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

Abstract

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.