Kevin F. Cassidy

Late-Archean granitoid-hosted lode-gold deposits, Yilgarn Craton, Western Australia: Deposit characteristics, crustal architecture and implications for ore genesis

Granitoid-hosted lode gold deposits constitute a sub-group of late-Archean lode-gold deposits in the Yilgarn Craton of Western Australia. They share a number of common characteristics, including: (1) a strong structural control on a variety of scales; (2) in most instances, the presence of mineralisation in adjacent supracrustal sequence; (3) wallrock alteration and vein assemblages consistent with the addition of SiO2, K2O, CO2, S±Na2O, but dependent on the P–T conditions of the host environment and ore fluid; (4) a metal association comprising Au, Ag, As, Bi, Te, W±Mo and low concentrations of Cu, Pb and Zn; and (5) deposition from a low-moderate salinity, near-neutral H2O–CO2±CH4 fluid over a temperature and pressure range from 250 to ≈600°C and ≈1 to 4 kbar, which corresponds to a range of crustal environments from lower greenschist to upper amphibolite facies. These characteristics suggest that the granitoid-hosted lode-gold deposits represent a coherent genetic group that is equivalent to the more abundant greenstone-hosted lode-gold deposits of the craton. In combination, lode-gold deposits irrespective of their host rocks and host terrane, formed over a range of crustal levels and developed late in the tectonic sequence of the craton. Granitoids which host lode-gold mineralisation are older than the inferred age of mineralisation by at least 10 million years and, in combination with thermodynamic and isotope studies, indicate that the host granitoids are not the dominant source of the ore fluid. However, the host granitoids do provide a structural and, to a lesser extent, chemical trap for gold-bearing fluids. In combination, trace-element and isotope studies of greenstone- and granitoid-hosted deposits suggest that the ore fluid originated from a deep (mid to lower crustal) source and, at least, interacted with granitic rocks at some stage. The ore fluid composition is quite constant over the range of crustal levels from deep to shallow, implying that the extent of modification by interaction with fluid conduit rocks is variable but not substantial.

Archean komatiite volcanism controlled by the evolution of early continents

Significance Komatiites are rare, ultra-high-temperature (∼1,600 °C) lavas that were erupted in large volumes 3.5–1.5 bya but only very rarely since. They are the signature rock type of a hotter early Earth. However, the hottest, most extensive komatiites have a very restricted distribution in particular linear belts within preserved Archean crust. This study used a combination of different radiogenic isotopes to map the boundaries of Archean microcontinents in space and time, identifying the microplates that form the building blocks of Precambrian cratons. Isotopic mapping demonstrates that the major komatiite belts are located along these crustal boundaries. Subsequently, the evolution of the early continents controlled the location and extent of major volcanic events, crustal heat flow, and major ore deposit provinces. The generation and evolution of Earth’s continental crust has played a fundamental role in the development of the planet. Its formation modified the composition of the mantle, contributed to the establishment of the atmosphere, and led to the creation of ecological niches important for early life. Here we show that in the Archean, the formation and stabilization of continents also controlled the location, geochemistry, and volcanology of the hottest preserved lavas on Earth: komatiites. These magmas typically represent 50–30% partial melting of the mantle and subsequently record important information on the thermal and chemical evolution of the Archean–Proterozoic Earth. As a result, it is vital to constrain and understand the processes that govern their localization and emplacement. Here, we combined Lu-Hf isotopes and U-Pb geochronology to map the four-dimensional evolution of the Yilgarn Craton, Western Australia, and reveal the progressive development of an Archean microcontinent. Our results show that in the early Earth, relatively small crustal blocks, analogous to modern microplates, progressively amalgamated to form larger continental masses, and eventually the first cratons. This cratonization process drove the hottest and most voluminous komatiite eruptions to the edge of established continental blocks. The dynamic evolution of the early continents thus directly influenced the addition of deep mantle material to the Archean crust, oceans, and atmosphere, while also providing a fundamental control on the distribution of major magmatic ore deposits.

