Thanks Mode ,
I think it reads well , only looked a small way in and realised it was beyond my small minds comprehension .
Want to the summary and conclusion , even that BELOW a bit beyond me
BUT, Thanks, if you think it reads well I'm STOKED too
9.5 Summary
The Phantom Diorite is a 470 Ma calc-alkaline intrusion, comprising predominantly hornblendebiotite diorites and subordinate ‘interlayered’ **bronorites, all of which have affinities to arcrelated non-cumulate rocks (Beard, 1986). Mineral barometry provides average pressure ranges of
2.9 – 5.8 kbars for the Phantom Diorite, which equates to emplacement at relatively shallow crustal
levels (~10 – 20 km). Hybridization between a fractionated mantle-derived magma and crustal
rocks/ magmas is a reasonable hypothesis for the ultimate source of the Phantom Diorite, with
calculated two component mixing models suggesting genesis via the addition of 8 - 12% crustal
contaminant (igneous, >1500 - 2500 Ma crust) to a mantle-derived magma. The moderate degrees
of geochemical variability shown by the rocks comprising the Phantom Diorite, in addition to
petrological heterogeneity, suggest that although some diorites likely represent comagmatic liquids
related via closed-system fractionation, other diorites and particularly **bronorite units, more
likely represent separate batches of magma.
Based on the geochemical similarities (e.g. overlapping isotopic compositions and similar REE and
incompatible element profiles) between the 470 Ma Phantom Diorite and 430 Ma Dido Suite, it is
suggested that, like the Dido Suite MFM, the Phantom Diorite potentially represents the residual
magmas produced via the fractionation (at depth) and crustal contamination of mantle-derived arc
rift or back-arc tholeiites. Based on these findings it is proposed that there was little change in the
composition of the mantle beneath the Greenvale Province (Georgetown Region, N Queensland)
between the Mid-Ordovician and Silurian.
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10 Conclusions
This detailed field, petrological, geochronological and geochemical study of the Dido Batholith
resulted in the following conclusions:
(1) The Dido Batholith predominantly comprises hornblende-biotite tonalites and granodiorites,
although an intermediate phase comprising hornblende-biotite diorites and **bronorites dominates
the south-eastern quarter of the batholith. Four elongate, km-scale, and several smaller mafic to
ultramafic bodies (UMB) are hosted within this intermediate phase, and these are crosscut by
abundant, texturally variable felsic to mafic dykes. New U-Pb dates indicate that the dioritegabbronorite phase crystallised at ~470 Ma, whereas the tonalite-granodiorite phase (coined the
Main Felsic Mass or MFM), UMB and associated mafic dykes crystallised at ~430 Ma. As a result,
the older dioritic-**bronorite phase has been informally reclassified as the Phantom Diorite,
whereas the 430 Ma phases remain a part of the Dido Suite.
(2) The UMB are composed of layered, cumulate sequences and represent open-system intrusions
emplaced at shallow- to mid-crustal levels (15 – 25 km). They are divided into two
petrographically and geochemically distinct varieties, a low-Fe UMB and a high-Fe UMB. The
three low-Fe UMB comprise dunites, wehrlites, troctolites and olivine **bros which contain
variable amounts of olivine (Fo85 – 72), clinopyroxene (Mg# 0.87 – 0.73), plagioclase (An92-72) and
Cr-spinel. The high-Fe UMB consists of dunites, wehrlites and pyroxenites which lack Cr-spinel
but 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 lowFe UMB displays lower 87Sr/86Sr(430) (0.703836 – 0.705318) and more positive εNd(430) (-0.9 to
+3.7) values. Based on the systematic geochemical and isotopic trends defined by cumulates of the
individual UMB, they are interpreted as composite intrusions which were formed from multiple
injections of related magmas that may have differentiated from a common mantle-derived parent.
(3) The primary magmas of both the high- and low-Fe UMB are interpreted as mantle-derived arc
rift or back-arc tholeiites. 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.% each of MgO and
FeOt and up to 2% H2O, whereas the high-Fe UMB parent magmas were more evolved, having
higher FeOt (12 – 16 wt.%) and H2O (~4%) and lower MgO (6.2 – 8.2 wt.%), Ni and Cr contents.
Crustal contamination during ascent is suggested to be responsible for the LREE-enriched and Nb-
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266
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 the 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 differentiation trend recorded by the high-Fe UMB is more akin to calc-alkaline
fractionation. The higher volumes and relatively late addition of crustal contaminants to the highFe UMB magmas, i.e. after they had experienced strong Fe-enrichment, is suggested to have
resulted in this apparent shift in LLD.
