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Distribution, petrology, geochemistry and geochronology of carbonate assemblages at the Olympic Dam deposit

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Apukhtina, O (2016) Distribution, petrology, geochemistry and geochronology of carbonate assemblages at the Olympic Dam deposit. PhD thesis, University of Tasmania.

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Abstract

The supergiant Olympic Dam Cu-U-Au-Ag deposit in South Australia is the type example of the iron-oxide copper-gold (IOCG) deposit family. Hosted entirely within heterogeneous breccia zone developed within 1.593 Ga Roxby Downs Granite (RDG) the deposit contains a volumetrically important and mineralogically diverse component of carbonate minerals. Carbonate minerals are almost always associated with ore minerals (e.g., Cu-Fe sulfides, uraninite, coffinite, brannerite), implying a genetic relationship. A study of the gangue carbonates may therefore help improve our understanding of ore genesis at Olympic Dam, yet there is very little published data. Most earlier workers favoured a broadly single-stage genetic model, with deposition of metals (and gangue minerals, including carbonates) in the same magmatic-hydrothermal cycle at ~1.59 Ga. In contrast, recent radiometric dating indicates that mineralization and gangue minerals formed episodically over a long period, requiring a re-evaluation of observed parageneses, mineral ages, depositional mechanisms and sources of metals and other components (e.g., carbon).
This study provides the first detailed and comprehensive petrographic, geochemical (e.g., major, minor and trace elements and stable C-O isotopes) and geochronological study of Olympic Dam gangue carbonates. The study is based on a large (totalling 197) set of samples representative of the entire deposit and includes samples from lithologies for which very limited or no drillcore was available for the studies carried out in the late 1980s and early 1990s.
Carbonate minerals are observed in all lithologies present at Olympic Dam: weakly to strongly brecciated granite, mafic to ultramafic dykes of various ages, felsic volcanics and clastic and carbonate sediments. Carbonates (Ca-Fe-Mg-Mn) occur as matrix in breccia, conglomerates and sediment, as breccia clasts, in veins crosscutting ore-rich breccia and other rock types, in amygdales and oolites, and in the form of massive laminated carbonate. Siderite and siderite-rhodochrosite-magnesite solid solution are by far the most common carbonate types, while calcite, dolomite-ankerite solid solution and REE-fluorocarbonates are locally abundant. Individual carbonate grains typically show some form of compositional zoning (simple or oscillatory) and replacement textures (including mutual replacement of carbonates with other carbonates and with hematite) are common. The complexity of the breccia zone prevented compilation of a paragenetic sequence; instead the carbonates were assigned to 7 associations based on the host rock lithology, mineralogy and texture: (1) coarse-grained calcite veins in weakly-brecciated granite and rhyolite, (2) carbonates in strongly-brecciated granite, (3) carbonate veins in bedded sediments, (4) carbonates in mafic and ultramafic igneous rocks, (5) massive barite-fluorite-dominated veins with minor carbonate, (6) laminated siderite, and (7)carbonate matrix in a conglomerate-breccia and sandstone.
Strong textural evidence for the multistage nature of Olympic Dam carbonates is supported by radiometric dating(Rb-Sr, Sm-Nd, Pb-Pb, and Lu-Hf) of Ca-Fe-Mg-Mn carbonates and other minerals which indicates carbonate formation in at least 3 stages (~1.59-1.55, ~0.8 and ~0.6-0.5 Ga), possibly more. Sr-Nd isotope systematics in carbonate minerals are consistent with carbonate- and ore-bearing fluids being derived in large part from the host 1.593 Ga granite and associated polygenetic breccia.
Stable isotope (C-O) data for carbonates are difficult to interpret. δ13C values (-6.5 ‰ to -2 ‰) show a relatively limited range while δ18O is more variable (+9.4 ‰ to +20.9 ‰). The data overlap the fields of several major carbon-oxygen reservoirs (magmatic, sedimentary) suggestive of mixed fluid sources possibly including recycling of older carbonate for which there is abundant textural evidence. Carbon sources in local granite, felsic volcanics, older banded iron-formations (BIFs) and sedimentary rocks (including Nuccaleena Dolomite related to the Cryogenian Marinoan glaciation, part of the ‘Snowball Earth’ event, e.g., Kendall et al., 2004) are all possible at different stages of carbonate deposition.
