CONSTRAINTS ON THE TIMING OF LODE-GOLD MINERALIZATION IN THE WILUNA GREENSTONE-BELT, YILGARN CRATON, WESTERN-AUSTRALIA

Ar-40-Ar-39 ages of hydrothermal muscovites constrain the age of gold mineralisation in the Wiluna greenstone belt, Western Australia. A sin gle muscovite sample from the Matilda M1 deposit, hosted in the,greens chist-amphibolite facies Matilda domain, has a Ar-40-Ar-39 plateau-lik e segment corresponding to an age of 2623 +/- 12 Ma (2 sigma). This is interpreted as the probable age of mineralisation and is similar to r adiometric ages of mineralisation from other parts of the craton. Two muscovite samples from an altered microdiorite dyke from the East Lode deposit, located within the prehnite-pumpellyite facies Wiluna domain have Ar-40-Ar-39 plateau-like segment ages of 2565 +/- 12 Ma and 2563 +/- 12 Ma. A third sample, consisting of muscovite from an altered hi gh-MgO basalt, has an enigmatic spectrum with no obvious plateau-like segment and the highest temperature steps have apparent ages of ca 260 0 Ma. Two alternative interpretations are considered for the data from the East Lode deposit: (i) mineralisation in the Wiluna and Matilda d omains occurred during a craton-wide gold-mineralisation event at ca 2 630 Ma and the younger Ar-40-Ar-39 ages for muscovites from the Wiluna East Lode deposit have been isotopically reset; and (ii) mineralisati on at the East Lode deposit occurred significantly after that at Matil da and elsewhere in the Yilgarn Craton, and represents a discrete hydr othermal episode that occurred some 90 million years after the ca 2630 Ma mineralising event. If the latter is the case then this is further evidence that discrete episodes of gold mineralisation occurred withi n the Yilgarn Craton after the widespread ca 2630 Ma event. A 2749 +/- 7 Ma (95% confidence limits) U-Pb zircon date from a pre-mineralisati on porphyritic microdiorite dyke in the Wiluna domain is a minimum age for the formation of the komatiite-bearing stratigraphic sequence and maximum age for subsequent gold mineralisation in the Wiluna domain.