Critical testing of Earth's oldest putative fossil assemblage from the ∼3.5 Ga Apex chert, Chinaman

48 Pages·2005·4.78 MB·English

PrecambrianResearchxxx(2005)xxx–xxx
Critical testing of Earth’s oldest putative fossil assemblage from
the 3.5Ga Apex chert, Chinaman Creek, Western Australia
∼
Martin D. Brasiera, , Owen R. Greena, John F. Lindsayb, Nicola McLoughlina,
∗
Andrew Steelec, Cris Stoakesd
aEarthSciencesDepartment,UniversityofOxford,SouthParksRoad,OxfordOX13PR,UK
bLunarandPlanetaryInstituteandJohnsonSpaceCenter,Houston,TX77058,USA
cGeophysicalLaboratory,CarnegieInstitutionofWashington,5251BroadBranchRoadNW,Washington,DC20015-1305,USA
dC.AStoakesandAssociatesPtyLtd.,3185VictoriaRoad,Hovea,WA6071,Australia
Received10December2004;receivedinrevisedform10June2005;accepted24June2005
Abstract
Structuresresemblingcyanobacterialmicrofossilsfromtheca.3465MaoldApexchertoftheWarrawoonaGroupinWestern
AustraliahaveuntilrecentlybeenacceptedasprovidingtheoldestmorphologicalevidenceforlifeonEarth,andhavebeen
takentosupportanearlybeginningforoxygen-releasingphotosynthesis.Elevenspeciesoffilamentousprokaryote,principally
distinguishedbyshapeandgeometry,havebeenputforwardasmeetingthecriteriarequiredofauthenticArchaeanmicrofossils.
Theywerecontrastedwithothermicrofossilsthatweredismissedaseitherunreliableorirreproducible.Theaimofthispaper
istoprovideadetailedaccountofresearchrecentlyreportedbyusonthetypeandrecollectedmaterial,involvingopticaland
electronmicroscopy,digitalimageanalysisandothertechniques.Allpreviouslyfiguredholotypematerialsareillustratedhere,
andthecontextforallthepublishedmaterialsisre-evaluated.
TheApexchert‘microfossils’occurnearthetopofa1.5-kmlongchertdykecomplexassociatedwithmajorsynsedimentary
growthfaults.Highlylocalised,glassyfelsictuffseruptedexplosivelyfromthisandotherfissuresduringtheearlystagesof
volcanism,andwerefollowedbythedepositionofessentiallyhydrothermalblackandwhiteBaSO richchertsthatinfiltrated
4
thefeederdykes,underplatinganddilatingadjacentstratiformchertsbeforethestartofthenextvolcaniccycle.TheApexchert
‘microfossils’occurwithinmultiplegenerationsofthesemetalliferoushydrothermalveinchertssome100mdownthedyke
system.Comparablestructuresoccurinassociatedvolcanicventglassandinhydrothermalchertsatleast1kmdeep.Wefindno
supportingevidenceforaprimarybiologicalorigin.Wereinterpretthepurportedmicrofossil-likestructuresaspseudofossilsthat
formedfromthereorganizationofcarbonaceousmatter,mainlyduringrecrystallizationfromamorphoustospheruliticsilica.
©2005ElsevierB.V.Allrightsreserved.
Keywords: Apexchert;Archaeanfossils;Earlylife;Warrawoona;WesternAustralia
∗ Correspondingauthor.Tel.:[phone];fax:[phone].
E-mailaddress:[email protected](M.D.Brasier).
0301-9268/$–seefrontmatter©2005ElsevierB.V.Allrightsreserved.
