Yilgarn Research Articles

Active dislocations in granitic terrains of the Gawler and Yilgarn cratons, Australia, and some implications

Examination and monitoring of bornhardts and of quarry exposures on the Gawler Craton exposed on northwestern Eyre Peninsula, South Australia, and in the Yilgarn Craton of the southwest of Western Australia have shown, first, that the bornhardts of those regions are complex features, with each topographic form including several structural domes; second, that the rock masses are in compression along one axis so that one set of steeply-inclined fractures has not been penetrated by water and hence is not weathered; third, and in contrast, sheet fractures, which are tectonic features, continue to be arched and thus are readily infiltrated by groundwaters so that the adjacent rock is consistently weathered; fourth, the rock masses continue to be stressed so that neotectonic forms are still developing. These observations confirm that bornhardts are developed on rock compartments which are in compression and that they are two-stage or etch forms, and that sheet fractures and A-tents, or popups, are tectonic in origin.

Numerical modelling of orogenic processes and gold mineralisation in the southeastern part of the Yilgarn Craton, Western Australia

We use numerical modelling codes to simulate aspects of some current hypotheses for the origin of gold deposits and hydrothermal systems in the Yilgarn Craton of Western Australia. In particular, we investigate conceptual models advocating vertically continuous hydrothermal systems as well as those invoking extensive lateral flow and possible links with advection of heat by late orogenic granitic magmatism. Numerical models of part of the Eastern Goldfields Province and Southern Cross Province have been built with FLAC3D, to simulate crustal‐scale coupled interaction between deformation and fluid flow. These illustrate the potential for fluid focusing and mixing in shear zones, including downflow of meteoric water, lateral fluid flow driven by topographic elevation and upwards flow of fluids derived from melting and metamorphism in the deep crust. In some cases, downflow also occurs within the middle crust, at depths where fluid influx might trigger melting if the geothermal gradient were appropriate. The models indicate that tectonic wedging within a layered crust and diverging thrust systems that generate ‘pop‐up’ wedges may be important in facilitating efficient fluid upflow and downflow during uplift, while topographic elevation related to asymmetric thrust migration and loading tends to promote lateral fluid flow. However, the effect of topography appears more important than the precise depth or location of the site of fluid production in the deep crust. The effects of thermal convection and fluid‐fluid interaction have also been numerically modelled for a simplified section across the Kalgoorlie Terrane. Modelling under both hydrostatic and lithostatically overpressured pore‐pressure gradients has effectively delineated domains of convective fluid flow within the middle and upper crust, and has identified two generic sites that are favourable for fluid mixing, notably hangingwall and footwall environments in major shear zones, such as the Bardoc Shear, and in broad antiforms, such as the Goongarrie ‐ Mt Pleasant Antiform. The thermal effect of small plutons embedded in a regional metamorphic regime can cause significant lateral displacement of fluid convection patterns, over distances greater than pluton diameter, as well as more proximal effects on precipitation and dissolution of mineral species. However, these results are highly dependent on the pore‐pressure gradient and the permeability structure of the crust, and require magmatic and metamorphic fluid generation to be precisely timed with respect to deformation, thus reinforcing the dynamic feedback between deformation, magmatism and fluid production and migration.

Did late Palaeoproterozoic assembly of proto-Australia involve collision between the Pilbara, Yilgarn and Gawler Cratons? Geochronological evidence from the Mount Barren Group in the Albany–Fraser Orogen of Western Australia

The Mount Barren Group, which crops out on the southern margin of the Yilgarn Craton, has been considered an allochthonous synorogenic sequence to a Mesoproterozoic (1300 Ma) collision between southwestern Australia, southeastern Australia and eastern Antarctica during assembly of East Gondwana. Reassessment of its stratigraphic geometry confirms the historical perspective that nonconformable and faulted contacts are preserved on autochthonous Archaean orthogneiss of the Yilgarn Craton, emphasising that it is not an allochthonous sequence. SHRIMP UPb dating of early diagenetic xenotime establishes a Late Palaeoproterozoic (1696±7 Ma) depositional age for the Mount Barren Group, thereby negating a Mesoproterozoic synorogenic origin. The Mount Barren Group now correlates in time with strike-slip sequences in the Capricorn Orogen approximately 1000 km to the north, and which were a post-collisional response to ∼1750 Ma sinistral transform collision between the Pilbara and Yilgarn Cratons. SHRIMP UPb detrital-zircon geochronology of the Mount Barren Group, therefore, has an important role in establishing if Pilbara–Yilgarn transform collision were either independent of or a component in the Late Palaeoproterozoic assembly of a proto-Australian continent. Detrital-zircon age-data from four samples of different sandstone beds in the Mount Barren Group define complex age spectra composed of seven significant provenance age-subpopulations. Comparison to age spectra from provinces with a possible geotectonic link to the Yilgarn Craton indicates that either the Pilbara Craton or the Gawler Craton was the source terrain. A barrier to a dispersal-system link to the Pilbara Craton points to the Gawler Craton as the source, a provenance link that was also made for the now-redundant 1300 Ma scenario. The 1696±7 Ma depositional age and zircon provenance analysis support a contentious hypothesis involving coeval collisions between the Pilbara–Yilgarn and Yilgarn–Gawler Cratons during assembly of a proto-Australian continent some 500–400 million years earlier than envisaged by other reconstructions.

