School of Earth Sciences - Theses
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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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ItemGeochemistry of sedimentary opal, Hebel, Southern Queensland( 2001)Previous studies on Australian opal were geologically uncontrolled and analysed specimens were commonly from an unknown source. This research is the first integrated geologically-controlled study of sedimentary opal in Australia with the aim of refining models for opal genesis. The work was undertaken in the recently-discovered Hebel field of southern Queensland. Mapping, underground sampling and RAB drilling specifically for this project were undertaken. After petrography, weathered host rocks and opal were analysed by XRF, XRD, INAA, LA-ICP-MS mass spectrometry. Geologically-constrained precious and non-precious opal samples were analysed by using mass spectrometry, EMPA, IR, NMR, SEM and bioluminescence. Opal is hosted by porous, low density kaolinite-bearing Early Cretaceous argillaceous sediments overlain by Tertiary rudites and arenites. The sequence is highly weathered and mapping, mineralogical, geochemical and isotope studies show that weathering becomes less intense with depth. Deep weathering took place in the Late Cretaceous and silicification associated with tropical weathering took place in the Early Tertiary. Later Tertiary cooler more arid weathering features overprint the earlier weathering. Geochemical and isotopic studies show that during the long period of weathering, there was constant re-equilibration in an open dynamic evolving system as it still is today. Opal occurrences are stratigraphically controlled by smectitic-bearing clay horizons. There are possible local structures such as faults, joints and bedding planes which allowed the plastic deformation of opaline gel by dehydration into the enveloping smectitic sediments. Opal underwent post-consolidation brittle deformation. Continued long term weathering produced bleaching of and trace element stripping from of the outer rims of opal. This study confirmed structural differences between nonprecious and precious opal and showed that opal operates as a molecular sponge. Hence it was not possible to obtain a RblSr age of formation of opal but it was possible to show, on the basis of 0 and H isotopes, that there were several periods of isotopic overprinting, probably related to different episodes of weathering. NMR, IR and EMP A studies showed that the molecular structure of opal is governed by impurities, water occurs as OH- and possible short chain aliphatic organic compounds occur in the structure. Short chain aliphatic compounds are of biological origin. SEM studies show bacterial microfossil silica pseudomorphs. No DNA was detected in opal. Opal formed from long-term intense tropical weathering of porous permeable clastic sediments. Descending silica-laden fluids were dehydrated by expanding-lattice clays and formed as gel in structural sites in favourable stratigraphic horizons. Ubiquitous microorganisms probably played a role in opal deposition, possibly providing the locus for structural distortion during colloidal precipitation thereby promoting the formation of nonprecious opal. It is suggested that precious opal has formed from colloidal silica precipitation in the absence of bacteria thereby explaining the rarity of precious opal.