Zincian dolomite from broken hill,New South Wales

Many Broken Hill (New South Wales) specimens labelled ‘zinco‐calcite’ in museum and private collections are snow‐white, globular forms with a sparkling appearance, on a coronadite or limonite matrix. X‐ray diffraction and microprobe analyses show the globules have a core of nearly pure calcite, overlain by colourless, drusy zincian dolomite with up to 4 mol % ZnCO₃ and up to 7 mol % excess CaCO₃. Although Zn is abundant in the Broken Hill orebody, it apparently entered carbonate, mainly as smithsonite, only during formation of the oxidised zone.     

Retrograde Schists of the Amphibolite Facies at broken hill,New South Wales

Aluminous, mafic, felsic, calcareous, and sulphide‐rich rocks have been involved in localized deformation and retrograde metamorphism at Broken Hill, western New South Wales, where retrograde schist‐zones intersect high‐grade, regional metamorphic rocks of the lower granulite facies (or the amphibolite‐granulite facies transition). Although technically retrograde, the schists contain mineral assemblages indicative of the lower amphibolite facies. The schist‐zones were formed by local folding, apparently as part of the third stage of deformation in the Broken Hill area.     

The Redan Geophysical Zone: Part of the Willyama Supergroup? Broken Hill,Australia

The Redan Geophysical Zone forms a regional magnetic high in contrast to the regional magnetic low defined by the main part of the Broken Hill Block. The magnetic rocks are interpreted to dip below the remainder of the Broken Hill Block and there has been speculation that they are significantly older than the Early Proterozoic Willyama Supergroup.

Evaluation of lithological mapping and aeromagnetic data permitted interpretation of a stratigraphic sequence within the Redan Geophysical Zone, consisting of three new formations: the Redan Gneiss, Ednas Gneiss and Mulculca Formation, plus the Lady Brassey Formation, part of the Thackaringa Group. The rocks are considered to belong to the lower part of the Willyama Supergroup and are not an older basement.

Although the Redan Geophysical Zone contains some rock types not found elsewhere in the Broken Hill Block, there are some lithological similarities with the lower part of the Willyama Supergroup: an abundance of albite‐rich rocks, the presence of quartz‐magnetite rocks with Cu and trace Co, and abundant amphibolite/ basic granulite in the Lady Brassey Formation.

The boundary between the Redan Geophysical Zone and the remainder of the Broken Hill Block appears to be conformable, with no evidence of major faulting. Similarly no evidence of unconformities or major displacement of stratigraphic boundaries has been found within the Redan Geophysical Zone. Structural history, fold style and orientation, and metamorphic grade within the Redan Geophysical Zone are similar to adjacent areas of the Broken Hill Block.

It is concluded that the Broken Hill Block contains no outcropping equivalent of the first cycle of sedimentary/ igneous rocks recognized in the Early Proterozoic of northern Australia.

Albite‐quartz‐hornblende‐magnetite rocks unique to the Redan Geophysical Zone most likely comprised detritus derived directly from an intermediate volcanic suite. Some were altered considerably, while other rocks retained the dacite/andesite composition, except for the addition of Na, an increase in the oxidation state, and partial leaching of some of the more mobile elements. These modifications could have taken place in shallow alkaline evaporitic lakes.

The Redan Geophysical Zone contains some of the elements of a foreland basin adjacent to a continental volcanic arc: a thick stratigraphic sequence, oxidizing evaporitic conditions, and intermediate volcanic detritus. The change from intermediate‐acid volcanism in the earliest formations, to bimodal acid/basic volcanism in the Thackaringa and Broken Hill Groups could correspond with a change from initial continental arc volcanism into bimodal rift volcanism. The case for the arc volcanism is weakened, however, by the relative scarcity of rocks with andesitic compositions and the lack of basaltic andesite compositions. The alternative is that the intermediate to acid volcanism represents only a variation on the later bimodal rift volcanism.     

