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

Albite-, quartz-, actinolite-, apatite-rich rocks with accessory titanite f orm a carapace that caps a small dome-like intrusion of Roxmere pluton, and provide evidence of the accumulation and release of magmatic fluids that m ay have contributed to regionally extensive Na-Ca alteration in the Protero zoic Cloncurry district, Australia. The Roxmere pluton was emplaced after t he peak amphibolite facies metamorphism, and into psammitic metasedimentary rocks that are stratigraphically younger and overlie the calc-silicate-ric h Mary Kathleen Group. Na-Ca hydrothermal assemblages closely spatially rel ated to the Roxmere pluton suggest that Na-Ca alteration, in this case, is inconsistent with excepted up-temperature metamorphic fluid circulation mod els. The carapace rocks comprise two variants, aplite and pegmatite, both o f which were affected by alteration and brecciation, Aplitic material predo minates and the carapace has a brecciated top, and a distinct textural zona tion, from an aplite-rich, pegmatite-poor base, to a relatively aplite-poor /pegmatite-rich top. This zonation is interpreted to represent the progress ive accumulation of volatile phases toward the top of the carapace, which c ulminated in fluid overpressuring and brecciation of the granitoid stock. T he convoluted and ptygmatic habit of the aplitic and pegmatitic material wi thin the carapace is similar in nature to the comb quartz unidirectional so lidification textures that are developed in the roof zones of some porphyry stocks. These complex textures are interpreted to have formed from a singl e phase of fluid saturation. The Roxmere pluton, or a deeper equivalent, ha s a Na-, Mg- and Ca-rich composition and a clinopyroxene-, amphibole-rich m ineralogy that is consistent with the formation of the contained albite-, a ctinolite-, apatite-rich phases. However, the source of the rocks within th e carapace is equivocal given that their contact relationships delta(18)O c omposition of minerals within the carapace has the following ranges: albite , 7.1-8.7 parts per thousand; actinolite, are obscured. The delta(18)O comp osition of minerals wthin the carapace has the following ranges: abilite, 7 .1%-8.7%; actinolite, 8.0-8.5 parts per thousand; and quartz, 8.9-11.4 part s per thousand. Albite and quartz delta(18)O equilibria suggest that these minerals in the pegmatitic and aplitic components formed at temperatures of similar to 510 degrees C to 540 degrees C. The calculated delta(18)O compo sition of a fluid in equilibrium with these minerals (similar to 6.0 parts per thousand to 8.0 parts per thousand), at temperatures between 450 degree s C and 550 degrees C, is consistent with a magmatic origin. These estimate d temperatures and delta(18)O fluid composition are comparable to estimates for regional Na-Ca alteration. In contrast, delta D analysis of actinolite (- 136 to - 150 parts per thousand) within the carapace demonstrates that these fluids have a significantly lower delta D signature compared to Na-Ca alteration assemblages associated with regional alteration (- 70 parts per thousand to - 90 parts per thousand). The low values in the carapace could either be interpreted as being produced from meteoric fluids or from degas sed magmatic fluids, although a meteoric fluid model is deemed improbable a s meteoric fluids in the district have a documented delta D composition of ca. 0.0 parts per thousand. AS a consequence, an open-system degassed magma tic-hydrothermal fluid origin is considered to be the most plausible explan ation for the low isotopic signatures, and is also consistent with the comp lex textural relationships described within the carapace. (C) 2000 Elsevier Science B.V. All rights reserved.