A modern structural regime in the Paleoarchean (3.64 Ga); Isua Greenstone Belt, southern West Greenland

The footwall gneisses beneath the western part of the Paleoarchean (3.8 Ga) Isua Greenstone Belt, southern West Greenland, are interpreted here in terms of a 3.64 Ga stack of mylonitic crystalline thrust-nappes, the oldest example known on Earth. In present coordinates, the kinematic history of the thrust-nappe stack is couched in terms of initial longitudinal (strike-parallel) thrusting towards the southwest, followed by transverse thrusting to the northwest, and subsequent extensional collapse of the thickened crust toward the southeast.Diorite and tonalite that form the western margin of granitoids, structurally overlying the western part of the Isua Greenstone Belt and its footwall, contain 3.5 Ga mafic dykes, some of which are deformed and/or truncated at fault contacts within the granitoids. Accordingly, a component of the deformation structurally above the Isua Greenstone Belt occurred after 3.5 Ga, significantly later than the formation of the underlying mylonitic nappes, probably during the Neoarchean.The structural regime of mylonitic thrust-nappe stacking is very similar to that in modern mountain belts. It would appear that the deformational behaviour, rheological constitution and overall strength of Paleoarchean and modern continental crust were similar.     

五台山新太古代绿岩带中高镁安山岩（HMA）的化学特征

采用高镁安山岩（HMA）新的分类方法,描述了五台地区新太古代绿岩带中HMA的化学特征,并与实验的HMA以及自然界中典型的HMA（日本的Chichijima地区HMA和Setouchi地区HMA）进行了对比,显示该区HMA具有与上述HMA类似的化学特征。初步认为五台新太古代HMA产于洋俯冲带的环境,进而表明板块构造理论的适用时间范围可推演到新太古代。     

The association boninite low‐ti andesite‐tholeiite in the heathcote greenstone belt,Victoria; ensimatic setting for the early lachlan fold belt

The Heathcote Greenstone Belt is composed mainly of Lower Cambrian metavolcanic rocks and is one of three outcropping belts of the apparent basement to the Lachlan Fold Belt in SE Australia. The greenstones may be assigned to two broad magma series. A younger tholeiitic series with mid‐ocean ridge basalt (MORB) affinities has intruded through, and been erupted upon low‐Ti, intermediate SiO² lavas. The latter were originally boninites (both clinoenstatite‐phyric and more fractionated orthopyroxene‐phyric varieties) and plagioclase‐phyric, low‐Ti andesites. They have partially re‐equilibrated to the lower greenschist facies and outcrop mainly in the central segment of the Heathcote Greenstone Belt, where deeper stratigraphic levels are exposed. Tholeiitic lavas and sills metamorphosed to the prehnite‐pumpellyite facies dominate the northern and southern segments. As the association boninite/low‐Ti lavas/MORB is known only from modern West Pacific‐type settings involving island arcs and backarc basins, the early history of the Lachlan Fold Belt is inferred to have taken place in a similar setting.     

Silicification in the Belt Supergroup (Mesoproterozoic), Glacier National Park, Montana, USA

Nodular cherts can provide a window on the original sediment composition, diagenetic history and biota of their host rock because of their low susceptibility to further diagenetic alteration. The majority of Phanerozoic cherts formed by the intraformational redistribution of biogenic silica, particularly siliceous sponge spicules, radiolarian tests and diatom frustules. In the absence of a biogenic silica source, Precambrian cherts necessarily had to have had a different origin than Phanerozoic cherts. The Mesoproterozoic Belt Supergroup in Glacier National Park contains a variety of chert types, including silicified oolites and stromatolites, which have similar microtextures and paragenesis to Phanerozoic cherts, despite their different origins. Much of the silicification in the Belt Supergroup occurred after the onset of intergranular compaction, but before the main episode of dolomitization. The Belt Supergroup cherts probably had an opal-CT precursor, in the same manner as many Phanerozoic cherts. Although it is likely that Precambrian seas had higher silica concentrations than at present because of the absence of silica-secreting organisms, no evidence was observed that would suggest that high dissolved silica concentrations in the Belt sea had a significant widespread effect on silicification. The rarity of microfossils in Belt Supergroup cherts indicates that early silicification, if it occurred, was exceptional and restricted to localized environments. The similarity of microtextures in cherts of different ages is evidence that the silicification process is largely controlled by host carbonate composition and dissolved silica concentration during diagenesis, regardless of the source of silica.     