doi:10.1016/j.precamres.2005.06.008
PRECAM-2583; No.ofPages48
2 M.D.Brasieretal./PrecambrianResearchxxx(2005)xxx–xxx
1. Introduction and Packer, 1987; Schopf, 1993; Buick et al., 1981;
Buick, 1984, 1988, 1990). Eleven putative species of
TheextremerarityofArchaean(3.8–2.5Ga)micro- microfossils from the Apex chert have, hitherto, pro-
fossils, in comparison with those from the Protero- videdtheoldestacceptedmorphologicalevidencefor
zoic(2.5–0.54Ga),issurprisinggiventhepresenceof life on Earth. These structures are nearly a billion
tufa-like carbonates (Grotzinger, 1994) and the abun- years older than putative cyanobacterial biomarkers
danceofsedimentarycherts(Buick,1990)atthistime. (Summonsetal.,1999),genomicargumentsfordating
Onepossibleexplanationforthisparadoxisthatmost theappearanceofcyanobacteria(Hedgesetal.,2001)
Archaean cherts had a lower preservation potential, and an oxygenic atmosphere (Catling et al., 2001;
beingformedbyhot,acidic,reducingandconcentrated LindsayandBrasier,2002)andmorethan1500million
hydrothermalfluids,incontrastwiththehigherpreser- yearsolderthananycomparablesuiteofmicrofossils
vationpotentialofrelativelycool,neutralanddiluted sofardescribed(Brasieretal.,2002;Knoll,2003).If
surficialfluidsofmostProterozoiccherts(Buick,1988, accepted,thismustimplythathighlevelsofbiological
1990).Butthiscannotexplaintheabsenceofbacterial diversity were achieved at a very early stage in Earth
moldsintufa-likecarbonatesofthisage.Anotherpossi- history(Schopf,1993),remarkablysoonaftertheend
bility,therefore,isthatArchaeanhabitatswerelargely ofmassivemeteoriticbombardmentoftheinnersolar
volcanogenicorhydrothermalandthattruecyanobac- systematca.3.8–3.9Ga(cf.Kamberetal.,2001),with
terial communities did not emerge until near the end little evidence for further diversification in the fossil
of the era (Brasier et al., 2002). Whatever the rea- record until the emergence of widespread eukaryotes
son, morphological evidence for life beyond 3.0Ga nearlytwobillionyearslater(Knoll,1994,2003).Fur-
is arguably of such great scientific significance, and thermore,thesizerangeofthesupposedcells(<20!m
now appears to be so rare, that rigorous criteria are in diameter) has been taken to suggest that oxygen-
needed for acceptance by the scientific community releasingcyanobacteriamayhavebeenpresentatleast
(Buick, 1990). As others have put it: extraordinary 3.45Ga (Schopf, 1992a,b, 1993, 1994, 1999), imply-
claims require extraordinary evidence. This caution- inganearlystartforthecontributionofphotosynthetic
arydictum,attributedtothelateCarlSagan,received oxygentotheatmosphere.
widepublicnoticeatthepressmeetingcalledbyNASA Thesecurityoftheseclaimsisnowopentoquestion.
onAugust7th1996(RaeburnandGolombek,1998,p. Thisisinpartbecausemajoraspectsofthepreservation
184) to announce and debate the putative microfos- andcontextofthispotentiallyimportantevolutionary
sils from Martian meteorite ALH 84001 (McKay et benchmarkhavereceivedlittleindependentordetailed
al.,1996).ItwasherethatthebiogenicityoftheMar- study,andinpartbecausenewtechniquesofanalysis
tian‘microfossils’wasfirstquestionedandcontrasted arenowavailable.Wehavethereforetakenafreshlook
withtheoldestevidenceoflifeonEarth—microfossils atEarth’soldest‘microfossils’,followinganintegrated
from the 3.46 billion-year-old Apex chert at China- and collaborative programme of research involving
man Creek, in the Warrawoona Group, near Marble field mapping over three field seasons, multiple sam-
BarinWesternAustralia(Schopf,1999;Raeburnand pling, petrography, optical and electron microscopy
Golombek,1998,p.184). coupled to computer-controlled digital image analy-
TheseworldfamousApex‘microfossils’havebeen sisplusgeochemicaltechniques.Wehavere-examined
described in a series of papers (Schopf and Packer, thepetrographicslicesofall‘microfossil’-bearingtype
1987; Schopf, 1992a,b, 1993, 1994; Schopf et al., material deposited at the Natural History Museum
2002a,b). They have rightly held a key position in (NHM)inLondon(Schopf,1993)andcomparedthem
Archaean palaeobiology because of their supposedly withnewslicesandthinsectionsofmaterialrecently
good state of preservation and their wide acceptance re-collected from the same horizon (deposited at the
by the scientific community (e.g. Buick, 1990; Knoll NHMandtheGeologicalSurveyofWesternAustralia,
and Walter, 1996; McClendon, 1999; Schopf, 1999). GSWA). Fabrics and mineralogy in both the original
This contrasts with preliminary reports of other pre- andtherecollectedsamplesaresimilarandbothcontain
sumed ‘microfossils’ from the Warrawoona Group, comparablemicrofossil-likestructures(seeBrasierand
dismissedaseitherunreliableorirreproducible(Schopf Green,2004,Fig.1a–o).Inthispaper,webeginbyre-
M.D.Brasieretal./PrecambrianResearchxxx(2005)xxx–xxx 3
Fig.1. WesternAustraliashowingtheArcheanPilbaraandYilgarnCratons,thedeformedsuturezoneoftheCapricornOrogen(Tylerand
Thorne,1990)andtheyoungersedimentarybasinsthatformedduringsuturing.TheChinamanCreekstudyareaislocatedclosetothetownof
MarbleBar.InsetshowsAustraliancrustalmega-elements(Shawetal.,1996).
