A vector of high-temperature paleo-fluid flow deduced from mass transfer across permeability barriers (quartz veins)

Quartz veins acted as impermeable barriers to regional fluid flow and not as fluid-flow conduits in Mesoproterozoic rocks of the Mt Painter Block, South Australia. Systematically distributed asymmetric alteration selvedges consisting of a muscovite-rich zone paired with a biotite-rich zone are centered on quartz veins in quartz–muscovite–biotite schist. Geometric analysis of the orientation and facing of 126 veins at Nooldoonooldoona Waterhole reveals a single direction along which a maximum of all veins have a muscovite-rich side, irrespective of their specific individual orientation. This direction represents a Mesoproterozoic fluid-flow vector and the veins represent permeability barriers to the flow. The pale muscovite-rich zones formed on the downstream side of the vein and the dark biotite-rich zones mark the upstream side. The alteration couplets formed from mica schist at constant Zr, Ga, Sc, and involved increases in Si, Na, Al and decreases in K, Fe, Mg for pale alteration zones, and inverse alteration within dark zones. The asymmetry of the alteration couplets is best explained by the pressure dependence of mineral–fluid equilibria. These equilibria, in combination with a Darcian flow model for coupled advection and diffusion, and with permeability barriers imposed by the quartz veins, simulate the pattern of both fluid flow and differential, asymmetric metasomatism. The determined vector of fluid flow lies along the regional foliation and is consistent with the known distribution of regional alteration products. The presence of asymmetric alteration zones in rock containing abundant pre-alteration veins suggests that vein-rich material may have generally retarded regional fluid flow