Fluid Chemical Evolution as a Factor in Controlling the Distribution of Gold at the Archean Golden Crown Lode Gold Deposit, Murchison Province, Western Australia

The Golden Crown lode gold deposit is one of a number of northwest-southeast– to north-south–trending quartz veins hosted by a steeply dipping, 500-m–thick, differentiated dolerite sill (the Great Fingall sill) and adjacent graphitic shale in the Day Dawn district of the Murchison province in the Archean Yilgarn craton, Western Australia. Quartz veins of the Golden Crown deposit are shear zone-hosted, crosscut the dominant host-rock stratigraphy at a high angle, and are laminated, partly boudinaged, and generally bifurcate at their extremities. Veins are dominated by dark gray or milky white quartz, with subordinate calcite and ankerite, and, in order of abundance, arsenopyrite, pyrrhotite, pyrite, galena, chalcopyrite, and minor sphalerite. The vein sulfides coexist with native gold and are most abundant at vein margins. Textural evidence indicates that precipitation of the majority of these minerals overlapped, and that most phases can be considered to have been in equilibrium during vein genesis. Mineral chemistry, fluid inclusions, alteration, and ore assemblages show that there are significant systematic gradients in intensive variables within the Golden Crown deposit. These are recorded over the approximately 450 m of vertical exposure of the vein system, and over up to 30 m into the wall rocks. Based on proximal alteration assemblages and zoning patterns, zoned wall-rock alteration haloes are divisible into four sequences, three of which are closely related to gold mineralization. Specific alteration sequences occur within different parts of the vein system. Biotite alteration predominates in the deeper portions, whereas muscovite alteration characterizes the upper and southern portions of the system. Higher-grade gold zones within the Golden Crown deposit are limited to the northern end of the system in the deeper portion of the veins, and expand along strike to the south in the upper portions. Fluid inclusions, interpreted to be trapped during gold mineralization, are low-salinity (<10 wt % NaCl equiv) H2O-CO2 ± CH4 inclusions with variable carbonic to aqueous contents that are interpreted to be due to fluid immiscibility. Inferred methane concentrations in the carbonic phase vary significantly from 19 ± 7 mole percent in the vicinity of the shale contact to 5 ± 2 mole percent in the shallower, southern end of the deposit. Carbon and oxygen isotope compositions of carbonates ( δ 13Ccalcite = –10.5 to –1.9‰; δ 13Cankerite = –5.3 to –4.1‰; δ 18Ocalcite = 7.0–18.3‰; and δ 18Oankerite = 7.7–9.2‰) and oxygen isotope compositions of vein quartz ( δ 18Oquartz = 10.2–12.5‰) are well within the range of known Archean orogenic lode gold deposits. Combining the results of all geothermometers and geobarometers, a temperature of 300° to 400°C and pressure of 2.5 ± 0.5 kbars are determined, with temperature variations over the exposed quartz vein of less than 40°C. The As content (at. %) of arsenopyrite and Au/Ag ratios of native gold show significant compositional variation, which appears to be controlled by parameters such as a H2S(aq) and f O2, not temperature gradients. On the basis of the variations in fluid composition determined in the reefs, the two major processes responsible for high-grade gold mineralization are interpreted to have been phase separation in the deeper portions of the ore-forming system (approximately below –400-m level) and, above these levels, mixing between two modified ore fluids, one reduced fluid that had reacted with graphitic shale and a relatively oxidized fluid that had partly equilibrated with dolerite.

Geochronological Constraints on Pre-, Syn-, and Postmineralization Events at the World-Class Cleo Gold Deposit, Eastern Goldfields Province, Western Australia

Recent geological and geochronological studies have led to a reevaluation of whether or not the majority of the lode gold deposits of the Yilgarn craton formed nearly synchronously during an Archean craton-wide hydrothermal event, as previously proposed. The majority of reliable data indicate that gold mineralization took place at ca. 2640 to 2625 Ma; however, recent work in the far north of the Eastern Goldfields province provides structural evidence for instances of earlier gold mineralization and geochronological data for interpreted postore rocks that are considerably older than 2640 Ma. The documentation of xenotime and monazite in association with ore minerals at the Cleo deposit provides a valuable means of determining the absolute age of gold mineralization. At the Cleo deposit, high-grade Western Lodes veins crosscut the Sunrise shear zone at a high angle with only centimeter-scale offset. Given that the stratigraphic sequence indicates at least several hundred meters offset across the shear zone, the minimal offset of the Western Lodes veins indicates that gold mineralization was late in the history of movement in the shear zone. The absolute ages of pre-, syn-, and postmineralization elements in the mine stratigraphic succession have been determined using a variety of geochronological techniques. Fuchsite micas formed in the Sunrise shear zone yield an 40Ar/39Ar isochron age of 2667 ± 19 Ma. Rhyodacite porphyry dikes postdate the formation of the shear zone but predate the main phase of gold mineralization. Dike intrusion is constrained to 2674 ± 3 Ma by SHRIMP II U-Pb analysis of zircons from these dikes. Rhenium-Os analysis of molybdenite from quartz + chalcopyrite + molybdenite veins that crosscut the porphyry dikes gives an age of formation at 2663 ± 11 Ma, consistent with their relationship to the porphyry dikes. Both the dikes and these veins are crosscut by gold-bearing Western Lodes veins. Mutually crosscutting relationships with ore minerals indicate that microscopic xenotime and monazite grains were deposited during ore formation in the Western Lodes ore zones. SHRIMP II U-Pb analyses of xenotime and monazite grains indicate that Western Lodes mineralization took place at 2654 ± 8 Ma, at least 9 m.y. after the intrusion of the porphyry dikes (95% confidence). Evidence for substantial temporal separation between intrusive activity and gold mineralization is recorded by other researchers for other deposits associated with granitic intrusions in the Leonora-Laverton area. Phlogopitic micas from a lamprophyre that crosscuts the whole succession, including the Sunrise shear zone, yield an Ar/Ar plateau age of 2080 ± 4 Ma. This study constrains the timing of multiple events at Cleo, including the main phase of gold mineralization, during a relatively short span of time. However, in the context of the larger debate about the existence of a dominant 2640 to 2625 Ma period of gold mineralization and the extent of a proposed widespread pre-2660 Ma mineralization event in the Yilgarn craton, the data from Cleo are equivocal, the 2654 ± 8 Ma age for mineralization being significantly older than 2640 Ma but not significantly pre-2660 Ma, even if the maximum possible age is accepted.

