School of Earth Sciences

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.

The geology, petrology and geochemistry of the Early Cretaceous Otway Formation have been investigated in detail and used to determ ine the nature of the source rocks and to develop a model for the diagenetic and low-grade metamorphic readjustments. The fluviatile Otway Formation was deposited in continental rift grabens that stretched some 1000 km along the southern coast of Australia during the Early Cretaceous. The main areas of deposition in the Otway, Gippsland and Bass Basins contain an estimated 100,000 cubic kilometres of detritus. The major part of this detritus was derived from pyroclastic material which has been shown by the fission track dating studies to have been derived from contemporaneous volcanism. The pile of volcanogenic material comprising the Otway Formation is at least 3 to 4 km thick in the main basins. The sediments are entirely non-marine and were deposited by large scale multichannel streams cut in extensive floodplains. The streams deposited thick multistorey channel sandstones in sheet-like bodies and a diverse spectrum of overbank mudstones and fine-grained sandstones. The complex channel sandstones fine upwards but have numerous erosional breaks indicating repeated flood cycles. Whereas the channel deposits have internal features consistent with braided stream channels the overall system has a large proportion of floodplain which was been considered in the past to have been a feature of meandering channels. The oversupply of volcanogenic detritus is considered to have been responsible for the development of the multiple channel depositional system in a climate of high seasonal rainfall. Whole rock chemical analyses of all lithologies in the sedimentary suite, recalculated i.nto a set of normative minerals, have proved useful in the distinction and description of sedimentary rocks in general. P20S was found to be useful for the identification of Early Cretaceous soil forming processes. The study of the chemical composition of detrital minerals has demonstrated the usefulness of this approach in the identification of the nature of the source magmas of volcanogenic sediments. For the Otway Formation, analyses of clinopyroxenes, amphiboles, feldspars and sphene in particular, have shown that high potassium dacitic to shoshonitic volcanism dominated during Early Cretaceous rifting. The new data on the geology and mineralogical and chemical features of the Otway Formation have application to the study of diagenesis and low-grade metamorphism in volcanogenic sediments in general. (From Abstract)

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