Oolitic iron formations in Northern Australia

Summary Recent exploration in northern Australia has discovered three shallow water oolitic iron formations, one of Permian age, and two of Upper Proterozoic age. The Permian formation resembles the oxidized oolitic chamositic formations of south-eastern England, but contains more detrital quartz and felspar.The two Precambrian iron formations, consist, below the zone of weathering, of oolites of hematite and of a chamosite-like substance, in a siderite cement, together with well-rounded quartz grains, and are interlayered with sandstones with a chamosite-like cement. Within the zone of weathering the siderite and chamosite are more or less completely replaced by quartz, but with preservation in detail of original textures.Published by permission of the Commonwealth Scientific and Industrial Research Organization (CSIRO).     

Aspects of Proterozoic stromatolite biostratigraphy in Western Australia

Stromatolites are abundant at many horizons in the Proterozoic of Western Australia. Recent advances in knowledge of Proterozoic stratigraphy of the state have provided a more detailed framework for interpreting the stromatolite data than has been available previously. In the 1.7 Ga Earaheedy Group of the Nabberu Basin a characteristic stromatolite assemblage occurs, and within the basin a biostratigraphic succession can be recognized. The assemblage contains several new forms which belong to new groups. The need to erect new groups for these early Proterozoic stromatolites is in agreement with recent studies in Canada, northern Europe and South Africa, and suggests that the problem of ‘younger’ or late Proterozoic stromatolite groups in early Proterozoic rocks mentioned by previous workers is a result of a lack of rigour in defining taxa. Examination of type material is necessary to determine how closely the Earaheedy forms resemble those described from these other regions.In Western Australia some stromatolite forms have a restricted vertical range and similar taxa occur in beds of approximately the same age in widely-separated areas: e.g. Kimberley Group and Earaheedy Group; Scorpion Group and Limbunya, Birrindudu, McArthur, Mt. Rigg and Mt. Albert Groups and Bungle Bungle Dolomite; Tolmer and Bullita Groups; Moora and Bangemall Groups; Kai Ki Beds, Louisa Downs, Mount House and Albert Edward Groups.Stromatolite diversity shows a decline in the number of taxa at about 1.1. Ga in the Bangemall Group. More data are required to determine whether this decline is universal or specific to the Bangemall Group. This study indicates that a stromatolite biostratigraphy for Western Australia is feasible and is consistent with data from other parts of Australia. Thus emphasis on correlation should be placed on the stromatolite form rather than the group, and intercontinental correlations should be attempted only when local biostratigraphic schemes have been firmly established.     

Banded iron formations in Proterozoic greenstone belts: Call for further studies

Banded iron formations occur in greenstone belts in which volcanic rocks are predominant. Greenstone belts are not restricted to the Archaean (>2500 Ma), as is commonly perceived, but they continued to form, albeit in lesser abundance, in the Proterozoic. Thus, banded iron formations which are closely associated with volcanic sequences occur in several well-documented early-mid Proterozoic greenstone belts. Examples are the Yavapai belts at Jerome in Arizona, the Trans-Amazonian belts in Guiana, and the Dalma belts of the Singhbhum region of NE India. Stratigraphic and sedimentological studies are needed to establish the similarities and differences of these iron formations with those in Archaean greenstone belts, and with the banded iron formations which were common in cratonic-shelf environments in the early-mid Proterozoic.     

Geochemistry, mineralogy and petrology of a new find of ultramafic lamprophyres from Bulljah Pool, Nabberu Basin, Yilgarn Craton, Western Australia