The composition of garnet in garnet-rich rocks in the southern Proterozoic Curnamona Province, Australia: an indicator of the premetamorphic physicochemical conditions of formation

Garnet-rich rocks occur throughout the Proterozoic southern Curnamona Province, Australia, where they are, in places, spatially related to Broken Hill-type Pb-Zn-Ag deposits. Fine-scale bedding in these rocks, their conformable relationship with enclosing metasedimentary rocks, and their enrichment in Mn and Fe suggest that they are metamorphosed chemical precipitates. They formed on the floor of a 1.69?Ga continental rift basin from hydrothermal fluids mixed with seawater and detritus. Garnet in garnet-quartz and garnet-amphibole rocks is generally light rare earth element (LREE) depleted, and has flat heavy REE (HREE) enriched chondrite-normalized REE patterns, and negative Eu anomalies (Eu/Eu*?<?1). Garnet in garnet-rich rocks from the giant Broken Hill deposit has similar REE patterns and either positive (Eu/Eu*?>?1) or negative Eu anomalies. Manganese- and Mn-Ca-rich, Fe-poor garnets in garnetite, garnet-hedenbergite, and garnet-cummingtonite rocks at Broken Hill have Eu/Eu*?>?1, whereas garnet in Mn-poor, Fe-rich quartz garnetite and quartz-garnet-gahnite rocks from Broken Hill, and quartz garnetite from other locations have Eu/Eu*?<?1. The REE patterns of garnet and its host rock and interelement correlations among REEs and major element contents in garnet and its host rock indicate that the Eu anomaly in garnet reflects that of its host rock and is related to the major element composition of garnet and its host rock. The value of Eu/Eu* in garnet is related to its Mn, Fe, and Ca content and that of its host rock, and the distribution of REEs among garnet and accessory phases (e.g., feldspar). Positive Eu anomalies reflect high amounts of Eu that was preferentially incorporated into Mn- and Mn-Ca-rich oxides and carbonates in the protolith. In contrast, Eu/Eu*?<?1 indicates the preferential discrimination against Eu by Fe-rich, Mn-poor precursor minerals. Precursors to Mn-rich garnets at Broken Hill formed by precipitation from cooler and more oxidized hydrothermal fluids compared to those that formed precursors to Mn-poor, Fe-rich garnet at Broken Hill and the other locations. Garnet from the Broken Hill deposit is enriched in Zn (> 400?ppm), Cr (> 140?ppm), and Eu (up to 6?ppm and positive Eu anomalies), and depleted in Co, Ti, and Y compared to garnet in garnet-rich rocks from other localities. These values, as well as MnO contents ?>?15 wt. % and Eu/Eu*?>?1 are only found at the Broken Hill deposit and are good indicators of the presence of Broken Hill-type mineralization.     

Stratiform and stratabound tungsten mineralisation in the Broken Hill Block,N.S.W.

The Early‐Middle Proterozoic Broken Hill Block contains three types of W occurrences, which show close stratigraphic control. All three types occur within a relatively narrow stratigraphic interval (the ‘Mine Sequence’ Suite of Stevens et al., 1980) comprising a highly variable group of metamorphosed silicic and mafic volcanics, clastic sediments, and exhalative and chemical sediments containing base metals. The first type includes occurrences of W and base metals in bedded calc‐silicate rocks. In the second type, W occurs in layered to non‐layered calcsilicate rocks associated with amphibolite; these are intimately associated in a narrow stratigraphic interval containing abundant, small, Broken Hill type deposits. The third type comprises stratabound, W‐bearing pegmatites, which have been remobilised from quartz‐feldspar‐biotite gneiss and bedded quartz‐tourmaline rocks. Tungsten has been mined only from the third type and only in small quantities. The three types of tungsten deposits show a close spatial relationship with stratiform and stratabound Pb‐Zn mineralisation, including the Broken Hill type. The Pb‐Zn and W deposits are inferred to be genetically related.     

