STRUCTURAL CONTROLS ON THE BASE-METAL VEIN DEPOSITS OF THE NORTHAMPTON COMPLEX, WESTERN-AUSTRALIA

The Northampton Complex is a Proterozoic inlier near the west coast of Western Australia, 450 km north of Perth. The copper, lead and zinc vein-type deposits occupy dilational sites along brittle-ductile shear zones. There are four types of structural settings of the ore deposits: (1) lens structures, (2) shear-breccia link-shear structures (extensional strike-slip duplexes), (3) intersecting brittle-ductile shears, and (4) curved brittle-ductile shears. A model involving N-S dextral wrenching along the Darling Mobile Belt best explains the various features of the deposits. In this model, Proterozoic granulite-facies paragneisses of the Northampton Complex were fractured with the development of predominantly extensional fractures and normal faults and, to a lesser extent, Riedel (both R and R'), restraint and principal displacement shears. Since previous isotopic dating has not resolved the age of mineralisation, crosscutting relationships between mineralisation, the older dolerite dykes (650-800 Ma) and younger 330-degrees-trending sinistral shears (500-650 Ma) are used to constrain the timing to between 650 Ma and 800 Ma. NE-SW shortening across the Paterson and King Leopold Mobile Belts, between 650 Ma and 800 Ma, may also have been associated with the dextral movements along the Darling Mobile Belt.