evaluatingtheevidencefortheseimportantmaterials. tioningSystems(GPS).Opticalpetrographyandfabric
Wethengoontofalsifytheclaimsforsyngenicityand mappingwasperformedatOxfordonpublishedandre-
biogenicityofthefilamentousstructuresfromtheWar- collectedmaterialwiththeuseof30,150and240!m
rawoonaGroup,andquestionwhethertheApexchert thicksections.Polishedslices,hydrofluoricacid(HF)-
dykeprovidedapotentialhabitatforearlylife,devel- etchedrockchipsandHFresidueswereexaminedina
opingargumentslaidoutbyusinBrasieretal.(2002, Jeol-840AandPhillipsXLS30Sfield-emissionSEM’s,
2004a,b).Ourabioticmodelforthegenerationofthe fitted with a light-element EDX system operating at
microfossil-like fabrics involves the recrystallization 1–15kV.Rockslices,polishedslices,hydrofluoricacid
ofcarbon-richchertinahydrothermalsetting.Wecon- (HF)-etched rock chips and HF residues were also
cludethepaperwithdetailedpetrographicdescriptions examinedatOxfordunderepifluorescence,cathodolu-
ofrockslicescontainingthe11holotypes. minescence, phase contrast and brightfield, polarized
transmitted and incident (reflected) light using Nikon
Optiophot-2 (biological) and Optiophot-pol (polariz-
2. Methods ing)andWild-M8microscopes.Imageswereobtained
usingasingle-chipCCDcamera,providingliveimages
FieldmappingoftheApexchertatChinamanCreek in full RGB colour, and processed using AcQuis and
(see Figs. 1–6) was undertaken by us as part of a Auto-Montage image capturing software. AcQuis is a
widerprogrammewiththeGeologicalSurveyofWest- single-frame image capturing, processing and archiv-
ernAustralia,Perth(VanKranendonk,inpress),sup- ingsystemdesignedspecificallyforopticalmicroscopy
plemented by a detailed collaborative programme of when using brightfield, fluorescent or polarized illu-
mapping and sampling undertaken jointly by Oxford mination. The user can apply a scale-bar and anno-
and NASA, Houston across an area of about 12km2. tation to the processed image, and store this infor-
Multiple samples were collected from the microfos- mation either engraved onto the original image or
sil locality around the site between 1999 and 2003, as a separate file that can be overlain on the orig-
locatedbymeansofsatelliteimagesandGlobalPosi- inal image. Auto-Montage is a more sophisticated
4 M.D.Brasieretal./PrecambrianResearchxxx(2005)xxx–xxx
software package, processing and combining multi- microfossil-likestructure.Allimagesarestoredinthe
ple source images, each obtained at a different focal Oxford digital database of Archaean microfossil-like
planewithinthesection.Processingalgorithmsenable structures.
