Structural evolution of the orogenic gold deposits in central Victoria, Australia: The role of regional stress change and the tectonic regime

摘要

This review synthesizes the main characteristics of the orogenic gold deposits in western and central Victoria, Australia, and presents new insights into the relationships between orogen-scale deformation and the 12 hard-rock gold deposits that have each produced>30 t Au. We examine five of these large deposits, Stawell, Ballarat, Bendigo, Fosterville and Costerfield. From the start of the Delamerian Orogeny at ca. 515 Ma to the end of the Tabberabberan Orogeny at ca. 370 Ma, the eastern continental margin of Gondwanan Australia experienced both accretionary growth and the development of the Lachlan Orocline. Central western Victoria lies on the western edge of this oroclinal structure, and is partly underlain by a Proterozoic continental fragment, the Selwyn Block. All of the>30 t orogenic gold deposits in the region lie either on or to the west of the Selwyn Block. From the Delamerian Orogeny onwards, the deformation front progressively moved eastwards and the principal stress direction rotated clockwise from NE-SW through E-W and ultimately to N-S, so that the initial faulting and folding created pathways and traps for the subsequent mineralizing fluids through the brittle to brittle-ductile crust. Thus, the 455-440 Ma Benambran Orogeny was the major mineralizing event at Stawell and resulted in some early mineralization in the Bendigo Zone. The 420-410 Ma Bindian Orogeny gave rise to significant additional mineralization at Ballarat, Bendigo and Fosterville, while the 385-370 Ma Tabberabberan Orogeny drove mineralization at Costerfield and a second, bonanza event at Fosterville. The largest gold deposits formed where the fluid pathways either remained open or reopened in this rotating stress field, allowing multiple pulses of gold-rich fluids.