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Petrology, geochemistry and geochronology of mafic lithologies at the Olympic Dam iron oxide Cu-U-Au-Ag deposit : implications for tectonic settings and ore-forming processes

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Huang, Q (2016) Petrology, geochemistry and geochronology of mafic lithologies at the Olympic Dam iron oxide Cu-U-Au-Ag deposit : implications for tectonic settings and ore-forming processes. PhD thesis, University of Tasmania.

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Abstract

The Olympic Dam deposit, South Australia, is a supergiant iron oxide Cu-U-Au-
Ag deposit currently containing the world’s largest uranium, fifth largest copper, and
third largest gold resource (10,100 Mt at 0.78% Cu, 0.25 kg/t U₃O8, 0.30 g/t Au, 1 g/t
Ag, BHP Billiton 2015 Annual Report). The exploration model leading to the discovery
of the deposit in 1975 was source-oriented: altered mafic lithologies were considered
as the potential source of copper and early exploration focused on defining and
searching for these lithologies. Not surprisingly, drilling and underground mining at
Olympic Dam have revealed the occurrence of more than one generation of mafic
lithologies. Previous studies have considered these mafic lithologies to be closely
related to brecciation, hydrothermal circulation (by providing heat), and mineralization
at Olympic Dam. One suite of mafic rocks, inferred to belong to the ca. 1590 Ma Gawler
Range Volcanics (GRV), has been proposed to be a major source of Cu (~50% of the
contained Cu) found in the deposit.
However, at over forty years since discovery of the deposit, questions remain as
to which groups the various mafic lithologies at Olympic Dam belong to regionally, as
to what kinds of tectonic settings there were when the mafic lithologies were emplaced,
and as to how the emplacement and alteration of the mafic lithologies were related to
the ore-forming processes at Olympic Dam.
This study endeavours to provide answers to these issues. The presence of two
generations of mafic lithologies at Olympic Dam has been confirmed by primary
accessory apatite U-Pb dating. The first group is found to be correlated with the ca.
1590 Ma Gawler Range Volcanics and the Gawler silicic large igneous province (SLIP),
consisting of intensely altered olivine-phyric basalt, and other mafic dykes of various
textures (aphanitic, porphyritic, and doleritic). The second group comprises basaltic to
mainly doleritic dykes (named the Olympic Dam dolerite), proved to belong to the ca.
820 Ma Gairdner Dyke Swarm and the Gairdner large igneous province (LIP).
The ca. 1590 Ma olivine-phyric basalt at Olympic Dam typically contains a higher
abundance of former olivine phenocrysts (~20 vol.%) and is more intensely altered thanequivalent mafic GRV outside the deposit at Kokatha, Mount Gunson and Wirrda Well.
Therefore, this suite of basalt represents the most ultramafic component in the GRV
recognized thus far. Compositions of a large number of Cr-spinel inclusions within
olivine pseudomorphs and olivine-phyric basalt samples of mafic GRV (including
Olympic Dam samples) reveal different magma types and indicate derivation of mafic
GRV from a heterogeneous mantle source that may have been modified by subduction,
implying a setting proximally to a continental margin or in a back-arc. This inference
is also compatible with the previous proposal that the formation of the Gawler SLIP
was associated with the assembly of the Laurentian supercontinent. The ca. 820 Ma
Olympic Dam dolerite shows similar petrographic features and comparable
compositional variations to the regional Gairdner Dyke Swarm. The correlation of the
Olympic Dam dolerite with the Gairdner Dyke Swarm thus extends the spatial
distribution and the compositional spectrum of the latter. Geochemical comparisons
among LIP and mafic suites in South Australia (including the Gairdner Dyke Swarm),
South China and North America, associated with the break-up of the supercontinent
Rodinia are in support of the “missing-link” model in which South China was situated
on top of a mantle plume and South Australia (including Olympic Dam) was affected
by the plume-induced rift magmatism at ca. 820 Ma. In conclusion, from the
perspective of mafic magmatism, the evolution of the supergiant Olympic Dam deposit
turned out to be linked to two supercontinent cycles: the assembly of Laurentia at ca.
1590 Ma, and the break-up of Rodinia at ca. 820 Ma.
Investigations of the alteration of the two generations of mafic lithologies at
Olympic Dam have shown that they contain similar major secondary minerals (chlorite,
sericite, and carbonate). Both are characterized by a mineral assemblage of magnetiteapatite
± chlorite ± quartz that is strikingly similar to the inferred early reduced iron
oxide alteration present in the periphery and at depth of the Olympic Dam Breccia
Complex that is the immediate host to ore. Secondary spongy apatite in the ca. 1590
Ma olivine-phyric basalt yielded an age that is broadly coeval with the basalt’s
emplacement. Sericite-altered basalt produced a post-primary Rb-Sr isochron age of ca.
1180 Ma, likely to indicate the most recent significant sericite alteration of these rocks.
Extreme concentrations of some components (e.g. up to 26 wt.% CO₂, 50 wt.% of Fe₂O₃,
and 6,000ppm of Cr) and extraordinary near linear positive correlations between Cr andhigh field strength elements (e.g. Ti, Nb, and Zr) revealed in drill core assays of the
basalt indicate significant whole-rock mass and/or volume loss due to hydrothermal
alteration, in accordance with its previously proposed role as a major Cu source. Results
obtained on the secondary apatite and titanite in the ca. 820 Ma Olympic Dam dolerite
also confirmed hydrothermal activities associated with the emplacement of the younger
dolerite. Pb isotope compositions of the dolerite as well as chalcopyrite and galena
within the dolerite indicate derivation of radiogenic Pb from hydrothermal fluids
circulating through the Olympic Dam Breccia Complex, implying that the dolerite was
a part of the active hydrothermal system at Olympic Dam at ca. 820 Ma. Elevated Zn,
Pb and depleted Cu concentrations of the ca. 820 Ma Olympic Dam dolerite compared
to dolerite worldwide suggest that the dolerite can even be an additional Cu source to
the Olympic Dam deposit. Moreover, the younger and less altered dolerite provides a
better opportunity to envisage Cu depletion processes that may also be anticipated for
the older olivine-phyric basalt, where such processes are no longer recognizable due to
superimposition of multiple hydrothermal events on the basalt. At last and most
importantly, magmatic-hydrothermal activities (ca. 1590 Ma, 1180 Ma, 820 Ma)
recorded in both generations of mafic rocks can be correlated with ages (spanning from
ca. 1590 Ma to 570 Ma) obtained on the Olympic Dam Breccia Complex. This
advocates the existence of an all-encompassing, multi-stage, hydrothermal system at
the supergiant Olympic Dam deposit.

