The “porphyry-gold” association in the Norseman-Wiluna Belt of Western Australia: implications for models of Archaean gold metallogeny

Abstract

Abstract Porphyritic minor intrusions (felsic/intermediate porphyries and lamprophyres) occur in the vicinity of many gold deposits in the Archaean Norseman-Wiluna Belt of Western Australia, but are not restricted to this environment. Unlike the Abitibi Belt of Canada, there is no correlation between the presence of such intrusions and the economic status of a deposit. The timing of intrusive activity with respect to spatially associated mineralization is variable, with mainly instances of pre- (sometimes > 10 Ma earlier) and some examples of post-ore emplacement being recorded. In many cases, the “porphyry-gold” association reflects the importance of structural control to the localization of mineralization. Intrusions may exploit the same zones of crustal weakness as the ore-fluids, although such a relationship need not imply contemporaneity, or may act as structural traps for mineralization owing to rheological contrast between intrusion and country-rock. The composition and petrogenesis of the minor intrusions are both variable and there is no evidence of a specific suite of intrusions preferentially associated with mineralization. Spatial, temporal and petrogenetic considerations preclude direct evolution of the gold-bearing fluids from the minor intrusions and militate against magmatic theories of ore genesis. Rather, both the igneous activity and the mineralization are viewed as expressions of a deep-seated tectono-thermal event that affected the Yilgarn Block in the late Archaean.