Contrasting structural styles of gold deposits in the Leonora Domain: evidence for early gold deposition, Eastern Goldfields, Western Australia

Abstract

The Leonora Domain contains seven significant gold deposits that display markedly different structural styles. Deposits such as Gwalia, Tower Hill and Harbour Lights are characterised by strongly folded and boudinaged veins. In contrast, the Tarmoola/King of the Hills deposit comprises gold lodes that have not experienced ductile conditions. The complex geometry of many of the deposits here has led to multiple interpretations for the timing and nature of gold deposition. The Leonora Domain, located along the margin of the Raeside Batholith 250 km north of Kalgoorlie, is dominated by a strong extensional fabric that wraps around the edge of the batholith and formed during D1 extension and exhumation of the granite body. In all localities the extensional fabric (S1b) is overprinted by upright folds and shears produced by the onset of east–west compression (D2 event) and sinistral transpression (D3 event). Subsequent east-northeast−west-southwest D4 shortening produced abundant dextral shears that overprinted and reactivated the D3 structures. Late orogenic collapse during the D5 event produced abundant steep planar normal faults.New structural observations presented here provide evidence that deposits such as Gwalia, Tower Hill and Harbour Lights formed at a very early stage of D1 extension, prior to the main exhumation event (D1b). In contrast, gold lodes at Tarmoola/King of the Hills are related to sinistral-reverse shears that developed in the deformed carapace of a large granite body during strong east–west compression (D3 event). The lodes have formed as a result of strong competency contrast between the granite and the surrounding ultramafic rocks and are more typical of gold deposits elsewhere in the Eastern Goldfields. The spatial distribution of the major early deposits in the Leonora Domain indicates that the first-order control on gold distribution is the major shear along the contact of the Raeside Batholith. The regular spacing of the deposits around the Raeside Batholith likely reflects the rheology and thickness of the extending sequence. Gwalia, Tower Hill and Harbour Lights deposits are located at an apparent bulge in the eastern side of the Raeside Batholith, which may have originally been the highest part of the batholith and, with the onset of extension, may have focused structures and rising hydrothermal fluids into this area.