Uranium ore-forming systems of the - Geoscience Australia

More documents

Recommendations

Info

Uranium ore-forming systems of the Lake Frome regionAbstractCenozoic basins of the Lake Frome region in South Australia contain most of the nation’s knownresources of sandstone-hosted uranium mineralisation. In addition to the currently operatingBeverley uranium mine, two other deposits have been approved for mining (Honeymoon, FourMile East), and discoveries continue to be made in the region (e.g., Pepegoona; HeathgateResources, announcement September 2009). While the known resources are significant, thepotential of the region for very large uranium deposits has not been well understood, in part due tolimited knowledge of the regional- and district-scale geological controls on uraniummineralisation.The multidisciplinary study reported herein applies a ‘mineral systems’ approach to identify andmap the principal geological controls on the location of known uranium mineralisation in the LakeFrome region. This new framework is aimed at providing a basis for refined exploration targetingof areas with potential for major undiscovered deposits, thus reducing risk in investment by theexploration industry.Mineral systems approach – Major ore deposits are the product of crustal- to deposit- to microscalegeological processes, and require the presence of (a) suitable sources of metals, fluids(aqueous, magmas, etc), ligands (for hydrothermal deposits), sulfur (for sulfide deposits), (b)permeable pathways or permeability architecture for transport of metals and for focussing andoutflow of fluids, (c) energy sources to drive the ore-forming system, and (d) chemical and/orphysical depositional gradients where the ore metals are precipitated. Suitable sources,permeability architecture, and physico-chemical gradients occur only in specific ‘fertile’ terranesthat have experienced favourable geodynamic and tectonic evolution, and where the sequence ofgeological events permits an efficient physical link (permeability architecture) between thesources regions and the ore depositional environments. The present study aims to identify and mapthe geological ‘proxies’ (i.e., observable geological features) for each of the four mineral systemcomponents. In addition, successful exploration for major uranium or other deposits requires thatthe deposits are preserved at depths accessible to exploration methods.Sources of U, fluids, ligands – It is widely assumed the uranium was sourced from the uraniumrichProterozoic basement in the region, or from sediments derived from such basement. TheBeverley deposit is hosted by sands of the Oligocene-Miocene Namba Formation, whereas theother known deposits in the region are hosted by sands of the Eocene Eyre Formation. Here wepresent an hypothesis for leaching of uranium from basement as it was exhumed during threemajor episodes of uplift since the late Mesozoic. The highly oxidised waters required to transportmajor quantities of uranium are assumed to have been meteoric waters and groundwaters, whichin our hypothesis were maintained at high oxidation state within zones of deeply weathered andoxidised basement.Energy and timing – The occurrence of three major uplift episodes and three periods of deepweathering since the late Mesozoic may have been crucial for uranium ore formation in the LakeFrome region. In this hypothesis, basement uplift triggered increases in gravitationally-drivenfluid flow through previously deeply weathered U-bearing source rocks and into adjacent basins.Numerical modelling confirms the greater fluid fluxes expected for geometries with higher relief(greater potential energy). As a consequence, we infer development of three uranium mineralsystems at different times since the late Cretaceous (see below). At least two of the periods ofuplift were probably an intracontinental response to changes in motion of the Australian platesince ~40-45 Ma.Permeability architecture – Permeable pathways for U-bearing fluids potentially include fracturedand weathered basement, faults, and primary and secondary permeability within sediments andPage 1 of 151