Microbial remains in some earliest Earth rocks: Comparison with a potential modern analogue

Carbonaceous matter (CM) from ca. 3.5 Ga hydrothermal black cherts of the Pilbara Craton of Western Australia and the Barberton Greenstone Belt of South Africa yielded transmission electron microscopy (TEM) images that are suggestive of microbial remains and possible remnants of microbial cell walls. These are compared to a potential modern analogue, the hyperthermophilic Methanocaldococcus jannaschii, derived from an active seafloor hydrothermal environment and cultured under similar conditions. A striking resemblance to the early Archaean forms was evident in wall structure and thermal degradation mode. Cell disintegration of the cultures occurred at 100 °C marking the limits of life. Complete disintegration, deformation and shrinkage occurred at 132 °C. A multidisciplinary approach to the characterisation of the CM was undertaken using organic petrology, TEM coupled with electron dispersive spectral analysis (EDS), high resolution TEM (HRTEM) to determine molecular ordering, and elemental and carbon isotope geochemistry. Reflectance measurements of the CM to determine thermal stress yielded a range of values corresponding to several populations, and pointing to different sources and processes. The δ¹³C values of Dresser Formation CM (−36.5 to −32.1‰) are negatively correlated with TOC (0.13–0.75%) and positively correlated with C/N ratio (134–569), which is interpreted to reflect the relative abundance of high R_o/oxidised/recycled CM and preferential loss of ¹²C and N during thermal maturation. TEM observations, inferred carbon isotopic heterogeneity and isotope fractionations of −27 to −32‰ are consistent with the activity of chemosynthetic microbes in a seafloor hydrothermal system where rapid silicification at relatively low temperature preserved the CM.     

Paleoarchean bedrock lithologies across the Makhonjwa Mountains of South Africa and Swaziland linked to geochemical,magnetic and tectonic data reveal early plate tectonic genes flanking subduction margins

The Makhonjwa Mountains, traditionally referred to as the Barberton Greenstone Belt, retain an iconic Paleoarchean archive against which numerical models of early earth geodynamics can be tested. We present new geologic and structural maps, geochemical plots, geo- and thermo-chronology, and geophysical data from seven silicic, mafic to ultramafic complexes separated by major shear systems across the southern Makhonjwa Mountains. All reveal signs of modern oceanic back-arc crust and subductionrelated processes. We compare the rates of processes determined from this data and balance these against plate tectonic and plume related models. Robust rates of both horizontal and vertical tectonic processes derived from the Makhonjwa Mountain complexes are similar, well within an order of magnitude, to those encountered across modern oceanic and orogenic terrains flanking Western Pacific-like subduction zones. We conclude that plate tectonics and linked plate-boundary processes were well established by 3.2-3.6 Ga. Our work provides new constraints for modellers with rates of a 'basket' of processes against which to test Paleoarchean geodynamic models over a time period close to the length of the Phanerozoic.     

Geochemistry and petrogenesis of volcanic rocks of the Neoarchaean Sukumaland Greenstone Belt, northwestern Tanzania

The late Archaean volcanic rocks of the Rwamagaza area in the Sukumaland Greenstone Belt consists of basalts and basaltic andesites associated with volumetrically minor rhyodacites and rhyolites. Most basalts and basaltic andesites yield nearly flat patterns (La/Sm_CN = 0.89–1.34) indicating derivation by partial melting of the mantle at relatively low pressure outside the garnet stability field. On primitive mantle normalized trace element diagrams, the basalts and basaltic andesites can be subdivided into two groups. The first group is characterised by moderately negative Nb anomalies (Nb/La_pm = 0.51–0.73, mean = 0.61 ± 0.08) with slight enrichment of LREE relative to both Th and HREE. The second group is characterised by nearly flat patterns with no Nb anomalies (Nb/La_pm = 0.77 ± 0.39). The observed Nb and Th anomalies in the Rwamagaza basalts and basaltic andesites, cannot be explained by alteration, crustal contamination or melt–solid equilibria. Rather, the anomalies are interpreted, on the basis of Nb–Th–La–Ce systematics, as having formed by partial melting of a heterogeneous mantle consisting of variable mixtures of components derived from two distinct sources. These sources are depleted mantle similar to that generating modern MORB and an LREE-enriched and HFSE-depleted source similar to that feeding volcanism along modern convergent margins.The rhyolites are characterised by high Na₂O/K₂O ratios (>1) and Al₂O₃ (>15 wt.%), low HREE contents (Yb = 0.24–0.68 ppm) leading to highly fractionated REE patterns (La/Yb_CN = 18.4–54.7) and large negative Nb anomalies (Nb/La_pm = 0.11–0.20), characteristics that are typical of Cenozoic adakites and Archaean TTG which form by partial melting of the hydrated basaltic crust at pressures high enough to stabilize garnet ± amphibole. The Rwamagaza basalts and basaltic andesites are geochemically analogous to the Phanerozoic Mariana Trough Back Arc Basin Basalts and the overall geochemical diversity of Rwamagaza volcanic rocks is interpreted in terms of a geodynamic model involving the interaction of a depleted mantle, a melting subducting oceanic slab in a back arc setting.     

