School of Earth Sciences - Theses

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    Geochemistry and mineralisation of primary and secondary platinum-group elements in the ultramafic "Alaskan-type" Owendale complex and laterites in the Fifield Region, New South Wales, Australia
    Shi, Bielin ( 1995)
    The Owendale Complex belongs to a family of ultramafic-mafic intrusions that is characterised by a zonal, nonstratiform arrangement of the principal ultramafic units. The ultramafic rocks of the Owendale Complex are virtually identical to many of the Alaskan-type intrusions, however the associated gabbroic rocks (wehrlites) are K-rich and Si-undersaturated, in contrast to the tholeiitic gabbroic rocks of the Alaskan examples. The intrusion history of the Owendale Complex is thought to have involved emplacement of a gabbroic intrusion that was invaded by an ultrabasic magma, possibly while the former was still only partly solidified. Emplacement of both magmas probably occurred during Late Devonian tectonism and deformation synchronous with emplacement and crystallisation is necessary to explain the present non-stratiform arrangement of the rock units. The most obvious linkage factor between the two proposed parent magmas (gabbroic and ultrabasic) of the Owendale suites is their mutual affinity with tholeiitic basalt magmas and the similarities of their products with intrusions of alkalic basalt derivation. This suggests the possibility that the Owendale Complex rocks and those of other tholeiitic intrusions of the regions are comagmatic products of an ancestral magma that may have also produced the widespread assemblage of complexes. Viewed from this perspective, the ultramafic rocks of Owendale Complex would thus represent a very minor product of a period of regional magmatic activity. Most alloys, erlichmanite, cooperite and some grains with exclusion texture of Pt-Os-Ir-Pd-Rh are considered to represent a primary high-temperature paragenesis. Concentration of PGE in pegmatoidal units of dunite-wehrlite is explained by the accumulation of platinum-rich alloys that segregated directly from the melt at an early stage in the evolution of the complex. The high-temperature PGM segregate directly from a silicate melt and were not generated by exsolution from spinels or magmatic sulphides. These suggest that fS2 was generally low (subordinate sulphide formation) and, after some influence at the beginning, has given way to rising fO2 (chromite, olivine and Pt-Fe-Cu-Ni alloys formation). After lithification, the ultramafic rocks become subject to "reducing" conditions, i.e., conditions of lower O2 and S2 activities. Ni-Fe alloys, native Fe and Bi formed in cracks which filled the serpentine matrixes. The former PGM (erlichmanite, cooperite and Pt-Fe alloys) were exposed to the reducing conditions via cracks were desulphurated to form porous cooperite with Pt-Fe alloys and multiphase textural Os-Ir-Ni, Pt-Ir aggregates. It is plausible that the veinlets and aggregates of unnamed Rh-Sb-S, (Pt, Ir)2(Fc, Cu)3(S, Sb, AS)3 in the dunites may also have been formed by reduction of Ni-rich sulphides and erlichmanite, Pt-Fe alloys or cooperite. Late PGM are dominated by sperrylite-geversite solid solution resulting from the reaction of early PGM with a fluid phase. A hydrothermal origin is also indicated for native Fe, native Bi and awaruite (NiFe) and the base-metal sulphides (pentlandite, chalcopyrite, sphalerite, arsenopyrite, pyrite, pyrrhotite, and some Ni-Co-Fe sulfide). The cause of the reducing conditions may have been related to H2 production accompanying hydrous alteration of the dunites and clinopyroxenites. The laterites overlying the ultramafic complexes in the Fifield region are exceptionally well-developed and well-preserved weathering profiles. Field, textural and geochemical data all support a chemical weathering origin for the profiles and compatible with meteoric and ground water origins. Meteoric water with intermediate Eh and pH and negligible dissolved species sinks into the laterite where these parameters are modified. The Eh rises and pH decreases to the conditions typical of lateritic soils and the concentration of dissolved species increases. In this state the water is able to take PGE and Au into solution from a finely disseminated form in the bedrock as a part of the process of lateritisation. When the soil solution transports the PGE and Au towards a transitional interface must exist between the ferruginous and saprolite zones with lower Eh, neutral pH and lower concentration of dissolved salts. At this transitional region, deposition of the PGE and Au occurred. The presence of magnetic Pt-Fe-Cu-Ni alloys suggests that hydrothermal solutions play a later role in the Fifield region, and the alloys have grown in situ in a lateritic soil by a process involving laterite water solution in the high Eh, low pH conditions prevalent in such soil, followed by deposition when the conditions become less extreme. Some examples of the Pt-Fe alloys from such an environment become frequently strongly magnetic with larger size. It is assumed that the temperature of the hydrothermal solution is in the range of 300° - 500° C (Bowles, 1990). PGE mineralisation in the primary rocks and laterite in this region has demonstrated a good example of multi-stage process mineralisation including primary high temperature magmatic formation; low temperature postmagmatic hydrothermal alteration and residual lateritic enrichment.