Uranium ore-forming systems of the Lake Frome regionsedimentary rocks. Previous published models depicting formation of the Four Mile and Beverleydeposits within a single west-to-east fluid flow system are not supported by numerical simulationsof simplified geological scenarios for this district. Either fluid flow occurred at different times toform the Beverley and Four Mile deposits, and/or the deposits formed in different west-to-eastfluid flow pathways. Geological evidence from this district and elsewhere in the region points tobroadly south-to-north or southwest-to-northeast direction of sediment transport in paleochannelswithin both the Namba Formation and Eyre Formation. Thus a viable alternative hypothesis isthat known deposits in the Namba and Eyre formations formed during broadly south-to-northpaleo-fluid flow, although present day groundwaters may have opposite flows in somepaleochannels. Mapping of regional faults in 3D suggests that basin architecture and paleochannelorientations were controlled by broadly north-south striking faults, some of which werereactivated through the Paleozoic to Cenozoic. Hydrocarbon seeps indicate that some of thesestructures tap hydrocarbon reservoirs in the Arrowie and/or Eromanga basins, from which mobilereductants may have been sourced.Depositional gradients – Reconnaissance petrography of relatively high-grade mineralisation inthe Four Mile East deposit shows that uranium is predominantly hosted by uraninite whereas atBeverley the major ore mineral is coffinite. Oxidation-reduction processes were undoubtedlyimportant in uranium deposition at both deposits, with evidence for the role of organic matter andreduced Fe 2+ (ilmenite, pyrite) in redox reactions. However, other mineralogical differencessupport the hypothesis of differing timing and/or fluid chemistries in the Four Mile and Beverleysystems. District- to regional-scale redox boundaries have yet to be mapped comprehensively,but their geometry will provide evidence of paleo-fluid flow directions. A new method ofmapping redox variations in 3D has demonstrated the likely presence of a north-south trendingpaleochannel system, and/or redox interface, in the upper Namba Formation to the east of LakeFrome.Exploration implications – Integration of results of this study supports a model of three potentialepisodes of basin-hosted uranium systems in the Lake Frome region. The Beverley deposit isproposed to have formed during the latest episode, whereas the Four Mile East deposit may haveinitially formed during either of the latter two ore-forming episodes.1. Late Cretaceous-Paleocene uranium mineral system: Post-Eromanga Basin uplift of basementthat was deeply weathered during the Mesozoic could have triggered gravitationally-drivenflow of uranium-rich groundwaters into permeable units of the Eromanga Basin during theLate Cretaceous or Paleocene. Where suitable reductants were available, uraniummineralisation could have formed in the late Cretaceous or Paleocene. These systemspotentially were large due to the effects of up to three episodes of oxidised fluid flow. Subbasinsisolated from present day groundwater flow in the Eromanga Basin may be mostprospective for preserved mineralisation.2. Late Eocene-Oligocene uranium mineral system: Post-Eyre Formation uplift and exhumationof the Mt Painter Inlier may have triggered fluid flow from basement areas (deeply weathered)into the Eyre Formation, utilising south-to-north oriented paleochannels. Fluids would havebeen confined by aquitards and aquicludes within the Eyre Formation, with mineralisationforming in the late Eocene to early Oligocene (~35-28 Ma). Large systems are possibleparticularly to the north, due to the opportunity for the Eyre Formation to have experienced atleast two episodes of oxidised fluid flow.3. Pliocene-Pleistocene uranium mineral system: Post-Namba Formation uplift and exhumationof basement rocks in the Lake Frome region occurred since ~5 Ma, producing most of thepresent day relief in the Northern Flinders Ranges and Mt Painter Inlier. Basement that wasdeeply weathered in the period ~10-20 Ma would now be available for leaching of uraniumand transport into permeable units of the Namba Formation, Eyre Formation or EromangaBasin. Larger uranium systems may exist to the north, where paleochannels converged or metlake systems.Page 2 of 151
Page 1: G E O S C I E N C E A U S T R A L I
Page 5: Uranium ore-forming systems of the
Page 9 and 10: Uranium ore-forming systems of the
Page 57 and 58:
Uranium ore-forming systems of the
Page 59 and 60:
Page 61 and 62:
Page 63 and 64:
Page 65 and 66:
Page 67 and 68:
Page 69 and 70:
Page 71 and 72:
Page 73 and 74:
Page 75 and 76:
Page 77 and 78:
Page 79 and 80:
Page 81 and 82:
Page 83 and 84:
Page 85 and 86:
Page 87 and 88:
Page 89 and 90:
Page 91 and 92:
Page 93 and 94:
Page 95 and 96:
Page 97 and 98:
Page 99 and 100:
Page 101 and 102:
Page 103 and 104:
Page 105 and 106:
Page 107 and 108:
Page 109 and 110:
Page 111 and 112:
Page 113 and 114:
Page 115 and 116:
Page 117 and 118:
Page 119 and 120:
Page 121 and 122:
Page 123 and 124:
Page 125 and 126:
Page 127 and 128:
Page 129 and 130:
Page 131 and 132:
Page 133 and 134:
Page 135 and 136:
Page 137 and 138:
Page 139 and 140:
Page 141 and 142:
Page 143 and 144:
Page 145 and 146:
Page 147 and 148:
Page 149 and 150:
Page 151 and 152:
Page 153:
Instructions for the CD-ROMUranium
show all

Uranium ore-forming systems of the - Geoscience Australia

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?