Geological evolution of the Ishpeming Greenstone Belt,Michigan, U.S.A.

The Ishpeming Greenstone Belt is an Archean belt in the southern part of the Canadian Shield in the Upper Peninsula of Michigan, U.S.A. Two volcanic cycles are preserved in it. The oldest formation, and basal to the first cycle (the Kitchi Schist), consists of mafic metavolcanics, has a major serpentinized ultramafic body near its base, and grades upward to a coarse felsic volcanic breccia at the top of the cycle. This unit in turn is overlain by a sequence of mafic flows that grades upward to interbedded mafic flows and exhalites of the Mona Schist. This sequence has been intruded by the Dead River Pluton.The Ishpeming Greenstone Belt probably represents the keel of a previously much more extensive Greenstone Belt.Gold mineralization occurs associated with mafic basaltic volcanic rocks and serpentinized ultramafics low in the succession, and with carbonate-rich quartz-chlorite-sericite schists and exhalites higher in the sequence. No mineral deposits are now being exploited here.     

PGE, Re-Os, and Mo isotope systematics in Archean and early Proterozoic sedimentary systems as proxies for redox conditions of the early Earth

Re-Os data and PGE concentrations as well as Mo concentrations and isotope data are reported for suites of fine clastic sediments and black shales from the Barberton Greenstone Belt, South Africa (Fig Tree and Moodies Groups, 3.25-3.15 Ga), the Belingwe Greenstone Belt, Zimbabwe (Manjeri Formation, ca. 2.7 Ga) and shales from the Witwatersrand, Ventersdorp and Transvaal Supergroups, South Africa ranging from 2.95 to 2.2 Ga. Moderately oxidizing conditions are required to mobilize Re and Mo in the environment, Mo fractionation only occurs in solution, and these parameters thus have potential use as paleoredox proxies for the early Earth.PGE + Re abundance patterns of Barberton Greenstone Belt sediments are uniform and very similar in shape to those of komatiites. This indicates (1) that the PGE came from a source of predominantly ultramafic composition and, (2) that PGE were transported and deposited essentially in particulate form. Sediments from the younger Belingwe Greenstone Belt show more fractionated PGE + Re patterns and have Re/Os ratios 10 to 100× higher than those of Barberton sediments. Their PGE abundance patterns and Re/Os ratios are intermediate between those of the mid-Archean shales and Neoproterozoic to Recent black shales. They reflect scavenging of Re from solution in the sedimentary environment.δ^98/95Mo values of black shales of all ages correlate with their concentrations. The Barberton Greenstone Belt samples have ∼1-3 ppm Mo, similar to a granitoid-basaltic source. This Mo has δ^98/95Mo between −1.9 and −2.4‰ relative to present day mean ocean water molybdenum, MOMO and is thus not isotopically fractionated relative to such a source. Similar to the PGE this indicates transport in solid form. Sediments from the Belingwe Greenstone Belt show in part enhanced Mo concentrations (up to 6 ppm) and Mo isotope fractionation (δ^98/95Mo up to −1.4‰ relative to MOMO). The combined PGE + Re and Mo data show mainly reducing conditions in the mid-Archean and suggest that by 2.7 Ga, the atmosphere and oceans had become more oxidizing.Substantially younger samples from the Transvaal Supergroup (to ca. 2.2 Ga) surprisingly have mainly low Mo concentrations (around 1 ppm) and show no significant Mo isotope fractionation relative to the continental source. Among possible explanations for this are a return to reducing atmospheric conditions after 2.7 Ga, reservoir effects, or Mo removal by sulfide precipitation following sulfate reduction in early Proterozoic oceans.     

