The Invincible deposit: Early gold mineralisation truncated by unaltered c. 2665 Ma conglomerate, St Ives, Eastern Goldfields, Western Australia

Abstract

Two distinct styles of mineralisation are recognised at the Invincible deposit, with bedding-parallel shear related gold mineralisation (Type 1) and shallow-dipping mineralised extension veins (Type 2). The different styles of mineralisation reflect formation during two discrete deformation events.Type 1 mineralisation is hosted in laminated mudstone of the upper Black Flag Group and mineralised quartz veins and breccia zones are distributed throughout the mudstone rather than being associated with a single shear. Mineralised zones are associated with intense sodic alteration. The early mineralised structures are strongly modified by east-west horizontal D2-3 compression. In contrast, Type 2 extension veins are undeformed and are best developed in the massive sandstone-conglomerate footwall, but also overprint Type 1 veins in the mudstone unit.Throughout the mine, the early Type 1 mineralisation is truncated by unaltered conglomerate of the c. 2665 Ma Merougil Formation. A 10–20° difference in strike/dip is commonly present and indicates that the main mineralising event at Invincible occurred prior to erosion and deposition of the Merougil Formation. The strong modification of the mineralisation by horizontal east-west D2-3 compression is also consistent with formation early in the deformation history, during the c. 2672–2660 Ma D1 event. Gold deposition in the EGT was episodic, occurring over a prolonged period during extension and compression.Bedding-vein relationships indicate southwest-side-down kinematics during the early mineralising event. In weakly mineralised zones sub-vertical extension veins cut across beds that dip 70° to the southwest. In strongly mineralised zones, bedding is sub-vertical and large quartz breccia veins develop along the bedding above moderately west-dipping shears that extend across the steep bedding. The location of the steep strongly mineralised zones where bedding is sub-vertical reflects enhanced dilation in these zones during southwest-side-down movement.Southwest-side-down movement could represent extensional slip on the laminated mudstone beds if the bedding was tilted into a developing basin (early Merougil Basin). The location of the Invincible system, at the eastern edge of the Merougil Basin likely reflects an association with early basin-controlling faults.