The Archean Cu-Zn Magnetite-Rich Gossan Hill Volcanic-Hosted Massive Sulfide Deposit, Western Australia: Genesis of a Multistage Hydrothermal System

The Gossan Hill volcanic-hosted massive sulfide (VHMS) deposit is an Archean Cu-Zn-magnetite-rich deposit located in the Warriedar fold belt of the Yilgarn craton, Western Australia. The deposit is hosted by redeposited rhyodacitic tuffaceous volcaniclastics of the Golden Grove Formation and is overlain by felsic volcanic rocks of the Scuddles Formation. The deposit consists of two separate subvertical ore zones. The stratigraphically lower Cu-rich ore zone (7.0 Mt @ 3.4% Cu) is strata bound and varies from podiform massive pyrite-chalcopyrite-pyrrhotite-magnetite to sheetlike massive magnetite-carbonate-chlorite-talc. The upper Zn-Cu ore zone (2.2 Mt @ 11.3% Zn, 0.3% Cu, 1.5 g/t Au, and 102 g/t Ag) contains strata-bound massive sphalerite- pyrite-chalcopyrite that overlies discordant, moundlike massive pyrite-pyrrhotite-chalcopyrite-magnetite. Sulfide stockwork connects the upper and lower ore zones. Metal zonation varies from Cu (+-Au) in the lower ore zone to Zn-Cu at the base of the upper ore zone, which grades upward and laterally to Zn-Ag-Au (+-Cu, +-Pb). Multiphase deformation and greenschist facies metamorphism overprint the deposit. The relationship between host rocks and sulfide indicates that mineralization was broadly codepositional with sedimentation of the Golden Grove Formation. However, gradational and interdigitating contacts between these volcaniclastic rocks and magnetite support the formation of massive magnetite by subsea-floor replacement. Sulfides replace and vein the massive magnetite with inferred synchronous formation of the upper and lower sulfide ore zones. Although these sulfide ore zones formed mainly by subsea-floor replacement, stratiform hydrothermal chert-sulfide-sediment layers within, and adjacent to, the upper Zn-rich ore zone attest to some local exhalation. The thickest development of massive magnetite, massive sulfide, and stringer stockwork occurs in the north of the deposit and supports a common feeder during massive magnetite and sulfide formation. Furthermore, local chlorite-quartz hydrothermal alteration, massive magnetite, massive sulfide, and stockwork all form asymmetric zones that thin southward. These attributes indicate strong synvolcanic structural control during mineralization; the relict synvolcanic growth structure is obscured and probably occupied and by a dacite dome of the hanging-wall Scuddles Formation. Thermodynamic considerations suggest that massive magnetite and sulfide formed from similar high-temperature (>300 degrees celsius), slightly acidic, low-fO2 hydrothermal fluids. Massive magnetite formed from H2S-poor fluids, whereas massive sulfide formed from relatively rich H2S fluids. Physicochemical changes associated with interaction between upwelling, H2S-bearing fluids and preexisting massive magnetite may have resulted in the subsea-floor precipitation of sulfide in the lower Cu-rich ore zone. The Gossan Hill VHMS deposit represents a multistage hydrothermal system within an environment characterized by rapid volcaniclastic sedimentation and changing structural and magmatic processes. The six main stages were as follows: (1) initiation of the hydrothermal system, (2) sedimentation, metasomatism, and progressive heating of convecting fluids, (3) deposition of massive magnetite from H2S-poor fluids by subsea-floor replacement above a buried synvolcanic conduit, (4) structural reactivation tapping deeper H2S- and metalbearing fluids, (5) subsea-floor replacement and minor exhalative sulfide mineralization, with (6) burial and preservation of the deposit resulting from proximal felsic volcanism.

Marymia: an Archean, amphibolite facies-hosted, orogenic lode-gold deposit overprinted by Palaeoproterozoic orogenesis and base metal mineralisation, Western Australia

