of this mafic crust produced tonalite-trondhjemite magma in the second 130stage. Finally, ensialic anatexis gave rise to LIL-rich granites-adamellites which formed 131‘hoods’ above tonalite-migmatite batholiths, together with the characteristic discrete 132intrusions. 133 Little is known about the tectonic processes operating on the early Earth, and 134explanations for the stress, strain and metamorphic regimes in Granite-Greenstone 135Terrains span the kinematic spectrum from extensional through vertical to compressional. 136Without exception, Archaean greenstone terranes have undergone multiple episodes of 137(both ductile and brittle) deformation. There is no general agreement as to the role and 138veracity of vertical forces accompanying pluton intrusion. Canonically, metamorphic 139grade ranges from greenschist (or locally prehnite-pumpellyite) to amphibolite facies, 140with higher grades occurring around margins of belts in pluton aureoles. Assemblages 141equilibrated at low pressures. 9 Pilgangoora Greenstone Belt

142 Early Archaean deformation, as found in the Barberton and the Pilbara, was 143classically attributed either to vertical tectonics through gravity-driven diapiric uprise of 144granitoids (Schwerdner and Lumbers, 1980), or to plate-tectonic induced far-field 145horizontal stresses effected on a layered granite-greenstone crust. Appeal has also been 146made to crustal thickening. 147 1481.3. Greenstones 149 Several authors have commented on the progressive development of volcanism 150within greenstone belts, with initial komatiitic flows progressing through more 151widespread tholeiitic basaltic units, and finally coming to be overlain by intermediate 152and/or felsic calc-alkaline units (Allard et al., 1985; Blackburn et al., 1985; Glikson and 153Jahn, 1985; Jensen, 1985; Padgham, 1985; Thurston et al., 1985). Accompanying this 154alleged trend of ultrabasic-to-acid volcanism are increases in the relative amount of 155volcaniclastics to lave flows, and in (still highly subordinate) layered chert. These parallel 156trends are thought to indicate the evolution from voluminous oceanic eruption of a mafic 157lava plain or plateau to the formation of more localized arc-like calc-alkaline and 158tholeiitic stratovolcanoes and intervening sedimentary basins. However, close inspection 159of greenstone belt stratigraphy shows that exceptions to this canonical trend are the rule. 160 Early Archaean sedimentation can be divided into detrital and (bio)chemical, with 161biology playing an uncertain role in the formation of the latter. Detrital sediments are 162dominated by volcanoclastics, with rare evidence for a terrigenous component (e.g. Isua 163Supracrustal Belt meta-turbidites, Chapter 4; Coonterunah Subgroup meta-pelites, 164Chapter 5). Mid- and Late- Archaean greenstone belts often contain significant 165thicknesses of clsatic turbidites and, in some instances, terrestrial fluvial to deltaic 166terrigenous sediments. 167 Chemical sediments comprise cherts, carbonates, evaporites and banded-iron 168formation.Silicification is widespread in Archaean volcano-sedimentary sequences. 169 170Kerogen 171 Insoluble hydrocarbon material (‘kerogen’) is a common component in Early 172Archaean greenstone-belt cherts, and is found in three different geological contexts: (i) 10 Pilgangoora Greenstone Belt

173Within bedded sedimentary units; (ii) Within cement-like pre-diagenetic chert; and (iii) 174Within cross-cutting post-diagenetic dykes. 175 Bedded sediment-hosted kerogen is found in a wide variety of sediments, but is 176most abundant in laminated carbonates (see thin-section photomicrographs of carbonate- 177hosted kerogen, Chapter 5) and cherts (outcrop photograph of laminated kerogenous chert 178in SPC, Figure 4 (j)). It can occur in a wide variety of textural associations, including as 179coatings on grains (thin-section photomicrographs of kerogen-coated pyrite in SPC, 180Figure 4 (a – c)), in association with magnetite, Concuding Chapter, Figure 1, as matrix 181material (Figure 4 (h, i), see also remobilized matrix kerogen in chalcedony in Figure 1828(m, n), Chapter 1), and in variably reworked detrital and breccia clasts. 183 Kerogenous cement-like pre-diagenetic chert is common in neptunian fissures 184underlying unconformity surfaces (Figure 4 (k)). It is less common within the bedded 185cherts, where it can be recognized by its production of soft-sediment deformation 186structures in associated sedimentary lithofacies prior to lithification (Figure 4 (f)). 187 Cross-cutting relationships show that much of the kerogenous chert in Early 188Archaean terrains likely post-dates sedimentation and fills cross-cutting chert dykes 189(outcrop photographs of cross-cutting kerogenous chert in the Hooggenoeg Formation, 190Barberton: Figure 4 (d, e); Schopf putative microfossil locality, Chert VII, Kelly Group, 191Pilbara: Figure 4 (g)). Great care is needed in discriminating between these kerogen 192sources when drawing biological inferences from the kerogen (Buick, 1984). 193 11 Pilgangoora Greenstone Belt

1942. Regional Geological Setting: The Pilbara 195 The ~193000 km2 Pilbara Craton comprises a ~ 3.655 – 2.85 Ga Archaean 196granitoid-greenstone terrane (‘GGT’) together with unconformably overlying volcano- 197sedimentary sequences of the Late Archaean to Early Proterozoic 2.77 – 2.40 Ga Mount 198Bruce Supergroup of the Hamersley Province (Trendall, 1990). The craton is bounded at 199its eastern margin by the Proterozoic Paterson Orogen, at its southern and southeastern 200margin by the Proterozoic Capricorn Orogen and Proterozoic Ashburton, Bresnahan and 201Bangemall Basins, at its western margin by the Permian Carnarvon Basin and at its 202northeastern and northern margins by the Silurian-Devonian Canning Basin and the 203Indian Ocean (Figure 5). 204 Hamersley rocks once overlying the northern third of the craton have been largely 205eroded to expose the underlying GGT lithologies. Archaean units of the craton lie in a 206belt stretching roughly 500 km east-west and 200 km north-south (Figure 6). North- to 207northeast-trending crustal-scale strike-slip faults dissect the craton, some of which are 208used to define the West-, Central- and East- Pilbara structural domains, with rocks older 209than 3300 Ma found only in the latter (Barley and Bickle, 1982). 210 Domical, ovoid, multi-component gneissic-to-granitic suites ~ 3655 - 2850 Ma in 211age comprise the ‘granitoid’ component of the terrane, visible on Figure 6 as paler areas. 212The term ‘granitoid’ is applied colloquially to designate this heterogeneous component, 213which comprises gneissic units of dominantly granodioritic composition, foliated 214supracrustal relics, and foliated-to-massive syn- to post-tectonic plutons exhibiting 215intrusive relationships with both surrounding gneisses and greenstones. Seismic and 216gravity studies reveal that these granitoids extend to nearly 15 km beneath the surface 217(Drummond, 1988). 218 The earliest granitoid phases are dominated by tonalite, trondhjemite and 219granodiorite, with granite sensu strictu gaining prominence over time. Granitoids record a 220polyphase and prolonged history of intrusion and deformation, with region-wide events at 2213470 - 3400 Ma, 3325 - 3290 Ma, 3270 - 3235 Ma and 2940 - 2920 Ma (Nelson et al., 2221999). Two older xenolithic phases, a ~ 3655 Ma gneissic xenolith in the Warrawagine 223Complex (Nelson et al., 1999) and a ~ 3578 Ma gabbroic anorthosite enclave in the Shaw 224Complex (McNaughton et al., 1988), have also been discovered. 12 Pilgangoora Greenstone Belt

2252.1. The Pilbara Supergroup 226 Granitoid-gneiss domes are separated by narrow, steeply dipping almost 227contemporaneous ~ 3515 – 2950 Ma volcanosedimentary greenstone belts of the Pilbara 228Supergroup, which are confined to the upper ~ 5 km of crust (Drummond, 1988). The 229relationship between granitoids and interspersed greenstones is complex. Intrusive 230granitoid contacts and broadly coincident felsic volcanism in greenstones are emphasized 231by workers favoring a role for vertical diapiric granitoid emplacement (Collins, 1989; van 232Kranendonk and Collins, 1998), while faulted or sheared contacts are put forward by 233others in support of a dominant role for horizontal tectonics in the juxtaposition of 234granitoids and greenstones (Bickle et al., 1980; Bettenay et al., 1981; Bickle et al., 1985). 235 The greenstone belts characteristically form tight, upright, shallowly plunging 236synclines with vertical to subvertical limbs oriented parallel to regionally trending belts. 237The McPhee and North Pole Domes, where sedimentary bedding steepens gradually and 238radially away from domal centers, provide important structural exceptions (Hickman, 2391983, 1984). Some workers argue that greenstone dips gradually decrease with 240stratigraphic height, which in their view suggests deposition in thickening wedge-shaped 241carapaces adjacent to growing granitoid diapirs (Hickman, 1975; Hickman, 1983, 1984; 242van Kranendonk, 2000; van Kranendonk et al., 2002). Greenstone metamorphism ranges 243from prehnite-pumpellyite to mid-amphibolite facies conditions, with the majority of 244rocks bearing assemblages indicative of peak metamorphism at greenschist facies 245(Terabayashi et al., 2003). 246 Greenstones comprise a restricted range of lithologies, overwhelmingly (~75 – 24790%) dominated by mafic volcanic rocks of an extrusive origin, many of which were 248lithified in a sub-aqueous environment. In the eastern Pilbara, is has been proposed that 249extensive sheets of mafic lavas gave rise to basins and shallow water platforms, in which 250partly sub-aerial calc-alkaline volcanic centres controlled major topographic relief 251(Barley et al., 1979; Barley, 1980). Pyroclastic and epiclastic volcanic sediments of calc- 252alkaline affinity formed sheets around these elevated volcanic centres and are interleaved 253with the mafic lavas. 254 Very low-grade magnesian basalts commonly exhibit relict igneous 255clinopyroxene, which is replaced by tremolite at higher grades. Komatiites (MgO > 18%) 13 Pilgangoora Greenstone Belt

