Crack–reaction veins from the Hodgkinson Formation, North Queensland, Australia

Abstract

AbstractThin, subplanar or parallel-sided syntectonic quartz veins from the Hodgkinson Province, North Queensland, Australia contain mica inclusions that are identical to wallrock mica textures. These ghost textures can only be produced by incomplete replacement of the wallrock by quartz. The planar form of the veins indicates that there was a planar control on their formation. We propose that the veins formed around syntectonic fractures that localized silica micrometasomatism of the fracture wall. The formation of the veins can be explained by a model which involves fracture of the wallrock, reaction between the fracture wall and the ambient metamorphic fluid and eventual sealing of the fracture by precipitation from the fluid. The new vein is then a locus of further fracture. Replacement of wallrock micas occurs via a two-stage process where biotite is replaced by muscovite, which is in turn replaced by quartz. We propose the term crack–reaction to describe the resulting cyclic process because it can be compared with the crack–seal model of vein formation. Crack–seal and crack–reaction are different only in the relative amounts of metasomatism and precipitation that occur subsequent to fracture and can be envisaged as end members of a more general vein-forming process.