Formation of magnesium‐smectite during lacustrine carbonates early diagenesis: Study case of the volcanic crater lake Dziani Dzaha (Mayotte – Indian Ocean)

The volcanic crater lake of Dziani Dzaha in Mayotte is studied to constrain the geochemical settings and the diagenetic processes at the origin of Mg‐phyllosilicates associated with carbonate rocks. The Dziani Dzaha is characterized by intense primary productivity, volcanic gases bubbling in three locations and a volcanic catchment of phonolitic/alkaline composition. The lake water has an alkalinity of ca 0·2 mol l^?1 and pH values of ca 9·3. Cores of the lake sediments reaching up to one metre in length were collected and studied by means of carbon–hydrogen–nitrogen elemental analyzer, X‐ray fluorescence spectrometry and X‐ray powder diffraction. In surface sediments, the content of total organic carbon reaches up to 20 weight %. The mineral content consists of aragonite and hydromagnesite with minor amounts of alkaline feldspar and clinopyroxene from the volcanic catchment. Below 30 cm depth, X‐ray diffraction analyses of the <2 μm clay fraction indicate the presence of a saponite‐like mineral, a Mg‐rich smectite. The saponite‐like mineral accumulates at depth to reach up to ca 30 weight %, concurrent with a decrease of the contents of hydromagnesite and organic matter. Thermodynamic considerations and mineral assemblages suggest that the evolution of the sediment composition resulted from early diagenetic reactions. The formation of the saponite‐like mineral instead of Al‐free Mg‐silicates resulted from high aluminum availability, which is favoured in restricted lacustrine environments hosted in alkaline volcanic terrains commonly emplaced during early stages of continental rifting. Supersaturation of the lake water relative to saponite is especially due to high pH values, themselves derived from high primary productivity. This suggests that a genetic link may exist between saponite and the development of organic‐rich carbonate rocks, which may be fuelled by the input of CO₂‐rich volcanic gases. This provides novel insights into the composition and formation of saponite‐rich deposits under a specific geodynamic context such as the Cretaceous South Atlantic carbonate reservoirs.     

Clay mineral,geochemical and Sr–Nd isotopic fingerprinting of sediments in the Murray–Darling fluvial system,southeast Australia

The Willyama Supergroup of the Broken Hill region in southern Australia consists of supracrustal sedimentary and magmatic rocks, formed between 1810 and 1600 Ma. A statistical analysis of nearly 2000 SHRIMP U–Pb zircon spot ages, compiled from published and unpublished sources, provides evidence for three distinct tectonostratigraphic successions and four magmatic events during this interval. Succession 1 includes Redan Geophysical Zone gneisses and the lower part of the Thackaringa Group (Cues Formation). These rocks were deposited after 1810 Ma and host granite sills of the first magmatic event (1710–1700 Ma). Succession 2 includes the upper Thackaringa Group (Himalaya Formation), the Broken Hill Group and the Sundown Group and was deposited between 1710 and 1660 Ma. These rocks all contain detrital zircons from the first magmatic event (1710–1700 Ma) and in some cases from the second magmatic event (1690–1680 Ma). The second magmatic event (1690–1680 Ma) was bimodal, resulted from crustal extension, and was coeval with deposition of the Broken Hill Group and deepening of the basin. With this event a mafic sill swarm focused in the Broken Hill Domain. Mafic sills lack any trace of inheritance, unlike the granitoids that commonly contain inherited zircons typical of the supracrustal sediments. Succession 3, the Paragon Group and equivalents were deposited after 1660 Ma, but before a regional metamorphic event at 1600 Ma. Metamorphism was closely followed by inversion of the succession into a fold‐and‐thrust belt, accompanied by a fourth late to post‐orogenic magmatic event (ca 1580 Ma) characterised by granite intrusion and regional acid volcanism (the local equivalents of the Gawler Range Volcanics in South Australia).     

