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《Gondwana Research》2013,24(4):1656-1658
The abundant and diverse assemblage of filamentous microbial fossils permineralized in the ~ 3465 Ma Apex chert of northwestern Australia — among the oldest records of life — are arguably the “best studied,” by the most workers using the most advanced techniques, in the history of science. Despite the extensive body of data establishing the biogenicity of the demonstrably cellular carbonaceous Apex fossils, Pinti et al. (2013) and Marshall and Marshall (2013) have raised issues regarding the interpretation of their studies of the Apex chert presented in our recent review article (Schopf and Kudryavtsev, 2012). We agree with the assessment of both of the relevant papers by Pinti et al. (2009, 2013): the observations they report do not apply to the bona fide microscopic fossils of the Apex chert. Similarly, like the minute objects reported by Pinti et al. (2009, 2013), the “quartz and haematite-filled fractures” discussed by Marshall and Marshall (2013) are mineralic pseudofossils that are not relevant to interpretation of the Apex fossil microbes and their suggestion that “multiple populations of carbonaceous material may be a wide-spread issue through out the Precambrian” is without merit. 相似文献
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Geochemical characterisation of 18 crude oils from the Potwar Basin (Upper Indus), Pakistan is carried out in this study. Their relative thermal maturities, environment of deposition, source of organic matter (OM) and the extent of biodegradation based on the hydrocarbon (HC) distributions are investigated. A detailed oil-oil correlation of the area is established. Gas chromatography-mass spectrometry (GC-MS) analyses and bulk stable carbon and hydrogen isotopic compositions of saturated and aromatic HC fractions reveals three compositional groups of oils. Most of the oils from the basin are typically generated from shallow marine source rocks. However, group A contains terrigenous OM deposited under highly oxic/fluvio-deltaic conditions reflected by high pristane/phytane (Pr/Ph), C30 diahopane/C29Ts, diahopane/hopane and diasterane/sterane ratios and low dibenzothiophene (DBT)/phenanthrene (P) ratios. The abundance of C19-tricyclic and C24-tetracyclic terpanes are consistent with a predominant terrigenous OM source for group A. Saturated HC biomarker parameters from the rest of the oils show a predominant marine origin, however groups B and C are clearly separated by bulk δ13C and δD and the distributions of the saturated HC fractions supporting variations in source and environment of deposition of their respective source rocks. Moreover, various saturated HC biomarker ratios such as steranes/hopanes, diasteranes/steranes, C23-tricyclic/C30 hopane, C28-tricyclic/C30 hopane, total tricyclic terpanes/hopanes and C31(R + S)/C30 hopane show that two different groups are present. These biomarker ratios show that group B oils are generated from clastic-rich source rocks deposited under more suboxic depositional environments compared to group C oils. Group C oils show a relatively higher input of algal mixed with terrigenous OM, supported by the abundance of extended tricyclic terpanes (up to C41+) and steranes.Biomarker thermal maturity parameters mostly reached to their equilibrium values indicating that the source rocks for Potwar Basin oils must have reached the early to peak oil generation window, while aromatic HC parameters suggest up to late oil window thermal maturity. The extent of biodegradation of the Potwar Basin oils is determined using various saturated HC parameters and variations in bulk properties such as API gravity. Groups A and C oils are not biodegraded and show mature HC profiles, while some of the oils from group B show minor levels of biodegradation consistent with high Pr/n-C17, Ph/n-C18 and low API gravities. 相似文献
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Maarten Haest Philippe Muchez Stijn Dewaele Adrian J. Boyce Albrecht von Quadt Jens Schneider 《Mineralium Deposita》2009,44(5):505-522
The Dikulushi Cu–Ag vein-type deposit is located on the Kundelungu Plateau, in the southeastern part of the Democratic Republic
of Congo (D.R.C.). The Kundelungu Plateau is situated to the north of the Lufilian Arc that hosts the world-class stratiform
Cu–Co deposits of the Central African Copperbelt. A combined petrographic, fluid inclusion and stable isotope study revealed
that the mineralisation at Dikulushi developed during two spatially and temporally distinct mineralising episodes. An early
Cu–Pb–Zn–Fe mineralisation took place during the Lufilian Orogeny in a zone of crosscutting EW- and NE-oriented faults and
consists of a sequence of sulphides that precipitated from moderate-temperature, saline H2O–NaCl–CaCl2-rich fluids. These fluids interacted extensively with the country rocks. Sulphur was probably derived from thermochemical
reduction of Neoproterozoic seawater sulphate. Undeformed, post-orogenic Cu–Ag mineralisation remobilised the upper part of
the Cu–Pb–Zn–Fe mineralisation in an oxidising environment along reactivated and newly formed NE-oriented faults in the eastern
part of the deposit. This mineralisation is dominated by massive Ag-rich chalcocite that precipitated from low-temperature
H2O–NaCl–KCl fluids, generated by mixing of moderate- and low-saline fluids. The same evolution in mineralisation assemblages
and types of mineralising fluids is observed in three other Cu deposits on the Kundelungu Plateau. Therefore, the recognition
of two distinct types of (vein-type) mineralisation in the study area has a profound impact on the exploration in the Kundelungu
Plateau region. The identification of a Cu–Ag type mineralisation at the surface could imply the presence of a Cu–Pb–Zn–Fe
mineralisation at depth. 相似文献
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