The detailed petrographic-geochemical work on gangue carbonates was augmented by two in-depth case studies. One of these focussed on magnetite-apatite veins hosted in dolerite dykes, the local equivalent of the regional 825 Ma Gairdner Dyke Swarm (GDS), which cut the mineralized Olympic Dam Breccia Complex (ODBC). Magnetite-apatite assemblages are a characteristic of many IOCG deposits, notably the well-known iron-oxide apatite (IOA) or Kiruna-type style. Several genetic models have been proposed to explain large-scale IOA-style mineralization but there appears to be no agreement. At Olympic Dam, there are many examples of dolerite dykes with intensive magnetite-apatite veining, without subsequent overprinting. U-Pb dating shows the magnetite-apatite assemblages formed very soon after the host dolerite, and the vein components (e.g., Fe, P) have been derived from an external fluid and from alteration of the dolerite along vein margins. The setting and spectacular colloform magnetite textures limit the range of metal transport and depositional processes for this IOA assemblage, providing a possible template for magnetite-apatite assemblages in magnetite-apatite-bearing IOCG systems and other occurrences in disparate geological settings.
The second case study focussed on a large-scale magnetite-apatite assemblage discovered in drillhole RD2773 deep within only weakly brecciated host granite and a rhyolitic unit and below the ODBC. By analogy with other IOCG prospects in the region, initial mineralization at Olympic Dam had been expected to be ~1.59 Ga magnetite-apatite-pyrite-siderite, without uraninite. Due to reaction with oxidized fluids, such early magnetite assemblages, if they ever existed at Olympic Dam, were replaced by hematite, with associated deposition of uraninite. In contrast, the magnetite-apatite (IOA) assemblage in RD2773 contains pyrite and quartz as major components, and U mineralization is present as uraninite whereas siderite is completely absent. U-Pb dating of magmatic zircon in the host rhyolite unit, and of hydrothermal apatite, uraninite and hematite (partially replaces magnetite) in the IOA assemblage shows consistent ages near 1.59 Ga. Sm-Nd dating of crosscutting calcite veins suggests these may have formed up to 50 Ma later. The results of this case study suggest that the deep root of the IOCG system is represented by ~1.59 Ga old IOA-type mineralization which was later modified and upgraded to a hematite-rich, highly mineralized paragenesis at shallower levels of the deposit.
The present work has helped to place diverse gangue carbonates and associated minerals within an emerging chronology for this multistage deposit. Carbonates appear to have formed at nearly every stage of the host deposit’s evolution, and their development and replacement history parallels that of other gangue minerals, and of the ore minerals themselves. The carbonates formed initially with the texturally earliest hydrothermal magnetite and apatite (~1.59-1.55 Ga) soon after emplacement of the host granite, followed by further periods at ~1.4-1.1 (REE-fluorocarbonates), ~0.8 and ~0.6-0.5 Ga. Thus, deposition of carbonates at Olympic Dam spans a period of more than 1 Ga. This chronology suggests that carbonate deposition and other aspects of the Olympic Dam deposit were related to major tectonic-magmatic events, beginning with the effects resulting from the breakup of Columbia (~1.6 Ga), followed by the tectonic, hydrologic and magmatic effects of the amalgamation and breakup of Rodinia (~1.3-1.1 and ~0.8 Ga, respectively), and renewed reworking, mineralization and mineral formation during amalgamation of Gondwana (~0.5 Ga). Carbonate gangue has been reported from many polymetallic IOCG deposits but few detailed studies have been published. Comparison with these studies suggests that carbonates at Olympic Dam are unique in their complexity and protracted depositional history. This could be the key to the formation of a supergiant IOCG deposit such as Olympic Dam.

Item Type: Thesis (PhD)
Keywords: ore deposits, genesis, carbonates, radiogenic isotopes, stable isotopes, geochronology
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Copyright 2016 the author

Additional Information:

Chapter 3 appears to be the equivalent of a post print version of an article finally published as: Apukhtina, O.B., Kamenetsky, V.S., Ehrig, K. et al., Postmagmatic magnetite–apatite assemblage in mafic intrusions: a case study of dolerite at Olympic Dam, South Australia, Contributions to Mineralogy and Petrology, (2016), 171: 2, 1-15, the final publication is available at Springer via http://dx.doi.org/10.1007/s00410-015-1215-7

Date Deposited: 06 Oct 2016 01:30
Last Modified: 11 Dec 2016 16:00
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