Abstract
Metasomatic alteration of fluorapatite has been reported in several iron-oxide apatite (IOA) deposits, but its effect on elemental and isotopic variations has not been well understood. In this study, we present integrated elemental, U-Pb, Sr, and O isotopic microanalytical data on fresh and altered domains in fluorapatite from the Taocun IOA deposit, Eastern China, to evaluate the timing and nature of the metasomatism and its effects on the ore-forming event. Orebodies of the Taocun deposit are spatially associated with a subvolcanic, intermediate intrusion, which displays zonal alteration patterns with albite in the center and increasing actinolite, chlorite, epidote, and carbonate toward the margin. Both disseminated and vein-type ores are present in the Taocun deposit, and fluorapatite commonly occurs with magnetite and actinolite in most ores.
Fluorapatite grains from the both types of ores have been variably metasomatized through a coupled dissolution-reprecipitation mechanism. Many trace elements, including Na, Cl, S, Si, Mg, Sr, U, Th, and (REEs+Y), were variably leached from the fluorapatite grains during this process and the Sr and O isotopic signatures of the grains were also modified. The altered fluorapatite grains/domains have in situ 87Sr/86Sr ratios (0.70829–0.70971) slightly higher than those of the fresh fluorapatite (0.70777–0.70868), and δ18O values (–3.0 to +3.4‰) variably lower than the primary domains (+5.3 to +7.5‰). The Sr and O isotopes of the primary fluorapatite are consistent with or slightly higher than those of the ore-hosting intrusion, implying that the early-stage, ore-forming fluids were magmatic in origin but underwent weak interaction with the country rocks.
U-Pb dating of the fresh and altered domains of the fluorapatite yielded indistinguishable ages of ~131 Ma, which are the same as the age of the ore-hosting intrusion. In combination with fluid inclusion data, we propose that the metasomatism of fluorapatite was induced by hydrothermal fluids at a late stage of the ore-forming event. The shifts to higher87Sr/86Sr ratios and lower δ18O values in the altered fluor-apatite indicate that the alteration was induced by fluids with more radioactive Sr and lighter O isotope signatures. The metasomatic fluids were likely dominated by meteoric waters that were mixed with the earlier magmatic fluids and interacted with sedimentary rocks. Our study highlights that elemental and isotopic compositions of fluorapatite can be significantly modified by hydrothermal fluids during ore-forming events. Thus, instead of traditional bulk-rock analysis, in situ microanalysis is important to provide accurate constraints on the magmatic and/or hydrothermal evolution of complex ore-forming systems.
Special collection papers can be found online at http://www.minsocam.org/MSA/AmMin/special-collections.html.
Acknowledgments
This study was supported by the 973 program (2012CB416802) and the Fundamental Research Funds for the Central Universities (CUG140618). We thank the local geologists, Minlin Rui and Zhangyan Shi, for their assistance in the field, Meijun Yang for the EMPA analyses, Congying Li for the LA-ICP-MS analyses, Tao Yang for the LA-MC-ICP-MS analyses, Xian-Hua Li and Xiaoxiao Lin for the SIMS O isotope analyses, and Rick Moscati for the bulk mineral oxygen isotopic analyses at the USGS. We appreciate Paul Robinson for polishing the English. We are grateful to David Lentz and an anonymous reviewer for their constructive comments, and Daniel Harlov for handling the manuscript.
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