Fluid migration and vein formation during deformation and greenschist facies metamorphism at Ormiston Gorge, central Australia

During the Alice Springs Orogeny, deformation at Ormiston Gorge, central Australia, occurred under lower‐ to middle‐greenschist facies conditions. Dolomites of the Bitter Springs Formation and quartzites. metagreywackes, and metapelites of the Heavitree Quartzite contain abundant early‐, syn‐, and post‐tectonic veins. However, though vein densities locally approach 15%, the distribution of veins and the oxygen isotope geochemistry of wallrocks and veins suggest that fluid movement was on a local scale. The Heavitree Quartzite contains quartz veins that, even along the main thrust plane, have similar δ ¹⁸ O values (13.5–16.9%o) to those of their wallrocks (13.6–16.9%o), with Δ ¹⁸ O(vein‐wallrock) values of ‐0.6 to 0.4%o. In contrast, the Bitter Springs Formation contains predominantly dolomite veins that have δ ¹⁸ O values of 23.4 to 27.7%o. These differences are observed even at the boundary between the Heavitree and Bitter Springs rocks, implying that significant fluid exchange between these rocks has not occurred, or that fluid flow was channelled through areas outside those sampled for this study. By contrast with the Heavitree Quartzite, δ ¹⁸ O values of wallrocks in individual samples of the Bitter Springs Formation are significantly higher (23.3–29.1%o) than those of the veins, with δ ¹⁸ O(vein‐wallrock) values up to ‐4%o (average of ‐2.1%o). These systematic differences in δ ¹⁸ O values most likely result from oxygen isotope fractionation caused by fluid immiscibility or disequilibrium dissolution. Smaller differences in δ ¹³ C values between some dolomite veins and wallrocks [δ ¹³ C(vein‐wallrock) up to ‐1.9%o, average of ‐0.5%o] are also explained by these processes. This study indicates that large volumes of veins may be produced by repeated fracturing and fluid migration within particular rock units, without involving large volumes of externally derived fluids.