256exhibiting extrusive spinifex textures are relatively scarce in Pilbara greenstone 257sequences, and where they occur are closely associated with magnesian basalt. Pilbara 258komatiites are characterized by the assemblage antigorite-tremolite-chlorite  olivine, and 259commonly also contain carbonate. Intermediate to felsic volcanic flows and gabbroic-to- 260doleritic sills comprise most of the remaining igneous outcrop. 261 Emphatically subordinate to these extrusive and hyperbyssal igneous units are a 262diverse array of relatively thin but laterally extensive and continuous rocks of (for the 263most part) uncontroversially sedimentary origin. These sedimentary lithofacies, which 264occur together in distinct packages within much thicker volcanic piles, have commonly 265undergone extensive chemical alteration but locally exhibit good textural preservation. 266 The stratigraphical nomenclature of the Pilbara Supergroup has changed 267appreciably over time, and has been complicated by the challenge of correlation across 268both structural domains and greenstone belts. (Stratigraphic table, Figure 7). Four stacked 269unconformity-bound stratigraphic intervals are presently recognized by the Geological 270Survey of Western Australia, consisting of (i) the ~ 3.515 – 3.427 Ga Warrawoona 271(formerly Coonterunah) Group; (ii) the ~ 3.40 - 3.315 Ga Kelly (formerly Warrawoona) 272Group; (iii) the ~ 3.24 Ga Sulphur Springs Group; and (iv) the ~ 3.2 – 2.0 Ga Gorge 273Creek Group (van Kranendonk, 2006). 274 Exposures reveal up to 10 km of autochtonous stratigraphy, although thickness 275varies considerably from belt to belt. Stratigraphic correlation between Warrawoona units 276is fraught with difficulty and the possibility that the Warrawoona Group may represent a 277regional amalgamation of broadly coeval but distinct greenstone successions cannot 278currently be ruled out (Krapez, 1993). 279 The oldest greenstones are found in the Coonterunah Subgroup, the lowermost 280subgroup of the Warrawoona Group. Lithologically, it is dominated by extrusive mafic 281rocks consisting mostly of tholeiitic to magnesian basalt, with lesser felsic volcanic units 282and but volumetrically minor cherts. Rare volcanoclastic and carbonate sedimentary units 283are locally preserved (Chapter 5). It has been recognized only in the Pilgangoora Belt, 284where it attains a maximum stratigraphic thickness of ~6500 m and a lateral extent of ~50 285km, and is unconformably overlain by the Strelley Pool Chert and Euro Basalt formations 286described below. The lower contact of the Coonterunah Subgroup, where exposed, is 14 Pilgangoora Greenstone Belt

287intruded by granitoids of the Carlindi Suite (outcrop photos of tectonized and intrusive 288contacts, Figure 18). 289 Elsewhere in the East Pilbara structural domain, the upper Warrawoona Group 290represents the lowermost expression of the Pilbara Supergroup. It is dominantly 291composed of tholeiitic and magnesian basalt and gabbro with significant felsic volcanic 292and volcanoclastic units, minor localized ultramafic units, and minor but widespread 293cherty sedimentary units. 294 The 3238 - 3235 Ma Sulphur Springs Group unconformably overlies the Kelly 295Group. It is confined to the supracrustal belts immediately adjacent to the Strelley 296Granite, and finds most prominent expression in the Soanesville and Pincunah Belts. An 297eruption age of ~3235 Ma, coeval with the adjoining Strelley Granite, has been obtained 298for the Kangaroo Caves Formation at the stratigraphic top of the Sulphur Springs Group. 299Magnesian to tholeiitic basalt, dacite, andesite, volcanic and volcaniclastic rhyolitic units, 300epiclastic sediments, and cherts predominate. 301 Ages intermediate between that of the Kelly Group and overlying Sulphur Springs 302Group have been obtained for the Golden Cockatoo Formation in the northeastern Yule 303Granitoid Complex, adjacent to the Pincunah Belt (van Kranendonk, 2000), for which the 304stratigraphical position remains unclear. 305 Overlying the Sulphur Springs Group is the dominantly sedimentary Gorge Creek 306Group. It is composed of fine- to medium-grained clastic sediments (Figure 11 (a, b)) and 307banded-iron formation (Figure 11 (c)), in addition to magnesian and tholeiitic basalt, 308commonly pillowed. It has been described as a sequence of platform to trough sediments 309(Eriksson, 1982). Alluvial platform deposits have been described in the eastern and 310western Pilbara (though these rocks are probably from the overlying De Grey Group), 311and turbidite troughs are found in the south-eastern and central Pilbara (Barley, 1980). 312Although the Gorge Creek Group is widespread throughout the Pilbara, its age and 313regional correlatives are poorly constrained. 314 Locally and unconformably overlying the Gorge Creek Group is the 2.99 Ga 315volcano-sedimentary Whim Creek Group and the ~2.95 Ga (Wingate, 1999) DeGrey 316Group, whose constituents are also of poorly constrained age. In the East Pilbara 317structural domain, the Whim Creek Group consists of mafic and felsic volcanics and fine- 15 Pilgangoora Greenstone Belt

318to coarse-grained epiclastic sediments, and can be distinguished from underlying units by 319the relative paucity of rocks of volcanic derivation. The DeGrey Group includes the 320conspicuous Lalla Rookh Sandstone, a thick package of fluvial and lacustrine sediments 321structurally emplaced between the Pilgangoora Belt and North Pole Dome. Deposition of 322both Gorge Creek- and DeGrey Group sediments in small restricted strike-slip basins 323(Krapez and Barley, 1987; Krapez, 1993) may account for the lack of Pilbara-wide 324correlations. 325 Deposition of the Pilbara Supergroup ceased by 2.85 Ga. Unconformably 326overlying the granitoids and greenstones of the Pilbara Craton are the generally flat-lying 327to shallowly-dipping supracrustal rocks of the Fortescue Group that form the stratigraphic 328base of the Mount Bruce Supergroup, deposition of which commenced at ~2757 Ma 329(Arndt et al., 1991; Blake et al., 2004). The Mount Bruce Supergroup is divided into the 330Fortescue, Hamersley and Turee Creek Groups, only the first of which is represented in 331the north Pilbara. The Fortescue Group is predominantly composed of subaerially erupted 332tholeiitic flood basalts and epiclastic sediments (Blake, 1984; Nelson et al., 1992; Blake, 3331993). 334 Regional metamorphism in the Pilbara is largely strain-controlled (Barley et al., 3351979). The overall range of metamorphism varies from prehnite-pumpellyite or sub- 336greenschist facies in low-strain domains to amphibolite facies in zones exhibiting intense 337deformation. It has long been recognized that both the metamorphic grade and the 338intensity of deformation within the greenstones increase towards the contact with 339granitoid or gneiss bodies (Hickman, 1972b; Bickle et al., 1980). 340 16 Pilgangoora Greenstone Belt

3413. Pilgangoora Belt Lithology 342 The Pilgangoora Greenstone Belt (‘PGB’) comprises the northern part of the 343irregularly-shaped Pilgangoora Syncline, which is bounded by Carlindi Granitoid 344Complex to the north and west, the Yule Granitoid Complex to the south, and the 345truncating Lalla Rookh-Western Shaw Fault to the east. Greenstones of the Pilgangoora 346Syncline have been divided into the geologically distinct northern Pilgangoora Belt and 347southern Pincunah Belt along the Mount York Deformation Zone. 348 Within the Pilgangoora Belt, emphasis here is placed on the oldest exposures, 349consisting of the Warrawoona and Kelly Groups, together with the basal contact with 350intrusive Carlindi granitoids. The ensemble forms a continuous ~50 km long belt in 351which ~6500 m of Warrawoona and up to ~3500 m of Kelly successions are exposed 352(Geological map, Figure 8). 353 The Carlindi Granitoid is the largest of the Pilbara granitoid-gneiss suites and 354complexes. Carlindi granitoid exposure has gentle relief and is most prevalent within 1 – 3552 km of the greenstone contact, where it is encountered in and near creek-beds or as 356isolated domes protruding above surrounding Quaternary cover. Isolated Carlindi 357granitoid outcrop has been reported as far as 30 km from greenstones (Green, 2001). 358Strong weathering has frequently given rise to friable and poorly preserved outcrop. 359Distinctly light-coloured soil conceals granitoid subcrop. 360 Warrawoona and Kelly Group exposure is under strong lithological control and 361more variably preserved than acidic plutonic rocks. In both, silicified sedimentary 362packages tilted to near vertical jut out as prominent ridges above the flat Carlindi 363landscape. Exposure of mafic extrusives and intrusives, markedly less silicified than chert 364units, occupy flatter valleys between chert ridges. With the exception of regional and 365local manifestations of several ductile and brittle deformation events, described below, 366sedimentary and volcanic bedding dips steeply (70 - 85º) away from the underlying 367contact with Carlindi granitoid, which is roughly parallel to bedding. 368 Bearing thin (< 5 m) chert ridges, Warrawoona Group outcrop is exposed as low, 369undulating hills. This gentler geomorphology terminates abruptly at the unconformable 370contact (Figure 10(a)) with the overlying pervasively silicified sub-aqueous and sub- 371aerial sediments of the 8 – 30 m Strelley Pool Chert (‘SPC’) of the Kelly Group, which 17 Pilgangoora Greenstone Belt