Four variously pipe or sill-like, Carboniferous ( ≈ 305 Ma) bodies have been located near the NE edge of the Archaean Yilgarn craton. The rocks comprise Ba---Ti-bearing tetraferriphlogopite-tetraferriannite, low Al---Ti-diopside, calcite, perovskite and groundmass titanomagnetite-chromite (up to 41.3% Cr₂O₃), with minor apatite, Mg---Mn ilmenite, rare-earth phosphate, K---Ba-feldspar (up to 17% BaO), baryte and an unidentified Ba---Zr silicate. The last three reflect very high whole-rock Ba (up to 5,652 ppm). Aegirine-rich pyroxenes occur in fenitic alteration assemblages. Together with high Si/Al and low Mg/Ca whole-rock geochemistry, these features are diagnostic of ultramafic lamprophyres (damkjernites and aillikites), although the rocks also show some affinities with classical kimberlites. Mineral concentrates from loam samples yield an array of minerals of mantle origin, including garnets (Dawson and Stephens' groups G1, 3, 5, 9 and 10), chromian diopsides (up to 6.2% Cr₂O₃), magnesiochromites (up to 20% MgO, 70% Cr₂O₃) and four compositional groups of ilmenites (low-Mn picroilmenites, Mn-rich, Mg-poor and two moderate Mn---Mg compositions). Actual spinel-lherzolite nodules are common in one body and the presence of spinel-and/or garnet-lherzolites can be inferred in the others from the concentrates. The Bulljah bodies are therefore of deep mantle origin, as confirmed by the recovery of a single microdiamond. They thus extend the field of potentially (if not necessarily economically) diamondiferous rocks beyond kimberlites and lamproites. When added to other recent lamprophyre finds, the Bulljah discoveries suggest that the Yilgarn craton could, like many other ancient cratons, be ringed and/or dotted by a diverse array of alkaline and lamprophyric rocks of varying ages which remain to be discovered.     

Proterozoic evolution and tectonic setting of the northwest Paterson Orogen,Western Australia

The Proterozoic igneous, deformation and metamorphic histories of the Palaeoproterozoic Rudall Complex in the northwestern Paterson Orogen can be linked to those of the Arunta Inlier in central Australia, and in part with the Capricorn Orogen in central Western Australia. The similarities in deformation and metamorphic histories for these widely separated regions indicate a Palaeoproterozoic continent–continent collisional event between the Palaeoproterozoic West Australian and North Australian cratons between c. 1830 and 1765 Ma. In the Paterson Orogen this Palaeoproterozoic collisional event resulted in the Yapungku Orogeny, which included thrust stacking of clastic sedimentary and volcanic rocks, deposition of the protoliths for the c. 1790 Ma siliciclastic paragneiss succession contemporaneous with granitic intrusion, and metamorphism up to granulite facies. During this 65-million-year period, the Arunta Inlier and Capricorn Orogen were deformed, metamorphosed at medium to high grades and intruded by granitoids during the Strangways Orogeny in the Arunta Inlier and the Capricorn Orogeny in the Capricorn Orogen.The Neoproterozoic Tarcunyah, Throssell and Lamil groups are clastic sedimentary sequences that were deposited after 1070 Ma in the northwestern Paterson Orogen, and deformed by the Miles Orogeny before 678 Ma. The Miles Orogeny produced a northwesterly trending fold and fault system of tight to isoclinal upright and overturned folds and thrust faults. The orogeny may have been coincident with the c. 750–720 Ma Areyonga tectonic movement affecting the Arunta Inlier and the lower Neoproterozoic part of the Amadeus Basin in central Australia. At c. 550 Ma the Paterson Orogeny, which is most likely equivalent to the Petermann Orogeny in the Musgrave Complex of central Australia, deformed the northwestern Paterson Orogen and was preceded by local intrusion of granites.The similarities of styles and timing of deformation in the northwestern Paterson Orogen, Arunta Inlier and Capricorn Orogen indicate that these three regions were probably linked during most of the Proterozoic.     

Geological history of the early Proterozoic Paraburdoo Hinge Zone,Western Australia

Palinspastic profiles across the Paraburdoo Hinge Zone, which forms the southwestern boundary to the Hamersley Shelf, indicate the existence of a fore-trough, adjacent to the Ashburton Trough, on the edge of the Hamersley Shelf. The fore-trough existed during Turee Creek Group and Lower Wyloo Group times of deposition and is characterized by unconformities or shallow-water facies on its margins. The distribution of mature iron-ores in the Hamersley Group suggests a relationship of this type of ore-body to the fore-trough margins.Continual uplift, or a gradual migration northeastwards of a geanticline, is suggested for the Ashburton Trough region from post-Hamersley Group times to pre-Ashburton Formation times of deposition.     

