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 H 2 O-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 ( 87 Sr/ 86 Sr) 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.

Apatite Texture, Composition, and O-Sr-Nd Isotope Signatures Record Magmatic and Hydrothermal Fluid Characteristics at the Black Mountain Porphyry Deposit, Philippines

“…Studies of fluid-mineral interaction in experiments and on natural mineralized rocks have shown that the apatite structure, chemistry and O-Sr-Nd isotopic composition can be modified by moderate to high temperature (300° to 900°C) hydrothermal fluids including H2O-CO2, acidic and alkalic fluids (Harlov and Fӧrster, 2003;Harlov et al, 2005;Migdisov and Williams-Jones, 2014;Li and Zhou, 2015;Zhao et al, 2015;Betkowski et al, 2016;Bouzari et al, 2016;Migdisov et al, 2016;Zeng et al, 2016;Anenburg et al, 2018;Li et al, 2018;Palma et al, 2019). Our previous studies at the Baogutu porphyry Cu deposit (China) have shown that low temperature (<300°C) fluids can modify the apatite structure, chemistry and Sr isotopic composition (Cao et al, 2019b). The altered rocks at Black Mountain are characterised by modifcation of plagioclase by epidote and of biotite by epidote and chlorite, as well as the widespread occurrence of calcite (Fig.…”

“…Three different models can explain the origin of distinct apatite characteristics in fresh and altered rocks: (1) inheritance from primary magmas (e.g., Belousova et al, 2002;Chu et al, 2009;Mao et al, 2016); (2) elemental diffusion (Ague and Baxter, 2007); and (3) fluid modification (Harlov and Fӧrster, 2003;Harlov et al, 2005;Migdisov and Williams-Jones, 2014;Li and Zhou, 2015;Zhao et al, 2015;Betkowski et al, 2016;Bouzari et al, 2016;Migdisov et al, 2016;Zeng et al, 2016;Anenburg et al, 2018;Li et al, 2018;Cao et al, 2019b;Palma et al, 2019).…”

“…The replacement apatite commonly has obvious dissolution-reprecipitation textures (e.g., Harlov et al, 2002Harlov et al, , 2005Harlov and Förster, 2003;Li and Zhou, 2015;Bouzari et al, 2016;Broom-Fendley et al, 2016;Zeng et al, 2016;Li et al, 2018;Cao et al, 2019b;Palma et al, 2019). In addition, strong dissolution-reprecipitation of apatite is typically accompanied by depletion of REE and other elements and the occurrence of REE-rich minerals (such as monazite; Harlov et al, 2002Harlov et al, , 2005Harlov and Förster, 2003;Li and Zhou, 2015;Zeng et al, 2016;Li et al, 2018;Palma et al, 2019).…”

“…The apatite in altered rocks from mineralized systems commonly shows replacement textures as well as significant geochemical (e.g., Mn, Mg, Fe, Sr, REE) and O-Sr-Nd isotope variations (Li and Zhou, 2015;Zhao et al, 2015;Bouzari et al, 2016;Zeng et al, 2016;Li et al, 2018;Cao et al, 2019b;Andersson et al, 2019;Palma et al, 2019).…”

Mixing of cogenetic magmas in the Cretaceous Zhangzhou calc-alkaline granite from southeast China recorded by in-situ apatite geochemistry

“…More recent studies on granite petrogenesis have benefited from advances in analytical techniques that enable in-situ analysis of minerals for stable and radiogenic isotopes, including O, S, Rb-Sr, Sm-Nd and Lu-Hf systems (e.g., Griffin et al 2002;Yang et al 2014;Economos et al 2017;Bruand et al 2019;Cao et al 2019). Such studies have shown that in-situ geochemical analysis on minerals (e.g., apatite, plagioclase, zircon and titanite) has many advantages over whole-rock chemistry in petrogenetic discussion (e.g., Yu et al 2018;Gros et al 2020;Zhang et al 2020).…”