Abstract
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 NS 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°-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.
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