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Magnetite-bearing granophyre and quartz dolerite are the evolved fractions of differentiated dolerite (diabase) sills and are an important host to Archean gold deposits because they are chemical traps for orogenic fluids. Despite their economic importance, there is a poor understanding of how melt composition, crystal fractionation, sill geometry, and depth of emplacement increase the volume of host rock that is most favorable for gold precipitation during orogenesis. We use drill core logging, whole-rock geochemistry, magnetic susceptibility, gold assay, and thermodynamic modeling data from 11 mineralized and unmineralized ca. 2.7 Ga differentiated dolerites in the Eastern Goldfields superterrane (Yilgarn craton, Western Australia) to better understand the influence of igneous and emplacement processes on gold prospectivity. Orogenic gold favors differentiated dolerites, derived from iron-rich parental magmas, that crystallize large volumes of magnetite-bearing quartz dolerite (>25% total thickness). Mineralized sills are commonly >150 m thick and hosted by thick and broadly coeval sedimentary sequences. Sill thickness is an important predictor for gold prospectivity, as it largely controls cooling rate and hence fractionation. The parental melts of gold-mineralized sills fractionated large amounts of clinopyroxene and plagioclase (possibly up to 50%) at depth before emplacement in the shallow crust. A second fractionation event at shallow levels (

Coarsely crystalline mafic rocks, commonly referred to as dolerite and diabase, make up a substantial proportion of Archean greenstone belts and are hosts to major orogenic gold deposits, yet their internal architecture and contact relationships remain poorly documented and their emplacement origins are assumed rather than supported with evidence. We present results from drill core logging of several late Archean coarsely crystalline packages from the Eastern Goldfields Superterrane of the Yilgarn Craton, Western Australia, including the Golden Mile Dolerite. These large mafic bodies comprise mostly medium- to coarsely crystalline mafic rocks, but also include a variety of other products of differentiation. The bodies range from ∼75 to 460 m thick and extend for tens of kilometers, are primarily hosted in mudstones and basalt and appear to be concordant to stratigraphy. As such, this study excludes mafic dykes and extremely thick and internally complex bodies such as the Bushveld Complex. From base to top, the internal stratigraphy generally consists of a lower unit of aphyric basalt, dolerite/gabbro and peridotite ± pyroxenite, then a voluminous middle dolerite/gabbro, and an upper quartz dolerite/gabbro, and a topmost aphyric basalt. Granophyric veins and domains are common in the upper third of the mafic body. Contacts are gradational between all lithofacies, with the exception of granophric veins, which have sharp contacts. Within this overall stratigraphic framework there is variability in the relative thickness of lithofacies, number of lithofacies, as well as repetition of some lithofacies (several examples also include basalt in the interior). In well exposed examples, the key evidence for distinguishing intrusions from extrusions is provided by the top contact. However, the top contact from these examples is ambiguous. To overcome this gap, we review the character of well-studied thick ponded lavas from the literature. Most aspects of the internal character are not unique to intrusions, although the maximum groundmass grain size (excluding granophyric veins) of the Archean examples (>3 mm) is significantly coarser than that for any known ponded lavas (≪1 mm). The ability to distinguish intrusions from extrusions has important implications for reconstructing Archean greenstone stratigraphy and mineral exploration.

Archean greenstone belts commonly consist of a stratigraphy that records subaqueous deposition capped by subaerial sedimentation, yet it remains unclear if the controls on this change were driven by horizontal or vertical processes. Clasts in conglomerates can provide important constraints on this question because they inform on the changing nature of sources. The 2.7 Ga Agnew Greenstone Belt, Yilgarn Craton, Australia, provides the ideal opportunity for such a study because it contains abundant and diverse conglomerate facies and its basal stratigraphy is well understood. This study focusses on the texture and composition of lithic clasts derived from three stratigraphic conglomerate units and their host successions. The lowermost conglomerate-bearing unit, hosted in the Vivien Formation, unconformably overlies the mafic-ultramafic lavas and intrusions of the Kalgoorlie Group (ca. 2720–2690 Ma) and is dominated by aphyric to porphyritic mafic and felsic volcanic clasts. The conformably overlying Maria Mine Formation conglomerates contain a higher proportion of felsic clasts with medium- and plutonic groundmass textures. The Vivien and Maria Mine Formations (ca. 2690–2670 Ma) are dominated by cherts, turbidites and conglomerates, and represent subaqueous sediments formed in the ring-plain surrounding an emergent volcanic island. The unconformably overlying Scotty Creek Formation (ca. 2665–2655 Ma) contains conglomerates with many plutonic felsic clasts, and is dominated by bedded and cross-bedded sandstones formed in a subaerial environment. Whole-rock geochemistry of mafic and ultramafic clasts matches the local Kalgoorlie Group rocks and indicates progressive erosion of the underlying supracrustal sequence. Undeformed conglomerates lack clasts with metamorphic foliations, indicating uplift by horizontal compression could not have been significant before ca. 2655 Ma, which is considered as the time of the first major compressional event in many published structural frameworks for the region. The systematic erosion of a felsic stratovolcano, its associated plumbing system, and any resultant upwarping of underlying sequences through vertically-driven diapirism, better explains the progressive changes observed in clast texture and composition, as opposed to horizontally-driven uplift, such as compression associated with subduction.