The tectonics and mineral systems of Proterozoic Western Australia:Relationships with supercontinents and global secular change

The cratonisation of Western Australia during the Proterozoic overlapped with several key events in the evolution of Earth. These include global oxidation events and glaciations, as well as the assembly,accretionary growth, and breakup of the supercontinents Columbia and Rodinia, culminating in the assembly of Gondwana. Globally, Proterozoic mineral systems evolved in response to the coupled evolution of the atmosphere, hydrosphere, biosphere and lithosphere. Consequently, mineral deposits form preferentially in certain times, but they also require a favourable tectonic setting. For Western Australia a distinct plate-margin mineralisation trend is associated with Columbia, whereas an intraplate mineralisation trend is associated with Rodinia and Gondwana, each with associated deposit types. We compare the current Proterozoic record of ore deposits in Western Australia to the estimated likelihood of oredeposit formation. Overall likelihood is estimated with a simple matrix-based approach that considers two components: The "global secular likelihood" and the "tectonic setting likelihood". This comparative study shows that at least for the studied ore-deposit types, deposits within Western Australia developed at times, and in tectonic settings compatible with global databases. Nevertheless, several deposit types are either absent or poorly-represented relative to the overall likelihood models. Insufficient exploration may partly explain this, but a genuine lack of deposits is also suggested for some deposit types. This may relate either to systemic inadequacies that inhibited ore-deposit formation, or to poor preservation. The systematic understanding on the record of Western Australia helps to understand mineralisation processes within Western Australia and its past connections in Columbia, Rodinia and Gondwana and aids to identify regions of high exploration potential.     

The Undoolya sequence: Late Proterozoic salt influenced deposition,Amadeus Basin,central Australia

Parts of the Late Proterozoic to Cambrian sequence along the northeastern margin of the Amadeus Basin were deposited under the influence of salt movement within the underlying Bitter Springs Formation. Later folding during the Devonian Alice Springs Orogeny and subsequent erosion has exposed salt‐influenced structures to provide a rare opportunity to observe the effects of diapiric growth on local facies and structure. Such effects are commonly only seen in seismic section. Salt withdrawal led to normal faulting and syn‐sedimentary thickening of adjacent units. The Undoolya Sequence, a previously undescribed 710 m section, was deposited within a salt‐withdrawal basin adjacent to a proposed diapiric structure. Periods of salt mobilization are recorded by syn‐depositional thickening and localized unconformities within units flanking the diapiric structure. This structure is representative of the influence salt movement had on deposition in the northeastern Amadeus Basin during the Late Proterozoic.     

Chronology and evolution of the Middle Proterozoic Albany‐Fraser Orogen,Western Australia

Geochemical and Sm‐Nd isotopic data, and 19 ion‐microprobe U‐Pb zircon dates are reported for gneiss and granite from the eastern part of the Albany‐Fraser Orogen. The orogen is dominated by granitic rocks derived from sources containing both Late Archaean and mantle‐derived components. Four major plutonic episodes have been identified at ca 2630 Ma, 1700–1600 Ma, ca 1300 Ma and ca 1160 Ma. Orthogneiss, largely derived from ca 2630 Ma and 1700–1600 Ma granitic precursors, forms a belt along the southeastern margin of the Yilgarn Craton. These rocks, together with gabbro of the Fraser Complex, were intruded by granitic magmas and metamorphosed in the granulite facies at ca 1300 Ma. They were then rapidly uplifted and transported westward along low‐angle thrust faults over the southeastern margin of the Yilgarn Craton. Between ca 1190 and 1130 Ma, granitic magmas were intruded throughout the eastern part of the orogen. These new data are integrated into a review of the geological evolution of the Albany‐Fraser Orogen and adjacent margin of eastern Antarctica, and possibly related rocks in the Musgrave Complex and Gawler Craton.     

Early Cambrian and latest Proterozoic stratigraphy,Desert Syncline,southern Georgina Basin

In the Desert Syncline of the southern Georgina Basin there is an Early and Middle Cambrian sequence unconformably overlying late Proterozoic sediments. Stratigraphic drilling and subsequent palaeontological studies have allowed the documentation of the sequence across the Proterozoic‐Cambrian unconformity. Earliest Cambrian green shales are bioturbated and contain distinctive acritarchs. These are overlain, probably unconformably, by sandstone with Diplocraterion burrows, in turn succeeded by archaeocyathan dolostone. Ordian and Templetonian (Middle Cambrian) shales and carbonates unconformably overlie the Early Cambrian sequence. The stratigraphic sequence is very similar to that in the Amadeus Basin and the Adelaide Geosyncline.     

