The onset of fluid-induced alteration of fluorapatite in iron oxide-apatite deposits: Insights from fluorapatite and their monazite inclusions at the Heiyingshan iron deposit, Beishan Metallogenic Belt, Northwest China

Abstract

Iron oxide-apatite (IOA) deposits generally undergo extensive late-stage metasomatic overprinting that modifies the texture and geochemistry of the magnetite, apatite, and other ore-forming minerals. However, the onset of metasomatism in IOA deposits worldwide generally remains poorly constrained. The Heiyingshan IOA deposit is located in the Beishan area of the Central Asian Orogenic Belt, NW China. Fluorapatite from this IOA deposit has undergone extensive fluid-aided alteration resulting in the formation of numerous monazite inclusions via a coupled dissolution-reprecipitation process (CDRP) in altered domains of the fluorapatite. Petrographic studies show that these monazite grains are mainly subhedral to anhedral in shape, have a topotaxial relationship with the parent fluorapatite, and co-exist with magnetite, xenotime, and other mineral inclusions. The crystallization age of this monazite constrains the metasomatic activity following the original IOA mineralization. This study presents EMPA and trace element LA-ICP-MS analytical data for fluorapatite and monazite from the Heiyingshan IOA deposit. It also includes in-situ U–Pb isotopic data for unaltered, altered domains of fluorapatite, and monazite. U–Pb data obtained for unaltered fluorapatite, altered fluorapatite and monazite inclusions within altered fluorapatite grains have similar U–Pb ages (∼325 Ma), consistent with the regional Carboniferous volcanic rocks. Furthermore, this study also presents Sr–Nd isotopic data for fluorapatite, the unaltered and altered fluorapatite domains in the Heiyingshan IOA deposit have similar initial Sr isotope ratios, 143Nd/144Nd ratios, and εNd(t) values, with the Sr isotope ratios consistent with the regional Carboniferous volcanic rocks. This implies that magmatism, mineralization, and metasomatism in the Heiyingshan IOA deposit were essentially contemporaneous, the metasomatizing fluids were derived from the evolution of mineralizing fluids. A comparison of the results from Heiyingshan with the available geochronological data from other IOA ore deposits reveals a general consistency in the timing between the mineralization and the metasomatism of IOA deposits due to evolved later-stage fluids. In contrast, dating of monazite metasomatically derived from fluorapatite in some older (e.g., Kiirunavaara) IOA deposits indicates that these IOA deposits appear to have undergone additional episodes of metasomatic alteration during various later geological events after the original IOA mineralization. We suggest that the formation age of monazite inclusions within apatite in IOA deposits should be consistent with the timing of the IOA mineralization, as the original formation and subsequent fluid-aided modification of IOA deposit occur in the same magmatic-hydrothermal event/system. However, some IOA deposits may have undergone additional late episodes of metasomatic events.