372crop out as a prominent and near-continuous ridge for most of the ~50 km strike-length 373of the belt. Units underlying this unconformity together represent the lowermost known 374exposure of the Pilbara Supergroup, and collectively comprise the Coonterunah Subgroup 375of the Warrawoona Group, which has been further subdivided into the magnesian basalts, 376tholeiites and mafic intrusives of the lowermost Table Top Formation, mafic and felsic 377volcanic units and lesser but well-exposed cherts of the intermediate Coucal Formation, 378and tholeiites and mafic intrusives of the upper Double Bar Formation. 379 Possible correlates of the SPC have been reported from the Coongan, North Shaw 380and Kelly Belts and the North Pole Dome. The SPC bears several distinct sedimentary 381lithofacies assemblages (Barley and Bickle, 1982; Lowe, 1983; Rasmussen and Buick, 3821999), including siliciclastic sediments, epiclastic volcanic sediments, carbonates, 383evaporites and carbonaceous muds. Evidence of local and intermittent sub-aerial 384exposure and erosion is provided by the interlayering of some sedimentary structures 385with mafic lavas (Barley et al., 1979). 386 Some of the oldest putative evidence of life comes from the SPC (Lowe, 1980, 3871983; Buick and Barnes, 1984; Dimarco and Lowe, 1989b, a). Stromatoloidal cones of 2 388– 15 cm diameter are common in the laminated SPC lithofacies in the Pilgangoora Belt, 389and have also been observed in other belts (Dunlop et al., 1978; Lowe, 1980; Walter, 3901980; Lowe, 1983; Buick and Barnes, 1984; Dimarco and Lowe, 1989a; van 391Kranendonk, 2000). They are frequently interpreted as ‘stromatolites’ (e.g. Allwood et 392al., 2006; Allwood et al., 2008) i.e. laminated benthic microbial deposits (Kalkowsky, 3931908; Riding, 1999). 394 Two volcanosedimentary meta-chert units (‘Chert VII’ and ‘Chert VIII’) and up 395to ~3500 m of mafic volcanics, hyperbyssals and intrusives of the Euro Basalt Formation 396overlie the SPC and represent the remainder of Kelly Group outcrop in the Pilgangoora 397Belt. Spherical μm-scale structures in black chert from Chert VIII (Figure 4 (g)) have 398been interpreted to be of cyanobacterial origin (Schopf and Packer, 1987) , although 399cross-cutting relationships suggest that this chert may not be of sedimentary origin.. 400 An unambiguous sub-horizontal to angular unconformity with the overlying 401siliciclastic units of the Gorge Creek Group (Figure 10(b)) marks the upper termination 402of the Kelly Group. 18 Pilgangoora Greenstone Belt

403 Several episodes of silicification have affected the Pilgangoora Belt. As in bedded 404meta-cherts, high quartz abundances and granular rather than cryptocrystalline quartz 405textures in mafic rocks are indicative of pre-peak-metamorphic silicification This has 406preferentially affected more permeable and porous units in mesoscopic alteration zones, 407commonly in the top ~ 5 m of volcanic flows (see Chapter 5). 408 All Pilgangoora Belt protoliths have been metamorphosed to at least lower 409greenschist facies, and the ‘meta’ prefix to lithologic names will therefore be taken as 410implied. Chert formed during Cenozoic silicification forms an important exception, and 411metamorphosed and non-metamorphosed chert will be therefore be designated with the 412traditional ‘meta-chert’ and ‘chert’. 413 Like Pilbara rocks elsewhere, Pilgangoora Belt outcrop has undergone variable 414and generally pronounced post-metamorphic alteration and weathering. Ferruginous 415surface outcrop bears thin oxidised mantles, and gossanous boxwork after pyrite and 416probably carbonate is common in the evaporite lithofacies of the SPC. Silcrete and 417ferricrete commonly alter metachert on elevated plateaux and rises, and in depressions. 418Depressions bear evidence of calcretization, probably altering calcic basalts. 419 4203.1. Coonterunah Subgroup 421Table Top Formation 422 The Table Top Formation is dominated by up to ~2000 m of variably silicified 423and metamorphosed mafic extrusive- and intrusive- units. Extrusive units consist almost 424entirely of fine-grained basalt, with grain sizes of ~0.5 mm in outcrop that has undergone 425the least metamorphic recrystallization. In zones of greater recrystallization in which 426volcanic textures are absent or have been obliterated, it is not always possible to 427distinguish between massive basalts and dolerite protoliths. The base of the Table Top 428Formation bears the only magnesian basalt in the Coonterunah Group, characterized by < 429250 m of variolitic flows with well-developed ocelli (Figure 12 (a, b)). Up to 20 m of 430pyroxene-phyric komatiitic basalt overlie this, with elongate needles completely altered 431to amphibole. This unit marks the only known occurrence of komatiite (sensu lato) in the 432Pilgangoora Belt. 19 Pilgangoora Greenstone Belt

433 The remainder of the mafic volcanic rocks in the Table Top Formation, and in the 434Coonterunah Group as a whole, are tholeiitic and generally massive in character. Pillows 435are uncommon, and vesicles and amygdales are rare to absent. Where present, pillows are 436well-preserved with distinct cores, margins and rinds, with long axes in the range of 50 - 437150 cm. Ripple cross-laminated volcanogenic sediment occurs on some flow-tops of 438amygdaloidal basalt (Figure 12 (c, d)). 439 Intrusive rocks exhibit a textural continuum (up to ~15 mm range in groundmass 440grain-sizes) from hyperbyssal (sub-volcanic) doleritic to gabbroic. Mafic intrusives occur 441as tabular conformable to semi-conformable bodies, commonly capped by extrusive 442flows. Intrusive intrusions occur as xenoliths in the lower Table Top Formation (Figure 44312 (e, f)). 444 Mafic intrusion pre-dated Carlindi plutonism and associated metamorphism. In 445coarser rocks at lower metamorphic grades, exsolution lamellae betray rare relict igneous 446assemblages of clinopyroxene and (even more rarely) green-pink pleochroic 447orthopyroxene, together with plagioclase and hornblende. However, both intrusive and 448extrusive mafic rocks are dominated by metamorphic assemblages of amphibole (25 - 44950% ferro-actinolite, with alumino-ferrotschermakite at amphibolite facies), albite (25 - 45050%), epidote (~ 10 %), chlorite (0 - 20%), and quartz (5 - 20%), together with trace 451amounts of leucoxene and opaque oxides (magnetite and ilmenite). Margarite (up to ~ 45210%) is sometimes present. Minor localized coarse ankeritic alteration is also present, 453particularly in more vesicular and amygaloidal outcrops. 454 455Coucal Formation 456 The Coucal Formation immediately overlies the Table Top Formation. Coucal 457Formation metasediments are described in detail in Chapter 5. For much of its lateral 458extent, two or three prominent metachert-BIFs near the base of the Coucal Formation 459represent the full complement of preserved Coonterunah sedimentation. Metachert units 460are up to ~ 8 meters thick at their eastern-most occurrence, and gradually thin to ~ 0.5 461meters or less towards the west-north-westerly fold closure of the the Pilgangoora 462Syncline. They exhibit mm- to cm- scale alternating, planar to gently undulating and 463anastomosing, dark and light banding. Black bands comprise either magnetite or black, 20 Pilgangoora Greenstone Belt

464variably kerogenous, chert. Overlying the intermediate metachert unit is a rarely 465preserved ~35 m section of interleaved bimodal tephra and planar laminated, variably 466silicified micritic-carbonate. Red, haematite-rich layers alternating with white bands of 467clear to milky white pure cryptocrystalline quartz occur towards the westward and 468eastward terminations of outcrop, and appear to represent alterations of the planar 469laminated carbonate. 470 The presence of Ca-Mg silicates within metachert bands, Fe-Ca-Mg silicates 471along the planar interfaces with ferruginous bands, and widespread metachert after 472laminated carbonate is highly suggestive of diagenetic silicification of an aragonite, 473calcite and/or dolomite precursor to at least some planar Coonterunah metachert bands. 474 Rocks overlying Coucal cherts are dominated by ~1000 m of fine-grained, 475occasionally pillowed, tholeiitic basalt and lesser hyperbyssals and intrusives texturally 476similar to tholeiites of the underlying Table Top Formation described above. 477 A laterally extensive unit consisting of various facies of felsic volcanic affinity 478occurs at the top of the Coucal Formation. This unit shows a distinctive brecciated 479appearance in outcrop, weathering into rounded blocks. Facies include highly vesicular 480and amygdaloidal hyaloclastic breccias of dacitic to rhyolitic affinity and flow-banded 481dacite. Where terminated by the Warrawoona-Kelly unconformity, one unit is over >200 482m thick. These felsic volcanics are dominantly composed of 1 - 5 mm quartz and heavily 483pyrophyllite-altered rectangular feldspar phenocrysts and stretched ~ 1.5 cm spheroidal 484amygales set in a highly silicified and sericitized beige to light-green groundmass 485assemblage of fine (< 0.1 mm) quartz and albite-altering muscovite with variable 486amounts of chlorite and trace quantities of opaque oxides. 487 U-Pb chronology of a brecciated hyaloclastic rhyolite sample from this unit has 488constrained of the age of the Coucal Formation to 3515 ± 3 Ma (Buick et al., 1995). The 489style of blocky pumiceous autobrecciation exhibited by this unit is typical of felsic 490volcanic deposition in deep aqueous environments (Fisher and Schmincke, 1984). 491 492Double Bar Formation 493 The Double Bar Formation, overlying the Coucal Formation, is dominated by 494~3500 m of fine-grained, occasionally pillowed, tholeiitic basalt and lesser hyperbyssals 21 Pilgangoora Greenstone Belt

495and intrusives texturally similar to that those of the lower Table Top Formation. Thin 496interbeds of volcanoclastic fine-grained mafic tuff are occasionally preserved. 497 4983.2. Carlindi Granitoids 499 The base of the Table Top Formation is intruded by the Carlindi Granitoid (Figure 50018(a – c)). Where exposed, this contact is variably tectonized, ranging from highly 501kinematic with contact-parallel foliation in greenstone country rock and gneissic 502granitoid (Figure 18(a)), to cross-cutting with massive igneous textures and plentiful 503tabular angular greenstone xenoliths in granitoid (Figure 18 (b, c)). 504 Quartz-feldspar ratios are compatible with a granodioritic or monzograntic 505composition for Carlindi Granitoids, although normative calculations instead indicate 506trondjemitic or granitic classification (Green, 2001). Massive outcrops with igneous 507textures consists of euhedral plagioclase (40 – 50%), quartz (30 – 40%) and microcline 508(10 – 20%) with lesser dark green prismatic hornblende (5 - 10%) and biotite phenocrysts 509(0 – 5%). Grain-sizes of these dominant minerals commonly fall in the range of 5 - 10 510mm, with some phenocrysts measuring up to ~40 mm. Accessory minerals, generally 511subhedral to euhedral, include epidote, clinozoisite, opaque oxides, muscovite, zircon, 512rutile and apatite. Hornblende and epidote have commonly undergone variable 513chloritization. On the basis of greater (~5 %) muscovite abundances and poikilitic 514microcline with plagioclase and quartz inclusions, (Green, 2001) distinguished a separate 515monzogranite member of the Carlindi granitoid suite. 516 A quartz-phyric microgranite bearing well-rounded spherical 1 – 2 mm beta- 517quartz and subhedral to euhedral 1 – 3 mm orthoclase phenocrysts intrudes the coarser 518Carlindi granitoid near the center of the Belt at Neptunian fissure locale (2) (see below). 519This has been dated by U-Pb zircon geochronology at 3464 ±x Ma (Buick et al., 1995), 520younger than other dated Carlindi granitoids by ~ 20 Ma. 521 5223.3. Kelly Group 523 Like the underlying Coonterunah Subgroup exposure, the Kelly Group consists in 524large part of volcanic rocks. Outcrop is dominated by pillowed tholeiitic lavas, 525interlayered with varied cherty sediments and volcanics of more calc-alkaline affinity. 22 Pilgangoora Greenstone Belt