The thick package of ∼2.7 Ga mafic and ultramafic lavas and intrusions preserved among the Neoarchean of the Kalgoorlie Terrene in Western Australia provides valuable insight into geological processes controlling the most prodigious episode of growth and preservation of juvenile continental crust in Earth's history. Limited exposure of these rocks results in uncertainty about their age, physical and chemical characteristics, and stratigraphic relationships. This in turn prevents confident correlation of regional occurrences of mafic and ultramafic successions (both intrusive and extrusive) and hinders the interpretation of tectonic setting and magmatic evolution. A recent stratigraphic drilling program of the Neoarchean stratigraphy of the Agnew Greenstone Belt in Western Australia has provided continuous exposures through a c. 7 km thick sequence of mafic and ultramafic units. In this study, we present a volcanological, lithogeochemical and chronological study of the Agnew Greenstone Belt, and provide the first pre-2690 Ma regional correlation across the Kalgoorlie Terrane. The Agnew Greenstone Belt records ∼30 m.y. of episodic ultramafic–mafic magmatism that includes two cycles, each defined by a komatiite that is overlain by units that become more evolved and contaminated with time. The sequence is divided into nine conformable packages, each consisting of stacked subaqueous lava flows and comagmatic intrusions, as well as two sills without associated extrusions. Lavas, with the exception of intercalations between two units, form a layer-cake stratigraphy and were likely erupted from a system of fissures tapping the same magma source. The komatiites are not contaminated by continental crust ([La/Sm]PM ∼ 0.7) and are of the Al-undepleted Munro-type. Crustal contamination is evident in many units (Songvang Basalt, Never Can Tell Basalt, Redeemer Basalt, and Turrett Dolerite), as judged by [La/Sm] > 1, negative Nb and Ti anomalies, and geochemical mixing trends towards felsic contaminants. Crystal fractionation was also significant, with early olivine and chromite (Mg# > 65) followed by plagioclase and clinopyroxene removal (Mg 100 km, which has implications for exploration targeting for stratigraphically hosted ultramafic Ni and VMS deposits.

The Crusader Complex, part of the Agnew gold camp of the Lawlers Anticline of the Yilgarn Craton, Western Australia, is located close to or along the contact between the Lawlers Basalt and Agnew Ultramafics units. Au mineralization within the four orebodies that form the Crusader Complex is dominated by very pure Au, containing less than 1 wt.% Ag, with Au variably associated with scheelite, Bi-tellurides and minor chalcopyrite within a magnetite and titanite gangue assemblage. Hydrothermal alteration associated with this style of mineralization is characterized by increasing concentrations of Mo, Be, Li, Sn and Fe and depletions in Na, Cu, Ba, Pb, Mn, Zn, Si, and K relative to protolith concentrations; these enrichments are more typical in orebodies associated with felsic intrusive-related mineralizing systems rather than the more well-known orogenic Au deposits found elsewhere within the Lawlers Anticline (e.g. at Waroonga) and within the greater Yilgarn Craton. We propose that flexures of the contact between the Lawlers Basalt and Agnew Ultramafic units acted as conduits for Au-bearing felsic intrusive-derived fluids and generated structural traps that enhanced fluid flow. The mineralizing fluids that formed the Crusader deposits were derived from the Lawlers granitoid pluton that intruded into the study area. Enhanced fluid flow promoted interaction between hydrothermal fluids and the reactive mafic–ultramafic rock sequence, augmenting the amount of Au that was precipitated during formation of the orebodies at Crusader. The magnetite-dominated quartz- and sulfide-poor intrusion-related mineralization at Crusader contrasts sharply with other late Archean intrusion-related deposits of the Yilgarn Craton that are usually sulfide- and/or quartz-rich. This may in turn suggest that the Crusader deposit represents a new class of under-explored intrusion-derived deposits, possibly opening new mineral exploration opportunities for the Agnew region, and potentially the wider Eastern Goldfields Superterrane. Enrichments in Mo and Sn and significant depletions in Cu suggest that other parts of the Lawlers batholith may also be prospective for base metal mineralization. Integration of stratigraphic interpretation with the identification of key structural fluid pathways and the presence of felsic intrusive bodies, as presented in this study, enables the delineation of the key elements that underlie mineralization at the Crusader Complex. We propose that these key elements provide vital information for future gold exploration models that can be used within other Archean terranes and within the Eastern Yilgarn Craton in particular.

