Geochemistry of color-zoned apatite from the Chadormalu iron oxide-apatite deposit, Central Iran: Insights into REE mobility during hydrothermal alteration

Abstract

Apatite in the ‘Kiruna-type’ iron deposits is known as potential resource of rare earth elements (REEs). The present study describes large color-zoned apatite crystals in the Chadormalu magnetite-apatite deposit, Central Iran, to obtain insights into the behavior of REEs and trace elements during apatite mineralization and subsequent hydrothermal alteration. The zoned apatite consists of a beige-colored core and surrounding reddish and white polycrystalline domains. The beige-colored core is optically single crystals, but shows a replacement texture of the early-stage apatite (Ap-I) by the later-stage apatite (Ap-II) with abundant monazite inclusions. Ap-I is characterized by higher REEs, Y, Na, Si, S and Cl contents than Ap II. Both Ap II and reddish/white polycrystalline domains show similar compositions with very low contents of REEs and trace elements. These textural and chemical relations suggest that initial apatite mineralization (Ap-I) was followed by an intensive hydrothermal event, which has resulted in dissolution of Ap-I and precipitation of Ap-II and surrounding polycrystalline domains. REEs in Ap-I were leached into the fluid and precipitated as monazite inclusions in Ap-II and interstitial monazite in the reddish domain of polycrystalline apatite. Parts of REEs leached in the fluid are also incorporated into cores of the polycrystalline apatite. It is concluded that, although the later hydrothermal event remobilized REEs, it did not significantly affect the total REE budget of the investigated apatite-magnetite ore deposit.