Mass and magnetic properties for 3D geological and geophysical modelling of the southern Agnew–Wiluna Greenstone Belt and Leinster nickel deposits,Western Australia

Physical property measurements provide a critical link between geological observations and geophysical measurements and modelling. To enhance the reliability of gravity and magnetic modelling in the Yilgarn Craton's Agnew–Wiluna Greenstone Belt, mass and magnetic properties were analysed on 157 new rock samples and combined with an existing corporate database of field measurements. The new samples include sulfide ore, serpentinised and olivine-bearing ultramafic host-rocks, granitoid, and felsic and mafic volcanic and volcaniclastic country rock. Synthesis of the data provides a useful resource for future geophysical modelling in the region. Several rock types in the region have sufficiently distinct physical properties that a discriminant diagram is proposed to facilitate a basic classification of rock types based on physical properties. However, the accumulation of emplacement, metamorphic, hydrothermal and structural processes has complicated the physical properties of the rocks by imposing duplicate and sometimes opposing physical property trends. The data confirm that massive sulfide and ultramafic rocks have the most distinctive mass and magnetic properties but with variability imposed by their complex history. Sulfide content imposes the strongest control on densities, but can only be identified when comprising >10 vol% of the rock. The pyrrhotite-rich Ni-sulfide assemblages generally have similar magnetic properties to the host ultramafic rocks, but can have much lower susceptibilities where the thermal history of the rocks has favoured development of hexagonal pyrrhotite over monoclinic pyrrhotite. In ultramafic rocks that contain <10 vol% sulfides, density and susceptibility are primarily controlled by serpentinisation, with olivine breaking down to serpentine and magnetite in the presence of water. Serpentinisation dramatically lowered densities and increased susceptibilities, but had limited influence on the intensity of remanent magnetisation. All ultramafic rocks contain multidomain magnetite, and most contain low coercivity grains prone to overprinting by in situ viscous remanent magnetisation or drilling-induced isothermal remanent magnetisation during extraction. Despite the low coercivities, Koenigsberger ratios of 1–20 are observed indicating that viscous remanent magnetisation aligned parallel to the present Earth field must be considered in any magnetic modelling. It is also noted that coarser-grained intrusive varieties of all rock types (e.g. granite, gabbro) show remanent magnetisation intensities 1–2 orders of magnitude greater than their extrusive equivalents (felsic and basaltic volcanics).     

Upper crustal structure of the Tamworth Belt,New South Wales: constraints from new gravity data

New gravity data along five profiles across the western side of the southern New England Fold Belt and the adjoining Gunnedah Basin show the Namoi Gravity High over the Tamworth Belt and the Meandarra Gravity Ridge over the Gunnedah Basin. Forward modelling of gravity anomalies, combined with previous geological mapping and a seismic-reflection transect acquired by Geoscience Australia, has led to iterative testing of models of the crustal structure of the southern New England Fold Belt, which indicates that the gravity anomalies can generally be explained using the densities of the presently exposed rock units. The Namoi Gravity High over the Tamworth Belt results from the high density of the rocks of this belt that reflects the mafic volcanic source of the older sedimentary rocks in the Tamworth Belt, the burial metamorphism of the pre-Permian units and the presence of some mafic volcanic units. Modelling shows that the Woolomin Association, present immediately east of the Peel Fault and constituting the most western part of the Tablelands Complex, also has a relatively high density of 2.72 – 2.75 t/m³, and this unit also contributes to the Namoi Gravity High. The Tamworth Belt can be modelled with a configuration where the Tablelands Complex has been thrust over the Tamworth Belt along the Peel Fault that dips steeply to the east. The Tamworth Belt is thrust westward over the Sydney – Gunnedah Basin for 15 – 30 km on the Mooki Fault, which has a shallow dip (～25°) to the east. The Meandarra Gravity Ridge in the Gunnedah Basin was modelled as a high-density volcanic rock unit with a density contrast of 0.25 t/m³ relative to the underlying rocks of the Lachlan Fold Belt. The modelled volcanic rock unit has a steep western margin, a gently tapering eastern margin and a thickness range of 4.5 – 6 km. These volcanic rocks are assumed to be Lower Permian and to be the western extension of the Permian Werrie Basalts that outcrop on the western edge of the Tamworth Belt and which have been argued to have formed in an extensional basin. Blind granitic plutons are inferred to occur near the Peel Fault along the central and the southern profiles.     

