Steve J. Barnes

Crustal evolution, intra-cratonic architecture and the metallogeny of an Archaean craton

Abstract The generation of the Earths continental crust modified the composition of the mantle and provided a stable, buoyant reservoir capable of capturing mantle material and ultimately preserving ore deposits. Within the continental crust, lithospheric architecture and associated cratonic margins are a first-order control on camp-scale mineralization. Here we show that the evolving crustal architecture of the Archaean Yilgarn Craton, Western Australia, played a key role in controlling the localization of camp-scale gold, iron and nickel mineralized systems. The age and source characteristics of Archaean lithosphere are heterogeneous in both space and time and are recorded by the varying Nd isotopic signature of crustal rocks. Spatial and temporal variations in isotopic character document the evolution of an intra-cratonic architecture through time, and in doing so map transient lithospheric discontinuities where gold, nickel and iron mineral systems were concentrated. Komatiite-hosted nickel deposits cluster into camps localized within young, juvenile crust at the isotopic margin with older lithosphere; orogenic gold systems are typically localized along major structures within juvenile crust; and banded iron formation (BIF)-hosted iron deposits are localized at the edge of, and within, older lithospheric blocks. Furthermore, this work shows that crustal evolution plays an important role in the development and localization of favourable sources of nickel, gold and iron by controlling the occurrence of thick BIFs, ultramafic lavas and fertile (juvenile) crust, respectively. Fundamentally, this study demonstrates that the lithospheric architecture of a craton can be effectively imaged by isotopic techniques and used to identify regions prospective for camp-scale mineralization.

Geochemistry and tectonic setting of basalts from the Eastern Goldfields Superterrane

A database of more than 650 whole-rock analyses on basaltic rocks from the Eastern Goldfields Superterrane has been compiled from the literature and from public-domain datasets. The data fall into three distinct geochemical categories: a High-Th Siliceous Basalt group, a Low-Th Basalt group and an Intermediate-Th Basalt group. The Low-Th Basalt group shows SiO2 values between 50 and 53 wt%; Al2O3 around 15 wt%; elevated Cr and Ni; MgO mostly between 5.5 and 8 wt%; and flat REE–HFSE patterns with slight depletion in Th and minor positive and negative Nb anomalies. The High-Th Siliceous Basalt group has high SiO2, commonly >54 wt%; MgO between 6 and 9 wt% with higher values reflecting presence of accumulated olivine or pyroxene; low Fe and Ti compared with Low-Th Basalt group; depleted Ni for given Mg#; enriched LREE and Th, combined with strongly negative Nb anomalies and mantle-like Zr/Nb, Nb/Y, Al/Ti and HREE ratios. The Intermediate-Th Basalt group is intermediate between these two end-members in almost all respects. All three groups are represented across the entire Eastern Goldfields Superterrane. The most widespread group, the Low-Th Basalt, is remarkably homogeneous across terranes and domains. It is interpreted as the Archean analogue of plume-head related Large Igneous Province basalts, showing a close match to flood basalts associated with late-stage continental rifting. The High-Th Siliceous Basalt group displays a distinctive geochemical signature, which is evidently unique to Archean greenstone terranes. Derivation by contamination during fractionation of komatiites, probably deep in the crust, and followed by mid-crustal homogenisation in magma chambers, is the most likely hypothesis. Basalts with characteristic island arc signatures such as Nb depletion and low Ni and Cr contents, have not been recorded in the Eastern Goldfields Superterrane. This poses a significant challenge to uniformitarian models, which attempt to explain the evolution of the entire east Yilgarn craton in terms of modern arc accretion tectonics. Distribution of mafic and ultramafic mafic magmatism across the superterrane at ca 2700 Ma can be explained by emplacement of a major driving plume under the ‘lid’ of the Youanmi Craton. The keel of thickened, buoyant lithosphere under the archon diverted the plume head towards the craton margin, where it induced continental rifting. Voluminous eruption of plume-tail komatiite was concentrated and focused through this zone of rifting along the eastern margin of the Youanmi craton in the Kalgoorlie Terrane, while plume-head basalts and less voluminous komatiites were erupted over a much wider area.

Atypical stratiform sulfide-poor platinum-group element mineralisation in the Agnew – Wiluna Belt komatiites, Wiluna, Western Australia

