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Precambrian Research
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rchean gravity-driven tectonics on hot and flooded continents: Controls onong-lived mineralised hydrothermal systems away from continental margins
. Thébauda,∗, P.F. Reyb
Centre for Exploration Targeting, University of Western Australia, School of Earth and Environment, M006, 35 Stirling Highway, WA 6009, Perth, AustraliaEarthbyte Research Group, School of Geosciences, University of Sydney, NSW 2006, Sydney, Australia
r t i c l e i n f o
rticle history:eceived 1 February 2011eceived in revised form 17 February 2012ccepted 1 March 2012vailable online 8 March 2012
We present the results of two-dimensional numerical modelling experiments on the thermal evolutionof Archean greenstones as they sink into a less dense, hot and weak felsic crust. We compare this thermalevolution to that obtained via the analysis of isotopic data and fluid inclusion microthermometry dataobtained in the Paleoarchean to Mesoarchean Warrawoona Synform (Eastern Pilbara Craton, WesternAustralia). Our numerical experiments reveal a two-stage evolution. In the first stage, cooling affectszones of downwelling as greenstone belts are advected downward, whereas adjacent domes becomewarmer as deep and hot material is advected upward. We show that this is consistent with stable isotopesdata from the Warrawoona Synform, which reveal an early episode of seafloor-like alteration (90–160 ◦C)strongly focused along steeply dipping shear zones. In a second long-lived stage, lateral heat exchangesbetween domes and basins dominate the system as domes cool down while downwelling zones becomeincreasingly warmer. In the Warrawoona greenstone belt, stable isotopes in gold-bearing quartz veinspost-dating the sagduction-related vertical fabrics reveal that rock–fluid interaction occurred at muchhigher temperatures (234–372 ◦C) than seafloor-like alteration. We propose that emplacement of thick
and dense continental flood basalts, on flooded hot and weak continental plates, led to conditions partic-ularly favourable to hydrothermal processes and the formation of mineral deposits. We further argue thatsagduction was able to drive crustal-scale deformation in the interior of continents, away from plate mar-gins. On largely flooded continents, sagduction-related shear zones acted as fluid pathways promotinggold mineralisation far away from active plate boundaries, continental rift zones or collisional mountainbelts.
. Introduction
Several features made the Paleo- to Mesoarchean landscape3.6–2.8 Gyr old) fundamentally different than it is today, andarticularly favourable to the formation and preservation of goldeposits. First, most Archean cratons are buried under up to 15 kmhick blanket of autochthonous Continental Flood Basalts (CFBs,.g. Maurice et al., 2009), which include high-magnesium basaltsnd komatiites that have only few equivalents on modern Earth.econd, despite their thicknesses most of these thick volcanic pilesere emplaced below, and remained below, sea level for most
f their Archean history (Arndt, 1999; Flament et al., 2008, 2011;ump and Barley, 2007). This characteristic is of great importance
s it implies that an infinite fluid reservoir was available to feedydrothermal circulations in the CBFs. Third, radiogenic heat pro-uction in the continental crust was much higher than it is today
which, in combination to the thermal insulation effect of CFBs,provided the heat engine, in the form of a strong thermal gradient,to power hydrothermal cells in the CFBs. Fourth, Paleoarcheanto Mesoarchean cratons throughout the world are characterisedby ovoid granitic domes, 40–100 km in diameter, encircled bygreenstone belts. Greenstone belts form strongly foliated ver-tical sheets connected through vertical triple junctions whereconstrictional fabrics dominate (Bouhallier et al., 1995; Chardonet al., 1996; McGregor, 1951). This dome and basin pattern, whichcharacterises many Archean cratons, is commonly interpreted interms of gravitational sinking (sagduction) of dense greenstonebelts into hot, and therefore weak, felsic crust (Chardon et al., 1996,1998; Collins et al., 1998; Dixon and Summers, 1983; Mareschaland West, 1980; McGregor, 1951). This process is often wronglydescribed as “vertical tectonics”. Indeed, during sagduction hor-izontal and vertical displacements are perfectly coupled. Domes
can rise and greenstone keels can sink because horizontal dis-placements provide the necessary space for vertical mass transfer(e.g. Mareschal and West, 1980). This process is also often wronglyopposed to plate boundary driven deformation. Indeed there is
o mechanical incompatibility between sagduction and plateectonics processes as both can coexist spatially and temporarilye.g. Bloem et al., 1997; Rey et al., 2003). While Archean lode goldeposits are interpreted traditionally as hydrothermal systemseveloped along convergent margins (Groves et al., 1998), sagduc-ion provided a mechanism to deform the interior of continents,ence greatly enlarging the domain where mineral deposits could
orm. Fifth, considerations on the rheology of Archean continentalithospheres suggest that continents in the Archean were unable toustain elevated mountain belts and high orogenic plateaux (Reynd Houseman, 2006; Rey and Coltice, 2008). Elevations > 3000 mould have been absent at the surface of the Earth, which wouldave strongly limited erosion, providing an explanation for theemarkable preservation of Archean supracrustal environments.
e suggest that these Archean specificities provide some ratio-ales to explain the exceptional endowment in mineral resources
n general, and in gold in particular, of most Archean cratons whenompared to younger terranes (Goldfarb et al., 2001).
