40 Ar- 39 Ar and U-Pb age constraints on the timing of gold mineralization in the Kalgoorlie gold field, Western Australia

Abstract

The 40 Ar- 39 Ar ages of three hydrothermal muscovite samples associated with auriferous gold stockwork systems at the Mount Charlotte deposit in the Kalgoorlie gold field have a weighted mean of 2602 + or - 8 Ma ( 40 Ar- 39 Ar age errors are 2Sigma ). At the nearby Golden Mile deposit 40 Ar- 39 Ar ages from two muscovite samples associated with shear-hosted lode gold mineralization have a weighted mean of 2629 + or - 9 Ma. On the balance of available evidence these ages are interpreted to represent 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 gigantic Golden Mile lode system. This is consistent with geologic relations 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 207 Pb/ 206 Pb zircon age for a postregional metamorphism biotite porphyry intrusion of 2674 + or - 6 Ma (95% confidence limits) provides a minimum estimate for the timing of regional metamorphism of the surrounding greenstone sequence and associated compressive deformation. Hypotheses for the formation of Archcan gold deposits which invoke fluids that are derived from regional metamorphic devolatilization of upper-middle crustal greenstone sequences or from intrusion of spatially related porphyries cannot account 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 of a widespread gold mineralizing event at ca. 2630 to 2640 Ma, supporting models in which crustal-scale hydrothermal circulation at this time is used to explain Archcan gold mineralization within the Yilgarn block. However, such models must now contain the caveat that discrete younger episodes of mineralization, such as that responsible for Mount Charlotte, may also produce significant gold deposits with features similar to typical Archcan gold deposits.