Manganese silicate minerals from broken hill,New South Wales

Sixteen unpublished analyses of Broken Hill manganese silicates (two pyrox‐mangite, one rhodonite, four hedenbergite, and nine bustamite) from the papers of the late Dr W. T. Schaller are presented. They have been complemented with additional analyses by the electron microprobe to define the compositional fields of these minerals. Data are provided on the relationship of these minerals with tephroite and garnet. The occurrence of roepperite (zincian tephroite) at Broken Hill is discredited, and its validity is doubtful. Dannemorite from Broken Hill has a composition close to Mn₂(Fe,Mg)₅Si₈O₂₂(OH)₂, and can be considered a valid amphibole subspecies.     

A structural metamorphic study of the Broken Hill Block, NSW, Australia

The prograde pressure–temperature (P–T) path for the complexly polydeformed Proterozoic Broken Hill Block (Australia) has been reconstructed through detailed structural analysis in conjunction with calculation of compositionally specific P–T pseudosections of pelitic rock units within a high‐temperature shear zone that formed early in the tectonic evolution of the terrane. Whilst the overall P–T path for the Broken Hill Block has been interpreted to be anticlockwise, the prograde portion of this path has been unresolved. Our results have constrained part of this prograde path, showing an early heating event (M1) at P–T conditions of at least c. 600 °C and c. 2.8–4.2 kbar, associated with an elevated geothermal gradient (c. 41–61 °C km^?1). This event is interpreted to be the result of rifting at c. 1.69–1.67 Ga, or at c. 1.64–1.61 in the Broken Hill Block. Early rifting was followed by an episode of lithospheric thermal relaxation and burial, during which time sag‐phase sediments of the upper Broken Hill stratigraphy (Paragon Group) were deposited. Following sedimentation, a second tectonothermal event (M2/D2) occurred. This event is associated with peak low‐pressure granulite facies metamorphism (c. 1.6 Ga) and attained conditions of at least 740 °C at c. 5 kbar. A regionally pervasive, high‐temperature fabric (S2) developed during the M2/D2 event, and deformation was accommodated along lithology‐parallel, high‐temperature shear zones. The larger‐scale deformation regime (extensional or shortening) of this event remains unresolved. The M2/D2 event was terminated by intense crustal shortening during the Olarian Orogeny, during which time the first mappable folds within the Broken Hill Block developed.     

A pediment survey at middle pinnacle,near broken hill,New South Wales

Extensive and well‐developed pediments and pediplains in western New South Wales have hitherto received less than due attention in the literature. This paper records the details of instrumental survey and of excavation at The Pinnacles, near Broken Hill. It presents observation and analysis of slope, rock‐type, structure, and surficial cover, and discusses the significance of a peripediment which contains multiple soil profiles.     

Petrographic and trace element analysis of uranium-rich tufa calcite, middle Miocene Barstow Formation, California, USA

An integrated petrographic and spectroscopic (X‐ray diffraction, phosphor imaging and synchrotron X‐ray absorption spectroscopy) study of tufas from the Miocene Barstow Formation, California, relates sample morphology, mineralogy and geochemical composition. The tufas, composed mainly of calcite that formed at the interface between an ancient alkaline lake and ground or spring waters, have textures similar to those of microbially mediated terrestrial stromatolites and travertines. The tufas have elevated concentrations of a number of trace elements including Mn, Fe, Sr and U. Synchrotron X‐ray fluorescence analyses show that U concentrations can exceed 500 p.p.m. X‐ray absorption spectroscopy indicates that the U in these samples is incorporated as U(IV). It is suggested here that alkaline lake waters had a high U/Ca ratio and tufa calcite formed where groundwater or (possibly epithermal) springs brought in Ca and trace elements such as Sr. The rapid, and possibly microbially mediated, precipitation of calcite allowed for incorporation of high concentrations of trace metals as either structural substitutes or extremely fine‐scale inclusions.     