In this paper, we present results of two-dimensional coupledhermo-mechanical numerical experiments on the sagduction ofreenstones into a hot and weak felsic crust. We compare the mod-lled thermal evolution of greenstones to that obtained throughtable isotopes analyses and fluid inclusions microthermometryn shear zones from the Warrawoona Synform, in the East Pil-ara Granite-Greenstone Terrane (EPGGT, Fig. 1a). Results point toprotracted two-stage hydrothermal history involving: (1) low-
emperature syn-deformation fluid–rock interactions during thearly stage of sagduction, and (2) mid-temperature syn- to post-eformation fluid–rock interactions during thermal relaxation andarming of sagducted greenstone sheets. We argue that sagduc-
ion of thick greenstone piles, with a large and near-permanenteawater reservoir above and a hot and weak basement below,ed to efficient, crustal-scale and long-lived plumbing systems thatavoured fluid–rock interactions and the genesis of gold deposits inhe interior of Paleoarchean to Mesoarchean cratons.
. The East Pilbara Craton: an example ofagduction-related “simple structural complexity”
The East Pilbara Granite Greenstone Terrane (Fig. 1a) is one ofhe best documented examples of dome-and-basin pattern inter-reted in terms of gravitational instability (Collins, 1989; Delort al., 1991; Hickman, 1983; Teyssier et al., 1990; Thébaud, 2006;an Kranendonk et al., 2004a), although alternative views exists
Blewett, 2002; Kloppenburg et al., 2001). The East Pilbara Granitereenstone Terrane provides a Paleoarchean to Mesoarchean geo-
ogic backdrop to study tectono-thermal processes and fluid–rocknteractions in a sagduction setting. It preserves a geologic historyften largely overprinted in many Neoarchean cratons. The bulkf the domes consist of 3324–3300 Ma old syn- to post-kinematicuites of high-K granitic suites that are derived from, and intru-ive in, older 3460–3430 Ma Tonalite–Trondjhemite–GranodioriteTTG) gneisses and greenstones of the Warrawoona GroupHickman, 1983; Hickman and Van Kranendonk, 2004; Smithiest al., 2003; Van Kranendonk et al., 2002, 2007). The domes arehemselves intruded by younger, mainly 3300–3240 Ma old, gran-tes. The older TTG gneisses formed the basement of greenstones,he emplacement of which is associated with two major volcanicycles starting with the deposition of Warrawoona Group at ca.490 Ma and followed by the deposition of the Kelly Group at ca.335 Ma (Fig. 2) (Van Kranendonk et al., 2007). The thicknesses
f the greenstone covers show significant lateral variation fromto 12 km for the Warrawoona Group, and 4–9 km for the Kellyroup (e.g. Hickman, 1983; Van Kranendonk et al., 2007). Con-iderations on both the present crustal thickness of the Pilbara
Research 229 (2013) 93–104
(35–37 km) and the average erosion level (ca. 7 km) suggest thatthe greenstones accumulated on top of a 30–35 km thick conti-nental crust. Ubiquitous pillow-lava, hydrothermal cherts, VMSmineralisation, epidote-chlorite-Ca–Na-plagioclase-Ca-amphibolesecondary mineral assemblages and intense silicification through-out the greenstone pile imply subaqueous emplacement (Barley,1984; Barley and Pickard, 1999; Buick and Barnes, 1984; DiMarcoand Lowe, 1989; Van Kranendonk, 2006).
The structural complexity of greenstone covers is a function oftheir position with respect to the domes. On the NE flank of theMount Edgar dome, the Marble Bar greenstone belt lies directlyon top of the Mount Edgar granitic complex and is structurallypart of the dome (Fig. 1b). In the Marble Bar greenstone belt, thestratigraphy of the Warrawoona and Kelly Groups is relatively wellpreserved and structurally simple with a monotonous dip of 20–60◦
to the NE. In contrast, in the Warrawoona synform (white star inFig. 1b1), greenstones are steeply dipping and strongly deformedagainst the near-vertical southwest margin of the Mount Edgardome (Fig. 3). At this location, the greenstone cover belongs toa basin pinched between the Mount Edgar dome to the northand the Corunna Down dome to the south. Compared to that ofthe Marble Bar greenstone belt, the structure in the Warrawoonasynform is more complex showing multiple phases of folding, aprominent horizontal to vertical stretching lineation, and numer-ous shear zones and quartz vein arrays (e.g. Thébaud et al., 2008).Contrasting, yet predictable structural complexity, with complexstructures above downwelling regions (e.g. Warrawoona syncline)and simpler structures above rising domes (e.g. Marble Bar belt) isa key attribute of sagduction settings (e.g. Bouhallier et al., 1995;Thébaud, 2006).
