Title: Genesis, tectonic setting- and exploration: considerations for Fe-oxide Cu Au deposits, Mount Isa Eastern Succession
Abstract: Based on the geochemistry of mafic rocks, the Palaeo-Mesoproterozoic
eastern margins of the North and South Australian Cratons can be classified into the
Eastern Domain (Mount Isa Eastern Succession, Curnamona Province and
Georgetown Inlier) and Western Domain (Mount Isa Western Succession, Kalkadoon-
Leichhardt Belt and McArthur River Basin). Basaltic magmatism of the Eastern
Domain was synchronous with back arc basinal development, while Western Domain
magmas were emplaced into a thicker continental crust. This difference is reflected in
the metallogenic nature of the domains, whereby Fe-oxide-Cu-Au (IOCG) and
Broken Hill Type (BHT-type) deposits dominates the Eastern Domain, and stratiform
Pb-Zn-Ag and Mount Isa Style Cu-Pb-Zn(Ag) are found in the Western Domain.
Based on the distinct evolutionary trends for mafic magmas of the domains of
Mesoproterozoic Australia, we suggest that the Mount Isa Western Succession and
McArthur River Basin continue to be recognised as part of the North Australian
Craton. While the Mount Isa Eastern Succession, Curnamona Province and the
Georgetown Inlier be referred to as the East Australian Craton. An actively or
formerly subducted slab sitting in the mantle lithosphere to the east of the eastern
margin of the East Australian Craton may have provided the appropriate mantle
chemistry to contribute to subsequent generation, in an extended continent, of
magmas and volcano-sedimentary input that led to the formation of Mesoproterozoic
IOCG and BHT deposits.
In the Mount Isa Eastern Succession, mafic rocks and magmas contributed
sulphur and metals to IOCG ore deposition over a protracted (~170My) period.
Between 1686 Ma and 1660Ma, S and metals (Cu, Au, Zn, Fe, Ni, Co) were exsolved
from crystallising strongly fractioned back-arc tholeiitic magmas into active
extensional faults, and surrounding country rocks. During Isan peak-metamorphism,
at ~1600Ma-1580Ma, significant amounts of S, Cu, Au, Zn, Ni, Co and Cr were
leached from mafic rocks and crustal accumulations, and led to the deposition of early
IOCG and base metal deposits. Subsequent albitic alteration associated with the
hydrothermal fluids of the ~1550Ma-1490Ma Williams-Naraku Batholith may also
have sequestered sulphide material from mafic rocks. This study highlights the
possibility that the previously held consensus that the Williams-Naraku Batholith of
felsic-intermediate magmas contributed the bulk of the metals to the Eastern
Succession mineral deposits, may not necessarily be the case, but rather, fluids
derived from these magmas remobilised previously existing mafic derived metal
accumulations.
Protracted metal and sulphur contributions to the Mount Isa Eastern
Succession Iron oxide-Cu-Au (IOCG) province occurred primarily as a consequence
of long-lived fluid and melt fluxes from the base of the crust, stimulated by initial
back-arc emplacement of voluminous mafic magmas. The concentration of sulphur,
iron, copper and gold into the presently observed mineral deposits involved a
significant component of remobilisation and reworking of early initial enrichments
(pre- to syn-Isan Orogeny) by later fluids (syn- to post-Isan and syn-Williams/Naraku
Batholith). Osborne (eastern domain) and Eloise-type ores formed or were strongly
remobilized at c. 1600 Ma by reduced, mafic-derived fluids, whereas oxidised brines
released by the Williams/Naraku granitoids overprinted magnetite ± sulphides at
Osborne (western domain) and Starra to produce younger (c. 1530 Ma) hematitechalcopyrite
associations. CO2-rich, potentially mantle-derived fluid may have
periodically pulsed through the system, manifest now as pyrrhotite-stable carbonate
veins and pods. Exploration for Ernest Henry and Starra style deposits should focus
on recognition of oxidised corridors in relation to mafic- proximal and structurally defined
targets, However, the possibility remains that large, early mafic rock related
Cu-Au ± (Fe, Co, Ni, Zn) deposits are preserved distal to the oxidising effects of the
Williams-Naraku hydrothermal system, and may also present exploration
opportunities.
Within the southern portion of the Mount Isa Eastern Succession, mafic rocks,
and faults that intersect areas of mafic rocks, exhibit the strongest spatial relationship
to IOCG mineralisation than any other geological unit. In contrast, felsic rocks, of
which both genetic and exploration models have relied heavily upon in the past in
order to explain the final localisation controls on IOCG deposits, do not display a
significant relationship to mineralisation. The results attained call for an immediate
review of exploration practices in the Eastern Succession, and call upon more mafic related
models in order to achieve sustainable IOCG mineral discoveries.
Publication Year: 2008
Publication Date: 2008-10-01
Language: en
Type: dissertation
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