The largest Neoarchean gold deposits in the world-class St Ives Goldfield, Western Australia, occur in an area known as the Argo–Junction region (e.g. Junction, Argo and Athena). Why this region is so well endowed with large deposits compared with other parts of the St Ives Goldfield is currently unclear, because gold deposits at St Ives are hosted by a variety of lithologic units and were formed during at least three different deformational events. This paper presents an investigation into the stratigraphic architecture and evolution of the Argo–Junction region to assess its implications for gold metallogenesis. The results show that the region’s stratigraphy may be subdivided into five regionally correlatable packages: mafic lavas of the Paringa Basalt; contemporaneously resedimented feldspar-rich pyroclastic debris of the Early Black Flag Group; coarse polymictic volcanic debris of the Late Black Flag Group; thick piles of mafic lavas and sub-volcanic sills of the Athena Basalt and Condenser Dolerite; and the voluminous quartz-rich sedimentary successions of the Early Merougil Group. In the Argo–Junction region, these units have an interpreted maximum thickness of at least 7,130 m, and thus represent an unusually thick accumulation of the Neoarchean volcano-sedimentary successions. It is postulated that major basin-forming structures that were active during deposition and emplacement of the voluminous successions later acted as important conduits during mineralisation. Therefore, a correlation exists between the location of the largest gold deposits in the St Ives Goldfield and the thickest parts of the stratigraphy. Recognition of this association has important implications for camp-scale exploration.

Granite-cored domes are associated with many of the larger gold deposits of the Archaean Eastern Yilgarn Craton of Western Australia. The Scotia-Kanowna Dome is eroded to sufficiently deep levels to provide insights into the role granite-cored domes play in controlling fluid flow and gold deposition. At the centre of the Scotia-Kanowna Dome is a granite batholith, which is surrounded by outward-dipping greenstone belts and associated shear zones. This upper-crustal dome sits above mid-crustal domes, providing a series of stacked geometries favourable to focussed fluid flow. A number of small- to medium-sized gold deposits occur on the limbs and the centre of the dome, and the world-class Kanowna Belle gold mine occurs on the nose of the dome. At least three separate gold mineralising events are defined, each of regional significance, which can be correlated with other well known gold deposits of the Eastern Yilgarn Craton. The palaeostress across this region was dominated by both maximum contractional and extensional vectors oriented perpendicular to the north-northwest trending grain. The resultant strain is seen in the architecture of the Scotia-Kanowna Dome, with its margins and associated shear zones controlling the distribution of stratigraphy, which is relatively linear and parallel to the margin. Throughout most of the tectonic history, the flanking shear zones were high-strain domains that recorded multiple stages of contraction, with thrust and mostly sinistral strike-slip shear kinematics. In contrast, the dome nose region was a relatively low-strain domain as it was located in the strain shadow of the regional contractional events. For example, the early extensional record has been preserved at the nose, with late-stage basins formed in the hangingwall to the extensional faults, adding to the stratigraphic complexity of the local region. These late basins also may have contributed to the fluid budget. In contrast, along the flanks of the domes there is no record of these late basins or the extensional faults that controlled their deposition. The architecture of the dome played an important role in controlling mineralisation during a significant deviation in the maximum contractional stress vector to an east-southeast-west-southwest orientation. At this time, basins in the nose region underwent inversion, with thrusting to the north and northwest. Shortening was accommodated along the dome flanks by sinistral strike-slip shearing, which was relatively free to move due to the mostly linear stratigraphy, combined with limited anisotropy and perturbations along the flank. However, the stratigraphic and geometrical complexity around the dome nose resulted in localised maximum dilation and gold deposition at this time. The dome nose area was also a favoured locus for gold-enriched, dominantly Mafic-type magmas, which are seen as the extensive porphyry swarms in the region. These felsic-intermediate rocks also may have contributed to the complex fluid budget and are considered as indicators of particular fertility for gold. They may have been the primary source for much of the metal endowment. The architecture of granite-cored domes has played a critical role in focussing magma as well as mineralising fluids into regions of maximum dilation. They did this by virtue of their inherent competence, and by influencing the position and geometry of shear zones (fluid pathways) and hence distribution of stratigraphy (depositional sites) across the region. The Scotia-Kanowna Dome is a good example of this effect, and inferences can be made in favour of similar controls by concealed domes, such as beneath Kalgoorlie.