Discovery of a layer of probable impact melt spherules in the Late Archaean Jeerinah Formation,Fortescue Group,Western Australia

In the high‐grade (granulite facies) metamorphic rocks at Broken Hill the foliation is deformed by two groups of folds. Group 1 folds have an axial‐plane schistosity and a sillimanite lineation parallel to their fold axes; the foliation has been transposed into the plane of the schistosity by these folds. Group 2 folds deform the schistosity and distort the sillimanite lineation so that it now lies in a plane. Both groups of folds are developed as large folds. The retrograde schist zones are zones in which new fold structures have formed. These structures deform Group 1 and Group 2 folds and are associated with the formation of a new schistosity and strain‐slip cleavage. The interface between ore and gneiss is folded about Group 1 axial planes but about axes different from those in the foliation in the gneiss. On the basis of this, the orebody could not have been parallel to the foliation prior to the first recognizable structural and metamorphic events at Broken Hill. The orebody has been deformed by Group 2 and later structures.     

Geochemistry and age of metamorphosed felsic igneous rocks with A-type affinities in the Willyama Supergroup, Olary Block, South Australia, and implications for mineral exploration

Leucocratic quartzofeldspathic gneisses form a significant proportion of the lower part of the Palaeoproterozoic Willyama Supergroup sequence in the Olary Block, South Australia and have correlatives in the adjacent Broken Hill Block. Field and geochemical data demonstrate that these rocks were originally rhyolitic volcanics and granite, with A-type affinities consistent with magma production during intracratonic rifting, supporting tectonic models proposed for the Willyama Supergroup in the Broken Hill Block. Although the rocks have characteristic high-field-strength element enrichment, many have undergone extensive pervasive pre- or syn-metamorphic sodic alteration and are typically rich in albite.

Sensitive high resolution ion microprobe (SHRIMP) U-Pb zircon data tightly constrain the depositional and early intrusive history. Zircons from an interpreted metavolcanic rock containing relict quartz phenocrysts yield an age of 1699 ± 10 Ma, whereas a metagranitoid sample has an age of 1703 ± 6 Ma. These results are compatible with recent geochronological data on felsic metavolcanic rocks from the Broken Hill Block (Page and Laing, 1992) and are indicative of widespread magmatism during deposition of the Willyama Supergroup. Nd signatures for the two Olary Block samples imply the presence of a significant component from a depleted mantle source.

The A-type metavolcanic rocks are locally associated with small iron formations, some of which grade into stratiform barite-rich horizons. Although potentially favourable for sediment-hosted exhalative PbZn mineralisation, the Fe- and Ba-rich units, along with transgressive vein and breccia occurrences of Fe oxides ± quartz ± pyrite cutting both the metavolcanic and metagranitoid rocks, may be more prospective for epigenetic Cu-Au mineralisation related to later metamorphic and/or magmatic events. Partial melting of the A-type suite during high grade regional metamorphism at ˜ 1600 ± 20 Ma led to the formation of local volumes of sodic granitoids and pegmatites containing U-Th-Ti-REE-F mineralisation.     

Identification of Uranyl Minerals Using Oxygen K‐Edge X‐Ray Absorption Spectroscopy

Although most of the world's uranium exists as pitchblende or uraninite, this mineral can be weathered to a great variety of secondary uranium minerals, most containing the uranyl cation. Anthropogenic uranium compounds can also react in the environment, leading to spatial–chemical alterations that could be useful for nuclear forensics analyses. Soft X‐ray absorption spectroscopy (XAS) has the advantages of being non‐destructive, element‐specific and sensitive to electronic and physical structure. The soft X‐ray probe can also be focused to a spot size on the order of tens of nanometres, providing chemical information with high spatial resolution. However, before XAS can be applied at high spatial resolution, it is necessary to find spectroscopic signatures for a variety of uranium compounds in the soft X‐ray spectral region. To that end, we collected the near edge X‐ray absorption fine structure (NEXAFS) spectra of a variety of common uranyl‐bearing minerals, including uranyl carbonates, oxyhydroxides, phosphates and silicates. We find that uranyl compounds can be distinguished by class (carbonate, oxyhydroxide, phosphate or silicate) based on their oxygen K‐edge absorption spectra. This work establishes a database of reference spectra for future spatially resolved analyses. We proceed to show scanning X‐ray transmission microscopy (STXM) data from a schoepite particle in the presence of an unknown contaminant.     

