Structural controls and geochemistry of hydrothermal fluid flow in an exhumed accretionary prism, Otago Schist, New Zealand

Abstract

This study examines the structural controls, geochemistry, and potential source of an ancient hydrothermal fluid-flow system within the Otago Schist, using excellent exposures at Bruce Rocks, near Dunedin, as a case study. Both active and ancient hydrothermal systems found in the Otago Schist have been linked to metamorphic dehydration within the Mesozoic accretionary prism and underlying slab, or to deformation related to the Cenozoic Australia-Pacific plate boundary, which generates fluid flow due to uplift of basement rocks. Most hydrothermal systems exposed within the Otago Schist are interpreted to have formed at several kilometres depth to mid-crustal levels, but there is currently a lack of understanding of systems formed at shallower crustal levels. Field observations show that the Bruce Rocks hydrothermal system belongs to the youngest post-metamorphic deformation and is dominated by carbonate breccia and vein networks that formed from CO2-bearing fluids. Data for this study includes detailed petrographic and structural characteristics of the vein networks and surrounding wall rocks, as well as analysis of the mineralogy, microstructure, and alteration style caused by the CO2-bearing fluids. Additionally, 87Sr/86Sr, δ13C, and δ18O isotope data and whole-rock geochemistry were collected from veins and wall rocks to ascertain a potential fluid source, the degree of fluid-wall rock reaction and element mobilisation, and to compare with other similar hydrothermal systems within the Otago Schist. The hydrothermal system is estimated to have an early Miocene age due to overlapping isotope data with other similar systems in the Otago Schist, e.g., Shotover River and Akatore Coast. The hydrothermal system shows a strong structural control: breccias and vein networks mainly formed along small-displacement (<meter) faults that reactivated pre-existing Late Cretaceous exhumation joints. As fluid travelled along the faults, it accessed the adjacent wall rocks along foliation surfaces where permeability was sufficiently high. Carbonate in breccias and veins is present as calcite, dolomite, ankerite and siderite which have 87Sr/86Sr isotope ratios of 0.7058 – 0.7064. The geochemical variation is from fluid-rock interaction with the wall rock mineralogy, causing a shift toward more radiogenic 87Sr/86Sr isotope ratios due to fluid interaction with highly radiogenic wall rock muscovite grains. A potential source of fluid is from a breakdown reaction of metamorphic epidote or carbonates, causing a release of both a Fe-rich CO2-bearing fluid resulting in the formation of siderite in the early stages, followed by Ca-rich CO2-bearing fluid causing the precipitation of calcite. The study suggests that the ancient hydrothermal system exposed at Bruce Rocks belongs to an early Miocene system formed at very shallow crustal levels, probably within a few kilometres of the paleo-surface, related to retrograde dehydration reaction from the formation of the Alpine Fault.