The Marymia gold deposit, comprising two orebodies, Keillor 1 and Keillor 2, is at the northern end of the Plutonic Well greenstone belt in the Marymia Inlier, in the southern Capricorn Orogen, just north of the Yilgarn craton. The Marymia Inlier is a discrete fault-bounded Archean gneiss–granitoid–greenstone domain surrounded by sedimentary basins that were formed and variably metamorphosed and deformed during several Palaeoproterozoic orogenic cycles. The greenstone sequence at Marymia is stratigraphically and geochemically similar to greenstone sequences in the Yilgarn craton, but was subjected to further deformation and metamorphism in the Palaeoproterozoic. Late Archean deformation (D1–D2) was ductile to brittle–ductile in style, whereas Palaeoproterozoic deformation was predominantly brittle. Equilibrium mineral assemblages indicate that peak amphibolite-facies metamorphism (540–575 °C, <3 kb) was overprinted by greenschist-facies metamorphism (300–360 °C). Petrographic textures indicate that prograde metamorphism was coeval with D1–D2, with peak metamorphism early to syn D2. Gold mineralisation at Marymia is hosted in metamorphosed tholeiitic basalts and banded iron formation. On a gross scale, the distribution of gold is controlled by D2 folds and shear zones. Lithological contacts with strong rheological or chemical contrasts provide local controls. Gold-related alteration comprises subtle millimetre- to centimetre-wide zones of silicification with variable amounts of quartz, hornblende, biotite, K-feldspar, plagioclase, calcite/siderite, scheelite, titanite, epidote, sulfide and telluride minerals. Quartz veins are generally narrow and discontinuous with low total volume of quartz. Gold is sited in the wall rock, at vein salvedges or within stringers of wall rock within veins. There are two distinct opaque-mineral assemblages: pyrite–pyrrhotite–chalcopyrite–galena and hessite–petzite–altaite–Bi-telluride–galena. Ore samples are variably enriched in Ag, Te, Pb, W, Cu, S and Fe reflecting heterogeneity of the ore mineralogy. Structural timing and temperature of formation of alteration and ore minerals support deposition of gold during late peak amphibolite-facies metamorphism from neutral to alkaline (pH=5–6), moderately oxidising (log ƒO2≈–21–22) and CO2-bearing (XCO2 ≈0.2) fluids. The total sulfur content of the fluid is estimated at 1mΣS. Lead isotope compositions support derivation of lead from within the local greenstone sequence. Gold lodes were deformed by faults and shear zones in the Palaeoproterozoic, with only limited remobilisation. Subeconomic, carbonate vein- and breccia-hosted base metal mineralisation is locally hosted within Palaeoproterozoic fault zones, which clearly cut gold lodes. Base-metal-related alteration is characterised by intense carbonatisation, chloritisation, and albitisation of the mafic host rocks. Mineral assemblages are consistent with formation at greenschist facies conditions. Lead isotope compositions support crystallisation at ca. 1.7 Ga from lead that is similar in composition to earlier gold-related galena.

Did global tectonics drive early biosphere evolution? Carbon isotope record from 2.6 to 1.9 Ga carbonates of Western Australian basins

The δ 13C carb record of well preserved carbonates in outcrop and core is here examined from the 2.6 to 1.9 Ga old basins of Western Australia. These data, which are constrained by a well defined stratigraphic and tectonic framework, and by U–Pb zircon ages, provide an insight into the variables coincident with the evolution of an oxidative atmosphere and the evolution of the early biosphere. In the latest Archaean (ca. 2.6 Ga) the secular δ 13C carb curve is flat much like that seen in the later Palaeoproterozoic basins of Northern Australia (<1.8 Ga). This implies that photosynthesis was a major component of the biosphere at that time and that the carbon mass balance was stable. In the early Palaeoproterozoic, beginning after 2.5 Ga and continuing until at least 1.9 Ga, the δ 13C carb the curve is much more dynamic, with significant positive and negative excursions, including a major positive excursion (+9‰ pdb) close to 2.2 Ga. These excursions can be correlated with the Lomagundi event identified in Africa, Europe and North America. Previously published studies of the overlying Meso- to Palaeoproterozoic Bangemall basin and of 1.8–1.5 Ga old basins in northern Australia suggest that the δ 13C carb curve became relatively monotonic again after ca. 1.8 Ga and remained so for most of the following Mesoproterozoic. Comparisons with data from other ancient cratons, especially Africa, suggest that the secular carbon curve may be even more complex than presently understood and probably comparable to the major excursions seen in the Neoproterozoic. When the Western Australian data are placed in their stratigraphic and tectonic framework we find that the monotonic latest-Archaean curve coincides with a tectonically quiescent period in which carbonates formed in an basinal setting on a craton surrounded by passive margins. The data are consistent with an earth in which the carbon mass balance was in equilibrium. The δ 13C carb curve began to oscillate following the onset of glaciation as the Pilbara and Yilgarn Cratons began to converge during the Capricorn Orogeny suggesting periods of rapid carbon burial during continental dispersal. However, the major positive excursion is preserved in carbonates from back-arc basins formed as the ocean closed and subduction began. Since similar tectonic processes can be recognised, not only in Northern Australia but also on other early cratons, it can be argued that the carbon excursions relate to supercontinent cycles and to major periods of mantle overturn and superplume development. We explain this coincidence of carbon isotopic excursions and tectonism by the sequestration of carbon during ocean closure with organic-rich passive margin sediments containing isotopically light carbon subducted into and stored in the lower crust and mantle. The global ocean thereby became enriched in isotopically heavy carbon, releasing oxygen to the atmosphere. A second stepwise increase in atmospheric oxygen in the Neoproterozoic may also have been connected with the assembly of Rodinia. This second event has been associated with the development of multicellular life and the evolutionary ‘Big Bang’. Between the two events the carbon cycle, and to some extent the biosphere, appear to have entered a period of prolonged evolutionary stasis. This implies that the evolution of both the atmosphere, and the biosphere, may have been driven forward by planetary evolution, implying that biospheric evolution has largely been driven by the dynamo of earth's tectonism and its long term survival depends upon these endogenic (thermal) energy resources. If this is so it has fundamental implications, not only for life on earth, but for the more general problems surrounding the likelihood of life having evolved on other planetary bodies. Small planets with insufficient endogenic energy resources to sustain the crust/mantle interactions of plate tectonics (such as Mars) seem to us unlikely to have allowed evolution beyond single-celled life forms. Once a planet's energy resources are expended the biosphere would most likely enter a prolonged stasis and ultimately face extinction.

