CHRONOLOGY AND TECTONIC FRAMEWORK OF TURBIDITE-HOSTED GOLD DEPOSITS IN THE WESTERN LACHLAN FOLD BELT, VICTORIA - AR-40-AR-39 RESULTS

Ar-40-Ar-39 data constrain the history of metamorphism, deformation an d mineralization in the western subprovince of the Lachlan Fold Belt. Rocks in the Mount Stavely Volcanic Complex and the Bushy Creek pluton give Ar-40-Ar-39 ages of about 500 Ma for hornblende and biotite. The dates from the Mount Stavely Volcanic Complex limit the age of minera lization in the metavolcanic rocks of the Stavely Belt. Whereas, the d ate for the Bushy Creek pluton extends the area of Cambrian-Ordovician plutons, of the Delamerian Orogeny, to east of the Grampians Mountain s and helps define the surface expression of the western border of the Paleozoic Lachlan Fold Belt. Ar-40-Ar-39 dating of metamorphic mica g rowth (phengite) in cleaved slates and phyllites within the Landsborou gh Fault, Avoca Fault zone, Heathcote Fault zone and Mount Wellington Fault zone gives ages of 453 +/- 2, 440 +/- 2, 426 +/- 4 and 410-390 M a, respectively. These data indicate a progression of deformation from west to east starting in Late Ordovician-Early Silurian time in the S tawell and Bendigo-Ballarat structural zones. The major phase of defor mation in the western subprovince of the Lachlan Fold Belt is therefor e Silurian and not Devonian. Sericites from major gold deposits in the Stawell zone and Bendigo-Ballarat zone give Ar-40-Ar-39 dates of abou t 440 Ma, that are coincident with folding and thrusting. A second epi sode of mineralization and fault reactivation at about 420 to 410 Ma i s found in some deposits in these zones, based on data from a previous study. In the Melbourne zone (including the Woods Point Dike Swarm) a nd locally in the Bendigo-Ballarat zone, some gold deposits give seric ite dates of about 380-360 Ma. These ages are consistent with the clos e association of many Melbourne zone gold deposits with Devonian dikes and plutons. The results indicate that gold mineralization in the Sta well zone, Bendigo-Ballarat zone and Melbourne zone was episodic and b oth temporally and spatially associated with crustal heating linked to regional metamorphism and plutonism. Initial stages of mineralization in the province, comprising many of the large deposits in western and central Victoria (e.g. Bendigo, Ballarat, Stawell, Wattle Gully, Tarn agulla), accompanied metamorphism, folding and thrusting. Subsequent m ineralization is widespread but produced mainly smaller deposits (< 10 ,000 kg, with the exception of Walhalla-Woods Point) and was associate d with the thermal events accompanying Devonian dike and pluton intrus ion and deformation. (C) 1998 Elsevier Science B.V. All rights reserve d.