themostsharplyfocussedareasofeachsourceimage
to be combined into a single well-focused montaged
image. This rendering facility is ideal for obtaining 3. Context
high-resolution images of 3D microscopical artefacts
alignedobliquelytothez-axiswithinaslide.Itissim- 3.1. Regionalsetting
ilartotheestablishedtechniqueofmanualmontage,in
which images of a microfossil-like structure are col- The Pilbara Craton of Western Australia (Fig. 1)
lectedbyopticalphotomicrographyandthenmanually contains some of Earth’s oldest and best-preserved
spliced in the darkroom or laboratory (e.g. Schopf, rocks.ThecratonispartofanArcheanproto-continent,
1992b,1993).Itdiffers,however,inthatselectedfocal comprising granitoid complexes that were emplaced
planes from the montaged image can be displayed into and are overlain by the Pilbara Supergroup, now
on the screen or print. Single-source images can be preserved as greenstone belts (Fig. 1). The Pilbara
viewed independently and, when combined, used to Supergroup consists of five unconformity-bound
scanthroughthez-planeofthestructureortomakea stratigraphic intervals (or groups) that formed as a
movingimagethroughthestructure.Thelatterallows volcanogenic carapace contemporaneous with the
observers to come to their own opinion about any intruding granitoid bodies between ca. 3.51 and
Fig.2. SimplifiedstratigraphyoftheArcheanrocksexposedintheChinamanCreekstudyarea(adaptedfromVanKranendonk,1999,2000)
showingconfirmedisochronagesandformationsthatcontainputativemicrofossilsandstromatolites.Arrowsindicatedatedperiodofactivityof
thegranitoidcomplexesasdistinctfromtheintrusionofsmallerplutonsthatareshownincartoonform.Majorperiodsofhydrothermalactivity
(H)tendtocoincidewithperiodsofintrusion.Putativefossils(1)AwramikandSemikhatov(1979),Awramiketal.(1983);(2and3)Schopf
andPacker(1987),Schopf(1993);(4)Buick(1984),Hoffmanetal.(1999);(5)Rasmussen(2000).
M.D.Brasieretal./PrecambrianResearchxxx(2005)xxx–xxx 5
2.94Ga. Components of the granitoid complexes are This study focuses on the Apex chert of the Apex
broadlycoevalwiththefelsicvolcanichorizonsinthe BasaltthatliesinthemiddleoftheWarrawoonaGroup
PilbaraSupergroup,withwhichtheycanhaveintrusive and is approximately 3.46Ga old (Van Kranendonk,
or sheared intrusive contacts. The five groups were 2000). The Apex chert is geographically limited to
deposited one above the other and they consistently the Panorama Belt of the central Pilbara Craton. The
dip away from the domal granitoids. The dips of the unit consists mainly of tholeiitic basalts and subordi-
beddinggraduallydecreasewithtime,suggestingthat nate high-Mg basalts (komatiitic basalts) that show
theyweredepositedasthickeningwedgesadjacentto abundant evidence for subaqueous eruption includ-
the growing granitoid diapirs (Hickman, 1975, 1983, ingpillows,chilledmarginsandhyaloclasticbreccias.
1984;VanKranendonk,1999,2000;VanKranendonk Doleritic flows and intrusions are also common and
et al., 2002). This succession is believed to have doleritic dykes are locally present (Van Kranendonk,
accumulated in a series of grabens developed in an 2000). The formation includes several thin (<30m
extensional setting that evolved above the intruding thick) chert horizons of which the Apex chert, close
anddominggranitoidcomplexes. toMarbleBarintheChinamanCreekarea,isthebest
The well-preserved Warrawoona Group is known.
10–15km thick and accumulated between ca. 3.5
and 3.4Ga. It consists largely of extrusive volcanic
rocks, with less common interstratified chert, barite, 4. Results
sulphide,carbonateandvolcaniclasticunits(Hickman,
1983). It is these interstratified units that contain the 4.1. Fieldmapping
putative microfossils and stromatolites that currently
provide the focus for much current research into OurmappingshowsthattheApexBasaltsandApex
Earth’searliestbiosphere(Fig.2). chert in the Marble Bar area (Fig. 3) are distributed
Fig.3. DetailedgeologicalmapoftheApexchertintheChinamanCreekarea.Theareaconsistsofthreestructuralblocks,north,centraland
south,definedbygrowthfaults.DykesarenumberedS1toS4inthesouthblockandN1toN4inthenorthblock.Themicrofossilsiteislocated
inthenorthblockapproximately100mpalaeodepthdowntheN1dyke.
