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Type: Theses
Title: The evolution of uraninite, coffinite and brannerite from the Olympic Dam iron oxide-copper-gold-silveruranium deposit: linking textural observations to compositional variability
Author: MacMillan, Edeltraud Irene
Issue Date: 2016
School/Discipline: School of Physical Sciences
Abstract: Interpretations of mineral textures have long been used to better understand the processes involved in the formation of mineral deposits. At the Olympic Dam iron-oxide-copper-gold (IOCG)-Ag-U deposit, South Australia, the genesis and evolution of the U-mineralization is difficult to reconstruct unequivocally. Uraninite, coffinite and brannerite are the dominant Uminerals, however previous studies have focussed on the parts of the deposit which have elevated U-grade and are dominated by massive- or vein-type uraninite. Few prior studies documented the textural and chemical variability of these minerals from a broad range of samples throughout the deposit. Based on detailed mineralogical and microanalytical analysis, this study has addressed some of these shortcomings. The data and interpretation thereof allow for models and hypotheses to be made about the formation and alteration mechanisms involved in forming the mineral textures as observed today. Two generations of uraninite have been identified, and these can be split into four main textural classes. The early generation consists of the primary, zoned and cob-web textural classes. These represent single uraninite crystals with high-Pb and ΣREE+Y (ΣREY), which have been progressively altered both chemically and texturally. The simplest cubic, euhedral morphology is displayed by the primary uraninites, which also often exhibit oscillatory and sectorial zonation of lattice-bound Pb and ΣREY, and commonly have elevated Th contents. Zoned uraninites are typically coarser, sub-euhedral to prismatic grains and contain unique zonation patterns defined by distinct zones of high- and low-Pb and ΣREY which differ to the zoning contained within the primary uraninites. The greatest heterogeneity is observed within the cob-web class, with variable hexagonal to octagonal morphologies, varying degrees of rounding, and rhythmic intergrowths of uraninite with Cu-(Fe)-sulfides ± fluorite from core to margin. There is also a late generation of uraninite which occurs in the highest-grade parts of the deposit and exists as μm-sized grains to aphanitic varieties which form larger (up to mmsized) aggregates and vein-fillings. Late uraninites typically have lower-Pb, but higher Ca±Si contents compared to the early generation. The early crystalline uraninites are only sparsely preserved, with the more massive-aphantitic uraninite representing the majority of the uraninite contained within the deposit. Nanoscale characterization of selected uraninite crystals from the early generation has revealed these have a defect-free fluorite structure, and contain lattice-bound Pb+ΣREY within chemically distinct zones or domains. Micro- and nanoscale inclusions of galena, Cu- (Fe)-sulfides and REY-minerals are also present within the cob-web uraninites. The presence of both lattice-bound Pb within distinct zones and domains, as well as inclusions of galena within these uraninites, are attributed to healing of radiogenic damage via solid-state traceelement mobility, and subsequent fS₂-driven percolation of a Cu-bearing fluid allowing for inclusion nucleation and recrystallization. Crystal-structural formulae for uraninite have been calculated, and the key underlying assumption for these formulae is that lattice-bound radiogenic Pb is present, at least in part, in the tetravalent state. To distinguish the two uraninite generations, in addition to the textural and chemical differences, the oxidation state [U⁶⁺/(U⁴⁺+U⁶⁺)] was calculated and it was revealed that these potentially experienced different formation conditions. The early uraninites are thought to have formed from higher temperature, granite-derived hydrothermal fluids, with later hydrothermal alteration of the zoned and cob-web types; whereas the late uraninites have formed hydrothermally at lower temperatures (<250 °C). Additional characterization of the zoned and cob-web uraninite using electron backscatter diffraction (EBSD) has further developed our understanding of the processes involved in their evolution. Zoned uraninite has been interpreted to have formed as a result of multiple superimposed effects, including alteration of initial oscillatory zoning (as displayed by the primary uraninite) from interaction with hydrothermal fluids and/or from self-annealing of radiation damage. Zones of weakness were created within uraninite as a result of the accumulation of defects and dislocations into tilt boundaries that correlate to one of the active slip systems in uraninite. High diffusivity pathways were generated along these zones of weakness, and aided in element mobility and exchange between uraninite and the