Juvenile lithospheric mantle enrichment and the formation of alkaline ultramafic magma sources: Re–Os, Lu–Hf and Sm–Nd isotopic systematics of the Norseman melnoites, Western Australia

Several 850-Ma melnoites occur in the Norseman region at the southeastern margin of the Archaean Yilgarn Craton, Western Australia. Major, trace element and Rb–Sr, Sm–Nd and Re–Os isotopic data from whole rocks and Re–Os and Lu–Hf isotopic data from mineral separates, including Mg-ilmenite macrocrysts and groundmass magnetite, are used to characterise their mantle source. Compositions of whole rocks and groundmass phlogopite and clinopyroxene show that although these rocks contain abundant Mg-ilmenite macrocrysts, they are not kimberlites. Macrocryst Mg-ilmenite geochemistry, coupled with the paucity of lithospheric mantle minerals, suggests that these macrocrysts are early-formed phenocrysts rather than xenocrysts. Mg-ilmenite geochemistry and estimated fO 2 are correlated with proximity to the cratonic margin, suggesting that the melnoite source was laterally heterogeneous on a scale of ∼100 km. Whole rock initial ε Nd values and 87Sr/ 86Sr ratios range from +3.3 to +5.4 and from 0.70181 to 0.70307, respectively, and Mg-ilmenite initial ε Hf values range between −4.8 and +1.7. These data are consistent with the interpretation that the mantle source experienced the time-integrated enrichment of incompatible elements. Initial 187Os/ 188Os ratios for the uncontaminated on-craton melnoites, ilmenites and magnetites are radiogenic and range from 0.2133 to 0.2599 ( γ Os=+76 to +115), whereas initial 187Os/ 188Os ratios for the Mg-ilmenite at the cratonic margin are significantly more radiogenic ( 187Os/ 188Os=0.3918 and 0.4338; γ Os=+223 and +258). In contrast, the Lu–Hf isotopic data from Mg-ilmenite do not vary at the level of analytical uncertainty obtained between the on-craton and craton margin samples, suggesting the decoupling of Re and incompatible element enrichment through the 100-km section. The Lu–Hf, Sm–Nd and Re–Os isotopic systems also yield Mid-Proterozoic (1000–1400 Ma) model ages that are indistinguishable from the age of the Albany–Fraser Orogen (1100–1300 Ma) which truncates the Yilgarn craton at its southern margin. It is postulated that this episode of plate collision provided the necessary components (e.g. pyroxenites–websterites), via intra-mantle melt addition, to the lithospheric mantle to produce the observed geochemical and isotopic features in the melnoite source. This study highlights the ability of the Re–Os isotopic system, as opposed to the Lu–Hf and Sm–Nd isotopic systems, to fingerprint juvenile lithospheric mantle enrichment events and has important implications for the generation of ocean island-like Nd and Hf isotopic signatures in kimberlites and carbonatites.

SHRIMP U–Pb geochronology of magmatism and thermal events in the Archaean Marymia Inlier, central Western Australia

Zircons from porphyry and granitoid samples collected in and around the Marymia Gold Mine in the Marymia Inlier, Western Australia, record a complex history. The results of U–Pb studies confirm that the Plutonic Well greenstone belt, and the surrounding granitoid envelope (including a 2,721±6 Ma intrusion), represent an Archaean terrain, which was intruded by high-level, felsic to intermediate porphyries at 2,694±7 Ma and potentially also at 2,660±4 Ma. Zircon xenocrysts (≥ca. 3.35, 2.93 and 2.74 Ga) indicate that there was older crust within, or below, the greenstone belt at the time of porphyry emplacement. Zircons from the granitoid envelope and intrusions within the greenstone belt record subsequent metamorphism and/or hydrothermal activity coeval with magmatism in the Late Archaean (ca. 2.66–2.63 Ga), and peak metamorphism, magmatism and gold mineralisation in the Yilgarn Block. A later period of metamorphism and hydrothermal activity at ca. 1.72 Ga is coeval with orogenesis in the southern Capricorn Orogen. Both the Late Archaean and Palaeoproterozoic thermal events have altered zircons, redistributed trace elements and caused zircon recrystallisation, which is distinctive in its isotope chemistry (in particular Th/U ratios >1) and morphology (e.g. homogeneous in transmitted light and back-scattered electron images, but sector-zoned in cathodoluminescence).

Proterozoic Events in the North China Craton

The 1.21 Ga Marnda Moorn large igneous province (LIP) of the Yilgarn Craton is important for understanding the final breakup of the Nuna (Columbia) supercontinent. However, its petrogenesis is poorly understood owing to the lack of geochemical data. We conducted geochemical analyses of the Gnowangerup–Fraser Dyke Suite, a major part of the Marnda Moorn LIP, and report the first geochemical and Nd isotope data for this LIP. Results of a complementary paleomagnetic study of these dykes will be published elsewhere. Most of the studied dykes consist of predominately tholeiitic and OIB-like dolerite (Group 1) and one arc-like and more felsic dyke (Group 2). Group 1 samples have incompatible trace element compositions similar to those of tholeiitic Hawaiian plume-induced OIB and typical asthenospheric mantle-derived Nd isotopes with ɛNd(t) varying from +3.7 to +7.5, produced mainly within the spinel stability field (<75 km depth). Their source region most likely contains recycled oceanic crust. Samples from the Group 2 dyke are characterized by extremely unradiogenic Nd isotopes with ɛNd(t) of about −12, strong depletion of Nb–Ta–Zr–Hf–Ti, chondritic Nb/Ta ratios (20–18), oversaturated silica, and strong deficiencies in CaO, FeOt, TiO2, and Ni. This implies that the dyke was produced by partial melting of enriched sub-continental lithospheric mantle. The coexistence of OIB- and arc-like end-members but mainly Hawaiian OIB-like tholeiitic mafic dykes, interpreted large-scale asthenosphere upwelling in a very short time, and the large volume of mafic magma, favour a plume origin for the Marnda Moorn LIP. The geochemical and emplacement characteristics are attributed to relief of the lithosphere–asthenosphere boundary across the Yilgarn craton and a complex interplay between the plume, heated lithosphere, normal asthenosphere, and recycled components. We propose a two-stage melting model to explain the geochemical composition and emplacement of the Marnda Moorn LIP. Our plume-lithosphere interaction model is consistent with the occurrence of synchronous ultrahigh-temperature events in the Musgrave Province of central Australia.