6 M.D.Brasieretal./PrecambrianResearchxxx(2005)xxx–xxx
thenorth.Thisvolcaniclasticunitcanbeseentooverlie
vesicularpillowbasaltsofunit1.ThestratiformApex
chert (unit 4) is underlain by numerous black chert
dykes that invade these faults, with the largest dyke
chertsmarkingboththeS1andS4faults.Thestratiform
Apexchertisherecappedbyapyroclasticbrecciabed
(unit 5), ca. 3m thick and thinning northwards, with
clasts (<1.0m across) that also fine along the strike
northwards.Thisunitcontainslargeclastsoftheunder-
lying lithologies, including both stratiform and dyke
chert lithologies, and pumice. It is taken to mark the
startofthenextvolcaniccycle(units5–9,Fig.3).This
secondvolcaniccycleislargelyobscuredintheSouth
and Central Blocks by an erosional unconformity at
thebaseoftheoverlying 2700MaFortescueGroup
∼
(Fig.3).
TheCentralBlocklacksvolcaniclasticunits2and
5 (Fig. 3), implying uplift and non-deposition or ero-
siononthisblockduringthesephases.TheApexchert
is also very thin here and the black chert dykes are
correspondinglyscarceandsmall.TheNorthBlockis
defined along its southern margin by the ca.1500m
long N1 fault and its associated chert dyke system
Fig. 4. Photographs of field relationship of Apex chert. (a) View (Figs. 3, 4 and 6) that yields the questionable Apex
looking SW of dykes S1, S2, S3, S4 (outlined in yellow) in the
‘microfossils’ (Schopf, 1993; Brasier et al., 2002). A
southern fault block, feeding up into a ridge of stratiform Apex
wedge of grey-green tuff and ignimbrite with devit-
chert(outlinedalongtheskylineingreen);(b)sub-roundedclasts
ofmegaquartzinblackchertfromdykeS4;(c)stratiformApexchert rified glass shards occurs near the top of this dyke
exposedinthebedofChinamanCreekshowingimbricate,tabular (unit 3, Figs. 3 and 6) here interpreted as an ini-
chertclaststhatcouldbeconfusedwithcross-bedding;(d)viewlook- tial volcanic eruption from the fissure now occupied
ingsouthofN1dykefeedingupintothestratiformApexchertridge
by the dyke chert. Similar felsic tuffaceous mate-
withChinamanCreekintheforeground—astarmarksthefamous
rial also occurs within the stratiform chert unit (unit
‘Microfossil’locality;scalebar5cm.
4, Fig. 3) where it has been silicified. This glassy
volcaniclastic unit pinches out northwards to disap-
acrossthreedistinctstructuralblocks,eachdefinedby pear beneath the stratiform Apex chert (Fig. 3). The
majorfaultsystems.Eachblockcanberecognisedby overlyingpyroclasticbrecciamarkerbed(unit5)that
distinctive patterns of sedimentation and volcanicity seemingly originates from the S1 fault (see above),
withinthefollowingsuccessionfromthebaseupwards: reappears in the North Block but continues to atten-
unit1,pillowbasaltsandkomatiites;unit2,grey-green uate in both thickness and grain size towards the
tuffs;unit3,felsictuffsandignimbrites;unit4,strat- north. A distinctive bed of fine grey tuff (unit 8,
iform Apex chert; unit 5, pyroclastic breccia, fining Figs. 3 and 6) thickens towards the N1 fault and
up into 6, green tuffs; units 7 and 9, komatiitic pil- also wedges out rapidly towards the north. This unit
lowbasalts;unit8,fine-grainedgreytuffwithinpillow is not seen in the Central and South Blocks and is
basalts. inferredtohaveeruptedinthevicinityoftheN1fault
IntheSouthBlock(Figs.3–5)thestratiformApex system.
chert (unit 4) is underlain by a wedge of grey-green Stratiform units within the Apex chert of this area
to bright green tuffs (unit 2, Fig. 3) whose outcrop aremainlycharacterizedbyplanarbedding,withmod-
pinches and swells in relation to local faulting, being eratetogoodlocalgrainsortingandgrainorientation.
