Open Apatite Sr Isotopic System in Low-Temperature Hydrous Regimes

Abstract

Single crystal in situ Sr and Nd isotopic analysis of apatite permits more accurate interrogation of a variety of geoscience questions, including provenance, petrogenesis, and ore system evolution. However, the degree to which the apatite Sr and Nd isotopic systems remain closed under extended low-temperature hydrous conditions is not well constrained. Granitoids at the Baogutu deposit in NW China contain veins of low-temperature H2O-rich epistilbite and thus provide an excellent opportunity to evaluate isotopic system integrity under low-temperature hydrous conditions. Apatite from the Late Carboniferous diorite and granodiorite porphyry show variable (87Sr/86Sr)i ratios (0.7042 to 0.7062) but restricted εNd(t) values (+7.0±0.4 and +6.5±0.4, 1σ). The apatite Sr isotope signatures are transitional between whole rock (0.7035) and vein epistilbite (~0.7062), indicating apparent incorporation of radiogenic Sr from the epistilbite-related hydrothermal fluid into the igneous apatite, whereas εNd values closely match those of the whole rock (+6.9±0.2 and +6.5±0.2, 1σ) and epistilbite (+6.2±0.2, 1σ). Obvious metasomatic textures of apatite were revealed by cathodoluminescence images. The 260 to 300°C formation temperatures for earlier quartz-sulfide veins and chlorite alteration and homogenization temperature (~190°C) of fluid inclusions in epistilbite-calcite vein calcite constrain the temperature of the later epistilbite-related hydrothermal fluid to 190 to 260°C. These results indicate that Sr and likely Nd isotopes in apatite are mobile under low-temperature (<300°C) hydrous conditions and implies that Rb-Sr signatures in apatite from hydrothermally altered rocks reveal the presence and characteristics of modifying fluids.