Cockade breccia: Product of mineralisation along dilational faults

Abstract

Cockade breccias are fault fills in which individual clasts are completely surrounded by concentric layers of cement. They occur particularly in low-temperature near-surface hydrothermal veins. At least six mechanisms have been proposed for the formation of cockade breccia-like textures, but only two – repeated rotation–accretion, and partial metasomatic replacement of clast minerals – have been supported by detailed evidence. A typical example of cockade breccia from the Gower Peninsula (South Wales) shows clear evidence for the rotation–accretion mechanism: in particular, overgrown breakage points in cement layers – where cockades were previously touching each other – and rotated geopetal infills of haematitic sediment. Based on the available evidence, it is proposed that cockade textures result from low rates of cement growth compared to high rates of dilational fault slip. Seven criteria are given for the correct identification of cockade breccias.