Reconstruction of an extensive Archaean dacitic submarine volcanic complex associated with the komatiite-hosted Mt Keith nickel deposit, Agnew-Wiluna Greenstone Belt, Yilgarn Craton, Western Australia

Abstract

Felsic volcanic rock units characterise a large portion of the 2.7 Ga. Agnew-Wiluna Greenstone Belt (Western Australia) stratigraphy and form the hangingwall and footwall sequences to several komatiite-hosted nickel sulphide deposits in the region. The Mt Keith region of the Agnew-Wiluna Greenstone Belt (AWB) comprises a sequence of felsic, mafic and ultramafic extrusive and intrusive units. The felsic volcanic dominated footwall and hangingwall sequences to the Mt Keith nickel deposit comprise cyclical packages of coherent dacite lavas, with associated monomictic in situ to resedimented breccias. Clasts in the monomictic breccia facies are identical to the coherent dacite and the two facies are often in gradational contact. The dacites are interpreted as submarine lavas and the breccias as associated autoclastic facies (hyaloclastite, autobreccia). These dacite lavas are inferred to have been emplaced into a deep water environment which retarded volatile exsolution suppressing explosive fragmentation and increasing lava mobility, resulting in large tabular flow units. The AWB sequence has undergone multiple periods of deformation and metamorphism subsequent to emplacement resulting in strain and alteration in large sections of the coherent lava facies. A notable textural variant within the lavas is a set of pseudo-textures/structures resembling tuffaceous and brecciated dacitic units. The process of pseudo-clastic texture/structure generation is related to a combination of polyphase alteration, brecciation and development of a secondary structural/metamorphic layering. Understanding the origin of these pseudo-clastic textures/structures has allowed recognition of the lateral and vertical facies variation of the felsic sequence and reconstruction of the original volcanic architecture.