THE NATURE AND DIMENSIONS OF REGIONAL AND LOCAL GOLD-RELATED HYDROTHERMAL ALTERATION IN THOLEIITIC METABASALTS IN THE NORSEMAN GOLDFIELDS -THE MISSING LINK IN A CRUSTAL CONTINUUM OF GOLD DEPOSITS

The Archaean lode-gold deposits at Norseman, Western Australia, consis t of auriferous quartz veins in dextral-reverse ductile-brittle shear zones within tholeiitic metabasalts of upper-greenschist to amphibolit e facies metamorphic grade. Three types of deposits (Northern, Central , Southern) are delineated on the basis of their spatial distribution, veining style, alteration mineralogy and metamorphic grade of host ro cks. Northern deposits, hosted in upper-greenschist to lower-amphiboli te facies rocks, comprise massive to laminated quartz veins with selve dges of quartz-chlorite-calcite-biotite-plagioclase assemblages. Centr al deposits, hosted in lower-amphibolite facies rocks, consist of lami nated to massive quartz veins with selvedges of quartz-actinolite-biot ite-plagioclase-calcite assemblages. Southern deposits, hosted in midd le-amphibolite facies metabasalts, consist of banded quartz-diopside-c alcite-microcline-zoisite veins. All deposits exhibit variable ductile deformation of veins and contiguous alteration haloes, consistent wit h a syn-deformational genesis at high temperatures. From Northern to S outhern deposits, the alteration assemblages are indicative of higher temperatures of formation, and there are progressively greater degrees of dynamically recovered textures in alteration and gangue minerals. These observations imply that a thermal variation of gold-related hydr othermal alteration exists within the Norseman Terrane over a distance of 40 km, with T(Northern) < T(Central) < T(Southern). This thermal z onation is corroborated by T-X(CO2) phase relations between vein selve dge assemblages, which signify formation temperatures of approximately 420-degrees-475-degrees-C, 470-degrees-495-degrees-C and > 500-degree s-C for Northern, Central and Southern deposits, respectively. The sum of structural, petrographic and mineral chemistry data indicates that the alteration assemblages formed in high-temperature, open hydrother mal systems and have not been subsequently metamorphosed. The thermal differences between the deposit groups may reflect (1) a temperature g radient, at relatively constant P, corresponding to the proximity of t he deposits to regional granitoid complexes, or (2) formation of the d eposits at progressively deeper crustal levels from north to south. In either case the deposits represent a continuum of gold deposition fro m upper-greenschist to amphibolite facies, now exposed in an oblique s ection through the Archaean crust at Norseman.