Wallrock alteration associated with turbidite-hosted gold deposits. Examples from the Palaeozoic Lachlan Fold Belt in central Victoria, Australia

The study of wallrock alteration phenomena around slate belt-hosted gold deposits can provide valuable information regarding the physico-chemical conditions of the ore-bearing fluids and the timing relationships between deformation, peak metamorphism, granitoid emplacement and mineralisation. Thus, studying the processes involved in the interaction of the hydrothermal fluids with the host rock is critical in order to constrain genetic models and, ultimately, as a tool for exploration. Largely due to the characteristics of the host rocks, turbidite-hosted mesothermal gold deposits were historically believed to lack such diagnostic alteration haloes and their study has long been neglected. However, recent studies in the Palaeozoic Lachlan Fold Belt in southeastern Australia and in similar tectonic settings elsewhere have demonstrated that alteration around many of these deposits is more widespread and pervasive than previously thought. A review of existing information and new data strongly support the viability of studying hydrothermal alteration as a guide to turbidite-hosted mineralisation and as an indicator of the character of the solutions associated with ore deposition. Characteristically, the auriferous reef structures are surrounded by broad bleached zones, while disseminations of carbonate spots, pyrite and arsenopyrite porphyroblasts are the most obvious features of alteration. Depending on parameters such as fault geometry, fluid pressure, and reactivity and permeability of the host rocks, these disseminations can occur up to a kilometre away from the auriferous structures. Sericitisation and carbonatisation are commonly discernible within 3- to 10-m-wide haloes. Wallrock alteration chemistry of turbidite-hosted gold deposits in the Lachlan Fold Belt shows consistent enrichments of K(2)O, CO(2), S and As, and the anomalous values of these elements also provide the most useful dispersion haloes for turbidite-hosted gold exploration due to both their regularity and extent. (C) 1998 Elsevier Science B.V. All rights reserved.