The consensus view is that the O 2 concentration of the Archean atmosphere was very low and that it rose to its present level of 21% in a series of steps, two of which dwarf the others in importance. The first, known as the Great Oxidation Event, occurred at ∼2.4 Ga. It involved an increase in the relative abundance of O 2 , which has been estimated at three orders of magnitude, and it is important because it led to the first surface weathering. The second, although less important in relative terms, involved the addition of 9 × 10 17 kg of O 2 to the atmosphere, at least ten times as much as that required to produce the Great Oxidation Event. Its importance lies in the fact that it correlates with the rise of animals in the Ediacaran and Early Cambrian periods. Although it is widely accepted that an increase in atmospheric O 2 facilitated the appearance of animals at ∼575 Ma, followed by the Cambrian Explosion ∼50 Myr later, the cause of this increase remains controversial. We show that the surge in the O 2 level near the Precambrian–Cambrian boundary correlates with major episodes of continent–continent collision associated with Gondwana’s amalgamation, including convergence between East and West Gondwana, which produced the 8000-km-long Transgondwanan Supermountains. The eroded roots of these mountains include the oldest lawsonite-bearing blueschists and eclogites, and ultra high-pressure metamorphic rocks. The sudden appearance of these low-thermal gradient, high-pressure metamorphic rocks implies that the Gondwanan orogenic zones were cooler and stronger than those associated with the assembly of earlier supercontinents and therefore capable of supporting higher mountains. There is a log-linear relationship between relief and erosion rate, and a linear relationship between sedimentation rate and organic C burial. Taken together these two relationships imply a log-linear relationship between relief and C sequestration. We suggest that the Gondwanan supermountains were higher than those produced during the assembly of earlier supercontinents and that rapid erosion of these mountains released a large flux of essential nutrients, including Fe and P, into the rivers and oceans, which triggered an explosion of algae and cyanobacteria. This, in turn, produced a marked increase in the production rate of photosynthetic O 2 . Rapid sedimentation during this period promoted high rates of burial of biogenic pyrite and organic matter generated during photosynthesis so that they could not back react with O 2 , leading to a sustained increase in atmospheric O 2 .

The ability to effectively target ore shoots requires a clear understanding of the numerous parameters that control their formation. However, identification of the nature and timing of these parameters is difficult, particularly in polydeformed deposits with complex stratigraphic and hydrothermal histories. For example, ore shoots at Stawell gold mine’s Magdala gold deposits in southeastern Australia are hosted by rocks that experienced at least three ductile and two brittle deformation events in the 70 m.y. prior to the principal mineralization event, and have a close spatial relationship to strongly chlorite ± stilpnomelane–altered sedimentary units near the apex of numerous >5-m-thick reentrant basalt lobes. Whereas the ore shoots at Stawell are located only on the southwestern flank of the 1-km-thick Magdala basalt, about 15 km along strike at the Wildwood prospect the ore shoots are predominantly restricted to the northeastern flank of the basalt. Here, the important issue of controls on ore shoot formation and distribution at Stawell is addressed by presenting the results of a multidisciplinary review of the characteristics and architecture of the volcanic and sedimentary facies, the spatial distribution of alteration assemblages, and the variations in intensity and vergence relationships of the cleavages associated with ductile deformation. This investigation, undertaken at both a regional and shoot scale, shows that the structures hosting the ore shoots at Magdala formed during preferential and localized reactivation of preexisting faults following rotation of the stress field. Preexisting structures, such as Central Lode fault, host many of the ore shoots and were initially generated during east-northeast–west-southwest compression at ca. 510 to 488 Ma. The geometries of these faults were also influenced by the irregular paleotopography of the basalt lobes, which represent flows of pillow and massive basalt separated by interflow sedimentary units. Coincident with this deformation, locally intense chlorite ± stilpnomelane alteration indurated the mud-rich units between the basalt flow-lobes and the Central Lode-type structures. Those thick, mud-rich sedimentary units are present on the southwestern flank of the basalt at Stawell, but predominantly near the top and on the northeastern flank at Wildwood. The switch to east-west compression at ca. 440 Ma caused localized failure and focusing of mineralized fluids in the indurated sedimentary rocks near the apex of the northwest-plunging portions of the basalt lobes. A second, subordinate mineralizing event at ca. 425 to 420 Ma utilized the same architecture under a different stress regime. During both mineralizing events, the high-grade ore shoots formed in zones of high fluid flow defined by the complex preexisting architecture.

