geochemistry
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Raman spectroscopy has been used for the identification of both common and uncommon minerals in melt inclusions in Group-I kimberlites from Siberia, Canada, SW Greenland and South Africa. The melt inclusions all contained high abundances of alkali-Ca carbonates, with varying proportions of cations, and Na-Ca-Ba sulphates. However, no hydrated carbonates or sulphates were detected in melt inclusions from the Udachnaya-East kimberlite which is in agreement with its dry matrix mineralogy. In contrast, the melt inclusions in kimberlites from Canada, South Africa and SW Greenland were found to contain bassanite, pirssonite, and hydromagnesite suggesting that greater amounts of water were present in their residual magmas. This suggests that enrichment in alkali carbonates and sulphates is widespread across a range of Group-I kimberlites and implies that they commonly have an alkali-, and sulphur-rich residual kimberlite melt.
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Legacy product - no abstract available
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Legacy product - no abstract available
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This study focuses on the stable carbon (13C) and hydrogen (D) isotopic compositions of bulk, saturated and aromatic hydrocarbons in 17 Paleozoic marine crude oils from the Canning Basin, Western Australia. The stable carbon isotopic composition of crude oils is primarily dependent upon the source of the organic matter. Comparison to other Australian and global marine oils and source rocks demonstrates systematic changes in the bulk stable carbon isotopic composition throughout the Paleozoic. From the Early to the Late Paleozoic, Australian oils have become isotopically more enriched in 13C. The most depleted 13Csat value (-32.0') is from the saturated hydrocarbon fraction of a Cambrian oil-stain in the Arafura Basin whereas in the Canning Basin the oldest oils are Ordovician with 13Csat values of about -31'. Late Devonian marine oils from this basin exhibit slightly more enriched values (mean 13Csat = -29.2'), and Mississippian (early Carboniferous) marine oils from both the Canning and Bonaparte basins have mean 13Csat values in the order of -28'. Carbon and hydrogen isotopic compositions of individual C7+ n-alkanes obtained for the three major oil families from the Canning Basin, as determined by biomarker analyses, corroborate previous findings and emphasise both facies variations and differences in the level of thermal maturation attained by their source rocks. The n-alkane-specific ?13C isotopic profiles of the Paleozoic marine oils from the Canning and Bonaparte basins characteristically follow the same trend as the bulk 13C isotopic values. The n-alkane-specific D isotopic profiles of the oils typically complement those of the carbon isotopic profiles; however, there are differences because organic matter produced by photosynthetic organisms in the source rocks is influenced by both water chemistry and thermal maturity, and as a result the oils show a greater range of values than the carbon isotopic data. The isotopic data has been used to refine the characterisation of oil families and petroleum systems of the Canning Basin. The similarities of the n-alkane-specific D isotopic profiles of the Ordovician-sourced Canning Basin oils with those of Early Triassic-sourced oils of the Perth Basin demonstrates that the typing of oil families should not be undertaken exclusively on a single parameter.
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Chemical modeling of gold mineralisation in the Lachlan Fold Belt shows that gold can be precipitated over a wide temperature range (from 320 to 200 ?C in this study) from CO2-bearing, low salinity, aqueous fluid flowing upwards through faults in turbiditic sequences. In agreement with field observations, the veins are predicted to be mostly quartz (> 93 vol.%) with minor amounts of pyrite, arsenopyrite and muscovite (sericite) precipitating above 230 ?C. The predicted alteration assemblage contains pyrite, arsenopyrite, calcite, muscovite (sericite), chlorite and feldspar. Varying some of the chemical characteristics of the initial fluid has resulted in the following changes to the model: Preventing the fluid from boiling stops gold precipitating below 310 ?C but has little effect on the vein mineralogy or the mineralogy of the surrounding alteration assemblage. Removing CO2 from the fluid also prevents gold precipitation in the veins below 300 ?C. The modeling also generates an alteration assemblage with a number of Ca-rich minerals as less calcium carbonate exists in this system. Removing sulfur species from the initial fluid decreases the amount of gold precipitated by more than a factor of ten, which is to be expected if sulfur ligands are the main species for gold transport. However, the vein assemblage and the lack of sulfide minerals in the surrounding alteration assemblage also suggest that sulfur species are important in this mineral system. Increasing the initial oxidation state (?O2) of the fluid inhibits gold precipitation in the veins above 260 ?C and leads to a high proportion of dolomite in the surrounding alteration assemblage. On the other hand, decreasing the initial oxidation state of the fluid lead to gold precipitation over a range of temperatures below 310 ?C but predicts that mainly graphite ? quartz