hydrothermal fluid/s. The rhythmic intergrowths of uraninite and Cu-(Fe)-sulfides, of which the cob-web uraninites comprise, are attributed to replacement of uraninite by bornite. Replacement is thought to be controlled by the inherent chemical zoning (of Th) within the uraninite crystal, and part of the replacement occurs via coupled dissolution-reprecipitation (CDR) reaction. Initially, the bornite inherits the crystallographic orientation of the parent uraninite, but different orientations of bornite are possible due to epitaxial nucleation. Based on the presence of Cu-(Fe)-sulfide ± fluorite inclusions and the chemistry of the proposed replacement, it is suggested that replacement was driven by a F-rich hydrothermal fluid that was also enriched in Cu, S, Fe and Ca. This is the first known study which integrates the use of EBSD and other micro- and nanoscale characterization techniques to study uraninites and associated minerals. The application of CDR-driven replacement to systems which have no common chemical constituents is also at present unique. The combined use of various microand nanoscale characterization techniques has therefore provided some fresh insights into the reactions and enhanced our knowledge about the evolution and progressive in-situ alteration of uraninite at Olympic Dam. Much of the past work conducted on the U-minerals at Olympic Dam has indicated that there were numerous cycles of U dissolution and reprecipitation, but few studies have further explored this hypothesis. Both brannerite and coffinite have also been characterized in the present study. Brannerite has a diverse morphology which ranges from complex irregular shaped aggregates, irregularly-shaped blebs, replacement bands, and discrete elongate seams. The internal structure of brannerite consists of randomly orientated hair-like needles and blades to a mix of uniform-massive or bleb-like irregular masses. Compositions range between that of uraniferous rutile and stoichiometric brannerite. The more uniform-massive brannerite blebs, typically have higher ΣREY, Pb, Nb ± As contents compared to the more needle-like, irregular-shaped, aggregated brannerite which contains elevated Fe, Mg ± Mn ± Na ± K. Based on chemical and textural observations, brannerite has been grouped into four distinct groups. Coffinite is typically globular to collomorph in appearance, and is often found on the margin of quartz grains and nucleates from a range of minerals including Cu-(Fe)- sulfides, galena, brannerite, uraninite, and chlorite. Variations in Ca, ΣREY, P ± As ± Nb appear to be responsible for much of the chemical heterogeneity. Three different coffinite groups have been identified based on chemical variability and textural observations, however there are some textural differences and variable mineral associations within these groups. It is likely that the textural heterogeneity is due to local variation in fluid-rock interactions. It is concluded that brannerite and coffinite are a result of a late-stage U-event(s), and this may have involved the dissolution and/or reprecipitation of earlier precipitated uraninite, or may have involved a fresh influx of U. Factors which support late-stage formation of both brannerite and coffinite include their low-Pb contents and the occurrence of coffinite on the edges of uraninite or brannerite, indicating that the coffinite may have formed after either of these minerals. Additional features like banding, scalloped edges, alteration rinds, variable compositions etc. are also indicative that these minerals may have formed as a result of alteration and by processes which occur after initial deposition of the mineral on which they occur. The precipitation of uraninite, brannerite and coffinite all require different conditions and chemical components, thus it is unlikely a single fluid could precipitate all of these minerals at one time. It is clear that some of the uraninites precipitated early in the formation of the deposit, but deciphering the subsequent generations of U-minerals is somewhat subjective. The results of this study will clearly document the range of textures and compositions of uraninite, brannerite and coffinite found within the Olympic Dam deposit and will provide evidence for a number of mechanisms which have contributed to their textural appearance. But, the genetic implications of these findings and what they mean for the genesis of the deposit remains unconstrained and will undoubtedly form the basis for future research.
Advisor: Pring, Allan
Cook, Nigel J.
Ehrig, Kathy
Foden, John David
Dissertation Note: Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Physical Sciences, 2016.
Keywords: uraninite
mineral textures
coupled dissolution-reprecipitation
Olympic Dam
IOCG deposits
Research by Publication
Provenance: Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text.
This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at:
DOI: 10.4225/55/59a61fde17204
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