Early Proterozoic regolith at Pasvik, NE Norway: palaeoenvironmental implications for the Baltic Shield

Regolith occurs at a number of localities along the northern margin of the early Proterozoic Pechenga Greenstone Belt marking a profound stratigraphic unconformity at the base of the 10 km thick Petsamo Supergroup. The age of regolith formation can be bracketed between 2.453 ± 42 Ga (Sm-Nd) and 2.330 ± 36 Ga (Rb-Sr). Regolith is developed on a varied substrate of Archaean rocks including early Proterozoic layered gabbros (2.45 Ga). The best example of the regolith occurs in the Pasvik valley at Brattli, Norway. Analogues are widespread on the Baltic Shield at the base of early Proterozoic 'greenstone belts' showing a low degree of chemical weathering and carbonatisation. Fluviatile deposits overlying the Brattli regolith preserve unweathered pyrite-magnetite boulders and terrigenous pyrite grains. These features are considered to relate an arid or semi-arid palaeoenvironment, possibly with some deficiency of oxygen in the atmosphere-hydrosphere system. A systematic and well-preserved regolith or palaeosaprolite at the base of the Petsamo Supergroup is proof that the northern boundary of the Pechenga Greenstone Belt is a fundamental first-order unconformity. This places constraints on geotectonic modelling of this early Proterozoic rift-basin.     

The Paleoarchean Northern Mundo Novo Greenstone Belt,São Francisco Craton: Geochemistry,U–Pb–Hf–O in zircon and pyrite δ34S-Δ33S-Δ36S signatures

Greenstone belts contain several clues about the evolutionary history of primitive Earth. Here, we describe the volcano-sedimentary rock association exposed along the eastern margin of the Gavião Block, named the Northern Mundo Novo Greenstone Belt (N-MNGB), and present data collected with different techniques, including U–Pb–Hf–O isotopes of zircon and multiple sulfur isotopes (³²S, ³³S, ³⁴S, and ³⁶S) of pyrite from this supracrustal sequence. A pillowed metabasalt situated in the upper section of the N-MNGB is 3337 ± 25 Ma old and has zircon with ε_Hf(t) =  ?2.47 to ?1.40, Hf model ages between 3.75 Ga and 3.82 Ga, and δ¹⁸O = +3.6‰ to +7.3‰. These isotopic data, together with compiled whole-rock trace element data, suggest that the mafic metavolcanic rocks formed in a subduction-related setting, likely a back-arc basin juxtaposed to a continental arc. In this context, the magma interacted with older Eoarchean crustal components from the Gavião Block. Detrital zircons from the overlying quartzites of the Jacobina Group are sourced from Paleoarchean rocks, in accordance with previous studies, yielding a maximum depositional age of 3353 ± 22 Ma. These detrital zircons have ε_Hf(t) =  ?5.40 to ?0.84, Hf model ages between 3.66 Ga and 4.30 Ga, and δ¹⁸O = +4.8‰ to +6.4‰. The pyrite multiple sulfur isotope investigation of the 3.3 Ga supracrustal rocks from the N-MNGB enabled a further understanding of Paleoarchean sulfur cycling. The samples have diverse isotopic compositions that indicate sulfur sourced from distinct reservoirs. Significantly, they preserve the signal of the anoxic Archean atmosphere, expressed by MIF-S signatures (Δ³³S between ?1.3‰ to +1.4‰) and a Δ³⁶S/Δ³³S slope of ?0.81 that is indistinguishable from the so-called Archean array. A BIF sample has a magmatic origin of sulfur, as indicated by the limited δ³⁴S range (0 to +2‰), Δ³³S ~ 0‰, and Δ³⁶S ~ 0‰. A carbonaceous schist shows positive δ³⁴S (2.1‰–3.5‰) and elevated Δ³³S (1.2‰–1.4‰) values, with corresponding negative Δ³⁶S between ?1.2‰ to ?0.2‰, which resemble the isotopic composition of Archean black shales and suggest a source from the photolytic reduction of elemental sulfur. The pillowed metabasalt displays heterogeneous δ³⁴S, Δ³³S, and Δ³⁶S signatures that reflect assimilation of both magmatic sulfur and photolytic sulfate during hydrothermal seafloor alteration. Lastly, pyrite in a massive sulfide lens is isotopically similar to barite of several Paleoarchean deposits worldwide, which might indicate mass dependent sulfur processing from a global and well-mixed sulfate reservoir at this time.     

Petrogenesis and geochemical characteristics of plagiogranites from Naga Ophiolite Belt,northeast India: Fractional crystallization of MORB-type magma