(4) No Ni-Cu-PGE mineralisation has been found associated with the investigated km-scale
UMB, and geochemical discriminators suggest that the magmas that formed these were chalcophile
element-depleted, having undergone previous S-saturation events and hence significant sulphide
segregation at depth. Although this finding implies that the parent magmas which formed the UMB
were infertile, many economic Ni-Cu-PGE intrusions are interpreted as forming from scavenged
PGE-rich sulphides which were carried to accessible crustal levels by later injections of magma, as
in the case of the Platreef. Therefore, although there is no evidence to suggest that the magmas
which formed any of the km-scale UMB were carrier magmas for large amounts of transported
PGE-rich sulphides, based on the minimal exploration completed to date and the likely presence of
significant volumes of PGE-rich sulphides at depth, the UMB cannot be ruled out as potential
targets for Ni-Cu-PGE deposits.
(5) The Dido MFM is classified as a medium-K calc-alkaline intrusion. It is isotopically similar to
cumulates of the high-Fe UMB [displaying relatively low 87Sr/86Sr(430) (0.706746) and negative
εNd(430) (-4.8)], has a TDM age of ~1390 Ma and contains magmatic zircons with negative εHf(430)
values (-0.9 to -5.9). Based on these geochemical characteristics, and isotopic modelling results, the
MFM is interpreted to have originated from a fractionated mantle-derived melt which assimilated
~12% of 2000 - 2500 Ma igneous protocrust during ascent.
(6) It is difficult to establish the exact relationship between the different phases of the Dido Suite
although it is thought probable that most of the differences in magmatic trends between the
individual phases of the Dido Suite were induced by the physical conditions of magmatic evolution
in the crust, rather than differences in initial magma types: (a) isotopic differences between the
individual UMB and MFM can be explained by variations in the amount of crustal contaminants
assimilated into a common, mantle-derived magma, and the timing of this assimilation; (b)
fractionation of cumulates similar to those comprising the low-Fe would cause the residual
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267
tholeiitic magma to become more enriched in Fe and H2O. Later crustal contamination of such an
evolved, Fe-enriched magma, and fractionation of FeTi oxide-rich cumulates to produce the highFe UMB, may have generated a calc-alkaline LLD, eventually producing magmas with MFM-like
compositions and isotopic signatures akin to the high-Fe UMB. Given that 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, and
the low-Fe and high-Fe UMB are suggested to only represent a very small proportion of the
cumulates produced by mafic magmas involved in the formation of the MFM.
(7) 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, 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: (a) the Phantom Diorite and Dido Suite originated from the
mixing of geochemically similar mantle-derived and crustal components; and (b) that there was
little change in the composition of the lithosphere beneath the Greenvale Province between the
Mid-Ordovician (470 Ma) and Early 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%).
(8) The 470 Ma Phantom Diorite and 430 Ma Dido Suite are isotopically distinct from the other
Pama Province (Silurian) granitoids in the Georgetown Region, having more radiogenic εNd and
younger TDM ages. The majority of the Pama Province granites in the Georgetown Region are
interpreted as originating from the melting of 2000 - 2500 Ma igneous protocrust, with no addition
of mantle-derived magmas. Conversely, the 430 Ma Dido Suite and 470 Ma Phantom Diorite are
interpreted as originating from mantle-derived magmas which assimilated <12% crustal
components, similar to those from which remaining Pama Province granites were derived.
(9) 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 due 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 indicates genesis in a continental arc setting, probably in an arc that
was actively rifting, given the tholeiitic nature of the primary magmas of the Dido UMB (and
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268
possibly the Dido MFM and Phantom Diorite). These findings support the tectonic model by
Henderson et al. (2011) for the Palaeozoic evolution of the Greenvale Province.
(10) The finding that the Palaeozoic rocks along the eastern margin of the Georgetown Region
were probably generated in an arc environment deems these rocks unfavourable for Ni-Cu-PGE
deposits as most of the world-class magmatic sulphide deposits are located in continental riftrelated tectonic environments unrelated to subduction. However, rare examples, such as
Aguablanca (Spain), indicate that feeders or conduits to intrusions emplaced in local extensional
regimes within subduction-related settings 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 ores and the
remaining, unexplored Dido UMB (including the high-Fe varieties), along with any other
contemporaneous mafic-ultramafic intrusions in the surrounding area, should be evaluated on a
case by case basis
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