Basin setting and age of the Late Palaeoproterozoic Capricorn Formation, Western Australia

The Palaeoproterozoic Capricorn Formation near Ashburton Downs in northwestern Australia formed during the latter stages of the convergence of the Pilbara and Yilgarn Cratons. Palaeocurrent and facies analyses show that the southwesterly derived sediments were deposited in terrestrial environments and in a lake or shallow sea with a shoreline trending southeast. Intraformational debris flows suggest instability during sedimentation. Zircon grains from an accretionary lapilli tuff, dated at 1804 ± 7 Ma by the SHRIMP U—Pb method, show that the Capricorn Formation was deposited at the same time as granitic plutons were intruded in the Gascoyne Complex to the south and west. Although the Capricorn Formation was deposited with marked angular unconformity over the turbiditic Ashburton Formation, both formations could have been deposited in a foreland basin on the northeast flank of the growing Ashburton Fold Belt.

A Thrust Ramp Model for Gold Mineralization at the Archean Trondhjemite-Hosted Tarmoola Deposit: The Importance of Heterogeneous Stress Distributions around Granitoid Contacts

The trondhjemite-hosted Tarmoola gold deposit is situated in the Leonora district in the Eastern Goldfields province and is the largest granitoid-hosted gold deposit in the Yilgarn craton of Western Australia (>100 t Au). The massive trondhjemite pluton intrudes a northwest-striking, shallowly northeast-dipping, supracrustal sequence that comprises greenschist facies metamorphosed basalt, komatiite, chloritic siltstones, and a volcani-clastic unit. Several east-northeast-striking, quartz diorite dikes cut the trondhjemite and supracrustal sequence. Major ore zones (>1 g/t Au) occur (1) within the trondhjemite as discontinuous west-northwest-striking zones, (2) along the steeply dipping, eastern trondhjemite contact in the supracrustal and trondhjemite rocks, and (3) subparallel to the shallowly dipping, western trondhjemite margin in supracrustal rocks. Ore zones in the trondhjemite are controlled by west-northwest-striking, steeply south-west- and northeast-dipping, conjugate, quartz-carbonate veins that contain minor pyrite, chalcopyrite, sphalerite, galena, gold, and rare scheelite. Gold-bearing veins in supracrustal rocks have a similar vein mineralogy and timing to that of the auriferous veins in trondhjemite. Deformation at Tarmoola occurred during progressive, regional east-west-directed shortening. The first deformation event (D 1 ) caused early barren, isoclinally folded, quartz-carbonate veins and a pervasive foliation in supracrustal rock to be aligned subparallel to the steeply dipping eastern margin of the pluton. During the gold mineralization event (D 2 ), the trondhjemite pluton acted as a competent body within ductile supracrustal country rocks. Subhorizontal, northwest-southeast shortening caused auriferous conjugate veins in trondhjemite and the reactivation of the eastern contact and a preexisting subparallel foliation in the supracrustal rock. The reverse movement of supracrustal rock along the sheared western trondhjemite contact caused the formation of west-dipping, gold-bearing shear zones in the overlying supracrustal sequence. The reactivation of the eastern trondhjemite contact caused the development of low mean stress zones adjacent to the contact. Hy-drothermal ore fluids were focused into these regions to form ore zones. The third deformation event (D 3 ) produced brittle-ductile shear zones, quartz-carbonate veins, and brittle faults that offset lithological contacts and gold-bearing veins.

Alteration Characteristics of the Archean Golden Grove Formation at the Gossan Hill Deposit, Western Australia: Induration as a Focusing Mechanism for Mineralizing Hydrothermal Fluids