The Naga Ophiolite Belt is a part of the Naga-Arakan-Yoma flysch trough that occurs along the Indo-Myanmar border. It is represented by peridotites, mafic-ultramafic cumulates, mafic volcanics, mafic dykes, plagiogranites, pelagic sediments and minor felsic to intermediate intrusives. Minor plagiogranites, gabbros and thin serpentinite bands occur juxtaposed near Luthur, with the slate-phyllite-metagreywacke sequence (Phokpur Formation) adjacent to the contact. The development of tonalites, trondhjemites and diorites in the oceanic crust, which is grouped as plagiogranites, offers an opportunity to study the process of formation of silicic melts from mafic crust. Plagiogranites from Naga Ophiolite Belt contains moderate SiO₂ (51.81–56.71 wt.%), low K₂O (0.08–1.65 wt.%) and high Na₂O (4.3–5.03 wt.%). The Naga Ophiolite Belt plagiogranites like ocean-ridge granites contain low K₂O, high Na₂O and CaO. The rocks investigated from Naga Ophiolite Belt contain TiO₂ concentrations above the lower limit for fractionated Mid Oceanic Ridge Basalt which is above 1 wt% of TiO₂ and the ternary plots of A (Na₂O + K₂O) F(FeO^T) M(MgO) and TiO₂-K₂O-SiO₂/50 indicate that the plagiogranite are tholeiitic in character and gabbro samples are calc-alkaline in nature. The plagiogranites are enriched in Rb, Ba, Th, U, Nb and Sm against chondrite with negative anomalies on Sr and Zr whereas Y and Yb are depleted to Mid Oceanic Ridge Basalt. The chondrite normalized REE patterns of the plagiogranite display enrichments in LREE (La_N/Sm_N: 2.37–3.62) and flat HREE (Eu/Eu*: 0.90–1.06). The Mid Oceanic Ridge Basalt normalization of gabbro is characterized by strong enrichment of LILE like Ba and Th. The REE pattern is about 50–100 times chondrite with slight enrichment of LREE (La_N/Sm_N = 2.21–3.13) and flat HREE (Eu/Eu*: 0.94–1.19). The major-element and trace element data of the NOB plagiogranites and their intrusive nature with host gabbroic rock suggest that the plagiogranites were produced by fractional crystallization of basaltic parental magmas at Mid Oceanic Ridge.     

Atypical stratiform sulfide-poor platinum-group element mineralisation in the Agnew – Wiluna Belt komatiites,Wiluna, Western Australia

A new style of komatiite-associated sulfide-poor platinum-group element (PGE: Os, Ir, Ru, Rh, Pt, Pd) mineralisation has been identified at Wiluna in the strongly nickel sulfide (NiS) mineralised Agnew – Wiluna Greenstone Belt, Western Australia. The komatiite sequence at Wiluna is ～200 m thick and comprises a basal pyroxenite layer, a thick ortho-to-mesocumulate-textured peridotite core, which is overlain by rhythmically layered wehrlite, oikocrystic pyroxenite and thick upper gabbroic margins. Pegmatoid and dendritic (harrisitic) domains are common features, whereas spinifex-textured horizons and flow-top breccias are absent. The presence of anomalous PGE-enriched horizons (ΣPt – Pd = 200 – 500 ppb) in the oikocrystic pyroxenite and in the layered melagabbro and gabbronorite horizons directly overlying the wehrlite unit is due to the presence of fine-grained (1 – 10 μm) platinum-group minerals (PGMs). More than 70 PGM grains were identified, and a considerable mineralogical variability was constrained. However, only Pd – Pt-bearing phases were identified, whereas no Ir – Ru-bearing PGMs were found in any of the sections examined. Interestingly, all PGMs are not in paragenetic association with sulfides, and only sulfide-poor/free intervals contain significant PGM concentrations. The whole-rock PGE sequence largely reflects the PGM distribution. It is hypothesised that the Pd – Pt enrichment in the oikocrystic pyroxenite and melagabbros and the overall Ir – Ru depletion in the upper mafic section of the sequence are the result of extensive olivine and chromite crystallisation in the basal ultramafic section. PGE saturation was driven by extensive crystallisation of silicate and oxide phases in a sulfide-undersaturated environment. The crystallisation of clinopyroxene in the oikocrystic pyroxenite horizon may have triggered the formation of Pt – Pd-bearing alloys and arsenides, which were the first PGMs to form. Stratiform sulfide-poor PGE mineralisation at Wiluna is more similar in stratigraphic setting, style and composition to PGE-rich sulfide-poor mineralisation zones in thick differentiated intrusions, rather than to other PGE-enriched zones in komatiite-hosted systems, where PGE enrichment is directly associated with accumulations of magmatic sulfides.     