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    Late Cainozoic climatic and eustatic record from the Loxton-Parilla Sands, Murray Basin, Southeastern Australia
    Kotsonis, Andrew ( 1995)
    A series of ancient shoreline ridges in the western Murray Basin of southeastern Australia preserve a detailed legacy of Pliocene marine retreat. The 157 subdued NNW trending coastal ridges of the Loxton-Parilla Sands, mapped using conventional techniques and night-time thermal imagery from the NOAA and the ERS-l satellites, extend in a parallel series from 400 km inland to the present coastline, and provide a virtual contour plan of the Pliocene landscape. Coastal ridges of the Loxton-Parilla Sands range in age from 6:6 Ma in the east, to 3.5 Ma towards the west, where they are tectonically deformed by the uplift of the Pinnaroo Block. The deposition of the Loxton-Parilla Sands at 6.6 Ma is correlated with high global sea levels, with the distribution of the sands suggesting deposition at a topographic level comparable to an ice-free earth (i.e. complete deglaciation of the polar regions). Coastal ridges consist of beach-barrier and near-shore sediments deposited in conditions of fluctuating sea levels. The absence of aeolian sediments within the ridges implies a significantly weaker wind-wave regime and/or permanent vegetation cover existed throughout the Pliocene. Eustatic oscillations recognized within the shoreline sequence correlate well with glacio-eustatic changes modulated by the axial precession of the earth with a periodicity near 20, 000 years. Following retreat of the sea, the Loxton-Parilla Sands were subject to deep weathering, with the resultant profile termed the Karoonda Regolith. Following cessation of coastal deposition the Karoonda Regolith developed diachronously, with the oldest pedogenic exposures in the east to the youngest towards the west. Ferric and silicic weathering profiles developed in late Miocene to Plio-Pleistocene times. Pedogenic silcretes formed by downward movement of acidic soil waters with saturation and deposition at the soilwater-groundwater interface under alternating wet and dry conditions. High water tables probably ensured accumulation of silica in the near surface environment. By the Mid Pliocene (3.5 Ma) weathering changed from predominantly silica to iron mobilization with development of ferricrete profiles. Late Pleistocene (0.7-0.4 Ma) ferricrete development ceased when arid climates developed as represented by calcareous soils across the basin. Addition of calcareous parna on the Karoonda Regolith buffered soil water pHs, and switched off ferricrete development. Extensive opaline silica dissolution under alkaline conditions resulted in the development of karstic-type solution pipes that were infilled with pisoliths and clasts of sandstone. Lowered groundwater tables probably contributed to the removal of silica from the near-surface permitting transfer to deep aquifers within the Loxton-Parilla Sands. The change from ferricrete to calcrete formation marks the onset of arid climates in Australia. Correlatives can be drawn between this continental record of sea level changes with those of the deep sea oxygen isotope curves which reflect Milankovitch-type changes in the ice budget of the world.