3. Numerical experiment setup
In order to interpret the thermal history derived from isotopicstudies, and to understand fluid flow in a sagduction setting, wehave performed a series of numerical experiments to documentthe thermal and mechanical history of sagducted greenstones.The process of sagduction has been tested through numerical andphysical experiments (de Bremond d‘Ars et al., 1999; Dixon andSummers, 1983; Mareschal and West, 1980; Robin and Bailey,2009; West and Mareschal, 1979). This process is driven by theneed for minimisation of internal gravitational potential energyof a system involving a density inversion (denser layer above alayer of lower density). It is resisted by the viscosity of the sys-tem, in particular by that of the stronger layer involved in thesagduction. Hence, the timing of sagduction is inversely propor-tional to the density contrast and proportional to the viscosity ofthe stronger layer. The emplacement of greenstones increases thegeothermal gradient (e.g. Rey et al., 2003; Sandiford et al., 2004;West and Mareschal, 1979), which in turn reduces the viscosityand accelerates sagduction. To model this process, we use Ellip-sis, a Lagrangian integration point finite element code capable oftracking time dependent variables in combination with an Eule-rian mesh. This coupled Lagrangian/Eulerian approach allows forthe accurate tracking of density interfaces during large deformation(Moresi et al., 2001, 2002). We use viscoplastic rheologies mim-icking standard rheological profiles for the continental lithosphere(e.g. Brace and Kohlstedt, 1980). Our experiments include realisticgeotherms with self-radiogenic heating and partial melting withfeedback on viscosities and densities.
In our numerical experiments, the greenstone cover is 15 km
thick in average, consistent with the thickness of many Archeangreenstone covers, and in particular the average cumulative thick-ness of the Warrawoona and Kelly Groups in the EPGG (e.g. VanKranendonk et al., 2007) to which we will confront our results.
N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104 95
Fig. 1. (a) Simplified Geological Map of the North Pilbara Terrain modified after Van Kranendonk et al. (2002). EPGGT, East Pilbara Granite-Greenstone Terrane; CPTZ, CentralPilbara Tectonic Zone; WPGGT, West Pilbara Granite-Greenstone Terrane; MB, Marble bar greenstone belt. Legend: (1) Greenstones, (2) Granitoid complexes, (3) Sedimentarybasins, (4) Hamersley basin, (5) Phanerozoic cover. (b1) Simplified structural sketch of Mount Edgar and Corunna Down granitic domes complexes and surrounding regions.(b2) The first derivative of the topography shows that domes (thick dashed lines) are characterised by gentle topography and include both granitic rocks (crossed region ont charaW hexag
StGdtr7AtV
he right panel) and greenstone cover (grey region). In contrast a rough topographyarrawoona Synform (white star), and the mining district of Bamboo Creek (white
mall thickness variations are introduced to allow for the initia-ion of sagduction. Gravity modelling in the East Pilbara Granitereenstone Terrane (Blewett et al., 2004) constraints the bulkensity of our CFBs (2840 kg m−3). These CFBs are emplaced inhree successive 2.5 km thick layers at t0, t0 + 30 myr (lower War-awoona Group), t0 + 60 myr (upper Warrawoona Group), plus a
.5 km thick layer (the Kelly Group) emplaced at t0 + 140 myr.ssuming t0 = 3490 Ma, this emplacement history approximates
hat of the Warrawoona and Kelly Groups (e.g. Bagas et al., 2002;an Kranendonk et al., 2007). Our model CFBs are deposited on top
cterises the basins surrounding the domes. The mining districts of Klondike in theon) are both located above downwelling regions.
of a 30 km thick basement to which we assign a depth-independentdensity of 2720 kg m−3. In comparison to Robin and Bailey (2009),our experiments use a smaller density contrast between greenstoneand basement (120 kg m−3 vs 200 kg m−3), a thicker greenstonecover (15 km vs 10 km), and the emplacement of the greenstonecover follows a stepwise history over 140 myr rather than an instan-
taneous emplacement.
Deformation in our models follows the mechanism that requiresthe least differential stress amongst: frictional faulting (Coulombcriteria), plastic faulting and viscous creep. The frictional faulting is
96 N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104
Fig. 2. Lithostratigraphy of the Pilbara Supergroup. Geochronology references: a (Buick et al., 1995), b (Thorpe et al., 1992), c (McNaughton et al., 1993) and d (Nelson, 1999,2000, 2001).
Table 1List of parameters used in the models.
