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Uraninite from the Olympic Dam IOCG-U-Ag deposit; linking textural and compositional variation to temporal evolution

Edeltraud Macmillan, Nigel J. Cook, Kathy Ehrig, Cristiana L. Ciobanu and Allan Pring
Uraninite from the Olympic Dam IOCG-U-Ag deposit; linking textural and compositional variation to temporal evolution
American Mineralogist (June 2016) 101 (6): 1295-1320

Abstract

The Olympic Dam IOCG-U-Ag deposit, South Australia, the world's largest known uranium (U) resource, contains three main U-minerals: uraninite, coffinite, and brannerite. Four main classes of uraninite have been identified. Uraninite occurring as single grains is characterized by high-Pb and Sigma REE+Y (Sigma REY) but based on textures can be classified into three of these classes, typically present in the same sample. Primary uraninite (Class 1) is smallest (10-50 mu m), displays a cubic-euhedral habit, and both oscillatory and sectorial zoning. "Zoned" uraninite (Class 2) is coarser, sub-euhedral, and combines different styles of zonation in the same grain. "Cobweb" uraninite (Class 3) is coarser still, up to several hundred micrometers, has variable hexagonal-octagonal morphologies, varying degrees of rounding, and features rhythmic intergrowths with sulfide minerals. In contrast, the highest-grade U in the deposit is found as micrometer-sized grains to aphanitic varieties of uraninite that form larger aggregates (up to millimeter) and vein-fillings (massive, Class 4) and have lower Pb and Sigma REY, but higher Ca. Nanoscale characterization of primary and cobweb uraninite shows these have defect-free fluorite structure. Both contain lattice-bound Pb+Sigma REY, which for primary uraninite is concentrated within zones, and for cobweb uraninite is within high-Pb+Sigma REY domains. Micro-fractured low-Pb+Sigma REY domains, sometimes with different crystal orientation to the high-Pb+Sigma REY domains in the same cobweb grain, contain nanoscale inclusions of galena, Cu-Fe-sulfides, and REY-minerals. The observed Pb zonation and presence of inclusions indicates solid-state trace-element mobility during the healing of radiogenic damage, and subsequent inclusion-nucleation + recrystallization during f (sub 82) -driven percolation of Cu-bearing fluid. Tetravalent, lattice-bound radiogenic Pb is proposed based on analogous evidence for U-bearing zircon. Calculating the crystal chemical formula to UO (sub 2) stoichiometry, the sum of cations (M*) is approximately 1 for most classes, but the presence of mono-, di-, and trivalent elements (Sigma REY, Ca, etc.) drive stoichiometry toward hypostoichiometric M*O (sub 2-x) In the absence of measured O and constraints of hypostoichiometric fluorite-structure, charge-balance calculations showing O deficit in the range 0.15-0.36 apfu is compensated by assumption of mixed U oxidation states. Crystal structural formulas show up to 0.20 apfu Pb and 0.25 apfu Sigma REY in Classes 1-3, while for Class 4, these are an order of magnitude less. Low-Pb and Sigma REY subcategories of Classes 2 and 3 are similar to massive uraninite with approximately 0.2 apfu Ca. Other elements (Si, Na, Mn, As, Nb, etc.), show two distinct geochemical trends: (1) across Classes 1-3; and (2) Class 4, whereby low-Pb+Sigma REY sub-populations of Classes 2 and 3 are part of trend 2 for certain elements. Plots of alteration factor (CaO+SiO (sub 2) +Fe (sub 2) O (sub 3) ) vs. Pb/U suggest two uraninite generations: early (high-Pb+Sigma REY, Classes 1-3); and late (massive, Class 4). There is evidence of Pb loss from diffusion, leaching and/or recrystallization for Classes 2-3 (low-Pb+Sigma REY domains). Micro-analytical data and petrographic observations reported here, including nanoscale characterization of individual uraninite grains, support the hypothesis for at least two main uraninite mineralizing events at Olympic Dam and multiple stages of U dissolution and reprecipitation. Early crystalline uraninite is only sparsely preserved, with the majority of uraninite represented by the massive-aphanitic products of post-1590 Ma dissolution, reprecipitation, and possibly addition of uranium into the system. Coupled dissolution-reprecipitation reactions are suggested for early uraninite evolution across Classes 1 to 3. The calculated oxidation state [U (super 6+) /(U (super 4+) +U (super 6+) )] of the "early" and "late" populations point to different conditions at the time of formation (charge compensation for Sigma REY-rich early fluids) rather than auto-oxidation of uraninite. Late uraninites may have formed hydrothermally at lower temperatures (T < 250 degrees C).


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 101
Serial Issue: 6
Title: Uraninite from the Olympic Dam IOCG-U-Ag deposit; linking textural and compositional variation to temporal evolution
Affiliation: University of Adelaide, School of Physical Sciences, Adelaide, South Aust., Australia
Pages: 1295-1320
Published: 201606
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 64
Accession Number: 2016-079194
Categories: Economic geology, geology of ore depositsMineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables
S30°26'60" - S30°25'00", E136°50'60" - E136°52'60"
Secondary Affiliation: BHP Billiton, AUS, AustraliaFlinders University, AUS, Australia
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201638
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