The Archean Golden Grove Formation is a 550-m-thick rhyodacitic tuffaceous volcaniclastic succession that hosts the Cu-Zn-rich Gossan Hill volcanic-hosted massive sulfide (VHMS) deposit in the Yilgarn craton, Western Australia. The Golden Grove Formation consists of volcanic quartz and altered pumice and shards, which were deposited during successive episodes of subaqueous mass flow. Coherent volcanics are absent from the Golden Grove Formation at Gossan Hill but form the main rock type in the hanging-wall Scuddles Formation. A massive dacite dome that overlies its volcanic feeder dike is inferred to occupy a synvolcanic structure that focused mineralizing fluids during the formation of the Gossan Hill deposit. The Gossan Hill deposit consists of two stratigraphically separate ore zones interconnected by stockwork. The Cu-rich lower ore zone and the Zn-rich upper ore zone occur in the middle and upper parts of the Golden Grove Formation, respectively. Podiform zones of massive magnetite occur in the Cu-rich ore zone, where the formation of magnetite predates massive sulfide. The asymmetry of massive sulfide, massive magnetite, and alteration zones at the Gossan Hill deposit attest to synvolcanic structural control during mineralization. The principal lithofacies of the Golden Grove Formation are sandstone and pebble breccia, which have a regionally extensive quartz, Fe-rich chlorite, and lesser muscovite alteration. At Gossan Hill, this alteration has resulted in near-complete replacement of tuffaceous components, causing substantial chemical modification of the primary lithologies. Quartz-chlorite (±muscovite) alteration is characterized by severe K2O, Na2O, and CaO depletion, with rocks consisting principally of SiO2, FeO, Al2O3, and MgO, along a quartz-chlorite mixing trend. Widespread preservation of pumice and shard volcanic textures within the Golden Grove Formation indicates that quartz-chlorite (±muscovite) alteration occurred soon after, or possibly during, sedimentation. The absence of diagenetic compaction textures further suggests induration of the succession during this early alteration stage, with the tuffaceous succession largely sealed from the texturally destructive effects of subsequent hydrothermal alteration, except where mineralizing fluids were locally channeled along synvolcanic feeder conduits. Local intense hydrothermal alteration zones surround the Gossan Hill deposit and overprint earlier quartz-chlorite (±muscovite) alteration. These local alteration zones have the same extent as the sulfide vein envelope and represent hydrothermal alteration formed during sulfide-magnetite mineralization. Intense Fe-rich chlorite (ankerite-siderite) alteration occurs as a strata-bound envelope around massive magnetite, Cu-rich veins, and massive sulfide in the lower ore zone. This chlorite-rich alteration has strong FeO and MgO enrichment with minor chloritoid and andalusite that reflect intense acid leaching during hydrothermal alteration. Iron chlorite (ankerite-siderite) alteration grades upward into discordant to strata-bound intense quartz alteration. Intense quartz alteration forms an envelope around Zn-rich veins and massive sulfide in the stockwork and upper ore zone. The trend from Fe chlorite-ankerite-siderite to quartz alteration toward the top of the deposit is consistent with the cooling of hydrothermal mineralizing fluids nearing the sea floor. Rhyodacite and dacite volcanics of the hanging-wall Scuddles Formation have a pervasive muscovite-calcite alteration. Muscovite-calcite alteration led to Na2O depletion and CaO and K2O enrichment associated with burial of the Gossan Hill mineralizing system. We propose that the Gossan Hill sulfide-magnetite VHMS deposit formed during an evolving Archean hydrothermal system that began as part of a regional-scale, low-temperature seawater convection-alteration system. Initially, this system caused extensive replacement of the Golden Grove Formation by quartz and Fe chlorite (±muscovite); a process that sealed and indurated the volcaniclastic rocks by infilling of primary porosity and permeability structures. Due to subsequent impermeability of the host-rock succession, later and hotter mineralizing fluids that generated alteration and massive magnetite and sulfide at the Gossan Hill deposit were constrained to, and focused upward along, a synvolcanic feeder structure.

Orogenic gold and geologic time: a global synthesis

AbstractOrogenic gold deposits have formed over more than 3 billion years of Earth’s history, episodically during the MiddleArchean to younger Precambrian, and continuously throughout the Phanerozoic. This class of gold deposit is characteristi-cally associated with deformed and metamorphosed mid-crustal blocks, particularly in spatial association with major crustalstructures. A consistent spatial and temporal association with granitoids of a variety of compositions indicates that melts andfluids were both inherent products of thermal events during orogenesis. Including placer accumulations, which arecommonly intimately associated with this mineral deposit type, recognized production and resources from economicPhanerozoic orogenic-gold deposits are estimated at just over one billion ounces gold. Exclusive of the still-controversialWitwatersrand ores, known Precambrian gold concentrations are about half this amount.The recent increased applicability of global paleo-reconstructions, coupled with improved geochronology from most ofthe world’s major gold camps, allows for an improved understanding of the distribution pattern of orogenic gold in spaceand time. There are few well-preserved blocks of Middle Archean mid-crustal rocks with gold-favorable, high-strain shearzones in generally low-strain belts. The exception is the Kaapvaal craton where a number of orogenic gold deposits arescattered through the Barberton greenstone belt. A few )3.0 Ga crustal fragments also contain smaller gold systems in theUkrainian shield and the Pilbara craton. If the placer model is correct for the Witwatersrand goldfields, then it is possiblethat an exceptional Middle Archean orogenic-gold lode-system existed in the Kaapvaal craton at one time. The latter half ofthe Late Archean ca. 2.8–2.55 Ga was an extremely favorable period for orogenic gold-vein formation, and resulting oresŽ.preserved in mid-crustal rocks contain a high percentage of the world’s gold resource. Preserved major goldfields occur ingreenstone belts of the Yilgarn craton e.g., Kalgoorlie , Superior province e.g., Timmins , Dharwar craton e.g., Kolar ,Ž. Ž . Ž.Zimbabwe craton e.g., Kwekwe , Slave craton e.g., Yellowknife , Sao Francisco craton e.g., Quadrilatero Ferrifero , andŽ. Ž . Ž .Tanzania craton e.g., Bulyanhulu , with smaller deposits exposed in the Wyoming craton and Fennoscandian shield. SomeŽ.workers also suggest that the Witwatersrand ores were formed from hydrothermal fluids in this period.The third global episode of orogenic gold-vein formation occurred at ca. 2.1–1.8 Ga, as supracrustal sedimentary rocksequences became as significant hosts as greenstones for the gold ores. Greenstone–sedimentary rock sequences nowexposed in interior Australia, northwestern Africarnorthern South America, Svecofennia, and the Canadian shield were the

