The Structural Setting and Controls of Giant Lithium Pegmatite Deposits in the Archean Pilbara Craton, Western Australia

Abstract

The giant lithium pegmatite deposits of the Archean Pilbara craton in Western Australia are located within the Turner River district, where they are associated with D4 structures and granitic intrusions of the Split Rock Supersuite, all of which are an expression of the final phase of orogenic activity in the granite-greenstone nucleus of the craton between 2.89 and 2.82 Ga. Progression of the D4 orogenic event involved early D4a wrench faulting, followed by emplacement of the granitic intrusions, and finally onset of D4b ductile shear zone deformation and the formation of the lithium pegmatite deposits. The giant deposits at Pilgangoora and Wodgina comprise vertically stacked sets of lithium pegmatite intrusions that were emplaced episodically during ductile fracturing, at lower amphibolite facies, within strained supracrustal wall rocks in D4b shear zones. Closely spaced D4a oblique faults are a feature of the deposit settings and, although they are an expression of earlier brittle deformation, an important role is recognized for these structures as passive attractors of coaxial strain during D4b shear zone deformation. The coaxial strain manifested as constriction and vertical extrusion in the deposit settings, and this is unique within the broader D4b shear zone array, which mostly expresses noncoaxial strain in the form of oblique extrusion. Together, the partitions of vertical and oblique extrusion are considered the product of regional transpression. Migration of low-viscosity melts that formed the lithium pegmatite intrusions was clearly favored within the partitions of D4b coaxial strain. This is attributed to the faster rates at which coaxial strains accumulate, which enhanced the potential for melt migration and intrusion emplacement, most likely through a combination of intergranular percolation and ductile fracturing. The convergence of D4a faults and D4b shear zones, as well as the association of such with lithium pegmatite intrusions, is an important criterium for exploration targeting. The structural disruption arising from D4a faults is perhaps the most distinctive feature expressed in satellite and airborne magnetic images, and therefore mapping, sampling, and/or drilling should focus within regions of overlap between shear zones and fault disruption.