Crustal evolution, intra-cratonic architecture and the metallogeny of an Archaean craton

Abstract The generation of the Earths continental crust modified the composition of the mantle and provided a stable, buoyant reservoir capable of capturing mantle material and ultimately preserving ore deposits. Within the continental crust, lithospheric architecture and associated cratonic margins are a first-order control on camp-scale mineralization. Here we show that the evolving crustal architecture of the Archaean Yilgarn Craton, Western Australia, played a key role in controlling the localization of camp-scale gold, iron and nickel mineralized systems. The age and source characteristics of Archaean lithosphere are heterogeneous in both space and time and are recorded by the varying Nd isotopic signature of crustal rocks. Spatial and temporal variations in isotopic character document the evolution of an intra-cratonic architecture through time, and in doing so map transient lithospheric discontinuities where gold, nickel and iron mineral systems were concentrated. Komatiite-hosted nickel deposits cluster into camps localized within young, juvenile crust at the isotopic margin with older lithosphere; orogenic gold systems are typically localized along major structures within juvenile crust; and banded iron formation (BIF)-hosted iron deposits are localized at the edge of, and within, older lithospheric blocks. Furthermore, this work shows that crustal evolution plays an important role in the development and localization of favourable sources of nickel, gold and iron by controlling the occurrence of thick BIFs, ultramafic lavas and fertile (juvenile) crust, respectively. Fundamentally, this study demonstrates that the lithospheric architecture of a craton can be effectively imaged by isotopic techniques and used to identify regions prospective for camp-scale mineralization.

Archean gold mineralization synchronous with the final stages of cratonization, Yilgarn Craton, Western Australia

Sm-Nd ages of pegmatite dikes that crosscut gold-bearing structures in the southern Yilgarn Craton, Western Australia, provide minimum age constraints of 2640 6 11 Ma, 2628 6 10 Ma, and 2620 6 36 Ma for gold mineralization at the Westonia and Nevoria (Yellowdine Terrane) and Scotia (Norseman Terrane) gold deposits, respectively. Similarly, a post‐gold mineralization dike at Westonia has a U-Pb zircon age of 2637 68 Ma. Theseconstraintsareconsistentwith,andprovidefurthersupportfor,suggestionsthatthe majority of gold deposits in the Yilgarn Craton formed during a regionally extensive gold mineralization event at ca. 2630 Ma (i.e., 2630 6 ’10 Ma). In combination with recent geochronological results, these data also provide further evidence that, although mineralization appears to significantly postdate the majority of magmatic and metamorphic activity at higher crustal levels, widespread thermal reworking of the lower-middle crust, involvingpartialmelting,amphibolitetogranulitefaciesmetamorphism,andlowercrustal granitoid intrusion, occurred concurrently with gold mineralization at ca. 2630 Ma. It is probable that the large-scale hydrothermalfluidflow that produced widespread gold deposition was also part of this tectonothermal event. Models developed for other Archean terranes whereby gold mineralization postdates formation and cratonization of host granite-greenstone terranes, and is therefore unrelated to these processes, are not required to explain the timing of the majority of gold deposits in the Yilgarn Craton.

Gold mineralisation at the Lady Bountiful Mine, Western Australia: An example of a granitoid-hosted Archaean lode gold deposit

The Lady Bountiful granitoid-hosted lode gold deposit, located in the mid-greenschist facies metamorphosed Ora Banda greenstone sequence, is hosted predominantly by the late-tectonic Liberty Granodiorite. Gold mineralisation is localised along quartz-veined, sinistral, brittle fault-zone(s) that transect the boundary between the Liberty Granodiorite and Mt Pleasant sill. Quartz vein textures indicate two stages of a single gold-related vein-development event, with high-grade gold mineralisation restricted to the second stage. Ore minerals include pyrite, chalcopyrite, pyrrhotite, galena, sphalerite, Au−Ag−Bi−Pb-tellurides, and native gold. Fluid infitration has resulted in narrow (<1 m) bleached wallrock alteration envelopes to the fault zones comprising albite-K-mica ±chlorite±calcite±rutile assemblages. Temperature-pressure conditions varied from Stage I (300°±50°C, ≈2 kbar) to Stage II (250°±50°C, ≈0.5 to 2 kbar), with the hydrothermal fluid in both stages characterised by X(CO2)≤0.15 and moderate salinity (≈1.28 m NaCl). Intermittent phase separation of Stage II mineralising fluids, initiated by pressure fluctuations in dilational sites, and/or fluid-dominated fluid: wallrock interaction, are invoked as the dominant depositional mechanisms. The granitoid-hosted Lady Bountiful lode gold deposit shares many features with other granitoid-hosted lode gold deposits in the Yilgarn Craton and the Superior Province. Granitoid-hosted lode gold deposits, such as the Lady Bountiful deposit, provide additional evidence that the dominant control on the localisation of gold mineralisation within a granitoid host is structure, with competency contrasts playing a significant role. Furthermore, the hydrothermal wallrock alteraction and orefluid chemistry characteristics of the granitoid-hosted lode gold deposits are comparable to those established for greenstone-hosted lode gold mineralisation.