Correlation between the Wonominta and the Broken Hill Blocks in western New South Wales: Evidence from Nd isotope studies of metasediments

Nd isotope studies of the oldest metasedimentary rocks from the Wonominta Block, western New South Wales reveal that these samples have a model age (T_DM) of 1780–2010 Ma, slightly younger than that of low‐grade Willyama Supergroup metasediments (1920–2160 Ma), and significantly younger than those ages previously reported from high‐grade rocks of the Broken Hill Block (2200–2300 Ma). These differences have important implications for tectonic reconstruction in this region and support a model of transitional tectonics from the Broken Hill to Wonominta Blocks, as suggested by earlier geochemical studies of mafic rocks. Those studies revealed that the mafic rocks from the basal sequence of the Wonominta Block may have formed in a back‐arc basin, developed from a propagating rifting, an environment contiguous to that in which Willyama Supergroup was deposited. These results also carry significant implications for tectonic reconstruction of eastern Australia.     

Chronostratigraphic basin framework for Palaeoproterozoic rocks (1730–1575 Ma) in northern Australia and implications for base‐metal mineralisation

Sequence‐stratigraphic interpretations of outcrop, drillcore, wireline and seismic datasets are integrated with SHRIMP zircon and palaeomagnetic determinations to provide a detailed chrono‐stratigraphic basin framework for the base‐metal‐rich Palaeoproterozoic rocks of the southern McArthur, Lawn Hill and Mt Isa regions. The analysis forms a basis for future correlations across northern Australia. Nine second‐order unconformity‐bounded supersequences are identified. Supersequences have a duration of 10–20 million years; some hitherto‐unrecognised unconformity surfaces record up to 25 million years of missing rock record. The second‐order supersequences contain a series of nested third‐, fourth‐ and fifth‐order sequences many of which can be correlated across the Mt Isa, Lawn Hill and southern McArthur regions. The analysis relates accommodation history to major intraplate tectonic events evident on the apparent polar wander path for northern Australia. Major tectonic events at approximately 1735 Ma, 1700 Ma, 1670 Ma, 1650 Ma, 1640 Ma, 1615 Ma, 1600 Ma and 1575 Ma impacted on accommodation rates and basin shape in northern Australia. Sub‐basin depocentres, the hosts for major sulfide mineralisation, are attributed to reactivated faults that controlled local subsidence. Pb/Pb model ages of 1653 Ma, 1640 Ma and 1575 Ma for the Mt Isa, McArthur River and Century Pb–Zn–Ag deposits, suggest that changes to intraplate stresses at tectonic events of like age resulted in the migration of metal‐bearing fluids into the sub‐basins. A Pb/Pb model age of 1675 for the Broken Hill deposit suggests that intraplate stresses manifest in northern Australia also affected rocks of similar age further south. Magmatic events close to 1700 Ma (Weberra Granite) and 1675 Ma (Sybella Granite) coincide with times of regional incision and the formation of supersequence‐bounding unconformity surfaces.     