Parameters Value(s) Unit
(a) Mechanical parametersg: Acceleration of gravity field 9.81 m s−2
�atm: Air density 2.0 kg m−3
�cfb: CFB density 2840 kg m−3
�cc: Crustal density 2720 kg m−3
�m: Mantle density 3310 kg m−3
Ccfb: CFB cohesion 10 MPaCcc: Crustal cohesion 10 MPaCm: Mantle cohesion 40 MPaεa: Strain weakening factor 0.2ε0: Strain from which weakening is maximum 0.5εn: Strain weakening sensitivity to accumulated strain 0.5�cfb: CFBs maximum yield stress 100 MPa�cc: Crustal maximum yield stress 250 MPa�m: Mantle maximum yield stress 400 MPa�cfb: CFBs internal angle of friction 5 ◦
�cc: Crustal internal angle of friction 15 ◦
�m: Mantle internal angle of friction 25 ◦
�air: Air viscosity 5 × 1018 Pa sAcfb: CFBs pre-exponential constant 5 × 10−5 MPa−n s−1
(c) Partial melting parametersLatent heat 250,000 JCFBs: Solidus (P) = 993–1.2 × 10−7·P + 1.2 × 10−16·P ◦CLiquidus (P) = 1493–1.2 × 10−7·P + 1.2 × 10−16 aPBasement: Solidus (P) = 983–9.37 × 10−8·P + 6.32 × 10−17·P ◦CLiquidus (P) = 1393–9.37 × 10−8·P + 6.32 × 10−17·Pa Non listed values for the CFB are taken equal to crustal values.
N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104 97
998; K
difsa
i
aptdTs1
Fig. 3. Geological map of the Warrawoona Synform (after Collins et al., 1
escribed by a yield stress: �yield = (C0 + tan (˚) �n) f(ε), in which C0s the cohesion at atmospheric pressure; ˚ is the angle of internalriction, �n is the normal stress. Strain localisation is achieved viatrain weakening function f(ε), which reduces the yield stress as theccumulated strain (ε) increases. The strain weakening function
s given by: f (ε) ={
1 − (1 − εa)(ε/ε0)εn , ε < ε0εa, ε ≥ ε0
where ε is the
ccumulated strain, taken as the second invariant of the deviatoriclastic strain tensor, and ε0 is the “saturation” strain from whichhe yield stress is reduced by a proportion εa. εn modulates the
ependency between accumulated strain and strain weakening.he yield stress has an upper limiting value �crit, which describesemi-brittle deformation independent of pressure (Ord and Hobbs,989). For differential stresses that attain the yield stress, the
loppenburg et al., 2001), localisation on the bottom right panel in Fig. 1.
material fails and deformation is modelled by an effective viscos-ity: nyield = �yield/2E in which E is the second invariant of the strainrate tensor. In our model, viscous rheologies are based on bothdislocation creep and diffusion creep (Bürgman and Dresen, 2008).This choice seems justified given that (i) diffusion creep is favouredby relatively small gravitational differential stress such as the oneinitiating sagduction, and (ii) diffusion creep is an important flowmechanism in feldspar-bearing assemblages prevalent in TTG.Therefore, viscous creep in our model crust is modelled usingthe fastest of dislocation creep and diffusion creep mechanisms
−1
(Turcotte and Schubert, 1982) following: �eff = (1/�dis + 1/�diff) .Both creep regimes can be described by an Arrhenius formulation(Karato and Wu, 1993): � = (N0Exp[Q/(RT)]E1−n)1/n with E thesecond invariant of the strain rate tensor, Q the activation energy
9 brian
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8 N. Thébaud, P.F. Rey / Precam
f the creep mechanism, and n the power law stress exponent.or dislocation creep viscosity: N0dis = 1/(2ndisAdis), with Adis there-exponential factor in the power law (parameter values fromrace and Kohlstedt, 1980, Table 1). In the crust, diffusion creepecomes important under low differential stress, high tempera-ure, and when partial melt is present (Mecklenburgh and Rutter,003). For many rocks, parameters Q and A for diffusion creepre unknown. However, they must lead to faster strain rate atifferential stress lower than a given transition differential stresshere �trs = 30 MPa) at which both mechanisms result in the sametrain rate. In our models, we assume that the activation energiesor dislocation creep and diffusion creep are the same and we drophe N0 factor for diffusion creep to a value for which the dislocationnd diffusion viscosities are equal at the transition differentialtress. For diffusion creep, viscosity n = 1 and N0 is obtained byatching both diffusive and dislocation strain rates at the transi-
ion differential stress (�trs): N0dif = N0dis = 1/(2ndisAdis)·�trs1−ndis.
e disregard diffusion creep in the mantle because the transitionifferential stress between dislocation creep and diffusion creep is
ow (ca. 0.5 MPa, Turcotte and Schubert, 1982). In all experiments,iscosities are clipped beyond a minimum of 5 × 1018 Pa s and aaximum of 5 × 1022 Pa s. All parameters are reported in Table 1.In most CFBs, magmatic mineral assemblages are deeply altered
nto micas and talc-bearing retrogressive assemblages. This alter-tion reduces both the density and strength of mafic rocks (deremond d‘Ars et al., 1999). Hence, we assume an effective viscos-
ty one tenth of that of the basement. This is achieved by imposinghat: N0cfb = 0.1N0Crust (i.e. Acfb = 10ACrust, Table 1).
