Timing of gold mineralisation in western and central Victoria, Australia: New constraints from SHRIMP II analysis of zircon grains from felsic intrusive rocks

Age constraints provided by felsic intrusive rocks indicate that gold mineralisation in western and central Victoria, Australia occurred over a prolonged period of time ranging from prior to the Late Silurian to the Late Devonian. An early phase of gold mineralisation associated with regional deformation pre-dated the emplacement of Late Silurian felsic porphyry dykes at 413 +/- 3 Ma in the central lode system of the Magdala deposit, Stawell. Gold mineralisation associated with the final stages of the main regional deformation also pre-dated the intrusion of Early (ca. 400 Ma) to Late Devonian (ca. 370 Ma) plutons at Stawell (Wonga deposit), Mt. Bute, Tarnagulla, Tallangalook, Maldon and Heathcote in both the Stawell, Pyrenees and Ballarat metallogenic provinces, consistent with preliminary dating studies of hydrothermal micas from major gold deposits. Later phases of gold mineralisation throughout western and central Victoria were either temporally associated with, or post-dated the intrusion of Late Silurian to Late Devonian felsic dykes. Examples of this style of mineralisation include the Wonga deposit at Stawell, Mafeking, Fosterville, Maryborough, minor occurrences near Linton and the Woods Point region. Preliminary constraints from Ballarat West indicate that Late Devonian dykes intruded at ca. 386 Ma were pervasively altered by interaction with an auriferous fluid similar to that responsible for gold deposition within the Guiding Star Lode, but that this was probably related to remobilisation during emplacement of the dykes into a pre-existing mineralised structure. With the exception of the Woods Point region and Ballarat West, these later phases often have a magmatic affiliation, indicated by an association with Cu, Mo, Bi, Sb and W. These complex timing relationships reflect the diachronous nature of deformation, metamorphism and magmatism in the western Lachlan Fold Belt of southeast Australia and suggest at least two discrete periods of gold mineralisation in the region. (C) 1998 Elsevier Science B.V. All rights reserved.