526Strelley Pool Chert 527 The basal unit of the Kelly Group in the Pilgangoora Belt, which concordantly 528overlies the basal unconformity with the Coonterunah Subgroup, is the the Strelley Pool 529Chert. This unit gradually thins from a maximum thickness of ~ 30 m in the eastern 530segment, through ~ 20 m to ~ 8 m in the central segment and disappears altogether in the 531highly strained western segment. Lateral continuity suggests that a gradually thinning ~ 532150 - 30 cm fuchsitic chert unit in the region of the Pilgangoora Syncline closure is 533correlative with the Strelley Pool Chert. 534 Up to five differentiable lithofacies assemblages make up the SPC. The basal 535assemblage, immediately and unconformably overlying Coonterunah Subgroup 536volcanics, consists of well-sorted and well-rounded 0 – 6 m thick orthoquartzitic 537sandstone with minor chert-pebble conglomerate. This assemblage is sometimes absent, 538and cannot be recognized in the western, higher grade quarter of strike. Clast diameters 539range from 1.5 – 2.0 mm, and are thoroughly cemented in a sericitized quartzitic matrix. 540In addition to well-rounded spherical quartz (~75 – 90%), clasts include sub- to well- 541rounded pyrite, chloritized mafic volcanics, magnetite, anhedral to euhedral ilmenite, 542rounded completely sericitizied felsic volcanics, and sub-hedral zircon, in decreasing 543order of abundance (Figure 13(a, b)). Sedimentary textures include normal grading and 544both planar and trough cross-bedding. 545 The most prominent SPC assemblage is the overlying planar- to wavy- laminated 546meta-chert unit, locally stromatoloidal, that overlies the quartzitic sandstone or 547Coonterunah rocks where no basal sandstone underlies (Figure 15 (c)). This unit is 548locally brecciated. Lamination consists of incompletely silicified dolomite in isolated 549low-grade outcrops, but is completely replaced by either or both pre- and post- 550metamorphic chert everywhere else (thin-section photomicrographs of protolith and 551metamorphic assemblages, Figure 9, 11; outcrop photographs, Figure 10). Dolomite is

552variably ferruginous but sub-ankeritic (< 20 wt.% FeCO3). Stromatoloidal cones occur on 553some bedding surfaces and have typical diameters of ~ 2 cm, with similar heights. 554 A crystal-fan lithofacies is occasionally found near the top of, but always within, 555the laminated lithofacies assemblage. Crystals occur as elongate 2 – 8 cm long and 0.5 – 5561.0 cm wide digitate shapes, showing six-sided cross-sections and square terminations 23 Pilgangoora Greenstone Belt

557suggestive of an aragonite precursor (Chapter 1, Figure 7 (c)). Dark-brown dolomitic 558mud fills intercrystalline spaces where incompletely silicified. 559 Two distinctive volcanoclastic meta-cherts mark the upper SPC lithofacies 560assemblage, and are characterized by varying degrees of apple-green fuchsitic alteration. 561Such chromium alteration appears to be a typical feature of the upper lithofacies of Early 562Archaean sedimentary cherts where they are overlain by magnesian volcanics. It is also 563seen at the top of meta-cherts in the lower Barberton stratigraphy (Figure 4 (l)), and is not 564obviously associated with ultramafic units at either locale. 565 The lower unit consists of a silicified greenish grain-supported anatase-rich 566sandstone, massive to normally graded (Figure 15 (e – g)). Unit thickness varies from 567~270 cm to absent, but where encountered is typically ~ 20 - 40 cm thick. Clast diameters 568are also highly variable, from as large as ~15 mm to as small as ~0.5 mm, but medium to 569fine sand is typical. The majority of clasts are variably chloritized, highly angular and 570distinctly arcuate, representing silicified pseudomorphs after unwelded shards of mafic 571glass. 572 The upper unit consists of silicified greenish-grey to grey siltstone-to-lutite 573(Figure 15 (i)). This unit is plane-laminated at lower grades, with sedimentary bedding 574becoming increasingly massive with metamorphic recrystallization. Lamina thicknesses 575are on the order of ~ 0.1 – 0.5 mm. A zebraic texture resulting from pressure dissolution 576is common. Flame structures and intra-clast brecciation are common. Also locally present 577are thorougly silicified 2 - 8 mm euhedral crystals with ~ 0.5 – 1.0 mm pseudo-hexagonal 578cross-sections and oblique pyramidal terminations (outcrop photographs, Figure 13 (c)), 579that have been interpreted elsewhere as pseudomorphs of gypsum (Buick and Dunlop, 5801990) (thin-section photomicrographs, Figure 13 (d, e)). 581 Alternating planar-banded (1 – 10 mm) black and white kerogenous cherts are 582found throughout the Strelley Pool Chert. Kerogen is also present in massive black chert, 583which commonly occurs in a variety of textural associations, including as tabular 584fragments in megabreccia, as cross-cutting dyke-like structures with thicknesses on the 585order of 10 – 40 cm, and less commonly as an early cement-like matrix to metachert 586mega-breccia (outcrop photographs, Figure 4 (f, j, k) and Figure 15 (b, m)). 587 24 Pilgangoora Greenstone Belt

588Euro Basalt Volcanics and Cherts 589 The Euro Basalt Formation comprises up to 3500 m of pillowed (and more rarely 590massive) tholeiitic and magnesian basalt flows, with intrusive gabbro and lesser localized 591doleritic flows. The mafic stratigraphy of the Euro Basalt is interrupted by two 592conformable ridge-forming 0 – 5 m chertified volcanoclastic units, colloquially named 593‘Chert VII’ and ‘Chert VIII’. 594 Kelly Group mafic volcanism is distinguished from that of the underlying 595Warrawoona Group by the greater prevalence of pillow structures and amygdales, and 596greater abundance of variolitic magnesian basalt. Vesicles are larger, more abundant, and 597more densely concentrated, indicative of shallower eruption under lower confining 598pressures. Doleritic flows can be distinguished by sudden decreases in grain-size at upper 599contacts. Igneous assemblages of both magnesian and tholeiitic basalts of the Euro Basalt 600member are compositionally similar to that described for the underlying Coonterunah 601Subgroup, as also born out by geochemical similarities (Green et al., 2000). 602 Chert VII is dominated by a very fine-grained plane to cross-laminated silicified 603bluish-grey mafic lutite, variable in thickness. A massive mafic mudstone immediately 604overlies this unit. It contains abundant ~ 0.5 mm euhedral diagenetic gypsum 605pseudomorphs, hexagonal in cross-section and unambiguously monoclinic in transection, 606with thin white alteration haloes and occasional lithic cores. Gypsum orientations are not 607quite random, tending towards the bedding-parallel. This unit is more pyritic than the 608lutite unit at the top of the Strelley Pool Chert, with pyrite of varying roundness and 609grainsize, but generally fairly cubic. Black kerogenous veins cross-cut both meta-chert 610lithofacies. 611 Chert VIII is similar in many respects to Chert VII, and was also highly 612gypsiferous. It contains a graded silicified mafic sandstone-conglomerate unit at the top. 613Clasts are highly varied, with a high proportion of black and white banded chert clasts. 614 615Neptunian fissures 616 Dyke-like structures are associated with Kelly Group sedimentary meta-cherts at 6173 localities in the Pilgangoora Belt (map, Chapter 7, Figure 1). Structures at sites (1) and 618(2) occur in the eastern and central belt segments, respectively, and are associated with 25 Pilgangoora Greenstone Belt

619the Strelley Pool Chert (field photographs, Figure 11 (a – c)). They are interpreted to be 620neptunian fissures that formed through crustal rupture at the time of Strelley Pool Chert 621deposition, and were filled by material derived from above whilst opening. A final dyke- 622like structure at site (3) in the western segment of the belt, and is associated with Chert 623VII. Here, dyke-like material is dominated by kerogenous granular meta-chert that 624exhibits similar isotopic distributions to kerogen in the neptunian fissures (Chapter 6). 625However, because metamorphism has obliterated sedimentary structures and textural 626relationships, this particular structure cannot confidently be interpreted as a neptunian 627fissure. 628 At fissure site 1, a single ~ 220 m deep fissure passes from the Strelley Pool Chert 629downwards through the Warrawoona-Kelly unconformity into underlying Coonterunah 630rhyolite (geological map of fissure locale 1, Chapter 7, Figure 1). This fissure is 270 cm 631wide at its top within the Strelley Pool Chert, and gradually thins to 20 cm at its tail. At 632fissure site 2 in the central segment of the belt, five fissures (designated (a) through (e), 633from east to west, hereafter) also pass from the Strelley Pool Chert downwards through 634the Warrawoona/Kelly unconformity into underlying Carlindi granitoid (geological map 635of fissure locale 2, Chapter 7, Figure 4). Fissures (a) through (d) are 400, 300, 400, 700 636and 400 meters deep, respectively. At ~700 m, fissure 2(d) is the deepest observed in the 637Pilgangoora Belt, and has previously been interpreted as a feeder dyke to the SPC 638(Lindsay et al., 2005). 639 All fissures have near-vertical (80º – 90º) dips, and are sub-perpendicular to their 640associated chert units and the dominant regional orientation of the underlying greenstone- 641granitoid contact. Below bedded chert units, their proximal host rock has undergone 642variable silicification within 1 - 5 m of fissure walls, with alteration increasing towards 643the fissure contact (for example, outcrop photo of Fissure 2(e), Figure 14 (b)). 644 Contact relations between fissures and the Strelley Pool Chert are occasionally 645preserved. Where the top of the fissure can be identified within SPC, it is accompanied 646by slump structures and intense brecciation directed towards the fissure opening. Soft- 647sediment deformation structures are absent in underlying sediment, which instead shows 648terminated contacts with fissure walls (outcrop photographs and interpretative sketch of 649SPC – fissure contacts, Figure 16 (a – g)). 26 Pilgangoora Greenstone Belt

