High-Temperature Fluid-Rock Interaction and Scapolitization in an Extension-Related Hydrothermal System, Mary Kathleen, Australia

Abstract

In the polymetamorphic Mary Kathleen Fold Belt, at the centre of the Proterozoic Mount Isa Inlier, Queensland, an early phase of extension (1760–1730 Ma) resulted in intrusion of voluminous granitic and doleritic magmas into the carbonate-evaporite-dominated Corella Formation at ∼5–10 km depths. Widespread high-temperature metasomatism ensued, involving scapolitization in dolerite, formation of albite-scapolite shear zones in granite, exo- and endoskarn formation, and a zone of K−Na−Ca alteration in the lowermost Corella Formation. Granites and dolerites were altered to an unusual Na−Ca-rich bulk composition, reflecting high-temperature infiltration of highly saline, chemically reactive externally derived fluid. The alteration products and their distribution suggest not only reaction of magmatic/aqueous fluids with the country rocks but also extensive halite dissolution and recirculation of saline fluids back into the intrusive bodies. The bulk of fluid flow occurred at high temperatures (500–700 °C), and major element and isotopic fronts were generally not smoothed out by the effects of temperature gradients, with the exception of one part of the system which shows evidence for up-temperature fluid flow. Analysis of oxygen isotopic data and the position of isotopic and geochemical fronts reveals time-integrated fluid fluxes of up to 2 × 104 m3/m2 for the metasomatism. Although very high salinities (up to 50 mol% NaCl) were attained by evaporite dissolution, δ18O values of most alteration products are in the range 7–12%, reflecting a predominance of oxygen derived from an igneous fluid. The position and interrelationships of metasomatic and isotopic fronts indicate an earlier stage of infiltration dominated by fluid released from crystallizing granite (with δ18O 10–12%), and a later stage (δ18O 7–9%) in which fluid had already interacted with halite and a mixed mafic-felsic igneous source or was repeatedly circulated between these rock types during alteration. The data reflect only a minimal contribution from fluids produced by devolatilization of the abundant carbonate-bearing rocks in the Corella Formation, and there are substantial areas of Corella Formation rocks that have escaped metasomatism during this phase of intrusion-related hydrothermal activity and during the subsequent regional metamorphic overprint. Along with the requirement that the fluids dissolved large amounts of halite from the same sequence, and the structural observations, we favour a model where fluid was preferentially channelled along specific permeable conduits, including former evaporite layers, before interaction with the now exposed altered rocks. Fluid was probably driven by both convective circulation and dilatancy-related deformation accompanying emplacement of magmas into a major crustal extensional décollement.