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Abstract

The Dido Batholith is a large north- to northeasterly-trending batholith, ~90 km long and up to 30 km
wide, located along the southeastern margin of the Precambrian Georgetown Region (Greenvale
Province), northern Queensland, Australia. It overlaps the contact between the Georgetown Region
Precambrian rocks and the Early Palaeozoic volcano-sedimentary units of the northern end of the
Tasman Fold Belt System. Hornblende-biotite tonalites and granodiorites are the dominant lithologies
comprising this batholith, although an intermediate phase comprising hornblende-biotite diorites and
gabbronorites dominates the southeastern quarter. Four elongate, km-scale, and several smaller mafic to
ultramafic bodies (UMB) are hosted within this intermediate phase. All UMB are crosscut by abundant,
texturally variable felsic to mafic dykes. New U-Pb zircon dates indicate that the diorite-gabbronorite
phase crystallised at ~470 Ma, whereas the tonalite-granodiorite phase (named the Main Felsic Mass:
MFM), UMB and associated mafic dykes crystallised at ~430 Ma. These new dates have led to the
informal renaming of the older dioritic-gabbronorite phase as the Phantom Diorite, whereas the 430 Ma
phases remain a part of the Dido Suite.
The km-scale UMB, which form the focus of this study, are layered cumulate sequences which
represent open-system intrusions emplaced at shallow- to mid-crustal levels (15 – 25 km). They are
divided into two petrographically and geochemically distinct varieties: (1) the low-Fe UMB (3
intrusions), comprising dunites, wehrlites, troctolites and olivine gabbro which contain variable amounts
of olivine (Fo85 – 72), clinopyroxene (Mg# 0.87 – 0.73), plagioclase (An92-72) and chromites; and (2) the
high-Fe UMB (1 intrusion), comprising dunites, wehrlites and pyroxenites which lack chromites and
contain abundant, early crystallising Fe-Ti oxides and hornblende and less primitive olivines (Fo78-72)
and pyroxenes (Mg# 0.87 – 0.73) than the low-Fe UMB. The high-Fe UMB displays moderate
87Sr/86Sr(430) (0.705932 – 0.706231) and negative εNd(430) (-3.1 to -4.0), whereas the low-Fe UMB
displays lower 87Sr/86Sr(430) (0.703836 – 0.705318) and more positive εNd(430) (-0.9 to +3.7) values.
Using the composition of cumulus minerals and mafic dykes, it is estimated that the parent magmas of
the low-Fe UMB contained 8 – 10 wt.% of both MgO and FeOt , whereas the high-Fe UMB parent
magmas were more evolved, having higher FeOt (12 – 16 wt.%) and lower MgO (6.2 – 8.2 wt.%), Ni
and Cr contents. The primary magmas of both the high- and low-Fe UMB are interpreted as mantlederived
arc rift or back-arc tholeiites. Crustal contamination during ascent is suggested to be responsible
for the LREE-enriched and Nb- and Ti-depleted nature of the UMB parent magmas. Two-component
isotope mixing models suggest that the addition of variable amounts (<5% in the low-Fe UMB and 9-
10% in the high-Fe UMB) of 2000 – 2500 Ma igneous crustal contaminant to tholeiitic melts derived
from a slightly enriched mantle source can account for isotopic compositions of the UMB. Although
both the high-Fe and low-Fe UMB are interpreted as initially evolving along the tholeiitic liquid line of
descent (LLD), the ultimate differentiation trend recorded by the high-Fe UMB is more akin to calcalkaline
fractionation, leading to tonalitic compositions. The higher volumes and relatively late addition of crustal contaminants to the high-Fe UMB magmas (i.e. after they had experienced strong Feenrichment)
is suggested to have resulted in this apparent shift in LLD.
The 430 Ma Dido MFM is a medium-K calc-alkaline intrusion, with relatively low 87Sr/86Sr(430)
(0.706746), negative εNd(430) (-4.8) and a TDM age of ~1390 Ma. It shows fractionated REE profiles
([La/Yb]CN = 9.0 – 10.3) and contains magmatic zircons with negative εHf(430) values (-0.9 to -5.9). The
MFM is interpreted to have originated from a fractionated, mantle-derived melt which assimilated
~12% of 2000 - 2500 Ma igneous protocrust during ascent.
It is difficult to establish the exact relationship between the different phases of the Dido Suite although
it is thought likely that most of the differences in magmatic trends between the individual phases are
attributable to the physical conditions of magmatic evolution in the crust, rather than differences in
initial magma types as: (a) isotopic differences between the individual UMB and MFM can be
explained by variations in the amount of crustal contaminants assimilated into a common, mantlederived
magma, and the timing of this assimilation; (b) fractionation of cumulates similar to those
comprising the low-Fe UMB likely drove the residual tholeiitic magma to become more enriched in Fe
and H2O; and (3) later crustal contamination of such an evolved, Fe-enriched magma, and fractionation
of Fe-Ti oxide-rich cumulates to produce the high-Fe UMB, may have generated a calc-alkaline LLD,
eventually producing magmas with MFM-like compositions and isotopic signatures akin to the high-Fe
UMB. As the latter process would require large volumes of mafic magmas and produce large cumulate
sequences not seen at the current exposure level, these processes are required to have occurred at deeper
crustal levels.
