The Norseman-Wiluna greenstone belt in Western Australia contains extensive Archaean auriferous mineralization. There are at least three stages of gold emplacement; the first stage is represented by stratiform deposits in interflow sediments and 'iron formations'; the second and third stages are hydrothermal veins which postdate isoclinal folding of the greenstone sequences. These hydrothermal ores were deposited in structurally prepared sites, prior to, and after the peak of metamorphism.
At Kalgoorlie, the Fimiston lodes are replacement bodies along faults and shear zones, whereas the Mount Charlotte orebodies comrise stockworks of quartz veins which are mainly restricted to unit 8 of the Golden Mile Dolerite, adjacent to steeply dipping oblique faults. Analysis of the geometry of the Fimiston lodes suggests that their morphology and location were dominantly controlled by existing fluid conduits and hydraulic fracturing associated with high fluid pressures. This conclusion contrasts with the situation at Mount Charlotte, where previous work has shown that hydraulic fracturing is a short term process associated with movement on the oblique faults. Timing relationships for the Kalgoorlie deposits are consistent with their formation during uplift after the peak of metamorphism.
The quartz reefs at Norseman -are restricted to the flatter dipping sections of reverse faults and the gold mineralization is mainly strata-bound. Structural and petrographic data indicate that the reefs were emplaced prior to the peak of metamorphism, but gold distribution and wallrock mineralogy were modified by metamorphism and subsequent deformation events.
There are substantial differences in the nature of wallrock alteration around the Kalgoorlie and Norseman gold deposits. Lateral zonation of the alteration envelope is a feature of all the deposits, whereas overprinting of alteration styles is confined to the Crown and Nararoa reefs at Norseman. Patterns of metasomatism characterized by an increase in the K2O/Na2O wallrock ratio and the introduction of CO2 and S are a conspicuous feature of the Fimston lodes and the Norseman reefs. The alteration zoning of the gold-quartz stockworks resembles porphyry copper zonation in many respects; a central albitic zone is surrounded by an outer zone of sericitic and carbonate alteration.
There is an empirical relationship between the abundance of carbonate and the predominant silicate at the edges of the alteration envelopes. Sericite occurs -in the intensely carbonated assemblages which characterize the Kalgoorlie deposits, whereas, at Norseman, chlorite and actinolite are present in moderately carbonated and carbonate-poor wallrock assemblages, respectively. Carbonate chemistry also reflects the inferred mole fraction of CO2 in the cre fluid rather than the nature of the host rocks.
Silicate and sulphide mineralogy, stable isotopic geothermometry and previous fluid inclusion work indicate that the Norseman and Mount Charlotte orebodies were emplaced at temperatures of 400 ± 50°C and fluid pressures of 1 to 2 kb. The No. 4 lode was deposited at a lower ambient temperature of 250 ±50°C. Alteration assemblages and the isotopic data show that the Charlotte and Fimiston ore fluids were near-neutral and moderately reduced (Mount Charlotte, pH ~5.5, aO2 ~ 10-27 ; Fimiston, pH ~ 6, aO2 ~ 10-37 ). The Norseman sulphide assemblages also indicate fluids at low redox potential. Gold was probably transported as thio complexes and the primary mechanism of deposition was a decrease in the activity of reduced sulphur by formation of sulphidic wall rock assemblages.
The б18O values of the Norseman wallrocks are not significantly different from the country rocks, whereas the Kalgoorlie wallrocks have characteristic heavy б18O values. The distribution of б18O whole-rock values is dependant upon the convective morphology of the hydrothermal systems and accurately defines the zone of economic gold mineralization in the gold-quartz stockworks. Vein quartzes exhibit a narrow and uniform range of б18O values exceeding 10‰. Carbon, sulphur and hydrogen isotopic data suggest that the fluids involved in ore deposition and metamorphism at Norseman and regional metasomatism at Kalgoorlie were derived from the greenstone pile, whereas there was a significant magmatic component in the post-metamorphic ore fluids.
The model presented to account for the gold deposits in the Kalgoorlie-Norseman area is that the economic concentration of gold occurred because of the contemporaneity of a distinctive style of low pressure metamorphism and granitoid plutonism. Ore genesis is intimately related to the evolution of the greenstone belt and reflects the mantle disturbance which initiated mafic-ultramafic vulcanism, polyphase deformation, metamorphism and granitoid emplacement.