In our experiments, the radiogenic heat production in the crusts calculated backward in time to 3.5 Ga from the averaged crustaleat production of modern Archean crusts (Taylor and McLennan,995). The total crustal radiogenic production is partitioned intohe pre-greenstone basement and we assume little heat produc-ion in the greenstone cover and none in the mantle (cf. Table 1).ssuming a basal heat flux of 25 × 10−3 W m−2, the steady-stateoho temperature is at 710 ◦C before the emplacement of the firstember of the CFB. A free-slip boundary condition is applied to
orizontal and vertical boundaries and no plate boundary force ispplied. Deformation is achieved through gravity-driven processesnitiated by small variations in thickness of the greenstone coverFig. 4).
Partial melting impacts significantly on the thermo-mechanicalroperties of the system (temperature, viscosities and densities). Inur numerical experiments, viscosities decrease over three ordersf magnitude as the melt fraction increasing from 0.2 to 0.3.n nature, the viscosity drop reaches many orders of magnitudeClemens and Petford, 1999), however only the first two or threerders are likely to have mechanical significance. As the temper-ture increases from the solidus to the liquidus, the density ofhe partially melted region decreases by 13%, which increases itsuoyancy. Solidus and liquidus parameters are reported in Table 1.
. Results
Crustal flow: From t0 to t0 + 140 myr, the Moho temperaturencreases from 704 ◦C to 772 ◦C leading to onset of partial melt-ng, which is limited to only a few percent of melt at the veryase of the crust (Fig. 4b1). This temperature increase is due tohe thermal insulation of the radiogenic crust progressively buriednder the greenstone cover (Mareschal and West, 1980; Rey et al.,003; Sandiford et al., 2004; West and Mareschal, 1979). There is
o mechanical disturbance of the greenstone-basement interfaceFig. 4b1), which is consistent with the concordant contact betweenhe Kelly Group and the Warrawoona Group (Van Kranendonkt al., 2004b). At t0 + 150 myr, 10 myr after the deposition of the
Research 229 (2013) 93–104
7.5 km thick Kelly group, partial melting affects a growing portionof the lower crust facilitating the initiation of gravitational insta-bilities which strain the greenstone cover (Fig. 4b2). Convectivemotion affects the deep crust. From t0 + 150 myr to t0 + 158 myr Ma,a couple of gravitational instabilities develop, one at ca. 152 myr(Fig. 4b3), the other at ca. 158 myr (Fig. 4b6). Their length-scale (ca.100 km) and time-scale (10 myr) are compatible with those of thedome and basin pattern of the East Pilbara Granite Greenstone Ter-rane (Collins et al., 1998), and consistent with the results of Robinand Bailey (2009). In the upper crust, above downwelling regions, asmuch as 60% bulk horizontal shortening accommodates sagductiondownward flow. Upward motion in domes and downward motionin basins are coupled through horizontal motions in the upper andlower part of the crust. Notably, enclaves of greenstones are caughtinto the upward flow in domes and exhumed from the base of thecrust into the upper crust.
Thermal evolution: From the onset of sagduction at t0 + 148 myr,downward flow advects cool greenstone rocks in downwellingregions where the temperature at 10 km depth decreases from ca.210 to ca. 90 ◦C (Fig. 5a). In contrast, in rising domes the temper-ature at 10 km depth increases from ca. 210 to ca. 660 ◦C (Fig. 5a).In about 12 myr, partial convective overturn leads to the build-up of a long-wavelength (ca. 100 km) profound lateral thermalanomalies >500 ◦C through fast advective cooling and heating ofdownwelling and upwelling regions respectively (Fig. 5a). The max-imum horizontal temperature gradient reaches 26 ◦C/km. Fromt0 + 158 myr onward, following the main sagduction stage, tem-perature anomalies, around fully developed domes, decrease. Thissecond post-deformation stage, led to rapid conductive heating ofgreenstone basins and conductive cooling of domes (Fig. 5b). Thetemperature at 10 km depth in the basins increases from 90 to200 ◦C in ca. 8 myr, whereas the temperature at the same depthin the domes decreases from 660 to 310 ◦C (Fig. 5b). Our numeri-cal experiments also revealed that, because of the sheer size of thethermal anomalies, and in part because domes are diachronous,deformation and thermal relaxation last over 150–200 myr afterthe bulk of sagduction. This is compatible with the folding of theGorge Creek Group (<3240 Ma) as well as the hornblende Ar–Arages in the EPGGT that spread from 3350 to 3100 Ma (Davids et al.,1997; Kloppenburg et al., 2001; Wijbrans and McDougall, 1987).
