Thrusts and thrust-associated mineralization in the Lachlan Orogen

Abstract

The Lachlan orogen is characterized by a thin-skinned deformational style, with major thrusts and strike- slip faults soling into detachments in the upper and middle crust. Thrusts are divided into three types: type 1 thrusts that formed in apparently unconstrained sediment packages such as Early Ordovician turbidites; type 2 thrusts formed during inversion of mid-Silurian to mid-Devonian constrained sedimentary + or - volcanic basins; and type 3 major thrusts that bound the edges of rare upthrusted middle crustal complexes. All three types of thrusts are associated with mineral deposits. Mineralized type 1 thrusts occur in the gold fields of central Victoria. Mineralized type 2 thrusts occur both on the edges of deformed sedimentary and sedimentary + or - volcanic basins (e.g., in synrift packages at Cobar and Captains Flat, respectively) as well as within deformed sedimentary and sedimentary + or - volcanic basins (e.g., at Quidong and Hill End, and in synrift rocks at Woodlawn, respectively). Base metal deposits in synrift rocks at Benambra, on the edge of the deformed sedimentary + or - volcanic Cowombat rift, are the best examples of mineralization associated with a type 3 thrust. The spatial and geometrical associations between deposits and thrusts in these and other examples lead to the recognition of a class of thrust-associated mineral deposits which constitute a major resource within the Lachlan orogen. Thrust-associated deposits include deposits localized on thrusts, deposits above or below thrusts, and deposits in anticlines above inferred blind thrusts. Some thrust-associated deposits are syntectonic, with mineralization genetically related to thrusting and deformation. Such deposits are thrust related, and formed during (and after) deformation. Other thrust-associated deposits predate deformation and are called thrust-modified deposits. The orebody and wall-rock contacts of these deposits have been modified by.