Abstract A series of acid volcanic brines was collected at White Island, New Zealand, largely during the period 1974 to 1977, when the acid brine discharge rate was high. The chemical and isotopic compositions of these brines were compared to the compositions of fumarolic gases, seawater, and volcanic and the altered rocks in order to determine their origins. Isochemical dissolution of volcanic host rocks appears to account for the relative concentrations of many rock-forming elements in the acid brines with the exception of the enrichment in Na, which most likely derives from the influx of the seawater, and the depletion in Al, K, Ca, and Fe, which reflects formation of alunite, anhydrite, and pyrite during acid alteration. Magmatic gas is the major source of B, NH 3 , Cl, and S in the brines. As it passes through the hydrothermal liquids, B and NH 3 are quantitatively condensed into brine, whereas the HCl and S are partially absorbed into the brine. Brine sulfate shows a large variation in δ 34 S values, ranging from 0 to 17 per mil; the heavy sulfate comes from disproportionation of magmatic SO 2 , whereas light δ 34 S sulfate is derived from near-surface bacterial oxidation of elemental sulfur and pyrite. The δD and δ 18 O values of acid brines and fumarolic condensates reflect the formation of the volcanic-hydrothermal system, mainly by the seawater-magmatic water mixing. However, a significant amount of meteoric water is likely incorporated during brine formation, but its proportion cannot be quantitatively evaluated because δD and δ 18 O compositions of meteoric water are close to that of the seawater. Mass-balance calculations replicating the average White Island brine composition require dissolution and alteration of 0.15 kg of the host volcanic andesite by a solution made up of 1 kg of the seawater and 3 kg of the magmatic gas. The detailed examination of the brine compositions shows that infiltrating the seawater into the White Island hydrothermal system and nonisochemical dissolution of the source volcanic rocks are responsible for their formation.

ABSTRACT Late diagenesis records a common history of fluid flow in sub-Permian strata in the midcontinent, where fluid inclusion Th are higher than burial temperatures and Tm ice show evolving salinity. Most negative δ 18 O dolomite and highest Th are at the top of the Mississippian. Fluid inclusion and geochemical data point to advective fluid flow out of basins utilizing Cambrian–Ordovician–Mississippian strata as an aquifer for hydrothermal fluids. The Pennsylvanian was a leaky confining unit. This system evolved from: Stage 1 Pennsylvanian–early Permian pulsed hydrothermal migration of connate brine and gas; between Stages 1 and 2, low-temperature Permian brine reflux; Stage 2 mixing between high-temperature and low-temperature brines during the Permian; and Stage 3 large-scale migration of hydrothermal brines and oil later during the Permian or after. Stages 1–3 were the most important late processes affecting Mississippian reservoirs, and record an inverted thermal structure with most impact of hot fluids at the top of the Mississippian. Stage 4 shows radiogenic 87 Sr/ 86 Sr in calcite, supporting a transition to localized fault pumping from basement, likely driven by Laramide fault reactivation. Stage 5 is the current system, with Ozark and Front Range uplift-driven fluid flow and potential for small-scale sporadic fault pumping.