Magmatic–hydrothermal albite–actinolite–apatite-rich rocks from the Cloncurry district, NW Queensland, Australia

Albite-, quartz-, actinolite-, apatite-rich rocks with accessory titanite form a carapace that caps a small dome-like intrusion of Roxmere pluton, and provide evidence of the accumulation and release of magmatic fluids that may have contributed to regionally extensive Na–Ca alteration in the Proterozoic Cloncurry district, Australia. The Roxmere pluton was emplaced after the peak amphibolite facies metamorphism, and into psammitic metasedimentary rocks that are stratigraphically younger and overlie the calc-silicate-rich Mary Kathleen Group. Na–Ca hydrothermal assemblages closely spatially related to the Roxmere pluton suggest that Na–Ca alteration, in this case, is inconsistent with excepted up-temperature metamorphic fluid circulation models. The carapace rocks comprise two variants, aplite and pegmatite, both of which were affected by alteration and brecciation. Aplitic material predominates and the carapace has a brecciated top, and a distinct textural zonation, from an aplite-rich, pegmatite-poor base, to a relatively aplite-poor/pegmatite-rich top. This zonation is interpreted to represent the progressive accumulation of volatile phases toward the top of the carapace, which culminated in fluid overpressuring and brecciation of the granitoid stock. The convoluted and ptygmatic habit of the aplitic and pegmatitic material within the carapace is similar in nature to the comb quartz unidirectional solidification textures that are developed in the roof zones of some porphyry stocks. These complex textures are interpreted to have formed from a single phase of fluid saturation. The Roxmere pluton, or a deeper equivalent, has a Na-, Mg- and Ca-rich composition and a clinopyroxene-, amphibole-rich mineralogy that is consistent with the formation of the contained albite-, actinolite-, apatite-rich phases. However, the source of the rocks within the carapace is equivocal given that their contact relationships are obscured. The δ18O composition of minerals within the carapace has the following ranges: albite, 7.1–8.7‰; actinolite, 8.0–8.5‰; and quartz, 8.9–11.4‰. Albite and quartz δ18O equilibria suggest that these minerals in the pegmatitic and aplitic components formed at temperatures of ∼510°C to 540°C. The calculated δ18O composition of a fluid in equilibrium with these minerals (∼6.0‰ to 8.0‰), at temperatures between 450°C and 550°C, is consistent with a magmatic origin. These estimated temperatures and δ18O fluid composition are comparable to estimates for regional Na–Ca alteration. In contrast, δD analysis of actinolite (−136 to −150‰) within the carapace demonstrates that these fluids have a significantly lower δD signature compared to Na–Ca alteration assemblages associated with regional alteration (−70‰ to −90‰). The low values in the carapace could either be interpreted as being produced from meteoric fluids or from degassed magmatic fluids, although a meteoric fluid model is deemed improbable as meteoric fluids in the district have a documented δD composition of ca. 0.0‰. As a consequence, an open-system degassed magmatic–hydrothermal fluid origin is considered to be the most plausible explanation for the low isotopic signatures, and is also consistent with the complex textural relationships described within the carapace.