5. Comparison to temperature records derived frompetrological and isotopic data
To verify the 2-stage thermal history documented in ournumerical experiments we compare it to that derived from geo-chemical and isotopic data from the Warrawoona Synform (Hustonet al., 2001; Thébaud et al., 2008). In the Warrawoona Synform,hydrothermal alteration has been described as strongly partitionedinto ductile shear zones that accommodated the sagduction of thegreenstones cover (Fig. 3) (Thébaud et al., 2006). The Fielding’sFind Shear Zone, parallel to the northern border of the felsic vol-canic Wyman Formation (Fig. 3), has been recently the focus of adetailed fluid–rock interaction study. A section across this shearzone shows that the increase in strain correlates with enhancedhydrothermal alteration, including strong silicification, coupled toa pronounced zoning of major elements, trace elements and oxygenisotopes values (Thébaud et al., 2008). The bulk rock ı18O val-ues increase towards the shear zone from 19 to 25‰, indicating afluid-buffered system (Fig. 6). This zonation is consistent with theshear zones having acted as fluids pathways. Similar extreme (up
to 18‰) 18O-enrichments have been documented in metabasalts ofthe Superior Province (Kerrich et al., 1981), and in association withPaleoarchean to Mesoarchean hydrothermal vents in Barberton (deWit et al., 1982; Knauth and Lowe, 2003; Lowe and Byerly, 1986).
N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104 99
Fig. 4. Initial settings (top box) and snapshots of the numerical experiment from t0 + 140 to t0 + 158 myr following the emplacement of the Kelly Group at t0 + 140 myr. Thehistory from t0 is t0 + 140 myr is not shown. This period corresponds to the progressive emplacement of the greenstone cover, and to a warming up of the crust. Duringthis time there is with little to no deformation. Thickness variations of the lower Warrawoona greenstone speedup initiation of sagduction. Two circular red markersrecord the horizontal versus vertical motions as well as the magnitude of shortening in the greenstone cover. From t0 + 140 to t0 + 158 myr, gravity-driven shortening abovethe downwelling region is >60%. Blue shading shows post yielding plastic strain. Arrows pointing at passive vertical markers in the basement document the deformationpattern.
100 N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104
0 100 200 3000
200
400
600
800
t0 + 140 myr
t0 + 148 myr
t0 + 158 myr
T (
ºC)
Distance (km)
Upwelling Upwelling Upwelling
Downwelling Downwelling
0 100 200 3000
200
400
600
800
t0 + 158myr
t0 + 162myr
t0 + 166 myr
T (
ºC)
Distance (km)
a) Temperature at 10 km depth, from t0+140 to t0+158 myr, building up of a thermal anomaly
b) Temperature at 10 km depth, from t0+158 to t0+166 myr, relaxation of the thermal anomaly
26 ºC/km
26 ºC/km
Fig. 5. Thermal evolution at 10 km depth. Upper graph shows the evolution from t0 + 140 to t0 + 158 myr following the emplacement of the Kelly Group. During this stage,the temperature in the basin decreases from 210 to 90 ◦C, whereas the temperature in the dome rises from 210 ◦C to 660 ◦C. This leads to a horizontal temperature differenceo m. Froc in from
1
aptidadvi2tag
b2BaEdr2r(
f ca. 570 ◦C over ca. 65 km distance, with a horizontal gradients up to 26 ◦C per kooling of the granitic domes from 660 to 310 ◦C and heating in the greenstone bas
8O-enrichment up to ca 14‰ have been also described in ophiolitesnd modern-day seafloor alteration zones and is interpreted as theroduct of hydrothermal convection cells involving seawater at lowemperature (e.g. Putlitz et al., 2001). In Archean sagduction sett-ngs, seafloor-like alteration would have been strongly enhanced byeformation and the development of faults and shear zones abovend within downwelling regions. In the Warrawoona Synform, syn-eformation alteration process was driven by interaction with largeolumes of low temperatures (∼90–160 ◦C) seawater-derived flu-ds during the early stage of sagduction process (Thébaud et al.,008). This is consistent with our numerical modelling showinghat downward advection of greenstones maintains a low temper-ture environment (<100 ◦C) in the top 10 km of the downwellingreenstones.
In the Warrawoona Synform, ductile shear zones also host gold-earing quartz ± calcite ± sulphide ± ankerite veins (Huston et al.,001). These veins do not exist in the nearby less deformed Marblear Greenstone Belt, but they do in other downwelling regions suchs the Bamboo Creek mining district on the NE margin of Mountdgar (white hexagon in Fig. 1b1). These gold-bearing quartz veinsevelop largely in association with shear zones in downwelling
egions and have a syn- to post-shearing origin (Thébaud et al.,006). Across the Warrawoona Synform, quartz veins display aestricted range of ı18O values with a mean value of +13.2 ± 2‰Fig. 6). This suggests that quartz precipitated from a homogeneous
m t0 + 158 to t0 + 166 myr (lower graph), the temperature evolution changes with90 to 200 ◦C.
