Quartz vein 18O/16O ratios across a 500 km transect through the Lachlan fold belt of southeastern Australia are remarkably uniform (±1–1.5 permil) at both local (centimeter to meter) and regional (over 104 km2) scales. They define isotopic zones that correlate with the tectonic divisions of the Cambrian through Devonian (dominantly Ordovician) quartz-rich turbidites determined by regional mapping. From west to east these divisions are (1) Stawell, δ18O=14.7±1.1 (47 samples); (2) Bendigo-Ballarat, δ18O=17.5±1.3 (204 samples); (3) Melbourne, δ18O=19.0±1.6 (80 samples); (4) Tabberabbera, δ18O=16.3±1.9 (12 samples); and (5) Omeo, δ18O=14.4±1.0 (26 samples). Isotope profiles across zone boundaries, and intrazone fault zones particularly within the Bendigo-Ballarat zone, show steps indicating abrupt changes across faults with little or no evidence of fluid mixing within the fault zones. δ18O values of veins are insensitive to relative age, type of vein, and immediate host rock lithologies. The δ18O values of coexisting vein and host rocks show nonequilibrium relationships which can be explained in terms of rock buffering under conditions of low fluid/rock ratios (water/rock ≪ 1). Limited D/H determinations on fluid inclusions fall mainly in the range −70 and −100, with one value as low as −140. These low deuterium values when considered in the context of paleolatitude may have been inherited from deutrium-depleted detrital minerals and do not necessarily require the direct penetration of meteoric fluids to midcrustal depths. The work suggests that vein formation is possible in regions where the integrated water/rock (w/r) ratio is very low (w/r ≪ 1) as long as a pervasive fluid phase is present. This fluid appears to achieve a steady state isotopic composition on scales of hundreds of meters, and once a quasi steady state has been reached, the isotopic compositions of the resultant quartz veins are rather insensitive to the diachronous and nonisothermal conditions under which vein growth occurs. Advective cycling of the fluid on this scale is by episodic dilatancy “pumping” in fracture networks associated with localized faulting.