Early Mesoproterozoic metamorphism in the Barossa Complex,South Australia: links with the eastern margin of Proterozoic Australia

LA-ICP-MS U–Pb geochronological data from metamorphic monazite in granulite-facies metapelites in the Barossa Complex, southern Australia, yield ages in the range 1580–1550 Ma. Metapelitic rocks from the Myponga and Houghton Inliers contain early biotite–sillimanite-bearing assemblages that underwent partial melting to produce peak metamorphic garnet–sillimanite-bearing anatectic assemblages. Phase equilibrium modelling suggests a clockwise P–T evolution with peak temperatures between 800 and 870°C and peak pressures of 8–9 kbar, followed by decompression to pressures of ～6 kbar. In combination with existing age data, the monazite U–Pb ages indicate that the early Mesoproterozoic evolution of the Barossa Complex is contemporaneous with other high geothermal gradient metamorphic terranes in eastern Proterozoic Australia. The areal extent of early Mesoproterozoic metamorphism in eastern Australia suggests that any proposed continental reconstructions involving eastern Proterozoic Australia should share a similar tectonothermal history.     

A barred-basin marine evaporite in the Upper Proterozoic of the Amadeus Basin, central Australia

The Ringwood evaporite is part of the 900 m.y. old Bitter Springs Formation, a warm-water shallow-marine sequence of stromatolitic dolomite and limestone, microfossiliferous chert, red beds, quartzite, and evaporites. The evaporite at Ringwood comprises two parts: (i) a lower 127 m characterized by brecciated pyritic bituminous dolomite, together with smaller amounts of dolomite-gypsum breccia, friable chloritic dololutite, coarsely crystalline anhydrite, and satin-spar gypsum; and (ii) an upper 133 m which is similar except that bituminous dolomite forms only one bed, and the characteristic rock-type is dolomite-gypsum breccia. The evaporite is overlain by limestone breccia and massive stromatolitic limestone, interpreted as an algal reef. Gypsum is secondary after anhydrite, and the ratio of gypsum to anhydrite increases upwards. The evaporite shows none of the features of a sabkha or desiccated deep ocean basin deposit, and instead is interpreted as the filling of a barred basin which was cut off from the ocean by growth of an algal barrier reef. As circulation became restricted, bituminous dolomite deposited in the lagoon behind the reef, together with pyrite from the destruction by anaerobic bacteria of algal debris derived from the reef. With continued evaporation, brine concentration increased and gypsum precipitated. Occasional dust storms contributed wind-blown clay to the deposit. The barrier reef transgressed diachronously across the evaporite lagoon, and was eventually drowned when normal marine conditions became established. Burial of the evaporite to about 7000 m beneath the succeeding sediments of the Amadeus Basin converted gypsum to anhydrite, and formed chlorite by reaction of clay with dolomite. Late Palaeozoic tectonism folded and brecciated the rocks, and was followed by erosion which eventually exposed the evaporite to ingress of meteoric water. Hydration of anhydrite to gypsum ensued, the reaction becoming less complete with increasing depth from the ground surface.     