fluid under near-isothermal conditions. These veins are there-fore in isotopic disequilibrium with their silicified host, whichimplies an emplacement postdating the silicification (Thébaudet al., 2008). Quartz-hosted CO2–NaCl–H2O–CH4 fluid inclusionsyielded homogenisation temperatures between 234 and 372 ◦C(Huston et al., 2001; Thébaud et al., 2006). Using Zheng (1993)quartz/water fractionation equation, Thébaud et al. (2008) calcu-lated that the ı18O of the water from which these quartz veinsprecipitated is in the range of +2.4 to +12.1‰. This oxygen iso-topic composition, together with the significant enrichment inpotassium and base metals revealed by Synchrotron radiation X-ray fluorescence fluid inclusion analyses, support a metamorphicand/or magmatic source (Thébaud et al., 2006). These hotter fluidswere mobilised at a later stage of the sagduction process, possiblyduring the devolatilisation of the greenstones keels and/or duringthe large production of potassic melt (Thébaud et al., 2008).
In summary, field observation, isotopic and fluid inclusionsmicrothermometry data, and our numerical investigation pre-sented here, document a long-lived two-stage hydrothermalhistory involving: (1) low-temperature (90–160 ◦C) syn-deformation seafloor-like fluid–rock interactions strongly focused
along steeply dipping ductile shear zones during the early stage ofsagduction, and (2) moderate-temperature (234–372 ◦C) syn- topost-deformation fluid–rock interactions, strongly focused alongshear zones, leading to the formation of gold-bearing quartz veins
N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104 101
Fig. 6. Whole rock ı18O and wt% SiO2 and quartz vein ı18O plotted against a synthetic lithological section of the Fielding’s Find shear zone from the Warrawoona Synform(modified after Thébaud et al., 2008). Whole Rock analyses across the Fielding’s Find Shear Zone reveal that he bulk rock ı18O values together with silica content, from bothsides of the shear zone, increase progressively towards its centre. Such enrichment in heavy oxygen isotopes and silicification was proposed to be driven by interaction withlarge volumes of low temperature (∼90–160 ◦C) hydrothermal fluids (sea-water) (Thébaud et al., 2008). The quartz veins are in isotopic disequilibrium with their alteredhost, which suggest that a fluid circulation event followed the silicification around the Fielding’s Find Shear Zone. Vein quartz display a relatively restricted range of ı18Ovalues across the Fielding’s Find shear zone, with a mean value of +13.2 ± 2‰. Together with fluid inclusion analyses the quartz veins are regarded as the signature of ac , 2006
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omposition involving magmatic and/or metamorphic fluid sources (Thébaud et al.
ost-dating the vertical fabrics. The early alteration assemblagesan be linked to the early stage of greenstone sagduction and theo-development of (i) crustal-scale thermal anomalies, duringhich cooling affected the downwelling greenstone, and (ii)
ertical shear zones which acted as efficient fluid pathways forupracrustal porous flow (Fig. 7a). In the early stage of sagduc-ion, large volumes of fluid, trapped in hydrated minerals duringeafloor alteration, would have been transported deep into therust. The second hydrothermal stage can be linked to the releasef metamorphic and/or magmatic fluid in association to theevolatisation of sagducted greenstone covers, and crystallisation
f felsic magmas (Fig. 7b). These fluids would have been releasedt a time when sagduction-related lateral thermal anomaliesntered the relaxation phase during which the temperature inhe greenstone keels rapidly increased. In the strongly foliated
).
steeply dipping greenstone sheets, these hot crustal fluids wouldhave been efficiently channelled toward the surface through thewell-established network of shear zones formed at an early stage.This is consistent with field observations showing that quartzveins are restricted to downwelling regions. The slow conductiverelaxation of the thermal anomaly may have helped to maintainthis plumbing system for several 10 s of myr.
6. Discussion and implication for Paleoarchean toMesoarchean gold mineralisation
To form a gold deposit, the average crustal gold content of1.3 ppb must be concentrated by one thousand to one hundredthousand times (e.g. Walshe and Cleverley, 2009). Temperaturegradients above shallow magmatic intrusions typically drive
102 N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) 93–104
Fig. 7. Conceptual tectonograms illustrating Archean intracontinental mineral systems associated to gravity driven tectonics and fluid flow during sagduction of subaqueousgreenstone covers (green envelops) into their hot and weak basement (red envelops). During the early stage of sagduction (upper panel), early deformation enhances low-temperature seafloor-like alteration along downwelling regions. Sagduction transfers cooler supracrustal rocks and large volumes of hydrated minerals deep into the crust.As shown in our numerical experiments, dome regions become a few hundred degrees hotter than the intervening downwelling regions. These lateral thermal gradientscontribute to focus fluid circulations into downwelling regions. In a second stage (lower panel), the relaxation of this lateral temperature anomaly led to the rapid heatingand devolatisation of hydrated steeply dipping greenstone sheets. Released metamorphic and magmatic fluids focus into pre-existing shear zones, which act as crustal-scalefluid pathways to the surface where mineral deposits form. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of thea
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rticle.)
fficient hydrothermal systems in which fluid–rock interactions the main concentration mechanism (Eldursi et al., 2009). Its therefore not surprising that shallow magmatic activities inack-arc settings, as well as sub-seafloor volcanism associatedith continental rift systems, are the main settings of gold deposits
n the modern Earth (Goldfarb et al., 2001). In other words, miner-lised hydrothermal systems are strongly partitioned onto oceanoors and active continental margins. However, it is likely thatost Archean ocean floors and Archean continental plate margins
id not survive the past 3 Gyr. This suggests that the bulk of thereserved Archean gold deposits may have formed in the interiorf continents, away from plate margins. On the modern Earth,his setting is the least favourable for hydrothermal systems, buterhaps not so in the Archean.
