Microstructural analyses of a giant quartz reef in south China reveal episodic brittle-ductile fluid transfer

Abstract

A long-lived hydrothermal system at the Heyuan fault, South China, has led to the development of a giant quartz reef, now partially exhumed along its length for more than 40 km. Systematic analyses and focused microstructural studies have been undertaken to unravel a complex formation history of repeated fracturing, hydrothermal fluid flow and sealing cycles, resulting in a dynamic permeability across the fault zone. The change in morphology and decreasing grain size with time further indicates the move from slow ductile opening to fast seismic events. Quartz-reef formation has been estimated to occur within a range of ~200–350 °C, based on evaluation of (i) quartz deformation microstructures; (ii) chlorite and mica geothermometry; and (iii) review of comparable quartz-reef studies. Additionally, a set of physico-chemical formation conditions have been identified which compose the ‘quartz-reef window’. These are: (i) significant volume of fluid; (ii) fluid sources from meteoric, metamorphic and/or from mantle origin; (iii) considerable Time-Integrated Fluid Fluxes; (iv) SiO2 oversaturation due to (a) temperature change, (b) sudden pressure drop, or (c) chemical change e.g. fluid mixing; (v) accommodation space to ‘grow’ the reef; (vi) channel permeability; and (vii) cap rock/seal to trap the fluid flow. The mechanism of quartz-reef growth is here interpreted as the brittle-ductile analogue of the brittle fault-valve model.