AR-40-AR-39 AND U-PB AGE CONSTRAINTS ON THE TIMING OF GOLD MINERALIZATION IN THE KALGOORLIE GOLD FIELD, WESTERN-AUSTRALIA

The Ar-40-Ar-39 ages of three hydrothermal muscovite samples associate d with auriferous gold stockwork systems at the Mount Charlotte deposi t in the Kalgoorlie gold field have a weighted mean of 2602 +/- 8 Ma ( Ar-40-Ar-39 age errors are 2 sigma). At the nearby Golden Mile deposit Ar-40-Ar-39 ages from two muscovite samples associated with shear-hos ted lode gold mineralization have a weighted mean of 2629 +/- 9 Ma. On the balance of available evidence these ages are interpreted to repre sent the timing of gold mineralization in the two deposits. These data indicate that formation of auriferous gold stockwork systems at Mount Charlotte occurred at least 10 million years after formation of the g igantic Golden Mile lode system. This is consistent with geologic rela tions between the two different styles of mineralization and suggests that separate hydrothermal episodes were responsible for stockwork and shear-hosted gold mineralization. A weighted mean Pb-207/Pb-206 zirco n age for a postregional metamorphism biotite porphyry intrusion of 26 74 +/- 6 Ma (95% confidence limits) provides a minimum estimate for th e timing of regional metamorphism of the surrounding greenstone sequen ce and associated compressive deformation. Hypotheses for the formatio n of Archean gold deposits which invoke fluids that are derived from r egional metamorphic devolatilization of upper-middle crustal greenston e sequences or from intrusion of spatially related porphyries cannot a ccount for the gold mineralization in the Kalgoorlie gold field. These data indicate that the events occurred 30 to 40 and 60 to 70 million years prior to mineralization at the Golden Mile and Mount Charlotte, respectively. Formation of the Golden Mile appears to have been part o f a widespread gold mineralizing event at ca. 2630 to 2640 Ma, support ing models in which crustal-scale hydrothermal circulation at this tim e is used to explain Archean gold mineralization within the Yilgarn bl ock. However, such models must now contain the. caveat that discrete y ounger episodes of mineralization, such as that responsible for Mount Charlotte, may also produce significant gold deposits with features si milar to typical Archean gold deposits.