A new style of komatiite-associated sulfide-poor platinum-group element (PGE: Os, Ir, Ru, Rh, Pt, Pd) mineralisation has been identified at Wiluna in the strongly nickel sulfide (NiS) mineralised Agnew – Wiluna Greenstone Belt, Western Australia. The komatiite sequence at Wiluna is ∼200 m thick and comprises a basal pyroxenite layer, a thick ortho-to-mesocumulate-textured peridotite core, which is overlain by rhythmically layered wehrlite, oikocrystic pyroxenite and thick upper gabbroic margins. Pegmatoid and dendritic (harrisitic) domains are common features, whereas spinifex-textured horizons and flow-top breccias are absent. The presence of anomalous PGE-enriched horizons (ΣPt – Pd = 200 – 500 ppb) in the oikocrystic pyroxenite and in the layered melagabbro and gabbronorite horizons directly overlying the wehrlite unit is due to the presence of fine-grained (1 – 10 μm) platinum-group minerals (PGMs). More than 70 PGM grains were identified, and a considerable mineralogical variability was constrained. However, only Pd – Pt-bearing phases were identified, whereas no Ir – Ru-bearing PGMs were found in any of the sections examined. Interestingly, all PGMs are not in paragenetic association with sulfides, and only sulfide-poor/free intervals contain significant PGM concentrations. The whole-rock PGE sequence largely reflects the PGM distribution. It is hypothesised that the Pd – Pt enrichment in the oikocrystic pyroxenite and melagabbros and the overall Ir – Ru depletion in the upper mafic section of the sequence are the result of extensive olivine and chromite crystallisation in the basal ultramafic section. PGE saturation was driven by extensive crystallisation of silicate and oxide phases in a sulfide-undersaturated environment. The crystallisation of clinopyroxene in the oikocrystic pyroxenite horizon may have triggered the formation of Pt – Pd-bearing alloys and arsenides, which were the first PGMs to form. Stratiform sulfide-poor PGE mineralisation at Wiluna is more similar in stratigraphic setting, style and composition to PGE-rich sulfide-poor mineralisation zones in thick differentiated intrusions, rather than to other PGE-enriched zones in komatiite-hosted systems, where PGE enrichment is directly associated with accumulations of magmatic sulfides.

A fundamental dispute: A discussion of "On some fundamentals of igneous petrology" by Bruce D. Marsh, Contributions to Mineralogy and Petrology (2013) 166: 665-690

Marsh (Contrib Miner Petrol 166:665–690, 2013) again claims that crystal-free basalt magmas are unable to differentiate in crustal magma chambers and regards layered intrusions as primarily due to the repeated emplacement of crystal suspensions. He ignores an earlier critique of his unconventional inferences (Latypov, J Petrol 50:1047–1069, 2009) as well as a wealth of petrographic, geochemical and experimental evidence supporting the dominant role of fractional crystallization in the solidification of layered intrusions. Most tellingly, the cryptic variations preserved in the Skaergaard and many other basaltic layered intrusions would require an exceedingly implausible sequence of phenocrystic magmas but are wholly consistent with in situ fractional crystallization. A major flaw in Marsh’s hypothesis is that it dismisses progressive fractional crystallization within any magma chamber and hence prohibits the formation of crystal slurries with phenocrysts and melts that change systematically in composition in any feeder system. This inherent attribute of the hypothesis excludes the formation of layered intrusions anywhere.

Geochemistry of komatiites in the Southern Cross Belt, Youamni Terrane, Western Australia

The Southern Cross belt is located in the Youanmi Terrane of the Yilgarn Craton, directly north of the Forrestania belt, which contains economic komatiite-hosted Ni–Cu mineralisation. The mafic and ultramafic greenstone assemblage in the Southern Cross belt, encompasses a continuum from olivine-rich cumulates through to evolved high MgO komatiitic basalts and gabbros. Inspection of geochemical data shows that komatiites can be divided into a high-Al group with Al2O3/TiO2 17–19, slightly lower than typical Munro-type komatiites, and a low-Al group with Al2O3/TiO2 17–19, intermediate between typical Munro-type and Barberton-type komatiites. Parent magmas for the high-Al and low-Al groups had MgO contents ranging between 10 and 15 wt% and between 15 and 25wt%, respectively. Volcanic facies show a transition from distal thin flow facies in the north, changing to lava lakes and sills further south. Olivine-rich channel or conduit facies assemblages are absent from both suites, suggesting low prospectivity for nickel sulfide mineralisation. The high-Al group displays flat to very weakly LREE enriched patterns with flat HREE that are typical of Munro-type komatiites. The low-Al group is weakly LREE enriched, has distinctly negative sloping REE patterns from La to Lu, and minor HREE depletion relative to MREE. Direct comparison with the Forrestania belt shows that the Southern Cross komatiites overlap with the transitional Munro-type komatiites of upper Central and Takashi belts that represent the unmineralised portion of the Forrestania belt. The predominance of Munro-type komatiites in the Southern Cross belt implies that the Southern Cross komatiites were derived from shallower melting than the Barberton-type komatiites that host mineralisation in Forrestania greenstone belt. Furthermore, the variation between Al/Ti and V/Ti implies that the Southern Cross ultramafic suite may represent the transition from Barberton-type to Munro-type komatiites, and formed close to thelower pressure limit of stability of majorite garnet in the mantle residue. It is suggested that the transitional character of the Southern Cross komatiites may be associated with a fundamental difference in the crustal architecture and craton keel thickness underlying the southeastern Youanmi Craton.