Criteria for the recognition of metamorphosed or altered lamprophyres: A case study from the Archaean of Kambalda, Western Australia

The ability to distinguish ancient lamprophyres from other mafic and ultramafic intrusive rock types has become increasingly important. Lamprophyres have been used as marker horizons in the mapping of several Precambrian terranes, they have been recognized for decades as possible indicators of gold mineralization, and their presence has implications for the tectonic interpretation of ancient greenstone belts. Whilst fresh, undeformed lamprophyres are readily recognized, metamorphosed, altered and/or deformed examples are more problematical. The intrusive ages of Kambalda lamprophyres are constrained by available field and geochronological evidence, including direct dating of one kersantite, to the period 2684-2660 Ma. These rocks therefore belong to the world's oldest group of alkaline rocks and lamprophyres so far recognized, which also includes contemporaneous examples from Canada, Zimbabwe and, probably, Tanzania. Despite their extreme age and low- to medium-grade regional metamorphic history, these rocks still preserve characteristic lamprophyric textures, chemistry and mineralogy. Only where metamorphism has been overprinted by gold-related alteration and deformation do the lamprophyres cease to be recognizable petrographically. However, their unique chemistry persists and, despite losses of the most mobile elements from presumed magmatic compositions, altered and deformed specimens still retain far higher contents of Sr, K, Ba, Th, Nb, P, Zr and Ce than basalts of the local greenstone succession. Even lamprophyres which have been converted to biotite-plagioclase-carbonate-pyrite schists retain this distinctive chemical signature. During both alteration and metamorphism, ‘mobile’ elements (K, Rb, Sr, Ba) display far larger changes than archetypal ‘immobile’ elements (Ce, Nb, Zr, P, Ti), clearly sanctioning traditional preference for the latter in assessing altered rocks. However, the surprising loss of Nb during biotitization of hornblende indicates that negative Nb anomalies, conventionally regarded as indicators of subduction-related magmatism, can also be induced purely by alteration processes.