School of Earth Sciences - Theses
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ItemThe Permian glacial sediments of central Victoria and the Murray Basin: their sedimentology and geochemistry( 1986)This study investigates the sedimentology and geochemistry of Permian glacial sediments cropping out in the Bacchus Marsh and Derrinal areas in central Victoria and in the subsurface beneath the Cainozoic Murray Basin in Victoria, New South Wales and South Australia. Facies analysis of the Bacchus Marsh Formation, based on a critical review of literature on glacial sedimentary processes and environments, identifies the following major facies groups: 1. Subglacial tillites deposited beneath wet-based ice. Some of these tillites exhibit structures indicative of a number of subglacial processes such as frictional lodgement of large clasts, subglacial bed deformation, subglacial meltwater flow and subglacial size sorting of clasts. Other subglacial tillites are essentially structureless. 2. Bedded diamictites to sandstones deposited predominantly by ice-rafting of debris into standing water. 3. Fluvial outwash sandstone and conglomerate facies that are finer-grained than typical proglacial outwash facies. 4. Deltas and subaqueous outwash fans vary from sandy sediments deposited by proglacial and subglacial streams to coarse, poorly sorted complexes deposited as debris aprons close to the ice front. Abundant underflow deposits suggest that less than normal marine salinities prevailed in these water bodies, even if they were arms of the sea. 5. Supraglacial tillites consisting of sandy diamictites to pebble conglomerates. Facies in the thickest sequence in the Bacchus Marsh area suggests that the area was covered by a major ice mass at least 8 times. Minor glacial advances took place during predominantly ice-free periods. The Derrinal Formation consists of a basal unit of predominantly subglacial tillite deposited in shallow glacially excavated valleys overlain by a complex of subglacial and supraglacial facies deposited by about 8 minor advances of a small ice tongue. Facies relationships in this part of the sequence are confused by intense deformation of the sediment pile during the melting of buried ice and dewatering of saturated diamictons. A major ice advance then overwhelmed the area depositing thick subglacial tillite. The Urana Formation, beneath the Murray Basin, is dominated by marine ice-rafted diamictite and mudstone. Rhythmically bedded siltstone and claystone, sediment gravity-flow deposits, traction-current deposits, and, possibly, subglacial tillites are also present. Facies assemblages in some drill holes indicate areas that were never covered by grounded glacial ice. Sedimentological and palaeontological evidence suggests that the Urana Formation was deposited towards the end of the glaciation. Ice motion indicators and ice sheet limits inferred from the facies assemblages in the Urana Formation are used to estimate the thickness of the ice over central Victoria during glacial maxima. These estimates support the conclusion drawn from the facies analysis that the ice was a large ice sheet. Comparisons of ice movement directions for central Victoria and formerly adjacent parts of Gondwana suggest that a large ice sheet was centred in North Victorialand. Major and some trace elements analyses of the clay component of marine and non-marine diamictites were used to test a number of methods of distinguishing marine from nonmarine glacial diamictites. None of the methods were clearly successful because sediment detrital mineralogy dominates the geochemical composition though V/Cr ratios may be useful in some circumstances.
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ItemVolatile and precious metal geochemistry of the Mount Isa ores and their host rocks( 1986)Geochemical and petrographic investigations of Pb-Zn-Ag mineralization (12 orebody) and Cu-Co mineralization (1100 orebody) from Mount Isa were undertaken. Over one hundred and twenty carefully selected samples were analyzed for major and minor elements and for some or all of the following volatile metals: Au, Ag, Cd, As, Sb, Se, Bi, Co and Tl. A strong Tl enrichment is observed in (pyritic) unmineralized lateral equivalents of 12 orebody for several kilometers to the north of the mine sequence. The Se and As contents, S/Se ratios and S isotope relationships in the Pb-Zn ores and their host pyritic shales preclude a magmatic or deep-seated hydrothermal S Source. The data suggest that sulfide S in the Urquhart Shales was derived from reduction of a “seawater”/evaporitic/pore water sulfate source. Lateral variations in the thickness of mineralized intervals, the nature of the sulfide-gangue textures in the ores, the pervasive K and Tl enrichment in the host rocks and other chemical features of the Pb-Zn ores indicate that much of the Mount Isa mineralization formed epigenetically within the unconsolidated Urquhart Shales. The Pb-Zn-Ag ores contain very little Au and it is argued that this feature is best explained by the hydrothermal solutions that formed the Pb-Zn ores being cool (<<200°C) and moderately oxidized. The “silica dolomite” (the host to all the Mount Isa Cu mineralization) formed from “normal” Urquhart Shale as a result of intense fault-related hydrothermal activity (Perkins, 1984). The alteration has silicified the shales adjacent to the fault, and dolomite, phyllosilicates and “immobile” elements liberated during the silicification have been re-deposited at higher levels up-dip in the silica dolomite bodies. For the most part primary sulfide textures have not been preserved. It is argued that the distribution of several elements (notably Co, Bi, As, Fe and S) in 1100 orebody and its location down-dip from a strongly pyritic section of Urquhart shale are good evidence that stratiform Co (and Cu) mineralization was present in pyritic Urquhart Shales prior to formation of the silica dolomite. Chemical and isotopic evidence suggest that the Cu mineralization had a similar S-source and formed from similar solutions to the Pb-Zn-Ag ores. A new co-genetic model for the Mount Isa Cu and Pb-Zn-Ag deposits in which the mineralization formed from cool oxidized solutions in the upper few meters of the unconsolidated Urquhart Shales is presented. The metal-bearing solutions were expelled from their source rocks (oxidized clastic sediments lower in the Moust Isa Group) during the course of normal basin compaction and dewatering. Base metal sulfides were fixed by sulfate reduction processes occurring in the diagenetic environment of the Urquhart Shales. Weathered mafic volcanic detritus may have been and important component of the source.