Sedimentogenesis in the Amundsen Basin from geophysical data and drilling results on the Lomonosov Ridge

Studies in the Amundsen Basin have revealed six seismostratigraphic complexes (SSCs) in this region. The horizons bounding these complexes were dated by identifying the linear magnetic anomalies. The recognized SSCs are correlated with the seismostratigraphic and lithostratigraphic units of the Lomonosov Ridge. Based on these correlations, the lithological composition of SSCs in the Amundsen Basin is suggested. The formation of SSC2 is supposed to be due to the diagenetic processes associated with the transition of opal-A to opal-CT. It is found that, generally, the rate of sedimentation in the Amundsen Basin has consistently decreased since the beginning of its formation. However, in the Chattian time, the global regression resulted in a sharp increase in the rate of sedimentation in the basin. Arguments in favor of the duration of the Middle Cenozoic sedimentary hiatus on the Lomonosov Ridge reduced to 16.3 Ma are presented. It is supposed that the decrease in the intensity of oceanic crustal accretion in the Eurasian Basin, which is identified by the slowdown in the rate of its opening in the interval from 46 to 20–23 Ma might have resulted in a gradual sea level falling in the Arctic Ocean isolated from the World Ocean. This fact probably accounts for the Lomonosov Ridge having remained in subaerial conditions over the period from 36.7 to 20.4 Ma.     

Mineralogical and sulfur isotopic characteristics of Archean greenstone belt-hosted gold mineralization at the Tau deposit of the Mupane gold mine,Botswana

The Mupane gold deposit, which is one of the numerous gold occurrences in the Tati Greenstone Belt in the northeastern part of Botswana, consists of four orebodies, namely Tau, Tawana, Kwena, and Tholo deposits. The present research, which focuses on the genesis of the Tau deposit, was based on ore petrography, mineral chemistry of sulfides, and sulfur isotope data. Mineralogical characteristics of the host rocks indicate that banded iron formation at the Tau deposit includes iron oxides (magnetite), carbonates (siderite and ankerite), silicates (chlorite and amphibole), and sulfides (arsenopyrite and pyrrhotite). The deposit features arsenopyrite-rich zones associated with biotite-chlorite veins, which are indicative of the precipitation of arsenopyrite concomitant with potassic alteration. The replacement of magnetite by pyrrhotite in some samples suggests that sulfidation was likely the dominant gold precipitation mechanism because it is considered to have destabilized gold-thiocomplexes in the ore-forming fluids. Based on textural relationships and chemical composition, arsenopyrite is interpreted to reflect two generations. Arsenopyrite 1 is possibly early in origin, sieve textured with abundant inclusions of pyrrhotite. Arsenopyrite 1 was then overgrown by late arsenopyrite 2 with no porous textures and rare inclusions of pyrrhotite. Gold mineralization was initiated by focused fluid flow and sulfidation of the oxide facies banded iron formation, leading to an epigenetic gold mineralization. The mineralogical assemblages, textures, and mineral chemistry data at the Tau gold deposit revealed two-stage gold mineralizations commencing with the deposition of invisible gold in arsenopyrite 1 followed by the later formation of native gold during hydrothermal alteration and post-depositional recrystallization of arsenopyrite 1. Laser ablation inductively coupled plasma mass spectrometric analysis of arsenopyrite from the Tau deposit revealed that the hydrothermal event responsible for the formation of late native gold also affected the distribution of other trace elements within the grains as evidenced by varying trace elements contents in arsenopyrite 1 and arsenopyrite 2. The range of δ³⁴S of gold-bearing assemblages from the Tau deposit is restricted from +1.6 to +3.9‰, which is typical of Archean orogenic gold deposits and indicates that overall reduced hydrothermal conditions prevailed during the gold mineralization process at the Tau deposit. The results from this study suggest that gold mineralization involved multi-processes such as sulfidation, metamorphism, deformation, hydrothermal alteration, and gold remobilization.     

Structural controls of fluid flow and gold mineralization in the easternmost parts of the Karagwe-Ankole Belt of north-western Tanzania