Spatio-temporal constraints on lithospheric development in the southwest–central Yilgarn Craton, Western Australia

The Archean western Yilgarn Craton contains an extensive record of supracrustal formation from ca 3730 to ca 2675 Ma, as well as evidence of an ensialic crustal component as old as ca 4400 Ma. These features make the western Yilgarn Craton one of the oldest crustal provinces on Earth and ideal for the study of Archean crustal evolution. Spatial analysis of new and collated U–Pb age data define three broad pulses of granite emplacement at ca 3000–2820, ca 2805–2720 and ca 2720–2600 Ma, with a period of regional quiescence at 2820–2805 Ma. Within these pulses, major peaks in granite production are defined at ca 2920, ca 2890, ca 2845, ca 2790, ca 2750, ca 2690, ca 2665, ca 2655, ca 2630, and ca 2615 Ma; with lesser inherited material as old as 3670 Ma. In the western Yilgarn Craton, all terranes show evidence of granite activity at ca 3000–2820 Ma. The South West Terrane and Southern Cross Domain share granite pulses at ca 2950–2920, 2880–2820 and 2800–2720 Ma, although during these intervals granite magmatism tends to dominate in one terrane, i.e. ca 2805–2780 Ma granite activity predominantly occurs in the South West Terrane, while 2780–2720 Ma activity is focused in the Southern Cross Domain. Including the period of quiescence, granite production is relatively minor between ca 2820 and ca 2720 Ma relative to the 3000–2820 Ma and 2720–2600 Ma intervals, suggesting limited crustal development at this time. This period corresponds with widespread greenstone formation throughout the western Yilgarn Craton. The major pulse of granite emplacement and crustal evolution occurs at ca 2700–2600 Ma, with the main phases of activity at ca 2680–2650 Ma in the Southern Cross Domain and ca 2640–2620 Ma in the South West Terrane. These pulses coincide with a craton-wide transition in granite geochemistry from high-Ca to low-Ca at ca 2650 Ma and suggest significant variations in the method and timing of melt generation. Results from this study provide new constraints on the spatio-temporal evolution of the lithosphere in the western Yilgarn Craton. The spatial distribution of these age data suggest that existing terrane boundaries should be revised with the South West Terrane separated into at least two distinct domains, and the boundary between the Youanmi and South West Terranes moved westward to correspond with the eastern extent of charnockite granites.

Characteristics and geodynamic setting of the 2.7 Ga Yilgarn heterogeneous plume and its interaction with continental lithosphere: evidence from komatiitic basalt and basalt geochemistry of the Eastern Goldfields Superterrane

A database of 1075 high-precision geochemical analyses of least-altered ultramafic–mafic units, predominantly flows, was compiled for the Eastern Goldfields Superterrane. Samples are divided into a high-Mg population at MgO≥10–24 wt% and a basaltic population where 4≤MgO<10 wt%. There are eight groups based on (La/Sm)N and Nb/Th ratios. Five magma series are identified. Uncontaminated komatiitic basalts have MgO ∼11–23 wt% and Nb/Th≥8, whereas contaminated counterparts have Nb/Th<8 corresponding to silicious high-Mg basalts (SHMB). A distinct second magma series with MgO ∼5–18 wt% MgO has a narrow range of Nb/Th at 0.5–≤2 over a range of (La/Sm)N from 0.7–5.5, unlike contaminated suites where (La/Sm)N and Nb/Th are correlated; this series corresponds to the enriched Paringa Basalt representing shallow melts of heterogeneous domains of the plume with recycled ancient continental lithosphere, or an independent plume. Prevalent, crustally uncontaminated, tholeiitic basalt magma series three all have Nb/Th≥8, span Mg-rich to fractionated Fe-rich counterparts, and range from LREE-depleted to mildly LREE-enriched where high Nb/Th ratios stem from eclogite streaks in the asthenosphere plume; contaminated equivalents have Nb/Th<8. A fourth alkaline, high-Mg magma series has a narrow range of MgO at ∼13–16 wt%, extends to elevated TiO2 and Ni contents relative to komatiitic basalts at that MgO range, and features (La/Sm)N ≥2. Two additional uncontaminated tholeiitic basaltic groups are defined respectively by high-Nb to 20 ppm akin to alkaline ocean island basalts, and high-ΣREE relative to the other basaltic groups. The former, a fifth magma series, reflect melts of an eclogite-rich domain of the plume. Contamination of all groups, when present, was dominantly by interaction with continental mantle lithosphere with a minor crustal component. Komatiitic basalts are fractionation products of komatiites erupted from the hot axis of a mantle plume whereas prevalent tholeitic basalts are liquids derived from the cooler plume annulus. In all cases melting was in anhydrous peridotite. Ratios of Nb/Th in uncontaminated samples span 8–24 signifying that the Neoarchean mantle was as heterogeneous in terms of this ratio as Phanerozoic asthenosphere. In contrast, the fourth alkaline magma series stems from decompressional melting of metasomatised, hydrous, continental mantle lithosphere at >90km. Komatiites and komatiitic basalts are most abundant proximal to terrane boundaries because mantle plumes are ‘steered’ to the margins of thin, rifted, continental lithosphere. Given that mantle plumes melt on impingement at the base of the lithosphere, (Gd/Yb)N ratios are used as a proxy to ‘map’ the thickness of the contemporaneous lithosphere.