Some published tectonic reconstructions of the eastern Lachlan Orogen in New South Wales have shown Ordovician volcanic and volcaniclastic rocks of the Macquarie Arc conformably overlying or interfingering with a coeval Ordovician quartz-rich turbidite sequence. Re-examination of key contacts between the volcanic and quartz-rich successions has found no evidence to support this interpretation, and suggests that the two packages are separate tectonostratigraphic terranes. The contacts between these two coeval successions are generally marked by major faults containing mylonites, cataclasites and, at some locations, fragments of mid-ocean ridge-type pillow basalt and chert. The quartz-rich turbidites are generally highly deformed and of higher metamorphic grade than the adjacent volcanics. At Oberon and Mudgee, contacts are faulted but there are no mylonites or significant differences in metamorphic grade. At Palmers Oaky and Black Springs, Silurian quartz-rich sandstones overlying the Ordovician volcanics ...

Early Ordovician (Phase 1) magmatism in the Macquarie Arc was followed by a magmatic hiatus of ∼9 million years, between late Bendigonian and early Darriwilian (i.e. between ca 475 and ca 466 Ma). Resumption of magmatism in the Middle Ordovician produced Phase 2 rocks, recorded by three major rock suites: (i) medium-K calc-alkaline lavas in the Cargo block (Molong Volcanic Belt) have primitive Nd values (+6.9 to +7.8) and volcanic facies suggesting eruption in an intra-oceanic arc stratovolcano; lavas in the fault-bounded Parkes Volcanics in the Junee – Narromine Volcanic Belt are compositionally identical to those in the Cargo block, suggesting that similar Phase 2 Middle Ordovician arc-type lavas may underlie the Cowra Trough; (ii) medium- to high-K dioritic to monzodioritic intrusions in the Narromine and Cowal Igneous Complexes of the Junee – Narromine Volcanic Belt have ages that cluster in the 470 – 460 Ma interval, and intrude presumed Phase 1 lavas and volcaniclastics; and (iii) in all three main ...

The Ordovician volcanic and intrusive rocks of the Cadia – Neville region, in the southern Molong Volcanic Belt section of the Macquarie Arc in central-western New South Wales, display a temporal progression from shoshonitic basaltic volcanism (e.g. Mt Pleasant Basalt Member) in the late Darriwilian to Gisbornian (ca 460 – 453 Ma) to small-volume dacitic medium-K calc-alkaline magmatism (e.g. Copper Hill-type dacite) in the late Eastonian to early Bolindian (ca 450 – 445 Ma) to large-volume, high-K calc-alkaline to shoshonitic, mafic to highly evolved magmatism (e.g. Nullawonga Latite Member and Cadia Intrusive Complex) at about 445 – 440 Ma. The two episodes of shoshonitic magmatism are separated by at least 15 (but up to ∼20) million years, during which time limestones, associated with regional uplift, were deposited broadly coincident with emplacement of the medium-K calc-alkaline dacitic intrusions. Despite this, the lavas and intrusions associated with both episodes of shoshonitic magmatism have stri...