650 Fissure material is dominated by a diversity of silicified clastic lithologies, 651angular mega-breccia of varied composition, cement-like matrix chert, mm- to cm-scale 652banded chert, and chalcedonic quartz. Neptunian volcanoclastic sediment is dominated by 653silicified mafic siltstone and sandstone, which at site (1) can be traced directly to its 654source in stratigraphically overlying clastic meta-chert lithofacies in the SPC. Contacts 655between neptunian clastic material and bracciated clasts of silicified laminated carbonate 656often show impinging flame structures (Figure 17 (a, b)). 657 Mega-breccia fragments range from pebble- to meter- size, and include cohesive 658blocky angular clasts of Carlindi granitoid (Figure 5 (a, b)), variably silicified planar- and 659wavy- laminated dolomite (Figure 5 (c, d), silicified mafic sandstone (Figure 15 (e – h), 660and silicified mafic lutite (Figure 15 (i, j)). The combined ensemble of fissure breccia 661clasts represent all lithofacies identified in the Strelley Pool Chert. Despite the presence 662of silicified lutite texturally indistinguishable from that of the SPC, gypsum crystallites 663are absent, suggesting that their formation occurred during diagenesis after fissure filling. 664 Where present, chalcedonic quartz has grown along the outer fissure edges, 665parallel to fissure walls, and clearly post-dates neptunian sedimentation and breccia in-fill 666(Figure 17 (d, e)). Chalcedony formed during final phases of neptunian opening, perhaps 667during periods of restricted sedimentary input during low-stand(s) when the fissure 668mouths emerged above sealevel. 669 Observations suggest that at least some of the fissures opened during deposition 670of the laminated carbonate lithofacies, the lower section of which remained cohesive 671during fissure formation. Structural features are compatible with early syn-SPC fissure 672opening. None of the fissures pass into Euro Basalt volcanics immediately overlying the 673SPC, and all record Pilgangoora deformation events. For example, fissure (1) has

674undergone folding during Pilgangoora Syncline-related deformation (D7), while sinistral 675fissure-parallel shear-zones in host granitoid at sire (2) post-date prominent fissure 676cleavage fabric (Figure 13 (o, p)), and probably represent exploitation of the

677fissure/granitoid lithological discontinuity during regional N- to NNE- faulting (D5), 678rather than reactivation of an earlier fault-related fabric. 679 Structural relationships with underlying granitoid-greenstone contacts and the 680lack of fissure-related shearing suggest that fissures opened in response to flexural crustal 27 Pilgangoora Greenstone Belt

681tension, likely in response to diapiric uprise of underlying granitoid(s). It is noteworthy 682that felsic rocks are rare in the Pilgangoora Belt, and underlie the Warrawoona-Kelly 683unconformity in only two areas, both of which contain fissures. In contrast, fissures are 684never encountered within vastly more common (> 95 % of the Warrawoona-Kelly 685unconformity strike-length) mafic Coonterunah units, suggesting that fissure formation 686was under strong lithological control. Perhaps granitoid and rhyolite locales represented 687erosion-resistant topographic highs, atop a surrounding mafic plane or plateau, in which 688sequential alternation of sub-aerial exposure and drowning allowed for the episodic 689entrapment of sedimentation. 690 Superficially similar dyke-like structures at North Pole Dome bear kerogenous 691chert of similar (low) isotopic composition to Pilgangoora Belt neptunian fissures 692(Chapter 7, see also Concluding Chapter). Although they have received varied geological 693interpretations (Hickman, 1972a; Dunlop, 1976; Nijman et al., 1998a; Ueno et al., 2004), 694a neptunian origin has not been amongst them. 695 696 697 28 Pilgangoora Greenstone Belt

6984. Structural Geology 699 Regional deformation histories were proposed by Hickman (1983) and more 700recently by Blewett (2002), and a comprehensive sequence-stratigraphic model was 701proposed by Krapez (1993). The majority of structural studies, however, have focused 702specifically either on the West Pilbara (Kiyokawa, 1983) or the East Pilbara (Bickle et 703al., 1985; Collins, 1989; Zegers et al., 1996; van Kranendonk, 1997; Collins et al., 1998; 704Nijman et al., 1998b; van Haaften and White, 1998; van Kranendonk and Collins, 1998; 705Zegers et al., 1998; Kloppenburg et al., 2001). A five-fold deformation history was 706recently proposed to address the structural history and timing of gold mineralization in 707the northern part of the field area (Baker et al., 2002). These are compared with results 708from the present study in Table 5 below. 709 710Table 5: Structural geology of the Pilgangoora Belt compared with other studies. This study Baker et al. (2002) Blewett (2002) Brief description D1 D1 D1 Pre-Kelly foliation fabric, found in Coonterunah only D2 D2 D2 Early Kelly and Warrawoona foliation fabric Dn-f Not reported Not reported Seafloor rupturing and formation of Neptunian fissures D3 D3 NE-plunging isoclinal folding D4 D4 NW-plunging closed folding D5 D5 N- to NNE- striking sinistral shearzones D6 D6 S-plunging upright tight folding D3 Broad, open folding D7 ENE-WSW folding D7 D4 Regional open 29 Pilgangoora Greenstone Belt

folding, sinistral shearing, granitic pegmatite dyke emplacement D8 D8 Minor N-S shortening, chevron- like moderate to steep E-plunging fold hinges D9 D9 NW-SE striking crenulations Not recognized as a D5 Brittle faulting separate event Dtect Not reported Not reported Ongoing N-S directed gentle km- scale folding 711 712 The Pilgangoora Belt, taken as a whole, records a complex deformation history. 713However, in rocks of the Warrawoona and Kelly Group in the eastern and central 714segments of the Pilgangoora Belt, most of the deformation events find expression only in 715localized shear-zones, if at all. This situation changes rapidly within the region of the

716Pilgangoora Syncline closure, where all but three of D1 through Dtect deformation are 717recorded (Table 5).

718 In the lower-grade segment of the belt, D1, D2, D5 and D7 fabrics are dominant,

719with isoclinal F3 folding, associated with M3 talc in Kelly Group meta-cherts and M3 720cummingtonite-grunerite in Warrawoona Group magnetite meta-cherts, of local 721importance. All fabrics are overprinted by low-amplitude (~ 5 km-scale wavelength) 722gentle N-S directed folding, associated with on-going tectonism associated with

723subduction of the Australian plate under Indonesia (Dtect).

724 The first recognizable fabric in the field-area, D1, overprints primary bedding in 725the Coonterunah Group, and is developed as bedding-parallel slaty cleavage or 726schistosity. Localized upright folds, associated with dextral kinematics in Coonterunah 30 Pilgangoora Greenstone Belt

727meta-cherts, record pre-Kelly Group deformation. Because this fabric is highly variable 728and so cannot be easily interpreted, it is not treated seperately here.

729 D1 fabric pre-dates the unconformably overlying Warrawoona Group lithologies.

730The earliest recognizable non-primary fabric in the Warrawoona Group, D2, overprints S1

731schistosity and slaty cleavage in the Coonterunah Subgroup. S2 foliation is bedding- 732parallel in Kelly-Group volcanosediments, while giving rise to a composite fabric in 733obliquely striking Coonterunah Subgroup rocks.

734 Regional N- to NNE- trending D5 faults cut through volcanosedimentary horizons, 735and across the contact with the Carlindi granitoid. Many of these faults show evidence of 736more recent reactivation.

737 Post-D5 fabric is dominated by D7 deformation, characterized by shallowly south-

738plunging lineations and (~500 – 3000 m) macroscale folds (F7) that refold ductile and

739brittle fabrics of older events. The most prominent regional structures resulting from D7 740deformation are the Pilgangoora Syncline, two smaller folds immediately southwest of it,

741and open folds refolding earlier NE-plunging isoclinal F3 folds immediate south-east of 742the Iron Stirrup mine (stereo-net projection plot, Figure 22). 743 744 31 Pilgangoora Greenstone Belt

7455. Diagenesis, Metamorphism and Alteration 746 An understanding of the diagenetic, metamorphic and alteration history of the 747field area is imperative to the interpretation of depositional environments and 748biosignatures. 749 7505.1. Early Dolomitization and Silicification 751 Diagenetic dolomitization and both diagenetic and hydrothermal silification are 752described for Coonterunah Subgroup rocks in Chapter 5. Sedimentary chert samples of 753the Strelley Pool Chert also bear evidence of early (pre-metamorphic) dolomite formation 754and concurrent or closely antecedent silicification that is similar to the localized 755‘selective’ type described in deeper marine meta-sediments from the Coonterunah 756Subgroup. 757 Abundant isolated sparry dolomite rhombs are preserved within SPC meta-chert 758(Figure 19 (a – l)), commonly in association with kerogen. Diagenetic dolomite is readily 759distinguished from unbedded sugary metasomatic carbonate, typically void-filling, that 760grows perpendicular to surfaces (outcrop photograph of sugary metasomatic ankerite and 761calcite veins in chert, Figure 20 (a)). 762 SPC dolomite is euhedral, and saddle dolomite is absent. Isolated rhombs in a 763meta-chert matrix are remarkably invariable in size across the entire lateral extent of the 764Strelley Pool Chert metamorphic facies that bears pre-metamorphic carbonate, with 100 – 765120 μm planar crystal faces (Figure 19 (a - j)). In less silicified samples, spar of variable 766size occurs along distinct lamellae (Figure 19 (k, l)), becoming fully interlocking in 767isolated sites that have escaped silicification. 768 SPC dolomite is variably silicified to meta-chert and chert (Figure 20 (b- h), 769Figure 21 (a – f)). Early silicification of isolated spar is identified by grains with skeletal 770textures and rounded amoeboid quartz blebs of ~ 10 - 20 μm diameter containing 771remobilized kerogen. In carbonate-rich meta-cherts and some samples bearing 772interlocking carbonate cut by quartz veins, distinct dissolution-silicification fronts occur 773parallel to carbonate-metachert contacts (Figure 21 (a – f)), similar to those observed in 774localized metachert replacement of precursor carbonate in the Coonterunah Subgroup 775(compare dissolution-silicification front in Chapter 5, Figure 10 (k, l)). 32 Pilgangoora Greenstone Belt