Effects of Coal Rank and High Organic Sulfur on the Structure and Optical Properties of Coal-based Graphene Quantum Dots

Coal‐based graphene quantum dots (GQDs) were successfully produced via a one‐step chemical synthesis from six different coal ranks, from which two superhigh organic sulfur (SHOS) coals were selected as natural S‐doped carbon sources for the preparation of S‐doped GQDs. The effects of coal properties on coal‐based GQDs were analyzed by means of high‐resolution transmission electron microscopy (HRTEM), X‐ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, X‐ray photoelectron spectroscopy (XPS), ultraviolet‐visible (UV‐Vis) absorption spectroscopy, and fluorescence emission spectra. It was shown that all coal samples can be used to prepare GQDs, which emit blue‐green and blue fluorescence under ultraviolet light. Anthracite‐based GQDs have a hexagonal crystal structure without defects, the largest size, and densely arranged carbon rings in their lamellae; the high‐rank bituminous coal‐based GQDs are relatively reduced in size, with their hexagonal crystal structure being only faintly visible; the low‐rank bituminous coal‐based GQDs are the smallest, with sparse lattice fringes and visible internal defects. As the metamorphism of raw coals increases, the yield decreases and the fluorescence quantum yield (QY) initially increases and then decreases. Additionally, the surface of GQDs that were prepared using high‐rank SHOS coal (high‐rank bituminous coal) preserves rich sulfur content even after strong oxidation, which effectively adjusts the bandgap and improves the fluorescence QY. Thus, high‐rank bituminous coal with SHOS content can be used as a natural S‐doped carbon source to prepare S‐doped GQDs, extending the clean utilization of low‐grade coal.     

Constraints on the early metamorphic evolution of Broken Hill, Australia, from in situ U-Pb dating and REE geochemistry of monazite

The Broken Hill Pb-Zn deposit, New South Wales Australia, is hosted in granulite facies gneisses of the Southern Curnamona Province (SCP) that have long been known to record a polydeformational and polymetamorphic history. The details of this potentially prolonged tectonothermal history have remained poorly understood because of a historical emphasis on conventional (i.e. grain mount) U-Pb zircon geochronology to reveal details of the sedimentary, magmatic and metamorphic history of the rock that crops out in the vicinity of the city of Broken Hill. An alternative approach to unravelling the metamorphic history of the granulite facies gneisses in and around Broken Hill is to date accessory minerals, such as monazite, that participate in sub-solidus metamorphic reactions. We have taken advantage of the high spatial resolution and high sensitivity afforded by SHRIMP monazite geochronology to reconstruct the early history of the metamorphic rocks at Broken Hill. In contrast to previous studies, in situ analysis of monazite grains preserved in their original textural context in polished thin sections is used. Guided by electron microprobe X-ray maps, SHRIMP U-Pb dates for three distinct monazite compositional domains record pulses of monazite growth at c. 1657 Ma, c. 1630 Ma and c. 1602 Ma. It is demonstrated that these ages correspond to monazite growth during lower amphibolite facies, upper amphibolite facies and granulite facies metamorphism, respectively. It is speculated that this progressive heating of the SCP crust may have been driven by inversion of the upper crust during the Olarian Orogeny that was pre-heated by magmatic underplating at c. 1657 Ma.     

Quantitative micro‐X‐ray fluorescence scanning spectroscopy of wet sediment based on the X‐ray absorption and emission theories: Its application to freshwater lake sedimentary sequences