Burial Metamorphism in the Hamersley Basin, Western Australia

The low-grade metamorphic minerals prehnite, pumpellyite, epidoteand actinolite in rocks of basic and intermediate compositionhave a broad, systematic distribution in the Hamersley Basin.Assemblages of these minerals are wisespread in the FortescueGroup, the lowermost group in the Hamersley Basin. Because ofsunsuitability of rock type no relevant mineral assemblageswere observed in samples from the Hamersley Group. However,metamorphism of this group can be implied from mineral assemblagesin the younger Turee Creek Group, and because the HamersleyGroup conformably overlies the metamorphosed Fortescue Group. Unfolded stratigraphic cross sections show that depth of burialwas the dominant control of increase in metamorphic grade. Fourmetamorphic zones are defined over a relative depth of burialof 9 km. From lowest grade to highest these are: Zone I (ZI)prehnite–pumpellyite zone; ZII, prehnite–pumpellyite–epidotezone; ZIII, prehnite–pumpellyite–epidote–actinolitezone; and ZIV, (prehnite–epidote–actinolite zone.Laumontite, definitive of the zeolite fades is absent but thatpart of the sequence may coincide with rocks of unsuitable composition,or may have been removed by erosion. A large area of prehnite–pumpellyitefades (ZI and ZII) dominates the north side of the basin, whilegreenschist fades (ZIV) dominates the south. Separating thetwo is a curved central strip of pumpellyite-actinolite facies(ZIII). Microprobe data of pumpellyites from the three pumpellyite–bearingzones, ZI, II and III, show two systematic trends: extensivevariation in Al/Fe ratios at any one grade, and a general decreaseof Mg with increasing metamorphism. Consideration of the compositionsof the most abundant pumpellyites in the metabasic rocks showsthat these two trends spread about a more fundamental lineartrend towards AJ-enrichment with increasing metamorphism astotal Fe and Mg decrease. Epidote shows a wide range in Fe content in ZII and ZIII (Ps15to Ps40) crossing the miscibility gap proposed by Raith (1976).In ZIV epidote compositions are more aluminous and restrictedin composition (Ps11 to Ps20). Magnesium has entered the epidotelattice in ZII and ZIII (up to 0–17 ions Mg where £cations = 8) but to only half this in ZIV. Synthesis of the burial model with published experimental workputs constraints on the ancient thermal gradient that existedduring burial metamorphism. For the peak of metamorphic adjustmentfluid pressure appears to have been equal to load pressure.A relatively high gradient of 80 to 100 deg;C/km seems likelyfor the shallow part of the sequence, with a gradient of 40deg;C/km for the deeper part of the sequence, the change beingat about 2–5 km. The prehnite-pumpellyite facies correspondsto a fluid pressure of 0–5 to 1 kilobar and a temperaturerange of about 100 to 300 deg;C. The prehnite-bearing pumpellyite-actinolitefacies is interpreted to have developed at about 1–5 kbover a temperature range of 300 to 360 deg;C. This facies isprobably a low pressure subfacies of the pumpellyite-actinolitefades of Hashimoto (1966).     

Early Proterozoic Melt Generation Processes beneath the Intra-cratonic Cuddapah Basin, Southern India

Early Proterozoic tholeiitic lavas and sills were emplaced duringthe initial phase of extension of the intra-cratonic CuddapahBasin, southern India. ⁴⁰Ar–³⁹Ar laser-fusion determinationson phlogopite mica, from the Tadpatri Fm mafic–ultramaficsill complex, constrain the age of the initial phase of extensionand volcanism in the basin at 1·9 Ga. Despite their EarlyProterozoic age, the igneous rocks are unmetamorphosed, undeformedand remarkably fresh. They exhibit a wide range in MgO contents(4–28 wt %) and have undergone varying degrees of accumulationor crystal fractionation. Variable La/Nb ratios (1·2–3·7)and     

Young ores in old rocks: Proterozoic iron mineralisation in Mesoarchean banded iron formation,northern Pilbara Craton,Australia

The origin of bedded iron-ore deposits developed in greenstone belt-hosted (Algoma-type) banded iron formations of the Archean Pilbara Craton has largely been overlooked during the last three decades. Two of the key problems in studying these deposits are a lack of information about the structural and stratigraphic setting of the ore bodies and an absence of geochronological data from the ores. In this paper, we present geological maps for nearly a dozen former mines in the Shay Gap and Goldsworthy belts on the northeastern margin of the craton, and the first U-Pb geochronology for xenotime intergrown with hematite ore. Iron-ore mineralisation in the studied deposits is controlled by a combination of steeply dipping NE- and SE-trending faults and associated dolerite dykes. Simultaneous dextral oblique-slip movement on SE-trending faults and sinistral normal oblique-slip movement on NE-trending faults during initial ore formation are probably related to E–W extension. Uranium–lead dating of xenotime from the ores using the sensitive high-resolution ion microprobe (SHRIMP) suggests that iron mineralisation was the cumulative result of several Proterozoic hydrothermal events: the first at c. 2250 Ma, followed by others at c. 2180 Ma, c. 1670 Ma and c. 1000 Ma. The cause of the first growth event is not clear but the other age peaks coincide with well-documented episodes of orogenic activity at 2210–2145 Ma, 1680–1620 Ma and 1030–950 Ma along the southern margin of the Pilbara Craton and the Capricorn Orogen farther south. These results suggest that high-grade hematite deposits are a product of protracted episodic reactivation of a structural architecture that developed during the Mesoarchean. The development of hematite mineralisation along major structures in Mesoarchean BIFs after 2250 Ma implies that fluid infiltration and oxidative alteration commenced within 100 myr of the start of the Great Oxidation Event at c. 2350 Ma.     