776 Similar dolomite spar textures have been interpreted elsewhere to indicate the 777preferential silicification of calcite (or aragonite) subsequent to the inception of 778dolomitization, but before dolomitization was sufficiently advanced beyond formation of 779individual crystals (Laschet, 1984; Knauth, 1994). 780 781 5.2. Metamorphism 782 In the absence of assemblages allowing quantitative geothermobarometry, 783metamorphism had to be studied through calculation of P-T diagrams and 784pseudosections. Pseudosections were constructed for meta-mafics in the Al-Fe-Mg-Na-

785Si-CO2-H2O system (Chapter 5, Figure 14) using bulk analyses for meta-basalts (Figure 78626, 27) reported in Green. P-T diagrams were constructed for meta-cherts in the Ca-Fe-

787Mg-Si-CO2-H2O system and Ca-Mg-Si-CO2-H2O sub-system (Figure 28; see also Chapter 7885, Figure 16, 17). Although useful, the low grades of metamorphism restrict the 789applicability of these tools to qualitative assessment only. The resulting metamorphic 790zonation is summarized in Figure 28. 791 792Early metamorphism 793 In addition to post-depositional hydrothermal alteration and burial, Kelly and 794Warrawoona Group rocks have been metamorphosed to at least sub-greenschist and 795lower-greenschist facies, respectively. In both units, initial metamorphism undoubtedly 796accompanied early tilting to sub-vertical. Surviving early metamorphic textures in Kelly 797and Warrawoona mafic rocks that pre-date regional metamorphism include the 798widespread presence of chlorite after both olivine and augitic clinopyroxene, and the 799replacement of plagioclase by albite. The presence of early prehnite-pumpellyite in Kelly 800Group meta-basalt pillow rims and prehnite in meta-chert (thin-section photographs, 801Figure 24 (a - c)) in the eastern segment of the Pilgangoora Belt marks the lowest 802metamorphic grade encountered, and may date back to this nascent phase of 803metamorphism. No prehnite is encountered in any underlying Warrawoona rocks of any 804kind, suggesting that they experienced higher grades of pre-regional metamorphism. Any 805related planar fabric associated with early tiliting would have been obliterated by 33 Pilgangoora Greenstone Belt

806similarly oriented oriented foliation associated with later metamorphic events, and no 807evidence of an accompanying early lineation fabric survives. 808 809Carlindi-granitoid thermal metamorphism 810 Two prominent later metamorphic gradients can be distinguished on the basis of

811structural, textural and mineralogical evidence. The older (M1) is a ~ 100 - 350 m thermal 812metamorphic gradient increasing towards the intrusive contact with the ~3.48 Ga (Buick 813et al., 1995) Carlindi Granitoid complex to the north. The fabric associated with contact 814metamorphism varies from hornfels to a strong contact-parallel foliation, now with sub-

815vertical dip. Characteristic M1 assemblages are defined by chlorite-actinolite-epidote in 816meta-basalts, -dolerites and -gabbros, tremolite- or actinolite- quartz in metachert- 817carbonates, and grunerite- or cummingtonite-quartz in ferruginous meta-cherts. 818Frustratingly, these peak assemblages do not allow for a quantitative assessment of the 819depth of intrusion. However, on the basis of P-T pseudosections calculated for metamafic

820rocks with Coonterunah bulk compositions, M1 metamorphism occurred at low but 821poorly constrainable pressures. In particular, the absence of almandine-garnet, the 822absence of metamorphic calcic clinopyroxene (diopside-hedenbergite series) or pargasite 823in favour of tremolite, and presence of highly sodic plagioclase, together suggest contact 824metamorphism at somewhat unusually low pressures, well below 3500 bars and probably 825closer to ~ 2000 bars. This low metamorphic pressure, taken together with the

826observation that the M1 hornfels assemblage remains consistent for ~30 km of strike, 827suggest plutonism occurred from below.

828 M1 fabric is restricted to Coonterunah Subgroup rocks, and does not penetrate the 829Warrawoona-Kelly unconformity, even where the unconformity itself coincides with the 830upper stratigraphic termination of Carlindi granitoid. Similarly, thermal metamorphism 831has not affected neptunian fissures penetrating into Coonterunah rhyolite or Carlindi 832granitoid. 833 834Pilgangoora Syncline metamorphism

835 Overprinting M1 and earlier metamorphic fabrics is a pronounced regional fabric

836(M7) associated with metamorphic grades increasing westward, under strain control. M7 34 Pilgangoora Greenstone Belt

837fabric is sub-parallel to bedding for most of Coonterunah strike, until culminating 838abruptly in lower amphibolite facies assemblages associated with the ~ 2.88 Ga (Baker et

839al., 2002) Pilgangoora Syncline fold (D7) closure that marks the west-northwestward 840extent of both Warrawoona and Kelly Group volcano-sedimentary outcrop. Highest grade 841assemblages are defined by sillimanite in Gorge Creek Group meta-psammites, and one 842single local occurrence of almandine-andesine-ferrotschermakite amphibole, with post-

843M7 retrogressive chlorite, in silicified meta-volcanics (thin-section photographs, Figure

84424 (e – h)). Widespread renewed plutonism accompanies M7, including a garnet

845pegmatite dyke swarm intruded along D7 fold cleavage planes. 846 Within Warrawoona and Kelly Group lithologies, variations in the conditions of

847peak metamorphism associated with more localized individual deformation events (M4 –

848M6, M8 – M10) are, at best, limited to talc formation in carbonate-metacherts and chlorite

849in meta-mafics, and nowhere exceed those of associated with M1 or M7. 850 851Carbonate-chert assemblages 852 The bulk geochemistry of most outcropping meta-chert is overwhelmingly 853dominated by silica, with variable amounts of Ca, Mg and Fe rarely exceeding 1 wt.% in 854higher-grade meta-cherts and resilicified cherts. Significant occurrences of calcite and 855dolomite are largely restricted to localized zones of low silicification. Other than trace 856quantities of opaque oxides, and with the exception of localized occurrences of banded 857post-metamorphic jasperitic chert or clasts thereof (e.g. Figure 31 (g)) and detrital 858magnetite grains in the basal clastic unit, Fe-bearing phases are absent from the Strelley 859Pool Chert. Ferruginous minerals do occur in Coonterunah Subgroup assemblages in 860localized horizons that bear magnetite at low grade. In view of these low Ca, Mg and Fe

861abundances, Pilgangoora SiO2-dominated chert units offer little potential for constraining

862metamorphic conditions (P-T diagram of the system SiO2, Figure 25). In theory, the 863presence of even small amounts of dolomite with or without calcite in chert allows for 864qualitative geothermometry (consider univariant curves in Figure 27; see further Chapter 8655), although steeply sloping univariant P-T curves prevent constraint of metamorphic 866pressures. 35 Pilgangoora Greenstone Belt

867 Through talc- and tremolite-forming reactions, dolomite ceases to coexist with 868quartz. In more hydrous assemblages (thin-section photomicrograph of talc-quartz 869assemblage, Figure 23 (g)):

870 3Dol + 4Qtz + H2O → Tc + 3Cc + 3CO2 (1) 871

872At higher xCO2, tremolite is formed (thin-section photomicrographs of tremolite-quartz 873assemblage, Figure 23 (e, f)):

874 5Dol + 8Qtz + H2O → Tr + 3Cc + 7CO2 (2) 875 876Assuming that calcite does not get removed, the upper limit of tremolite is set by the 877appearance of diopside in quartz-rich rocks:

878 Tr + 3Cc + 2Qtz → 5Di + 3CO2 + H2O (3) 879 880While in dolomite-rich rocks, co-existing dolomite and tremolite react to give calcite and 881forsterite:

882 Tr + 11Dol → 8Fo + 13Cc + 9CO2 + H2O (4) 883 884Forsterite and diopside are nowhere observed in Strelley Pool Chert or Coonterunah 885Subgroup metacherts. On the basis of the forgoing, the following assemblages can be 886expected with progressive metamorphism in the Ca-Mg-bearing cherts (tabulated 887graphically in Figure 29): 888(i) Protolith assemblage: qtz + dol 889(ii) Lower greenschist facies assemblage:

890 Lower xCO2: qtz + tlc ± dol ± cc

891 Higer xCO2: qtz + trem ± dol ± cc 892(iii) Upper greenschist facies assemblage: qtz + trem ± cc 893(iv) Lower amphibolite facies assemblage: 894 qtz > dol: qtz + di ± cc 895 qtz < dol: qtz + trem ± cc 896 36 Pilgangoora Greenstone Belt

897In siliceous assemblages, the thermal stability limit of calcite is set by the formation of 898wollastonite (at ~640 ºC and 680 ºC at 1000 and 2000 bars, respectively):

899 Cc + Qz → Wo + CO2 (5) 900 901The near-absence of carbonate in quartz-tremolite assemblages after carbonate-chert 902protoliths suggests calcite-poor bulk compositions close to the quartz-dolomite join 903(consider stability fields in Ca-Mg-Si space in Chapter 5, Figure 17), compatible with the 904calcite-barren dolomitic-metachert assemblages encountered locally at lower grades. 905However, these inferences rely on system closure with respect to Ca, Mg and Si, which is 906an assumption that is far from secure. Siliceous fluids frequently accompanied 907deformation and metamorphic events, and dissolution textures at carbonate-quartz 908contacts preserved at low grade in cherts from both the Coonterunah Subgroup and 909Strelley Pool Chert (thin-section photomicrographs of carbonate dissolution-silicification 910textures on quartz contact, in Kelly Group: Figure 21 (a – f); in Warrawoona Group: 911Chapter 5, Figure 10 (k – n)) attest to the efficient removal of Ca and Mg from carbonate- 912bearing systems during Pilbara metamorphism. 913 Honey-coloured grunerite-amphibole is present in fibrous habit in higher-grade 914Coonterunah ferruginous cherts, sometimes in co-existence with lath-like amphiboles of 915the tremolite-actinolite series. The latter display systematic variation in optical properties, 916varying from apleochroic with low second-order blue birifringence, to mild greenish- 917yellowing pleochroism and upper first-order birifringence. In the system Ca-Mg-Fe-Si- 918fluid, grunerite can form through one or more of the following candidate reactions (Gole, 9191980):