Micro‐X‐ray fluorescence scanning spectroscopy of marine and lake sedimentary sequences can provide detailed palaeoenvironmental records through element intensity proxy data. However, problems with the effects of interstitial pore water on the micro‐X‐ray fluorescence intensities have been pointed out. This is because the X‐ray fluorescence intensities are measured directly at the surfaces of split wet sediment core samples. This study developed a new method for correcting X‐ray fluorescence data to compensate for the effects of pore water using a scanning X‐ray analytical microscope. This involved simultaneous use of micro‐X‐ray fluorescence scanning spectroscopy and an X‐ray transmission detector. To evaluate the interstitial pore water content from the X‐ray transmission intensities, a fine‐grained sediment core retrieved from Lake Baikal (VER99‐G12) was used to prepare resin‐embedded samples with smooth surfaces and uniform thickness. Simple linear regression between the linear absorption coefficients of the samples and their porosity, based on the Lambert–Beer law, enabled calculation of the interstitial pore spaces and their resin content with high reproducibility. The X‐ray fluorescence intensities of resin‐embedded samples were reduced compared with those of dry sediment samples because of: (i) the X‐ray fluorescence absorption of resin within sediment; and (ii) the sediment dilution effects by resin. An improved micro‐X‐ray fluorescence correction equation based on X‐ray fluorescence emission theory considers the instrument's sensitivity to each element, which provides a reasonable explanation of these two effects. The resin‐corrected X‐ray fluorescence intensity was then successfully converted to elemental concentrations using simple linear regression between the data from micro‐X‐ray fluorescence scanning spectroscopy and from the conventional analyzer. In particular, the calculated concentration of SiO₂ over the depth of the core, reflecting diatom/biogenic silica concentration, was significantly changed by the calibrations, from a progressively decreasing trend to an increasing trend towards the top of the core.     

Evidence for multiple Palaeoproterozoic thermal events and magmatism adjacent to the Broken Hill Pb---Zn---Ag orebody, Australia

Thermal events at 1690-1680, 1660-1640 and 1600-1570 Ma have been resolved by SHRIMP U---Pb geochronological study on zircons and monazites from seven localities near to the Broken Hill Pb---Zn---Ag orebody, Australia. The earliest-recognized thermal event included intrusion of now deformed granites such as Rasp Ridge Gneiss and Alma Gneiss and intrusion of gabbro at Round Hill. Previously these have been interpreted as volcanic in origin, and have been assigned to different stratigraphic units of the Palaeoproterozoic Willyama Supergroup. Because these rocks are intrusions, they should be removed from the Supergroup stratigraphic sequence. The 1640–1660 Ma thermal event reached upper amphibolite to granulite conditions and produced melt segregations in parts of the Rasp Ridge Gneiss. Granites of this age are the Purnamoota Road Gneiss, previously correlated with 1690-1680 Ma rocks assigned to the Hores Gneiss stratigraphic unit, and granitic veins within Sundown Group metapelites. The 1600-1570 Ma thermal event also reached upper amphibolite to granulite conditions. The only possible 1600-1570 Ma intrusive rock reported in this study is ‘Lf-leucogneiss’ (granite) at the Purnamoota Road locality. Melt segregations of this age have been found in the Round Hill gabbro and metamorphic segregations have been found in the Purnamoota Road Gneiss. The granite intrusions and segregations are absolute time markers for fabric development and therefore can be used to re-evaluate tectonothermal evolution of rocks close to the Broken Hill Pb---Zn orebody. Within the studied rocks several discrete high grade deformation phases have been observed. The earliest detected deformation is older than 1640–1660 Ma, but syn- or post 1690 Ma. A later deformation phase can be constrained to be pre-or syn 1640–1660 Ma and a yet later deformation phase to be syn- or post- 1600-1570 Ma. The current consensus classifies the Broken Hill Pb---Zn---Ag orebody as the metamorphosed equivalent of classic SEDEX (sedimentary-exhalative) deposits, deposited at ca 1690 Ma. This interpretation heavily relies on the Hores Gneiss being a volcanic marker horizon, because the orebody is situated, apparently conformably, within the Hores Gneiss. However, results of this study show that rocks assigned to the Hores Gneiss are of different age, thus do not form a reliable marker horizon. The present results suggest that in the Thackaringa and Broken Hill Groups in the vicinity of Broken Hill, true supracrustal rocks are ≥ 1690 Ma, rather than ca 1690 Ma as previously suggested. Large parts of rocks surrounding the orebody are intrusions and together with their host supracrustal rocks were metamorphosed and locally remelted at 1660-1640 and 1600-1570 Ma.