The Mesoproterozoic Abra polymetallic sedimentary rock-hosted mineral deposit,Edmund Basin,Western Australia

Abra is a blind, sedimentary rock-hosted polymetallic Fe–Pb–Zn–Ba–Cu ± Au ± Ag ± Bi ± W deposit, discovered in 1981, located within the easterly trending Jillawarra rift sub-basin of the Mesoproterozoic Edmund Basin, Capricorn Orogen, Western Australia. The Edmund Basin contains a 4–10 km thick succession of siltstone, sandstone, dolomitic siltstone, and stromatolitic dolomite. The age of the Edmund Group is between 1.66 and 1.46 Ga. The Abra polymetallic deposit is hosted in siltstone, dolostone, sandstone and conglomerate of the Irregully and Kiangi Creek Formations, but the mineralised zones do not extend above an erosion surface marking the change from fluvial to marine facies in the lower part of the Kiangi Creek Formation. The Abra deposit is characterised by a funnel-shaped brecciated zone, interpreted as a feeder pipe, overlain by stratiform–stratabound mineralisation. The stratiform–stratabound mineralisation includes a Red Zone and an underlying Black Zone. The Red Zone is characterised by banded jaspilite, hematite, galena, pyrite, quartz, barite, and siderite. The jaspilite and hematite cause the predominant red colouration. The Black Zone consists of veins and rhythmically banded sulphides, laminated and/or brecciated hematite, magnetite, Fe-rich carbonate and scheelite. In both zones, laminations and bands of sulphide minerals, Fe oxides, barite and quartz commonly exhibit colloform textures. The feeder pipe (Stringer Zone) merges with Black Zone and consists of a stockwork of Fe-carbonate-quartz, barite, pyrite, magnetite and chalcopyrite, exhibiting fluidised and/or jigsaw textures.The Abra mineral system is characterised by several overprinting phases of hydrothermal activity, from several stages of brecciation and fluidisation, barite and sulphide veining to barren low-temperature chalcedonic (epithermal regime) veining. Hydrothermal alteration minerals include multi-stage quartz, chlorite, prehnite, Fe-rich carbonate and albite. Albite (Na metasomatism) is an early alteration phase, whereas Fe-rich carbonate is a late phase. Fluid inclusion studies indicate that the ore fluids had temperatures ranging from 162 to 250 °C, with salinities ranging from 5.8 to about 20 wt.% NaCl. In the course of our studies, microthermometric and Raman microprobe analyses were performed on fluid inclusions in carbonate, quartz and barite grains. Fluid inclusions in quartz show homogenisation temperatures ranging from 150 to 170 °C with calculated salinities of between 3.7 and 13.8 wt.% NaCl.The sulphur isotopic system shows δ³⁴S values ranging from 19.4 to 26.6‰ for sulphides and from 37.4 to 41.9‰ for barite (Vogt and Stumpfl, 1987, Austen, 2007). Sulphur isotope thermometry between sulphides and sulphide–barite pairs yields values ranging from 219 to 336 °C (Austen, 2007).Galena samples were analysed for Pb isotope ratios, which have been compared with previous Pb isotopic data. The new Pb isotope systematics show model ages of 1650–1628 Ma, consistent with the formation of the host Edmund Basin.Re–Os dating of euhedral pyrite from the Black Zone yielded an age of ~ 1255 Ma. This age corresponds to the 1320–1170 Ma Mutherbukin tectonic event in the Gascoyne Complex. This event is manifested primarily along a WNW-trending structural corridor of amphibolite facies rocks, about 250 km to the northwest of the Abra area. It is possible that the Re–Os age represents a younger re-activation event of an earlier SEDEX style system with a possible age range of 1640–1590 Ma.A genetic model for Abra is proposed based on the above data. The model involves two end-members ore-forming stages: the first is the formation of the SEDEX style mineral systems, followed by a second multi-phase stage during which there was repeated re-working of the mineral system, guided by seismic activity along major regional faults.     

