Olympic Dam-type iron oxide (Cu-U-Au-LREE) constitute a distinct class of
ore deposits characterised by iron-rich, low-titanium rocks formed in
extensional tectonic environments.
These deposits formed primarily in shallow crustal
environments (<4-6 km) and are expressions of deeper-seated,
volatile-rich igneous-hydrothermal systems, tapped by deep crustal
The Olympic Dam Cu-U-Au-Ag ore deposit of South Australia occurs in
tectonic-hydrothermal breccia complex that is surrounded by
breccia is composed mainly of granite clasts and minor amounts of
Very thick (>350 m) sections of bedded sedimentary facies that
occur in the breccia
complex include laminated to very thin planar mudstone beds, thin to
graded sandstone beds, and thick conglomerate beds.
The bedded sedimentary facies extend
continuously across a 1.5 km × 0.9 km area and are not limited to
small separate maar craters.
Detrital chromite and volcanic quartz in the bedded sedimentary
facies cannot be matched with local sources, and imply that the
provenance extended beyond
the area of Olympic Dam.
The lateral continuity, provenance characteristics, great thickness,
below-wave-base lithofacies, and intracontinental setting suggest that
the bedded sedimentary
facies are remnants of a sedimentary basin that was present at Olympic
Dam prior to formation
of the breccia complex.
The Olympic Dam hydrothermal system operated
beneath and partly within an active sedimentary basin, was not
confined to maar craters,
and did not vent.
This view of the setting raises the possibility that sedimentary
were involved in ore genesis
Salient characteristics of this class are:
The majority of known deposits, particularly the larger examples, are
found within Early to mid-Proterozoic host rocks (1.1 - 1.8 Ga). However,
examples are recognised into the Tertiary.
The deposits are located in areas that were cratonic or continental margin
environments; in many cases there is a definite spatial and temporal
association with extensional tectonics. Most of the districts occur along
major structural zones, and many of the deposits are elongated parallel to
regional or local structural trends. The host rocks may be igneous or
sedimentary; many of the deposits occur within silicic to intermediate
igneous rocks of anorogenic type. However, mineralisation in many deposits
is not easily related to igneous activity at the structural level of
The ores are generally dominated by iron oxides, either magnetite or
hematite. Magnetite is found at deeper levels than hematite. CO3, Ba, P,
or F minerals are common and often abundant. The deposits contain
anomalous to potentially economic concentrations of LREEs, either in
apatite, or in distinct LREE mineral phases.
The host rocks are generally intensely altered. The exact alteration
mineralogy depends on host lithology and depth of formation, but there is
general trend from sodic alteration at deep levels, to potassic alteration
at intermediate to shallow levels, to sericitic alteration and
silicification at very shallow levels. In addition, the host rocks are
locally intensely Fe-metasomatised.
Regional Variation of deposits
In spite of these similarities, many variations occur between and within
individual districts, particularly in deposit morphology. Individual
deposits occur as strongly discordant veins and breccias to massive
concordant bodies. Both the morphology and extent of alteration and
mineralisation appear to be largely controlled by permeability along
faults, shear zones and intrusive contacts, or by permeable horizons such
as poorly welded tuffs. Local variations in mineralogy and geochemistry
may be largely attributable to wall-rock composition, and to P, T, and ƒO2
controls related to depth of formation.
Other Examples of Olympic Dam-type deposits
Examples of Olympic Dam-type deposits are found in the Wernecke
Mountains of eastern Yukon Territory and in the western portion of the
The Wernecke Mountains contain approximately 90 discrete breccia bodies
which are concentrated along the Richardson Fault Array, a major fault
zone that controlled block faulting from the mid-Proterozoic to the
Tertiary. The breccias cut the Proterozoic Wernecke Supergroup, a >4.5
km thick section of marine sediments.
All the breccia bodies contain minor to significant amounts of iron oxide.
Sulfide mineralisation appears to be a late-stage event in all the breccia
bodies. In the deeper breccias, chalcopyrite replaces magnetite and is
intergrown with, or replaces pyrite and hematite. In stratigraphically
higher breccias, chalcopyrite is veinlet-controlled and disseminated, as
either replacements of magnetite or as interstitial grains within
carbonate or specular hematite.
Uranium minerals and gold commonly occur on the periphery of breccia
bodies. LREEs in the Wernecke breccias are concentrated in apatite and
monazite. Isotopic studies suggest that the hydrothermal fluids
responsible for alteration and mineralisation in these breccia bodies had
near magmatic compositions.
Though the Olympic Dam-type deposits constitute major sources of iron
(Kiruna, Chilean Fe), only two deposits are currently exploited for other
metals (Olympic Dam - Cu, Au, U; Bayan Obo -LREE). The variability of
Cu-U-Au-LREE contents within this deposit class, combined with potentially
difficult metallurgy, makes these deposits a high risk exploration target.
However, these factors are partially offset by the large size potential of