Can the role of thermal erosion in strongly deformed komatiite‐Ni–Cu–(PGE) deposits be determined? Perseverance, Agnew‐Wiluna Belt, Western Australia

Abstract

Perseverance, Agnew‐Wiluna Belt, Western Australia, is the largest stratabound komatiitic dunite‐hosted Ni–Cu–(PGE) deposit on Earth. Perseverance is often reported as containing demonstrable evidence for thermal erosion (e.g. truncation of footwall stratigraphy), an essential component in prevailing ore genesis models. Perseverance is a highly strained and strongly altered deposit. The strong alteration and structural overprint have resulted in a metakomatiite unit with no primary volcanological or magmatic contacts with surrounding rocks. Strain is strongly partitioned around the komatiitic dunitic core and the surrounding rocks are largely mylonitic, annealed mylonitic gneisses or schists. The protolith of the surrounding rocks is difficult to assess because of alteration and strain. Previous work suggested that the surrounding stratigraphy was very complicated. The felsic units were described as felsic lapilli tuffs and were interpreted as subaqueous welded ignimbrites. Our reassessment of the footwall to the Perseverance deposit indicates that the footwall consists of a simple stratigraphy of shallow, but coherent, quartz‐ and feldspar‐phyric porphyries and basalt intrusions, which intrude a pelitic sedimentary sequence. Fragmental or pyroclastic units have not been observed nor can they be inferred. The complex stratigraphy described by previous workers and apparent from the present day metamorphic mineral assemblages is a pseudo‐stratigraphy, which is largely a function of deformation and metasomatism. The komatiitic dunitic unit of the Perseverance Ultramafic Complex appears to truncate the basal stratigraphy, in agreement with previous workers. However this basal pseudo‐stratigraphy is itself a function of strain partitioning around the isotropic dunitic unit. In such an environment, any thermal erosional descriptions are highly interpretative. We strongly caution against recent detailed mathematical modelling based on volcanological measurements from these highly altered and strained units.