A middle Proterozoic palaeokarst unconformity and associated sedimentary rocks, Elu Basin, northwest Canada

A major palaeokarst erosion surface is developed within the middle Proterozoic Elu Basin, northwestern Canada. This palaeokarst is named the sub-Kanuyak unconformity and truncates the Parry Bay Formation, a sequence of shallow-marine dolostones that were deposited within a north-facing carbonate platform under a semi-arid climate. The sub-Kanuyak unconformity exhibits up to 90 m of local relief, and also formed under semi-arid conditions when Parry Bay dolostones were subaerially exposed during a relative sea-level drop of about 180 m. Caves and various karren developed within the meteoric vadose and phreatic zones. Their geometry, size and orientation were largely controlled by northwest- and northeast-trending antecedent joints, bedding, and lithology. Near-surface caves later collapsed forming valleys, and intervening towers or walls, and plains. Minor terra rossa formed on top of highs. Karstification was most pronounced in southern parts of Bathurst Inlet but decreased northward, probably reflecting varying lengths of exposure time along a north-dipping slope. The Kanuyak Formation is up to 65 m thick, and partially covers the underlying palaeokarst. It consists of six lithofacies: (i) breccia formed during collapse of caves, as reworked collapse breccia and regolith; (ii) conglomerate representing gravel-dominated braided-fluvial deposits; (iii) sandstone deposited as braided-fluvial and storm-dominated lacustrine deposits; (iv) interbedded sandstone, siltstone and mudstone of sheet flood origin; (v) dolostones formed from dolocretes and quiet-water lacustrine deposits; and (vi) red-beds representing intertidal-marine mudflat deposits. Rivers flowed toward the northwest and northeast within karst valleys and caves; lakes were also situated within valleys; marine mudflat sediments completely cover the palaeokarst to the north. A regional correlation of the sub-Kanuyak unconformity with the intra-Greenhorn Lakes disconformity within the Coppermine homocline suggests that similar styles of karstification occurred over an extensive region. The Elu Basin palaeokarst, however, was developed more landward, and was exposed for a longer period of time than the Coppermine homocline palaeokarst.     

碳同位素与早元古代地层对比

本文首次较详细地研究了早元古代碳酸盐地层的碳同位素组成，样品采自华北早元古代标准地层单元──山西五台地区滹沱群，结果表明碳同位素组成随地质时代有明显的变化，并与地层层位有很好的对应关系。作者认为，碳同位素对于早元古代碳酸盐地层的对比是很有用的。     

Proterozoic biotite Rb–Sr dates in the northwestern part of the Yilgarn Craton,Western Australia

Rb–Sr dating of biotite in the northwestern corner of the Yilgarn Craton identified four areas with distinctive age ranges. Biotite in the northwestern area, which includes the Narryer Terrane and part of the Murchison Terrane, yields reset Rb–Sr dates of ca 1650 Ma. In the western area, along the margin of the craton, biotite has been reset to 629 Ma. Eastward of these areas, mainly in the Murchison Terrane, the modal biotite date is near 2450 Ma, though because of a skewed distribution the mean date is closer to 2300 Ma. Dates in a transition zone between the western and eastern areas range broadly between 2000 and 1000 Ma, averaging about 1775 Ma. The western area and the transition zone are continuous with analogous areas south of the limits of the present study. The 1650 Ma dates in the northwestern area are probably related to plutonic and tectonic activity of similar age in the Gascoyne Province to the north. They may represent cooling after thermal resetting during tectonic loading by southward thrust‐stacking of slices of Narryer Terrane and allochthonous Palaeoproterozoic volcanic arc and backarc rocks during the Capricorn Orogeny. This episode of crustal shortening resulted from the collision of the Yilgarn and Pilbara Cratons to form the West Australian Craton. The dates reflect cooling associated with subsequent erosion‐induced rebound. The 2450 Ma biotite dates of the eastern area are similar to biotite dates found over most of the Yilgarn Craton and represent a background upon which the later dates have been superimposed. The origin of dates in the western area is unknown but may be related to an associated dolerite dyke swarm or to possible thrusting from the west. There is some evidence of minor later intrusion of felsic hypabyssal rock between 2000 and 2200 Ma and localised shearing in the Narryer area at about 1350 to 1400 Ma. One small area near Yalgoo with biotite Rb–Sr dates near 2200 Ma may be cogenetic with the Muggamurra Swarm of dolerite dykes.