muchthickerneartotheS1faultandthinnertowards Several volcanic-sedimentary subcycles are present,
M.D.Brasieretal./PrecambrianResearchxxx(2005)xxx–xxx 7
Fig.5. DetailedgeologicalmapofS4dykeinthesouthblockoftheChinamanCreekstudyarea.Notethepresenceofroundedblocksof
stratifiedchertupto450mpalaeodepthdownthedykesystem.
showing rhythms from 1 to 5m thick that typically Dyke cherts are abundant in both the South and
begin with grey-green, planar bedded siliceous tuffs North Blocks (Fig. 3). They are characterised by a
plus red-brown jaspilitic banded chert (1–4cm thick cross-cutting geometry; by a lack of grain sorting or
layers), passing upwards into grey, black or black- grain orientation in the matrix; by multiple genera-
white planar bedded chert (1–10cm) and thence into tions of fracturing, spalling (with jigsaw puzzle fit of
black-whitebandedchertswithlocalisedsoftsediment clasts), fissure formation and fissure filling; by fault-
deformation and breccias, indicating zones of lique- rotatedgeopetalfabrics;byhydrocarbon-impregnated
faction,cavityformationandcollapse.Asimilarshift, botryoidal and spherulitic chalcedony and chert; by
from tuffaceous and jaspilitic chert near the base, to associatedmegaquartzveining.Thesedykechertstend
black and brecciated chert, can be seen in the strati- tofanupwards,andtobecomeboththickerandmore
form chert as a whole, especially to the north of N4. multiphase upwards. In the South Block, dyke cherts
Earlier reports of cross-bedding from the bed of Chi- appear to fan outwards from a depth of about 500m
namanCreek(e.g.Schopf,1993)arere-interpretedas beneath the paleosurface. At greater depth are found
imbricate slabs of chert that have slumped within a discontinuousdykesandhorizontalsillsofgreychert
zone of liquefaction (Figs. 4d and 5a). Such features plussillsofveinquartz(Fig.3).
are common. Even so, hummocky cross-stratification Twoblackchertdykesystemshavebeenstudiedby
fromthepossiblycoevalAntarcticCreekchertsofthe usindetail(seeFigs.3–6).TheS4dykecomplexhas
NorthPolearea,suggeststhatwaterdepthswereclose acurvedoutcropthatextendsforsome500mbeneath
tostormwavebaseinpartsofthebasin(VanKranen- the stratiform cherts, extending along the line of the
donk,2000). S4faultthatmarksthenorthernboundaryoftheSouth
8 M.D.Brasieretal./PrecambrianResearchxxx(2005)xxx–xxx
Block(Figs.3and4a).Thegrey-greenvolcanoclastics Acomparablepatternofrelationshipsisseenalong
ofunit2intheSouthBlockcannotbetracedacrossthis dykeN1(Figs.3,4cand6A),whichhoststhefamous
fault (Fig. 5), suggesting they were not deposited or ‘microfossil’SiteatChinamanCreek.Thecountryrock
werelaterremovedpriortodepositionoftheverythin of the North Block here comprises silicified pillow
anddiscontinuousstratiformApexchertoftheMiddle basalts of unit 1. Glassy tuffs of unit 3 occupy a tri-
Block.Bywayofcontrast,thereisamarkedthicken- angular outcrop near the top of the dyke (Fig. 6A),
ing of the stratiform Apex chert around the mouth of presumed to be similar in age to unit 2 of the South
theS4dykeontheSouthBlock.Thisthickeningmay Block(Fig.3).ThereisaslightthickeningoftheApex
in part be due to greater accommodation close to the chert immediately to the north of the dyke, due to
fault,andinparttothedilationaleffectsofblackchert greateraccommodationspaceplustheeffectsofdila-
sills that intrude southwards from the dyke (Fig. 5). tionbyblackchertsills.DistinctivefeaturesoftheN1
Rotated and scattered blocks of black chert are also dykeincludeitsgreatlength(over1600m),itsassoci-
found around the upper part of S4, presumably dis- ationwithamajoroffsetintheunderlyingMarbleBar
placed by faulting (Fig. 5). The upper part of the S4 chert(Fig.3)andwithnumerous,upwardlybranching
dyke complex comprises black cherts in which float anddivergingblackchertdykes,includingtheN2dyke
large (<1m), angular to rounded blocks of stratiform (Fig.3).