Item Type: Thesis (PhD)
Keywords: Geology; Petrology; Economic Geology; Olympic Dam; Ore Deposit; Geochronology; Geochemistry
Copyright Information:

Copyright 2016 the Author

Additional Information:

Chapter 3 appears to be the equivalent of a post-print version of an article published as: Q. Huang, V. S. Kamenetsky, J. McPhie, K. Ehrig, S. Meffre, R. Maas, J. Thompson, M. Kamenetsky, I. Chambefort, O. Apukhtina, Y. Hu. 2015. Neoproterozoic (ca. 820–830 Ma) mafic dykes at Olympic Dam, South Australia: links with the Gairdner Large Igneous Province, Precambrian research, 271, 160-172.

Chapter 4 appears to be the equivalent of a post-print version of an article published as: Q. Huang, V. S. Kamenetsky, K. Ehrig, J. McPhie, M. Kamenetsky, K. Cross, S. Meffre, A. Agangi, I. Chambefort, N. G. Direen, R. Mass, O. Apukhtina. 2016. Olivine-phyric basalt in the Mesoproterozoic Gawler silicic large igneous province, South Australia: examples at the Olympic Dam iron oxide Cu–U–Au–Ag deposit and other localities, Precambrian research, 281, 185-199.

Date Deposited: 30 Oct 2016 22:34
Last Modified: 12 Dec 2016 00:30
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