Gold mineralization in the Biharamulo region of western Tanzania is confined to the sheared, low-angle basement-cover contact between Archaean basement gneisses of the Tanzania Craton and the structurally overlying, low-grade metamorphic metasediments of the Mesoproterozoic Karagwe-Ankole Belt. Regional-scale fluid flow along this detachment is indicated by the pervasive silicification and retrogression of wall rocks to pervasively foliated phyllonites and pyritization of particularly metasediments, commonly graphite-rich, in the hanging wall of the shear zone. Gold mining centres on specific structural sites along the detachment, but also in stratigraphically higher sections in the structurally overlying metasediments. Zones of gold mineralization along the detachment correlate with NE trending ramp structures (dip angles 20°–35°) that are most ideally orientated for slip and reactivation within the low-angle phyllonitic detachment. Repeatedly overprinted auriferous quartz-vein stockworks in quartzofeldspathic gneisses immediately below the detachment indicate brittle fracturing of the competent footwall lithotypes during slip along the weaker detachment. In cases of massive silicification, up to 50 m thick quartz blows are formed along the contacts between detachment phyllonites and footwall gneisses. The multiple overprinting relationships of successive quartz-vein generations in these zones of massive silicification suggests that the quartz blows acted as competent blocks in the weak detachment, causing the repeated overprint of earlier silicification by later fracturing and quartz-veining events. Gold mineralization above the detachment and in stratigraphically higher metasediments is closely associated with fold structures that form part of the low-grade metamorphic fold-and-thrust belt. Veining is particularly abundant in competent lithotypes, such as quartzite and chemically reactive ferruginous mafic sills. Overprinting relationships between quartz vein sets illustrate fluid flow during fold amplification and, importantly, the final lock-up stage of folds, during which much of the mineralization was introduced. Oxygen isotope values for quartz veins indicate fluids were likely derived from clastic, mainly metapelitic sedimentary sequences of the Karagwe-Ankole Fold Belt. The data also implies that the partially reworked Archaean granitoid-greenstone basement of the Tanzania Craton has not contributed to the fluid evolution and possibly gold mineralization. The extent (> 100 km) of the basement-cover detachment and associated alteration is indicative for a regional-scale fluid system. Gold mineralization is, however, controlled by local structures and lithological contrasts that require the detailed mapping and sampling of the regional structure.     

Deformation and possible origins of the Cooma Complex,southeastern Lachlan Fold Belt,New South Wales

The western half of the Cooma Complex, New South Wales, consists of three thrust‐bound blocks that contain the same structural fabrics, but with different orientations and intensities, owing largely to heterogeneous strain late in the deformation history. Correlation of these fabrics with those found regionally outside the complex shows that a well‐developed, gently dipping crenulation cleavage (S₄) apparently has no regional counterpart. This cleavage may have formed by vertical shortening that was restricted to the complex and its development may have been assisted by the higher temperatures there. The Cooma Complex is one of five metamorphic complexes in what is known as the Eastern Metamorphic Belt, which stretches several hundred kilometres through the southeastern Lachlan Fold Belt. The complexes may have formed as local hot spots, possibly related to underplating of mafic magma or intrusion of hot tonalites at or near the base of the Ordovician metasediments (or both). Whether or not these complexes are exhumed portions of an extensive layer in the mid‐crust of the fold belt can be tested by evaluating Late Ordovician/Early Silurian thermal gradients in the ubiquitous Ordovician metasediments.     

A review of Palaeoarchaean felsic volcanism in the eastern Kaapvaal craton:Linking plutonic and volcanic records

In the Kaapvaal craton of southern Africa, as well as other Archaean cratons worldwide, the progression from dominant tonalite-trondhjemite-granodiorite(TTG) to granite-monzogranite-syenogranite(GMS)rock types is interpreted to reflect progressive reworking and differentiation of the continental crust.Here we re-evaluate the early Archaean evolution of the Kaapvaal craton and propose a unified view of the plutonic and volcanic records based on elemental and isotopic(Nd, Hf) data and zircon U-Pb ages.We also report new whole-rock major and trace element analyses, zircon U-Pb ages and Hf-in-zircon analyses of igneous clasts from a conglomerate of the 3.2 Ga Moodies Group of the Barberton Greenstone Belt. Many of these clasts are derived from shallow intrusive rocks of granitic composition, which are scarcely represented in outcrop. Despite alteration, the volcanic rocks can be classified based on their trace element contents into two main groups by comparison with plutonic rocks. One group has characteristics resembling TTGs: relatively low and fractionated rare earth element concentrations with no Eu anomaly and relatively low concentrations of high field strength elements(Nb mostly ≤12 ppm). The second group has GMS-like characteristics: less fractionated REE, marked negative Eu anomalies and HFSE-increasing trends with progressing fractionation(Nb ≤ 50 ppm or more, Th up to 30-40 ppm). In addition, igneous clasts of Moodies Group conglomerate have chemical, mineralogical and isotopic characteristics that link them to GMS. New analyses of some of these clasts indicate elevated high field strength elements(Nb up to 20 ppm) and_(εHf)(t)of zircon down to -3.5. These rocks imply the presence of an already differentiated felsic crust at 3.5 Ga, which has Nd and Hf model ages indicating mantle extraction ages extending back to the Eoarchaean. The combined record of plutonic and volcanic rocks of the Kaapvaal craton provides a more complex scenario than previously suggested and indicates that TTG and GMS-like felsic magmas were emplaced broadly coevally in multiple pulses between ~3.5 Ga and 3.2 Ga.