The 470 Ma Phantom Diorite is a medium-K calc-alkaline intrusion. It is geochemically similar to the
Dido high-Fe UMB, displaying relatively low 87Sr/86Sr(470) (0.705775 to 0.706100), negative εNd(430) (-
3.0 to -4.3) and TDM ages of 1333 – 1461 Ma, with fractionated REE profiles ([La/Yb]CN = 6.1 – 10.4)
and containing magmatic zircons with negative εHf(470) values (-1.64 and -8.69). These similarities
suggest that (1) the Phantom Diorite and Dido Suite originated from the mixing of geochemically
similar mantle-derived and crustal components and (2) that there was little change in the tectonic
setting, and composition of the lithosphere beneath the Greenvale Province between the MidOrdovician
(470 Ma) and Silurian (430 Ma). The striking geochemical similarities between the Dido
high-Fe UMB and Phantom Diorite suggest both underwent a similar degree of crustal contamination
(~8 – 12%).
Geochemistry and mineral compositions suggest that the 470 Ma Phantom Diorite and 430 Ma Dido
Suite originated in an arc-backarc setting. Furthermore, although no Proterozoic rocks outcrop east of
the Dido Batholith (the approximate position of the Tasman Line), the identified involvement of
Proterozoic crustal contaminants in the genesis of the Dido Suite supports the ascent of these magmas
through the Georgetown Region Precambrian crust of the North Australian craton. This, in turn,
indicates genesis in a continental margin (Andean- or Mexican-type) arc setting, probably in an arc that was actively rifting, given the tholeiitic nature of the primary magmas of the Dido UMB (and possibly
the Dido MFM and Phantom Diorite). This finding largely supports the presence of a west-dipping
subduction zone beneath the Greenvale Province during the Ordovician and Silurian.
As most of the large Ni-Cu-Platinum-group element (PGE) deposits worldwide have formed where
mantle-derived melts intersect or are focussed along large faults in continental rift zones or rifted
continental margins, Anglo American plc identified the southeast margin of the Georgetown Region,
and specifically the Dido UMB, as a target for magmatic Ni-Cu-PGE deposits. No mineralisation has
been found associated with the UMB. Rocks comprising the low-Fe UMB contain <1000 ppm Cu and
<40 ppb of both Pt and Pd, whereas the high-Fe UMB rocks have similar Cu concentrations to the
former but are generally more PGE-enriched (containing up to 160 ppb of both Pt and Pd). Geochemical
discriminators suggest that the magmas that formed the km-scale UMB were chalcophile elementdepleted,
having undergone a previous S-saturation event (i.e. significant sulphide-deposition) at depth.
Furthermore, based on their lower Cu/Zr ratios and highly Ni-depleted composition, the high-Fe
magmas are interpreted to have experienced greater chalcophile metal loss than the low-Fe magmas.
The PGE-enrichment in the high-Fe cumulates is therefore explained by the addition of PGE to the
high-Fe magmas from an external source. Many economic Ni-Cu-PGE intrusions are interpreted as
being formed from (a) the addition of batches of PGE-enriched magmas to resident, chalcophile
element-depleted magmas (e.g. the Maracás deposit); or (b) scavenged PGE-rich sulphides which were
carried to accessible crustal levels by later injections of magma (e.g. the Platreef). Either of these
mechanisms could potentially account for the minor PGE-enrichment in the high-Fe UMB.
Although there is no evidence to suggest that large volumes of PGE-enriched sulphides were added to
the Dido UMB magmas, considering the minimal exploration completed to date and the new PGE data
suggesting the likely presence of significant volumes of PGE-rich sulphides at depth in this magmatic
system, the UMB cannot be ruled out as potential targets for Ni-Cu-PGE deposits. Furthermore,
although convergent settings rarely host economic Ni-Cu-PGE mineralisation, important exceptions
such as the Aguablanca deposit (Spain), indicate that feeders or conduits to intrusions emplaced in local
extensional regimes within subduction-related environments can generate important Ni-Cu sulphide
deposits. Small mafic intrusions in convergent margin settings, particularly where local extension
regimes are evident, should therefore not be overlooked in exploration for new Ni-Cu sulphide deposits
and the remaining unexplored Dido UMB, including the high-Fe varieties and any other
contemporaneous mafic-ultramafic intrusions in the surrounding area, should be evaluated on a case by
case basis.

Item Type:	Thesis - PhD
Authors/Creators:	Best, FC
Keywords:	petrogenesis, magmatic, sulphides, batholith, tonalite, ultramafics, layered intrusions
Copyright Information:	Copyright 2012 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
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