Thematic Issue: Archean Evolution—Yilgarn Craton

The dominant geological processes responsible for the evolution of Archean cratons remain a subject of ongoing debate. The debate is no more robust than for the Yilgarn Craton in Western Australia where much of it has centred on the relative roles of plume-dominant and subduction-related processes. Contrasting positions on this issue have naturally resulted in contrasting geodynamic models (cf. Czarnota et al. 2010; Van Kranendonk et al. in press). This debate, however, occurs in the context of ongoing advances in the application of geochronology, geochemistry and tectonics to the understanding of the evolution of the Yilgarn Craton. These advances have broader implications and are critical to understanding important Early Earth processes such as those responsible for, among others, the development of a habitable planet (e.g. Lowe & Tice 2007) and unique mineral systems (e.g. Begg et al. 2010). The majority of papers in this thematic issue of the Australian Journal of Earth Sciences were presented at the 5th International Archean Symposium held in Perth, Western Australia in September 2010, part of a series held every decade by Geoconferences (WA) Inc. to present the advances in our understanding of the Early Earth. One of the principal drivers of research over the last decade has been the advances in analytical capability (e.g. Belousova et al. 2010), most importantly relating to that small mineral, zircon, that revolutionised our understanding of the age of the Earth in previous decades, as well as advances in understanding the physical and chemical processes that control generation, transport, crystallisation and structural modification of both mantle and crustalderived magmas (e.g. Hawkesworth & Kemp 2006; Arndt et al. 2008). The nine papers in this issue present some of these advances that directly relate to the Yilgarn Craton. While the very early tectonic history of the Yilgarn Craton remains largely unresolved, recent field studies, combined with application of geophysical, geochemical and geochronological datasets, provide new insights into the later history of the craton. The papers are arranged in a loosely thematic manner— from the application of geochronology and isotope geochemistry to propose new terranes and constrain the development of lithosphere, through the use of geochemistry to constrain possible tectonic models for the dominant mafic and ultramafic sequences, to advances in understanding the deformation history of greenstone belts and granite–gneiss regions. Importantly, many of the new findings relate to regions outside the historically better researched ‘Eastern Goldfields,’ allowing new insights into the evolution of the craton as a whole. The first three papers use U–Pb geochronology and Lu–Hf isotope geochemistry to constrain lithospheric evolution of regions of the Yilgarn Craton. Major thermal events after ca 2.95 Ga are consistent with the development of craton-scale mantle plumes. The consequences of these events are seen in the rock record for the next 300 Ma or so, with mafic–felsic greenstone cycles and voluminous granites in the central and far eastern parts of the craton involving substantial recycling of older crustal material at various times. Arc-like greenstone successions in the Eastern Goldfields, after ca 2.72 Ga, although punctuated by a major, discrete episode of plume-related basalt and komatiite magmatism at ca 2.71 Ga, provide evidence for subduction-related processes at least for parts of the craton. Pawley et al. in Adding pieces to the puzzle: episodic crustal growth and a new terrane in the northeast Yilgarn Craton, Western Australia, use field studies and geochronology to propose a previously unrecognised Yamarna Terrane in the northeast corner of the preserved craton that has affinities with the metallogenically endowed Kalgoorlie Terrane. On the basis of correlation of lithologies and ages of magmatism, the redefined Burtville Terrane shares a common history with the Youanmi Terrane. Mole et al. in Spatiotemporal constraints on lithospheric development in the southwest-central Yilgarn Craton, Western Australia use innovative spatio-temporal techniques to show that new and existing U–Pb geochronology data from the central and southwest Youanmi Terrane suggest a number of discrete lithospheric blocks at different time periods, each with discrete magmatic histories. Wyche et al. in Isotopic constraints on stratigraphy in the central and eastern Yilgarn Craton, Western Australia present Lu–Hf data that constrain crust formation events and indicate that the central and eastern Yilgarn share common elements post ca 2.95 Ga. Importantly, broadly contemporaneous, episodes of mantle extraction and crustal reworking are indicated by the datasets. In combination, these papers highlight the importance of understanding the long history of Australian Journal of Earth Sciences (2012) 59, (599–601)