Durney, David W1, Hood, David I A2
1Earth & Environmental Sciences, Macquarie University, Australia, 2R&D Dept., ARDEX Australia, Sydney, Australia
Different explanations have long been given for the formation of base-metal massive sulphide mineral deposits in the Lachlan Orogen of New South Wales. (1) One holds that combined syngenetic and epigenetic mineralisation is associated with either sedimentation (SedEx) or volcanism (VMS) during basin formation. (2) An alternative, sometimes labelled “epigenetic”, suggests that the mineralisation forms syn-deformationally from fluids released by orogenic regional metamorphism (Metamorphogenic) during thrusting and basin inversion. In the first, the tectonic environment is generally considered extensional while, in the second, it is necessarily convergent. Examples where these explanations have been suggested include the Cobar, Woodlawn and Clarke’s Reef sulphide deposits. The outcome has been either a lack of consensus or a tendency for Metamorphogenesis to be a partial or dominant paradigm.
New observations are reported which throw further light on the origin of the Clarke’s Reef Pb-Zn-Cu deposit and associated occurrences of massive sulphides in the Silurian Quidong Basin of far south-eastern New South Wales. These are part of a broader series of conclusions drawn from field observations of structures in the Quidong Basin by Hood and co-workers at this Conference, highlighted here as an example of how structural data may assist in resolving questions of this kind.
As the Quidong sulphides occur partly in pyritic sediments at the unconformable base of the Basin and partly on faults and veins which extend into the overlying limestones and mudstones, a key question is the nature of the faulting and whether it pre-dates, syn-dates or post-dates the folds. In agreement with earlier unpublished work, the observed faults within the Basin reported by Hood and co-workers are all normal, with or without a wrench component. This places them either before or after the convergent fold deformation which also affected the Basin. A newly recognised structure called incoherent fault-related minor folds, where subhorizontal thinly layered rocks show localised buckling against steep faults, provides, for the first time, clear structural evidence that the faulting occurred before folding in this area.
The observations therefore support a combined syngenetic–epigenetic origin for the Clarke’s Reef deposits, contemporaneous with extension during basin formation.
David Durney began geology at London, has taught structural geology and mapping, and maintains an interest in ore deposits.
David Hood obtained Hons in Earth Sciences at Macquarie University (1996), specialising in multiple deformations. Employment includes James Hardie and ARDEX Australia as an Industrial Chemist.