Four elongate, km-scale, mafic to ultramafic bodies (UMB) were identified within the Ordovician-Silurian Dido Batholith, north Queensland, and were assessed for Ni-Cu-PGE fertility. The UMB comprise layered cumulate sequences and represent open-system intrusions emplaced at mid-crustal levels. The UMB are divided into two petrographically and geochemically distinct types: (1) 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) high-Fe UMB (1 intrusion), comprising dunites, wehrlites and pyroxenites which lack chromites, contain abundant early crystallising Fe-Ti oxides and hornblende, and less primitive olivines (Fo78-72) and pyroxenes (Mg# 0.87-0.73). The calculated parent magmas of the low-Fe UMB contained 8-10wt% of both MgO and FeOt, whereas the high-Fe UMB parent magmas were more evolved, having higher FeOt (12-16wt%) and lower MgO (6.2-8.2wt%), Ni and Cr concentrations. The parent magmas of the Dido UMB are interpreted as mantle-derived arc rift or backarc tholeiites, and mineral compositions suggest the UMB are similar to arc-cumulate, Klamath-type intrusions. Crustal contamination during ascent is suggested to be responsible for the LREE-enriched and Nb- and Ti-depleted nature of the UMB parent magmas. A two-component Sr-Nd isotope mixing model suggests that the addition of variable amounts (<5% in the low-Fe UMB and 9-10% in the high-Fe UMB) of 2000-2500Ma igneous crustal contaminant to tholeiitic melts derived from a slightly enriched mantle source can account for isotopic compositions of the UMB. Although the mantle-derived, crustally contaminated nature of the parent magmas are positive factors for magmatic sulphide mineralisation, economic mineralisation has not been found associated with the UMB. Geochemical discriminators, such as Cu/Pd and Ni-depletion, suggest that the magmas that formed the km-scale UMB were chalcophile element-depleted, having undergone a previous S-saturation event (i.e., significant sulphide-deposition) at depth. The slight PGE-enrichment in the high-Fe cumulates (up to 160ppb of both Pt and Pd) is suggested to be due to the addition of small amounts of PGE to the high-Fe magmas from an external source. There is no evidence to suggest that large volumes of PGE-enriched sulphides were added to the UMB magmas. These findings, in addition to the knowledge that most large deposits are associated with intracontinental settings rather than convergent settings, determined that the investigated Dido UMB are low-priority Ni-Cu-PGE targets. However, the likelihood that the Dido UMB were emplaced in local extensional regimes within subduction-related environments, i.e., a similar geodynamic setting to economic deposits such as Aguablanca (Spain), requires that other UMB in the Dido area are assessed on an individual basis.
- Alaskan/Ural and Klamath intrusions
- Convergent margin
- Mafic-ultramafic cumulates
- Magmatic sulphides
- Ni-Cu-PGE exploration