Transpression and restraining jogs in the northeastern Yilgarn craton, Western Australia

The structural framework in the northeastern Yilgarn craton of Western Australia was largely shaped by transpression. Local shortening induced by sinistral strike-slip on right-stepping, NNW-trending regional shear zones led to the development of N- to NNE-trending folds and reverse faults within the Laverton, Duketon and Yandal restraining jogs, and at the northwestern termination of the Kilkenny shear zone. The restraining jogs formed late in the tectonic history of the area, and each contains a diagonal transfer shear zone that merges with the bounding shear zones. Sinistral strike-slip movement on the NNW-trending regional shear zones (D 3) followed regional folding and shortening (D 2). D 2 and D 3 were probably progressive and overlapping tectonic processes. The first deformation event (D 1) is poorly understood, but appears to have involved at least locally significant N–S thrusting. The style of deformation described herein has not been widely reported from other Archaean granite–greenstone terrains but is unlikely to be unique to the northeastern Yilgarn craton.

Superposition of gold mineralisation on pre‐existing carbonate alteration: Structural evidence from the Mulgarrie gold deposit, Yilgarn Craton

Carbonate alteration at the Mulgarrie gold mine in the Eastern Goldfields of Western Australia, is represented by porphyroblasts, veins and pervasive, texturally destructive, carbonatisation. Two foliations, S1M and S2M, were produced by two separate deformation events at the mine‐scale, D1M and D2M. D1M and D2M both occurred in response to regional D2 tectonism. Carbonate alteration was the product of two separate episodes of fluid ingress: the earlier produced magnesite and the latter Fe‐dolomite. Both periods of carbonate alteration occurred pre‐ to early syn‐D2M, when mafic to ultramafic komatiitic rocks reacted with fluids that moved along regional faults and pre‐date the alteration associated with regional peak metamorphism. Gold at Mulgarrie overprints pre‐ and late syn‐D2 quartz veins in zones of massive carbonate alteration, suggesting it has a late‐ to post‐D2 timing. This late timing agrees with the generally accepted syn‐D3 (and younger) age for gold mineralisation in the Eastern Goldfields. We suggest that carbonate alteration at Mulgarrie is not a product of the hydrothermal event responsible for the gold mineralisation. Rather, the different relative timing of magnesite, Fe‐dolomite and gold indicates there were two carbonate‐producing fluid systems and a fluid transporting the gold overprinted these. Similarly, early carbonate alteration may play a role in localising auriferous vein deposits throughout the Yilgarn and other Archaean cratons.

Geometry and kinematics of large arcuate structures formed by impingement of rigid granitoids into greenstone belts during progressive shortening

Regional-scale arcuate structures in the central part of the Archean Yilgarn craton, Western Australia, were generated by impingement of competent granitoid blocks into less competent greenstone belts during progressive east-west shortening. Sinistral shear zones developed along northwest-trending margins of the granitoid blocks, whereas dextral shear zones developed along northeast-trending margins. In apex regions, these northeast- and northwest-trending shear zones are linked by north-trending contractional zones along which shortening was accommodated by the formation of folds and reverse faults in the greenstone belts and a coaxial flattening fabric in granitoid rocks. Lateral escape of the greenstone belts is indicated by the progressive rotation of early macroscopic folds into parallelism with the strike-slip shear zones during granitoid impingement. This recently recognized deformation style may have important tectonic implications for other Archean granite-greenstone terranes and where rock heterogeneity and competency differences are significant.

Crustal signature of Late Archaean tectonic episodes in the Yilgarn craton, Western Australia: evidence from deep seismic sounding

Deformation in the greenstone supracrustal rocks of the Eastern Goldfields Province of the Archaean Yilgarn Craton in Western Australia is delaminated from the underlying basement along a regional detachment surface presently at 3–7 km depth. This might suggest that the history of crustal deformation cannot be inferred with any certainty from that of the greenstones. However, seismic images of the crust below the greenstones show structures that can be interpreted in terms of a series of tectonic events similar to those within the greenstones. The upper crust (below and to the west of the greenstones) is largely unreflective, with interpreted west-dipping reverse faults. The middle crust is reflective, with reflector geometry implying thickening by west directed thrust stacking, and the lower crust has a fabric indicative of ductile deformation. These reflection fabrics imply crustal shortening, probably during the Late Archaean regional D2 ENE–WSW shortening event. They were subsequently overprinted and disrupted by structures consistent with regional NNW–SSE strike slip D3 faulting, and probably younger, more localised D4 faulting. The seismic images of the crust therefore show that the crust suffered tectonic events in which both the order and direction of deformation are similar to those of the greenstones. This is evidence that the whole crust deformed when the greenstones deformed. However, the scale and style of deformation vary with depth through the crust, and include thrusting and probably folding in the upper crust, thrust stacking in the middle crust, and ductile deformation in the lower crust. The length scale (wavelength) of structures in the middle and lower crust is greater than that in the greenstones.

The in situ stress field of the Perth Basin, Australia

A total of 17 wells in the Perth Basin were interpreted to have 114 breakouts covering a combined length of 3.2 km and a mean σHmax orientation of 108°N. The inferred mean orientation of σHmax in the Perth Basin is broadly consistent with that in the adjacent basement, Yilgarn Block. The new data confirm that σHmax orientation in the region is consistent with other stress indicators, different depths and different geological provinces. The new data also confirm that the orientation of σHmax does not parallel the NNE direction of absolute plate velocity. Anomalous north‐south σHmax orientations identified in a number of wells in the Perth Basin can be attributed to local structural effects.