The explosive radiation of animals on Earth during the late Early Cambrian period (∼ 530–510 Ma) coincides with the deposition of enormous volumes of continentally derived sedimentary rocks throughout Gondwana. We show here, that these quartz-rich sedimentary units, collected from five continents, display remarkably similar detrital-zircon U–Pb age-patterns and propose that they were sourced from either side of a > 8000-km-long and generally > 1000-km-wide mountain chain (the Transgondwanan Supermountain), which formed following oblique collision between East and West Gondwana, commencing at ∼ 650 Ma. The depositional system supplied by this mountain chain was > 100 km3, which is equivalent to covering all 50 states of the USA with ∼ 10 km of sediment, and it lasted for at least 260 Myr. The enormous size of the vegetation-free mountain chain, its position close to the equator and the dramatic changes in global plate-motion in response to the cessation in continent–continent collision, together with the possible appearance of biota in the soils that promoted rapid chemical weathering, resulted in extreme erosion and sedimentation rates that are arguably the highest in the geological record. This led to an unprecedented flux of P, Fe, Sr, Ca and bicarbonate ions into the oceans. The addition of Sr was responsible for seawater 87Sr/86Sr building up to the highest levels in Earth's history, whereas the addition of P and Fe provided the essential nutrients that supported a bloom of primitive life that in turn provided abundant food to support the Cambrian explosion of life. The addition of Ca and bicarbonate ions increased CaCO3 supersaturation in the oceans, which allowed species in numerous phyla to simultaneously develop skeletons.

The Hangingwall reefs are high-grade auriferous quartz lodes that occur in the upper levels of the Magdala mine at Stawell, and thus represent an important deposit style in western Victoria. Structural relationships indicate that the Hangingwall Reefs formed coincident with the same metallogenic event that produced the adjacent Magdala deposit. However, the host-rocks to the quartz – gold veins that make up the Hangingwall Reefs display markedly different structural and stratigraphic relationships. Hangingwall Reef mineralisation occurred during east – west shortening in a muscovite-altered turbidite sequence that had little prior iron alteration (cf. Magdala deposit). Lithological and structural data show that the distribution of the quartz – gold veins is related to the geometry of the Stawell Fault and its associated fault splays, with the quartz veining being localised where the faults are discordant with the pre-existing structural fabrics. This discordant relationship produced the dilational sites a...

Lower Cambrian basalts in the Stawell Zone (e.g. Magdala Basalt) are intercalated with a thick package of continentally derived quartz-rich turbidites, whereas similar basalts in the Heathcote and Mt Wellington Greenstone Belts are underlain by broadly coeval boninites and overlain by Upper Cambrian cherts. These marked differences in the magmatic – stratigraphic associations of the Early Cambrian basalts in Victoria have led to highly varied interpretations of their tectonic setting, which include subduction zones that are east-dipping, west-dipping and even a combination of east- and west-dipping systems. Here, we present new geochemical data for Cambrian basalts from the Stawell Zone and reassess the palaeogeographic and palaeotectonic setting of the region. Our geochemical data show that basalts in the Stawell Zone may be broadly divided into two units: basalts that are LREE-depleted relative to N-MORB and have low (≤3.0 ppm) Nb contents; and basalts that are slightly LREE-enriched relative to N-MORB ...

The Magdala deposit in the Stawell goldfield in western Victoria was formed during the 440 Ma gold event of the Lachlan Orogeny and is hosted by Cambrian quartz-rich turbiditic sedimentary rocks (Albion Formation) that onlap a thick pile of tholeiitic basaltic lavas (Magdala Basalt). Detailed petrographic and geochemical analyses suggest that the host-rock (Stawell Facies) was originally a turbiditic sedimentary rock that was hydrothermally altered in response to seawater interaction with the hot basaltic pile. Subsequent regional greenschist metamorphism and ductile deformation that lasted at least 10 million years culminated in the formation of the Magdala mineralised system and produced a complex pattern of hydrothermal alteration. Evolution of this alteration occurred over six stages: Stage 1, Fe-enrichment of sedimentary rock adjacent to the basalt pre-D1; Stage 2, chlorite (metamorphism), orbicular carbonate and pyrite, syn-D2; Stage 3, muscovite, siderite, ankerite and pyrrhotite, syn-D3 – D4a – b;...