precipitates in the veins and that the surrounding alteration assemblage is dominated by feldspar proximal to the veins. This style of mineralogy is not commonly observed in gold deposits in the Lachlan Fold Belt. Increasing the initial pH of the fluid inhibits the amount of minerals that precipitate in the veins, which are dominated by calcite at high temperatures and graphite at low temperatures and corresponding minor amounts of gold. The proximal alteration assemblage is dominated by K-feldspar with amphibole, biotite and epidote. This mineral assemblage is not commonly observed in these deposits. Decreasing the initial pH of the fluid allows gold to precipitate below 280 ?C but generates a proximal alteration assemblage dominated by pyrophyllite, which again is not commonly observed. The results are in agreement with the widely accepted premise that gold is transported as bisulfide complexes and that the ore-bearing fluid is typically a low-salinity, mixed aqueous-carbonic fluid with low-moderate CO2 contents (Ridley and Diamond, 2000). However, the modeling has shown that the absence of certain physico-chemical processes or fluid constituents, such as boiling or lack of CO2 may inhibit gold precipitation in some environments. Large fluctuations in ?O2 or pH will also significantly change the vein and alteration mineralogy and generally reduce the amount of gold that is precipitated. This suggests that these fluids remain rock buffered during their journey from the source to the trap site.
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Proterozoic Granites in Australia crop out over at least 145 000 km2 (Table 1). To assess their metallogenic potential a systematic study was undertaken of all granites as well as the composition of rocks within five kilometres of the granite boundaries (Budd et al., 2001). For the granites, data on the field characteristics (presence of alteration, miarolitic cavities, presence or absence of pegmatites etc) as well as the mineralogical, major and trace element compositions of the granites were compiled. Individual granite plutons were then aggregated into suites and Supersuites on a province basis. Data were also assembled on the mineralogical composition of the host rocks, specifically the presence of reactive minerals such as carbonate, carbon, feldspar, magnetite and hematite. A GIS was constructed of all data, and simply proximity analysis was used to intersect the granite plutons as well as 5 km buffers around each pluton with known mineral deposits and occurrences. The commodities and ore deposit types were recorded around each pluton. Each occurrence was checked to ensure its age was ? the age of the related intrusion. On the basis of similarities between Suites/Supersuites of different provinces nine granite associations were identified based on their chemical characteristics, pressure/temperature conditions in their source region and their associated metallogeny. Due to insufficient data, 8.3% of exposed Australian Proterozoic granites could not be classified (Table 1). The differences between each Association and its metallogeny is believed to be controlled by first order differences in temperature and pressure conditions in their source regions. Second order changes that also influenced metallogeny, are imposed by interaction with their host rocks.
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No abstract available
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Legacy product - no abstract available
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The present report provides a compilation of thermodynamic data for geologically relevant uranium species suitable for geochemical equilibrium calculations from low to moderate temperatures (up to 300°C). It also reports a set of diagrams displaying the solubility of key uranium ore minerals (uraninite, coffinite and carnotite) and the stability of uranium and vanadium complexes at temperatures between 25° and 300°C. Further, it discusses mass-balance calculations of fluid-rock reactions at temperatures up to 200°C relevant to understanding the behaviour of uranium in unconformity-related uranium and sediment-hosted stratiform copper-uranium deposits.
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The Australian Geological Survey Organisation (AGSO) carried out a regional water column geochemical `sniffer' program on the north-west Australian continental margin, from 28 July to 02 September 1996. The survey acquired data on bottom water hydrocarbon concentration and composition on the Yampi Shelf, southern Vulcan Sub-basin and Sahul Platform. A primary goal was to detect natural hydrocarbon seepage. A total of 3535 line km (150 survey lines) of sniffer data were acquired during the program, using the RV Rig Seismic. In addition to the sniffer program, the survey tested a new high-resolution seismic cable. Also, a total of eight gravity cores were taken. The principal areas of seepage detected were on the Yampi Shelf where very high, dry, thermogenic gas concentrations (~1%wet) were detected where the regional seal thins onto the Kimberley Block basement (O'Brien et al.,1998a). Principal areas of seepage were located inboard from the Cornea trend. In addition, significantly lower concentrations of relatively wet gas were detected around the Skua field, in the southern Vulcan Sub-basin. Drier, low-level methane seepage was detected around the breached East Swan and Eclipse fields.