920 7Ca(Fe,Mg)(CO3)2 + 8SiO2 + H2O → (Fe,Mg)7Si8O22(OH)2 + CaCO3 + 7CO2

921 Fe-dol + qtz → grun + cc + CO2 922 (6) 923

924 8 (Fe,Mg)CO3 + 8SiO2 + H2O → (Fe,Mg)7Si8O22(OH)2 + 7CO2

925 sid + qtz + H2O → grun + CO2 (7) 926

927 7Fe3Si4O10(OH)2 → 3Fe7SiO22(OH)2 + 4SiO2 + 4H2O 37 Pilgangoora Greenstone Belt

928 minnesotaite → grun + qtz + H2O (8)

930 7Fe3O4 + 24SiO2 + 3H2O → 3Fe7Si8O22(OH)2 + 5O2

931 7 mt + 24 qtz + 3 H2O → 3 grun + 5O2 (9) 932 933Textural evidence was found only for reactions (6) and (9) (thin-section photographs of 934grunerite-forming reactions, Chapter 5, Figure 10 (j)). Silicates associated with high- 935grade banded-iron formations, such as pyroxene group minerals, riebeckite, and fayalite, 936are absent from the ferruginous cherts examined, as are the ferroan carbonates ankerite 937and siderite, and the low-grade ferruginous silicates greenalite and minnesotaite. 938 9395.3. Post-metamorphic Alteration 940Silicification 941 Pilgangoora Belt rocks have undergone extensive post-metamorphic silicification 942(discussed further in Chapter 5). In a high-precision isotopic examination of cherts from a 943330 meter-long drillcore through the Warrawoona-Kelly unconformity, measurable

944quantities (> 0.05 wt.% CO2) of Archaean carbonate were only encountered below ~ 142 945m (Table 6). The occurrence of carbonate in drillcore is restricted to more silicification- 946resistant dolomite spar. 947 948Haematization 949 The presence of ferric iron in primary minerals, such as magnetite and haematite, 950constitutes an important proxy for oxidation fugacities of palaeoenvironments. Ferric 951oxides in deep-marine sediments of the Coonterunah Subgroup are of post-metamorphic 952origin, as shown in Chapter 5. The shallow-marine sediments of the Strelley Pool Chert 953also contain occasional ferric oxide minerals, most particularly in the form of fine 954haematite granules, and a drillcore study was undertaken to determine their origin. 955 SPC haematite of demonstrably secondary origin is shown in Figure 30 (a – g). 956Secondary haematite occurs as finely dispersed granules lining mineral faces (Figure 30 957(f, g)), or in close association with cross-cutting quartz or chlorite veins (Figure 30 (a - 958e)), diffusing outwards from penetrative structures. 38 Pilgangoora Greenstone Belt

959 SPC haematite of a different textural association occurs in bedding-parallel 960lenses, also observed locally in SPC laminated carbonate lithofacies and breccia (drillcore 961and outcrop photographs, Figure 31 (a – g)). Frequently, this bedding-parallel haematite 962does not cross-cut any earlier fabric, and pre-dates multiple episodes of chlorite, quartz 963and pyrite veining. If primary, its presence would point to the existence of localized 964oxygenated conditions in shallow waters of Early Archaean near-shore environments. 965However, its very fine grain-size, its red colouration and the rather nebulous boundaries 966to the lenses suggests instead a secondary peak-metamorphic origin, because fine red 967haematite recrystallizes to coarser grey haematite well below sub-greenschist facies 968temperatures (Catling and Moore, 2003). As all rocks in the SPC examined here have 969experienced at least prehnite-pumpellyite facies metamorphism, with minimum 970temperatures of over 250 ºC, pre-metamorphic fine red haematite would not survive. 39 Pilgangoora Greenstone Belt

9716. Geological Synthesis 972 The geological history of the older sequences of the Pilgangoora Belt is depicted 973in Figure 32 (a – i). During deposition of the Coonterunah Subgroup, deep-water 974subaqueous eruption of low-viscosity basalts of increasingly tholeiitic character took 975place upon an unknown, possibly sialic, basement. Eruptive hiatuses were marked by 976silicified tops to volcanic piles and chert caps, which stand testimony to high seawater 977silica concentrations. Localized rhyolitic flows, flanked by silicified breccia, represented 978local topographic highs in a mafic plane. 979 An eruptive event gave rise to deposition of ~35 m of felsic tephra, temporarily 980changing the nature of hydrothermal seafloor alteration and diagenesis, and shifting the 981chemistry of the benthic environment, thereby allowing for the preservation of 982unsilicified micritic carbonates. Carbonate sedimentation and organic irrigation 983continued, but the return to a hot mafic substrate following renewed mafic seafloor 984volcanism re-initiated pervasive silification (or dissolution) of any carbonates. 985 Shallow intrusion and diapiric uprise of the Carlindi granitoids resulted in a large- 986scale domal topography and general uplift. Erosion levelling this topography resulted in 987the loss of at least 3000 m of volcanosedimentary Coonterunah and sialic Carlindi 988material (perhaps along with other younger units of the Warrawoona Group), some of 989which came to be deposited and preserved in the basal sandstone of the Strelley Pool 990Chert. Silicified peri-tidal sediments of the Kelly Group record a different environment 991from that of the deep-marine Coonterunah Subgroup, and provide evidence for stratified 992ocean hydrochemistry in the Early Archaean (Chapter 5). 993 Further doming caused flexural crustal rupture of sections of the Strelley Pool 994Chert during reef-like carbonate precipitation, resulting in the opening of neptunian 995fissures. Cohesive lithified rocks descended into the fissures as megabreccia in a soft 996carbonate and organic-matter matrix. Sequential shoaling and drowning ensured 997entrapment of unconsolidated mafic volcanoclastic silt and sand and provided a favorable 998environment for bacterial consortia (Chapter 7). 40 Pilgangoora Greenstone Belt

999Tables

13 18 1000Table 6.  Ccarb and  Ocarb analyses of chert-hosted carbonate from Warrawoona-Kelly 1001drillcore.

13 18 Sample Analysis Mass CO2 CO2 ± 1s  Ccarb ± 1s  Ocarb ± 1s (mg) (μg) (wt.%) (‰, PDB) (‰, PDB) 143.27m rep1 6.276 2.9 0.05 -3.90 0.22 -20.70 0.23 rep1 2.344 57.6 2.46 1.67 0.03 -17.77 0.04 rep2 2.221 52.9 2.38 1.64 0.02 -17.84 0.02 163.40m rep3 4.720 56.9 1.21 1.63 0.04 -18.31 0.05 rep4 1.896 50.9 2.68 1.65 0.01 -18.07 0.04 mean 2.18 0.66 1.65 0.02 -18.00 0.25 rep1 3.568 58.0 1.62 1.66 0.02 -18.49 0.03 rep2 1.535 49.0 3.19 1.68 0.02 -18.42 0.04 163.70m rep3 1.488 49.0 3.29 1.66 0.03 -18.40 0.03 rep4 3.473 57.2 1.65 1.62 0.03 -18.64 0.04 mean 2.44 0.93 1.65 0.03 -18.48 0.13 rep1 14.394 51.1 0.36 -0.68 0.03 -17.21 0.03 rep2 11.136 44.3 0.40 -0.57 0.03 -16.79 0.06 310.53m rep3 7.825 31.6 0.40 -0.62 0.04 -17.03 0.05 mean 0.39 0.03 -0.06 0.06 -17.38 0.21 rep1 10.747 57.6 0.54 -0.10 0.03 -16.62 0.04 rep2 11.430 57.6 0.50 -0.10 0.02 -16.64 0.06 311.64m rep3 5.878 37.4 0.64 -0.16 0.04 -16.98 0.03 mean 0.56 0.07 -0.12 0.04 -16.75 0.20 rep1 11.361 38.7 0.34 -0.72 0.02 -16.01 0.03 rep2 14.750 48.0 0.33 -0.73 0.03 -15.96 0.13 312.32m rep3 5.239 17.2 0.33 -0.81 0.03 -16.25 0.07 mean 0.33 0.01 -0.75 0.05 -16.07 0.15 rep1 14.971 52.4 0.35 0.15 0.04 -16.48 0.05 rep2 14.822 54.4 0.37 0.16 0.02 -16.28 0.02 313.37m rep3 6.488 28.2 0.43 0.07 0.02 -16.64 0.02 mean 0.38 0.04 0.12 0.05 -16.47 0.18 1002 41 Pilgangoora Greenstone Belt

1003Figure Captions 1004Figure 1. Biomarker sampling localities in the Pilgangoora Belt. 1005 1006Figure 2 (a – d). Macronutrient calcium-, phosphorus-, carbon- and sulphur- element 1007map of BIF-hosted graphitic apatite from the Isua Supracrustal Belt in southwest 1008Greenland. Sulphur occurs below detection levels, while the graphitic coating on apatite 1009cannot readily be distinguished from interference effects on crystal faces. Nitrogen was 1010not detected at all. 1011 1012Figure 3. Waveform output from Fourier-Transform Infrared Spectroscopy study of

+ 1013sheet-silicate hosted biogenic ammonia in Pilbara metapelites. NH4 was also not found 1014in Isua Supracrustal Belt metaturbidites. 1015 1016Figure 4 (a – m). (a – k) Textural associations of Early Archaean kerogen. (a – c): Plane- 1017polarized (a), cross-polarized (b) and reflected-light (c) photomicrograph of kerogen- 1018coated pyrite from the chert in the Coucal Formation, Coonterunah Subgroup, 1019Warrawoona Group, Pilbara. (d, e): Outcrop photographs of kerogenous chert cross-