The early Palaeozoic Macquarie Arc, southeastern Australia, hosts a variety of major late Ordovician to earliest Silurian subduction-related deposits (e.g., Cadia East, Ridgeway, Cadia Hill, Cowal and Northparkes). However, there is uncertainty about whether coeval high-sulfidation epithermal deposits, which occur in intra-oceanic metallogenic belts elsewhere in the West Pacific, (e.g., Lepanto and Chinkuashih), are also present in the Macquarie Arc. This has led to suggestions that their absence may be due to the poor preservation potential of deposits that form at relatively shallow crustal levels in ancient rocks. We present here an interpretation for evolution of the Peak Hill Au–Cu deposit based on the distribution of alteration facies, sulfur isotope data from several textural forms of pyrite and barite, and an assessment of the regional volcanic and sedimentary facies architecture. These data show that the Peak Hill deposit displays a distinct sub-vertical zoning with a pyrophyllite and vuggy-quartz core, that today extends about 350 m east–west and at least 550 m north–south, which grades out through paragonite+muscovite, kaolinite to a chlorite+epidote alteration zone at the margin. The alteration zoning reflects both lower temperatures and neutralisation of acid fluids with increasing distance from the core, which represents the conduit along which hot acidic hydrothermal fluids were channelled. Several temporally overlapping events of silicification, bladed-quartz-pyrite veining, brecciation and pyrite veining occurred during the last stages of hydrothermal alteration, although most appear to predate mineralisation. Au–Cu mineralisation was associated with late quartz-pyrite-barite veins, and the highest gold grades occur mainly in microcrystalline-quartz-altered rocks in the paragonite+muscovite alteration zone, generally within 50 m outward from the boundary of the pyrophyllite and vuggy-quartz core. Sulfur- and lead-isotope data, and the characteristic zoning of ore minerals and alteration assemblages support a magmatic source for the hydrothermal fluids. Similarities in many of the isotopic signatures between Peak Hill and deposits such as Northparkes support generation of the high-sulfidation mineralisation during the Late Ordovician to earliest Silurian (possibly ca. 440 Ma) metallogenic event. The Late Ordovician to Early Silurian volcanic and sedimentary facies associations at Peak Hill are consistent with alteration and mineralisation occurring in rocks deposited in a submarine setting.

Peperite involving basalt and polymictic volcanic conglomerate occurs in the Pliocene Ba Volcanic Group at Yaqara in northern Viti Levu, Fiji. Because the host sediment is coarse-grained and dominated by basalt clasts, the peperite could be easily overlooked and mistaken for another coarse volcaniclastic facies. However, the presence of groups of basalt clasts that show jigsaw-fit texture, fluidally shaped basalt clasts with complete glassy margins, gradational contacts with adjacent sedimentary facies and the absence of stratification indicate that molten basalt mingled with unconsolidated gravel. Using these criteria, we show that other superficially similar, coarse, polymictic facies with fluidal basalt clasts are not peperite. Both blocky and fluidal basalt clasts occur together in the peperite. The amoeboid basalt clasts in the fluidal peperite result from dismembering of ductile, low-viscosity, relatively hot magma. At this stage, propagating magma lobes were probably insulated from direct contact with the wet sediment by a vapour film. The angular, polyhedral basalt clasts in the blocky peperite indicate brittle disintegration of somewhat cooler, higher-viscosity magma. The presence of jigsaw-fit texture and polyhedral clasts with glassy margins suggest that quench fragmentation of the basalt was important in the formation of the blocky peperite. Although there is no positive evidence for steam explosivity, the presence of steam could be recorded by small quartz-filled cavities that occur within the host sediment. The co-existence of fluidal and blocky basalt clasts is interpreted to reflect successive ductile then brittle fragmentation of intruding magma. The change from fluidal to blocky peperite might have resulted from progressive cooling of the magma during intrusion, and also from the breakdown of fluidisation when the limited supply of fine sediment in the host gravel was exhausted.

The Lunnon Basalt is an Archaean, subaqueous, tholeiitic metabasalt succession, with a minimum inferred thickness of 1750 m. It forms the oldest exposed stratigraphic unit at Kambalda in the nickel sulfide‐rich Norseman‐Wiluna Greenstone Belt, Western Australia, and is dominated by variable proportions of massive basalt, pillow basalt, and basalt breccia. These facies form intimately inter‐layered massive, pillow and complex lava flow units, with an average thickness of about 20 m. The stratigraphy of the basalt can be subdivided into a ‘lower’ MgO‐rich member and an ‘upper’ less MgO‐rich member, these being separated by a sedimentary horizon. Of the possible palaeovolcanic and tectonic settings (layer 2 of oceanic crust, a large shield volcano, or a tectonically‐topographically ponded sea‐floor lava field), the Lunnon Basalt appears to be the remains of a ponded (?rift) lava succession. The presence of inherited zircon xenocrysts indicates that the subjacent crust at the time of formation was sialic, Arc...