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ItemGenesis of volcanogenic epithermal gold-silver mineralization, Budawang Rift, New South Wales, Australia( 1988)The genesis of four volcanogenic epithermal Au-Ag deposits located within the Budawang Rift of South Coastal New South Wales, Australia have been investigated. Co-genetic pyrophyllite deposits have also been studied. Mineralization is hosted within peraluminous rhyolites which comprise approximately 50% of the bi-modal (rhyolites and tholeiitic basalts) Budawang Volcanic Complex. All mineralization occurs within the confines of the Budawang Rift of early Late Devonian age, with which mineralization is temporally related. Based upon geochemical, isotopic, structural, and lithologic investigations, the former division of the intra-rift volcanic rocks into three units (Boyd, Comerong, and Yalwal Volcanics) has been abandoned, with the adoption of a new name to include all three co-magmatic rocks; the Budawang Volcanic Complex. The name Eden-Comerong-Yalwal Rift has also been abandoned, and the new name Budawang Rift applied. The Pambula, Wolumla, Grassy Gully, and Yalwal deposits are each located along the margins of separate rhyolite flow domes, located within cumulo flow dome complexes. All four deposits plus co-genetic pyrophyllite deposits occur along N-S trending faults of similar orientation and are probably genetically related to rift graben faults. The Pambula and Wolumla deposits, plus at least two proximally located pyrophyllite deposits are situated adjacent to E-W trending cross graben block faults which predate rifting but which were re-activated by that event. Fluid inclusion studies yield temperatures of mineralization of between 320° C and 380 ° C for the Au-Ag deposits, and 290 ° C for the pyrophyllite deposits. System fluids were highly saline, ranging from 12 to 17 wt. % NaCI equiv. for the Au-Ag deposits, to 9% for the pyrophyllite deposits. Salt species are NaCI dominant with variable amounts of CaC!. No carbon dioxide was documented in fluid inclusions. Depth calculations for mineralization using the salinity corrected critical path of boiling fluids (most of the intra-rift deposits display evidence of phase separation) yield depths of: Pyrophyllite deposits 800-1,000 m, Pambula 1,200 m, Yalwal 1,300 m, Grassy Gully 1,450 m, and Wolumla > 1,600 m. Ore mineralogy is dominated by electrum which displays a distinct Au:Ag compositional ratio for each deposit, and which conforms to a temperature-depth profile for the suite of deposits, with increasing Ag in the higher temperature deposits. At Wolumla, other Ag minerals identified include native Ag, acanthite, stephantite, antimonpearceite, arsenpolybasite, pearceite, and proustite. cerargyrite, and the very rare selenide minerals naumannite and aguilarite. At Grassy Gully, trace amounts of the telluride minerals hessite and petzite were also identified. Ore associate mineral assemblages include chalcopyrite, galena, sphalerite, chalcocite, chalcostibite, tetrahedrite, tennantite, and arsenopyrite. The bulk of these minerals are co-depositional to electrum. Multiple episodes of pyrite have been identified in all goldfields, and always occur post-brecciation and shearing, but pre-electrum and pre-electrum associate mineral deposition. Some pyrites display As as well as optical zonation patterns. The chemical composition and paragenetic sequence of all mineral species identified have been documented. The levels of Se substitution of S in the Ag sulphosalts, and composi tion of naumannite and aguilarite are also discussed in detail. Ore geochemistry yields a vertical metal zonation pattern among the deposits, with the deepest and highest temperature deposit containing higher concentrations of base metals, Se, and Ag. In the shallower deposits, correlation coefficients and metal ratios indicate a decoupling of base and precious metals, and a previously un-recognized behavioural aspect of Sand Se in boiling epithermal systems.......
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ItemThe geology, petrology and geochemistry of the Otway formation volcanogenic sediments( 1983)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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ItemNo Preview AvailableThe geology and petrology of the Lower Devonian Buchan Group, Victoria( 1981)This study is devoted to a detailed examination of the stratigraphy and petrology of the late Lower Devonian (Emisan) Buchan Group, an essentially carbonate sequence. The Buchan Group begins with the lenticular Spring Creek member of the Buchan Caves Limestone. This consists of terrigenous clastic sediments derived from erosion of the underlying Snowy River Rhyodacites. Lithologies range from conglomerates and breccias with rhyodacite pebbles, through arkosic sandstones to quartz sandstones and shales. Pyroclastics, previously identified in this unit, are absent. The Spring Creek member changed from non-marine to marine as the main transgression became established and was followed by a change to carbonate deposition.