1020cutting silicified basalt immediately below the black-white banded H3C chert in the 1021Hooggenoeg Formation, Onverwacht Group, Barberton. (f): Cement-like black 1022kerogenous chert deforming silicified laminated carbonate in the Strelley Pool Chert, 1023Kelly Group, Pilbara. Top arrow shows black chert-filled scour surface, lower arrow 1024shows silicified laminated lithofacies draping black chert (g): Black kerogenous chert 1025sub-parallel to and cross-cutting sedimentary bedding in Chert VII, Kelly Group, Pilbara. 1026(h, i): Reddish oxidation stains on kerogen granules in Coonterunah ferruginous meta- 1027chert at high metamorphic grade. (j): Laminated black kerogenous chert and carbonate in 1028the Strelley Pool Chert. (k): Silicified laminated carbonate breccia and cement-like 1029kerogenous black chert matrix in neptunian fissure. (l) Fuchsitic alteration in silicified 1030laminated meta-chert of the Komati Formation, Barberton Greenstone Belt. (m) 1031Carbonate alteration on accretionary lapilli horizons in ‘K2v’ Member, Kromberg 1032Formation, western limb of Onverwacht anticline, Barberton Greenstone Belt. 1033 42 Pilgangoora Greenstone Belt

1034Figure 5. Distribution of Archaean granite-greenstone and unconformably overlying 1035lithologies of the Late Archaean-Early Proterozoic Hamersley Basin in the Pilbara 1036Craton, along with bounding terrains (adapted after Tyler, 1990; Green, 2001). 1037 1038Figure 6. (a): Satellite image of granite-greenstone terrains in the northern Pilbara, with 1039Pilgangoora Belt field area highlighted. (b): Structural domains and distribution of 1040granitoid suites/gneiss complexes and greenstone belts in the northern Pilbara (adapted 1041after Green, 2001; Blewett, 2002). 1042 1043Figure 7. Stratigraphic divisions of the East Pilbara structural domain of Pilbara 1044Supergroup in the Pilgangoora Belt. 1045 1046Figure 8. Fold-out geological map of the Pilgangoora Belt. 1047 1048Figure 9. Stratigraphic section of the Pilgangoora Belt. 1049 1050Figure 10 (a, b). Field photographs showing regional-scale angular unconformity 1051between Warrawoona Group and Strelley Pool Chert of Kelly Group (a) and regional- 1052scale angular unconformity between Chert VIII of Kelly Group and clastic sediments of 1053the Gorge Creek Group (b). 1054 1055Figure 11 (a – c). Gorge Creek Group sediments. (a, b): Trough cross-bedding in fluvial 1056arenite, Lalla Rookh Formation, Gorge Creek Group. (c): Haematite-jasper banded-iron 1057formation, Cleaverville Formation, Gorge Creek Group. 1058 1059Figure 12 (a – f). Basalt features in the Pilgangoora Belt. (a, b) Occelli in magnesian 1060basalt, showing interlocking ridge-forming clinopyroxene rims with light-greenish pale 1061albite-chlorite centers. (c, d): Ripple-laminated tuffaceous sediment on flow tops in 1062tholeiitic Coonterunah basalt. (e, d) Gabbro and dolerite cumulate xenoliths in 1063Coonterunah Subgroup basalt. 1064 43 Pilgangoora Greenstone Belt

1065Figure 13 (a – e). Selected Strelley Pool Chert lithofacies. (a, b): Embayed zircon in 1066basal rounded orthoquartzite. (c – e): Silified diagenetic gypsum crystallites in outcrop 1067and thin-seciton. 1068 1069Figure 14 (a - c). Field photographs of neptunian fissures. 1070 1071Figure 15 (a – p). Carlindi granitoid and SPC sedimentary units compared with 1072allochtonous fissure breccia. 1073 1074Figure 16 (a – g). Stuctural relationships between SPC and neptuninan fissures. 1075 1076Figure 17 (a - e). Textural relationships within neptunian fissures. 1077 1078Figure 18 (a – c). Outcrop photographs of contact relationships between Carlindi 1079granitoid and amphibolite-facies Table Top metamafics. (a) Kinematic contact parallel to 1080dominant foliation in both rock types, gneissic fabric in Carlindi granitoid. (b) Intrusive 1081contact cross-cutting dominant foliation in metamafics, plutonic fabric in Carlindi 1082granitioid. Box shows area of photograph (c), which shows angular and tabular 1083brecciation of countryrock. 1084 1085Figure 19 (a – f). Thin-section photomicrographs of partially silicified early sparry 1086dolomite textures in Strelley Pool Chert silicified laminated carbonate lithofacies 1087obtained from drillcore. (a - f) Dolomite spar with kerogenous inclusions and 1088concentrated kerogenous outer rim. (e, f) Detail of dolomite in (c, d), showing rim (e) and 1089core (f). (g, h) Thoroughly silicified dolomite spar in drusy quartz. Note kerogen 1090displacement towards top-right of dolomite pseudomorph. 1091 1092Figure 20 (a – h). Outcrop photos of Pilgangoora Belt carbonate type occurences and 1093contact relations (see also basaltic carbonate metasomatism, Chapter 5, Figure 24(b, e, 1094f)). (a) Late, post-peak-metamorphic ankerite alteration enveloping chert in neptunian 1095fissure. (b - h) Outcrop photographs of Early Archaean shallow-water dolomitization and 44 Pilgangoora Greenstone Belt

1096silicification. (b) Heavily silicified dolomitic stromatoloids in the the wavy laminated 1097lithofacies of the Strelley Pool Chert. (c) Dolomite stylolitization, evincing 1098remobilization of pre-existing dolomite upon silicification. Chert now metamorphosed to

1099sugary metachert. (d) Tectonized sinistral S7 fabric in metachert-dolomite: dolomitization 1100and silicification precede Pilgangoora synclinaization. The sinistral fabric, which clearly 1101post-dates dolomite silicification, is associated with Pilgangoora Synline closure at ~2.8 1102Ga. Note talc formation on dolomite-metachert contacts, absent elsewhere. (e, f) 1103Laminated Strelley Pool Chert dolomite boulder in silicified neptunian fissure breccia. 1104The selective silicification of boulder lamellae appears to be associated with silicification 1105in the fissure matrix, suggesting that (most) boulder silicification post-dates formation of 1106the fissure. (g) Patchily silicified dolomite enclosing unsilicified stylolitic dolomite 1107domains. (h) Late high-relief quartz veins perpendicularly cross-cutting laminated 1108dolomite, bringing about selective lamellar silicification. Note relatively higher 1109susceptibility of bas-relief dolomite to weathering and erosion than chert. 1110 1111 1112Figure 21 (a – f). Carbonate dissolution textures. 1113

1114Figure 22. Stereonet projections of bedding planes to F7 folds, including the Pilgangoora 1115Syncline. 1116 1117Figure 23 (a - f). (a – d) Petrological evidence for early silicification in foliated Strelley

1118Pool Chert. (a, b) S7 crenulation cleavage cross-curring quartz bands. (c, d) S7 crenulation 1119and sinistral quartz boudinage. (e, f) Evidence for precursor carbonate in foliated 1120alternating quartz/kerogen Strelley Pool Chert. Box in (e) shows area of photomicrograph

1121of small tremolite crystals alligned parallel to S7 in quartz band (f). 1122 1123Figure 24 (a - h). Thin-section photomicrographs of assemblages delineating 1124metamorphic facies. (a – c) Classic ‘bow-tie’ texture in prehnite in Strelley Pool Chert.

1125(d) Incomplete reaction of dolomite and quartz to form tremolite on grain contacts in M1 1126contact aureole. (e – f) Almandine-albite-quartz in Coonterunah Subgroup silicified 45 Pilgangoora Greenstone Belt

1127metamafic. (g – h) Hornblende in equilbrium with almandine in lower amphibolite-facies 1128Coonterunah Subgroup metamafics. 1129

1130Figure 25. P-T diagram of stable and metastable SiO2 transitions. 1131 1132Figure 26. Ternary ACF phase diagram (Spear, 1995) with Pilgangoora Belt basalt 1133compositions (red crosses). Triangle shows average composition. FeO' = FeO + MgO. 1134 1135Figure 27. Ternary AFM phase diagram (Spear, 1995) with Pilgangoora Belt basalt 1136compositions (red crosses). A = 0.5 AlO1.5 - 0.75 CaO; F = FeO; M = MgO. 1137

1138Figure 28. Simplififed T-xCO2 diagram in the Ca-Mg-Si-fluid system at P = 2000 bars. 1139 1140Figure 29. Metamorphic mineral zonations for mafic and carbonate-metachert 1141assemblages in the Pilgangoora Belt. 1142 1143Figure 30 (a – g). Post-metamorphic haematite in drillcore. 1144 1145Figure 31 (a - f). Post-metamorphic haematite in drillcore. 1146 1147Figure 31 (g). Jasperitic bands in neptunian breccia clasts. 1148 1149Figure 32 (a – i). Geological synthesis: major structural and tectonothermal events in the 1150Pilgangoora Belt. (a) Deposition and extrusion of >6.5 km Coonterunah deep-marine 1151sediments and lavas at ~3517 Ma onto an unknown, possible sialic (Green et al., 2000) 1152basement; (b) Intrusion of Carlindi granitoid suite at 3468-3484 Ma. Contact 1153metamorphism to upper greenschist and local lower amphibolite facies in a ~500 m wide 1154aureole. Steepening of eastern Coonterunah units possibly dome-related; (c) Shallow 1155crustal emplacement of Carlindi-related microgranite at 3468-3484 Ma; (d) Exhumation, 1156leveling and erosion prior to deposition of unconformably overlying Warrawoona Group; 1157(e) Continued uplift and doming give rise to the development of 250-750 meter deep 46 Pilgangoora Greenstone Belt

1158Neptunian fissures autochtonous with deposition of the shallow marine and subaerial 1159sediments of the ≤ 3458 Ma Strelley Pool Chert, Warrawoona Group; (f) Upright tight 1160synclinal tilting at < 3235 Ma results in the present sub-vertical bedding of Coonterunah, 1161Warrawoona and Gorge Creek group, characteristic of greenstone belts in the Pilbara and 1162elsewhere. Tilting likely occurs in response to large-scale diapiric uplift; (g) Regional N- 1163to NNE- directed dominantly sinistral strike-slip faulting. Re-activation along 1164rheologically vulnerable planes, such as Neptunian fissures; (h) Development of steeply 1165NNE plunging Pilgangoora syncline at ~2890 Ma (Neumayr et al., 1998), associated with 1166steep lineations, strain-controlled lower amphibolite-facies metamorphism, Au- 1167mineralization and garnet-bearing pegmatite dykes along fold cleavage planes; (i) Long- 1168wavelength broad gentle north-south directed folding associated with continuing 1169subduction of Indonesia slab under Australia. 47 Pilgangoora Greenstone Belt

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