In the Archean, basaltic piles up to 15 km thick emplaced onlder felsic continental crusts were common (de Wit and Ashwal,997), and most of them emplaced onto flooded continents (Arndt,999; Flament et al., 2008; Kump and Barley, 2007). Higher sea
evel in the Archean would have maintained an infinite reservoirf water above the continental basalts, whereas higher surface
eat flow, due to enhanced radiogenic heat production in the crustnd due to CFBs thermal insulation, would have powered shallowydrothermal cells at the interface between continent and sea-ater, accounting for widespread low temperature alteration and
silicification observed in greenstone belts. Numerical and phys-ical experiments have shown that density inversions related tothe emplacement of thick continental basalts onto less dense andweak basement are large enough to drive the foundering of basalticpiles into the continental crust (Chardon et al., 1998; de Bremondd‘Ars et al., 1999; Dixon and Summers, 1983; Robin and Bailey,2009; West and Mareschal, 1979; this study). Our numerical exper-iments show that sagduction is capable of promoting significanthorizontal temperature gradients up to 26 ◦C/km. These horizontalgradients may have been large enough to force fluids, released dur-ing devolatisation of greenstones and crystallisation of domes, backinto the cooler greenstone keels, where steeply dipping shear zoneswould have channelised these hotter fluids toward the surface.Importantly, sagduction is a mechanism that explains crustal-scaledeformation within the inner-part of continental plates, away fromtheir margins. Therefore in the Archean, the association of volcanicrocks above a felsic crust but below sea level, in tectonically activeregions with high heat flow – critical to formation of gold deposits –was not restricted to continental rifting or active continental mar-gins. Sagduction on flooded continents would have promoted the
formation intracontinental hydrothermal systems expanding con-siderably the prospectivity of Archean cratons, compared to that ofmodern continents whose prospectivity is limited to active platemargins and continental rifts.
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. Conclusions
Using realistic rheologic and thermal parameters, our coupledhermo-mechanical numerical experiments confirm that, in therchean, sagduction was a natural response to density inversionsnd thermal weakening due to the emplacement of thick continen-al flood basalts onto radiogenic crusts. Our experiments show that,uring sagduction, the advection of cold rocks into downwellingasins and advection of hot rocks into rising domes lead to crustal-cale, horizontal, thermal anomalies in excess to 500 ◦C over anorizontal distance of ca. 30 km. Large temperature gradients up to6 ◦C/km and the availability of large volumes of fluids (i.e. (i) sea-ater above flooded continents, (ii) metamorphic fluids releaseduring dehydration of sagducting supracrustal greenstones, andiii) magmatic fluids released during crystallisation of magma inomes) would have powered crustal-scale hydrothermal systems.he formation around granitic domes of networks of sagduction-elated steeply dipping greenstone sheets and vertical shear zonesonnected through vertical triple junctions, would have facilitatedrustal-scale transfer of fluids, matter and heat. Fluid–rock inter-ctions in these tectonically active plumbing systems would haveverprinted low-temperature fluid–rock interactions that occurreduring the emplacement of CFBs onto flooded continents and dur-
ng the early stage of sagduction. This is consistent with structural,etrological and isotopic data. Based on our numerical experi-ents, field observations and existing petrological and isotopic
ata, we propose that in the Archean, the sagduction of thickreenstone piles with a large and near-permanent seawater reser-oir above, and a hot and weak basement below, allowed for theevelopment of efficient and long-lived plumbing systems. Thislumbing systems favoured crustal-scale fluid–rock interactionsnd the formation of craton-wide gold deposits in the interior ofontinents, far away from their margins, a context very differentrom the paleoproterozoic and Phanerozoic, where orogenic and
agmatic orebodies are closely associated with active margin sett-ngs.
cknowledgements
This work was supported by the Australian Research Council’siscovery funding scheme (ARC DP 0342933) and AUSCOPE-CRIS and Computational Infrastructure for Geodynamics software
nfrastructure. This paper has benefited from the reviews of Jeanébard and Laurent Aillères, and from countless discussions with. Flament, N. Coltice, P. Philippot, F. Wellmann, M. Fiorentini, K.essner, M. Van Kranendonk and many other colleagues from theniversity of Sydney, the University of Paris 6 and the Centre forxploration Targeting in Perth.
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