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ItemNeoproterozoic seas: ocean chemistry and marine carbonate mineralogy( 2014)The step-wise oxygenation of the ocean-atmosphere system is arguably one of the most profound processes in Earth history, affecting most surficial Earth processes. The last major oxygenation of the oceans is believed to have occurred in the Neoproterozoic Oxygenation Event (~800-540 Ma), and is implicated as a trigger for the rise of animal life. However, the timing of this event is not well constrained, both due to geochronological problems with Neoproterozoic stratigraphy; and because of the inherent uncertainty in ocean oxygenation proxies. Furthermore, there is now evidence for a more complex Neoproterozoic ocean chemical history, including return to strongly anoxic and ferruginous conditions. An additional complication in the understanding of Precambrian marine environments is the abundance of dolomite in Proterozoic successions. A recently discovered series of dolomitic reef complexes in the Neoproterozoic Adelaide Fold Belt, Australia, and Otavi Belt, Namibia, improve our understanding of Precambrian marine conditions. Stratigraphic and petrological analysis suggests that synsedimentary marine dolomite precipitation was pervasive within these reefs. Newly described dolomite cements have optical properties, chemical zonation and cathodoluminescent characteristics indicating that they were direct marine precipitates. Dolomite precipitation during marine diagenesis in these reef complexes suggests that the oceans of the Cryogenian were chemically different to those of the Phanerozoic. Marine dolomite precipitation appears to be linked to anoxic, magnesium-rich ocean conditions. These newly documented primary marine dolomite cements preserve information about conditions in the parent seawater via their petrographic properties and geochemistry. Being constrained by sedimentology, carbonate geochemistry provides a window into Cryogenian ocean chemistry and structure. Geochemical results reveal a pronounced chemical stratification where a thin veneer of oxic surface waters existed above a peritidal redoxcline with anoxic, strongly ferruginous seawater at depth. These conditions describe a ferro-sulfidic ocean and encompass some of the most extreme anoxia yet documented during the late Precambrian. A return to Archean-like ocean conditions at this time suggests large-scale disruption of the ocean system during the Neoproterozoic. These conditions may be linked to extreme climatic fluctuations at this time, perhaps induced by ocean stratification in this Neoproterozoic ‘Stagnant Earth’. When analysed in stratigraphic framework, variations in carbonate mineralogy provide a record of ocean oxygenation during the Neoproterozoic. New sedimentological and stratigraphic constraints for the Namibian Otavi Belt provides a context for this variation and has also led to the discovery of new Cryogenian reef complexes. When correlated with the Adelaidian succession, the distribution of marine cements in these sequences reflects changing seawater conditions. Pre-Sturtian, Neoproterozoic oceans precipitated both dolomite and aragonite and developed widespread marine anoxia prior to glaciation. Interglacial Cryogenian oceans were extremely anoxic and ferruginous, with widespread dolomite precipitation. In contrast, late Cryogenian and Ediacaran oceans hosted abundant aragonite precipitation recording a gradual decline in marine dolomitisation. The deepening of the oceanic chemocline during this interval suggests that these seas were likely to have been moderately oxygenated, paving the way for the large-scale radiation of animal life.
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ItemGeochronology and geochemistry of Cenozoic volcanism in relation to epithermal gold mineralisation in western Java, Indonesia( 2014)Western Java in Indonesia is a well known gold deposit district that contains a number of world class ore deposits. The district hosts mainly low-sulphidation epithermal gold deposits and the most important gold deposits occur in the Pongkor, Cibaliung, Cikotok, and Papandayan districts. Although Java has a long record of volcanic activity, little is known of its pre-Pleistocene arc history, particularly in relation to the timing and geochemical evolution of the volcanic successions associated with ore mineralisation. Moreover, much of the available geochronological data for volcanic rocks in the region are based on imprecise K-Ar results. Therefore a combination of 40Ar/39Ar dating techniques and elemental geochemical methods (major and trace element analyses) have been utilised in the current project. Most of volcanic samples from the current study are characterised by enriched LILE and LREE compositions, which are characteristic of calc-alkaline arcs. However, Papandayan basaltic samples exhibit depleted LREE contents, typical of island arc tholeiites. The more enriched LILE and LREE compositions present in some Pongkor samples and Papandayan intrusive rocks, are indicative of high-K calc-alkaline and shoshonite arcs. Trends in Nb/Y, Th/Nb, Ce/Yb, and Ce/La(N) ratios reflect temporal evolution of the arcs, from: i) a primitive arc (low Nb/Y, Th/Nb, Ce/Yb and high Ce/La(N) ratios) characterised by the tholeiite basaltic samples; ii) an evolved arc (high Nb/Y, Th/Nb, Ce/Yb and low Ce/La(N) values) typified by the high K – shoshonite volcanic samples; and iii) a mature arc (with intermediate Nb/Y, Th/Nb, Ce/Yb and Ce/La(N) values) represented by the calc-alkaline volcanic samples. Trace element signatures of La and Ce suggest a broadly similar magma source for all rocks in the region. 40Ar/39Ar dating of volcanic rocks that host gold mineralisation are representative of the western Java magmatic arc, give the oldest age of ca. 18 Ma for the Papandayan district; ages ranging from ~11 Ma to ~9.5 Ma for the Cibaliung district, from ~18 Ma to ~4.5 Ma for the Cikotok district, and the youngest ages from 2.74 ± 0.03 Ma to ca. 2 Ma for the Pongkor district. Adularia crystallisation that is associated with western Java gold mineralisation shows 40Ar/39Ar ages from the oldest to most recent: ca. 18 Ma for the Papandayan district; from 12.44 ± 0.19 Ma to 9.39 ± 0.75 Ma for the Cibaliung district; 5.36 ± 0.46 Ma and 3.43 ± 0.04 Ma for the Cikotok district; and from 2.02 ± 0.03 Ma to 1.80 ± 0.03 Ma for the Pongkor district. Some host volcanic rocks have been affected by hydrothermal alteration, which has resulted in partial resetting of ages towards the time of gold mineralisation. Variation of the mineralisation ages suggests multiple generation of adularia growth in the mineralised veins. The dating results suggest that the magmatic arc across western Java are most likely linked to Southeast Asia tectonic evolution, from Early Miocene counter clock wise rotation of Kalimantan to Late Miocene – Pliocene subduction of the Eurasian continental plate beneath the Indo-Australian oceanic plate. The new and existing age data allow for a reconstruction of the western Java magmatic arc, with three main events identified: an Early Miocene primitive tholeiite arc (20 – 18 Ma), a Middle Miocene mature calc-alkaline arc (13 – 9 Ma) and a Late Miocene – Pliocene evolved high-K calc-alkaline and shoshonitic arc (7 – 2 Ma). The reconstruction indicates that the Early Miocene Papandayan low sulphidation epithermal system (with some indications of high sulphidation activity) is related to a basement comprising thinned island arc crust. In contrast the Miocene – Pleistocene low-sulphidation epithermal mineralisation system of the Cibaliung, Cikotok and Pongkor districts, which is associated with a calc-alkaline arc, was constructed on Sundaland continental crust.