chert(Fig.4b).Theseclastsappeartohavefallendown None of the dyke systems in Fig. 3 are found
the dyke system from the palaeosurface, diminishing to extend to stratigraphic levels above the strati-
insizedownwards,beinginconspicuousinthebottom form cherts of unit 4 except, that is, for minor out-
200m of the black chert dyke. At greater depth are cropsclearlyassociatedwithmajorfaultdisplacements
found a few small isolated chert dykes plus a series (Figs. 5 and 6A). Black chert clasts are found within
ofnorth–southtrendingsillsofwhiteveinquartzand thepyroclasticbrecciaofunit5atthebaseofthenext
browncarbonate-filledveins(seeFig.3). volcaniccycle.Thesefeaturesareconsistentwithaccu-
Fig.6. (A)Detailedgeologicalmapofthemicrofossilsite(Schopf,1993)andtheN1dyke;(B)microprobeanalysesof(a)bariumsulphate
(barite),(b)chromiumoxide(chromite),(c)aluminiumsilicate,(d)titaniumoxide;(C)summarytableofmineralphasesdetectedbymicroprobe
andisotopicgeochemicaldata.
M.D.Brasieretal./PrecambrianResearchxxx(2005)xxx–xxx 9
Fig.6. (Continued).
10 M.D.Brasieretal./PrecambrianResearchxxx(2005)xxx–xxx
Fig.7. Endmemberchertmicrofabrics(a–f)andtheirtypicalpositionswithinstratiformunitsinan‘Apex-type’hydrothermaldykesystem
(schematiccentre).Images(g–l)showtypicalmicrofabricevolutionatdepthwithinthefeederdyke(seeFig.8fordetailofcarbonaceouschert
and‘microfossil’formation).Scalebar:1mm(a,f,kandl);400!m(b,c,d,e,g,h,iandj).
mulationofthedilationalblackchertsalongtheN1to rocks at depth. The jaspilitic cherts (Fig. 7b and c)
N4faultsystems,atatimebroadlycontemporaneous typicallyconsistofmm-scalelaminationsofhematite-
withthedepositionofthestratiformApexchertsofunit richchertwitha‘micropeloid’fabric,alternatingwith
4andpriortotheextrusionofunit5. hematite-poor chert containing small floating rhomb
ghostsinfilledwithsilicaafterironcarbonate(Fig.7b).
4.2. Chertmicrofabrics Suchchertsarebelievedtohaveformedontheseafloor
in relation to hydrothermal emanations from subma-
Petrographicmicrofabricsofthekindtypicallyseen rinevents(cf.Nijmanetal.,1998).Manyofthebanded
in and around the Apex chert dyke systems (S1 to black-and-whitechertsmaybesecondaryinnature(see
N4) are illustrated in Figs. 7–9. Stratiform beds of below) but others are certainly primary because they
theApexchertmainlyconsistofthreedistinctlitholo- show clear lamination, with grain orientation, sorting
gies:volcanoclastics,includingdevitrifiedglassytuffs andpacking(Fig.7d).Typically,thesegrainsareoftwo
(Fig. 7a); jaspilitic cherts (Fig. 7b and c); banded maintypes:irregular,lobatetofluffygrainsofcarbona-
black-and-white cherts (Fig. 7d). Of these, the devit- ceousmatter(Fig.7e);andelongategrainsoflaminated
rifiedglassytuffscommonlyshowcontortedshardsset pale silica and dark carbonaceous matter (Fig. 7d).
withinachalcedonicchertmatrix.Occasionally,asin Thesearesuperficiallysimilartofragmentsofputative
unit 3 at Chinaman Creek (Fig. 6A), the shards may microbiallaminae(cf.Schopf,1993,Fig.3;Walshand
have carbonaceous outer margins and voids infilled Lowe, 1999, Fig. 3c), but in this case are laminated-
with carbonaceous chert. Such cherts are assumed to black volcanogenic cherts, best seen immediately to
haveoriginatedfromtheexplosiveeruptionofviscous the south of Chinaman Creek near the ‘Microfossil’
andsiliceousmagmaontheseafloor,withthecarbon Site(Fig.6).NearthetopofN1dyke,theselaminated
providing evidence for eruption through carbon-rich fabrics are overprinted by later growths of rhomboid