Apatite-group minerals from nepheline syenite, Pilansberg alkaline complex, South Africa

Abstract

The nepheline syenites of the Pilansberg alkaline complex (South Africa) have undergone extensive subsolidus equilibration and alteration with a deuteric Cl-and Na-rich fluid phase. Complex assemblages of secondary minerals result from the replacement of primary aluminosilicates, rinkite, eudialyte and fluorapatite. The composition of apatite group minerals formed during these alteration processes reflects the Sr-and rare earth element (REE) content, Na/Cl ratio and pH of the deuteric fluids. Apatite-group minerals are observed to have formed in the following sequence: orthomagmatic fluorapatite; strontian britholite-(Ce); strontian fluorapatite; Sr-apatite; REE-rich Sr-apatite; Sr-Na-REErich minerals approaching the stoichiometry of belovite-(Ce) and deloneite-(Ce); britholite-(Ce). Increasing alkalinity of the deuteric fluids is reflected by increasing amounts of Sr replacing Ca in apatite and culminates in the formation of Sr apatite containing 62.1 wt.% SrO (~4.17 a.p.f.u. Sr). Pilansberg apatite-group minerals form a near-complete solid solution between fluorapatite and a fluorine analogue of Sr apatite with limited solution towards belovite-(Ce), Si-rich belovite-(Ce) and strontian britholite-(Ce).

Sr-rich apatite from the Dangzishan leucitite-ijolite xenoliths (Heilongjiang Province): Mineralogy and mantle-fluid metasomatism

“…The thickness of the Sr-apatite is highly variable, from several to tens of microns. This feature is similar to that of Sr-rich apatites found in the Pilansberg nephelite syenites of South Africa [6].…”

“…The Sr-rich apatites in the Pilansberg nephelite syenite possessed a Sr apfu of 4.24 (SrO = 62 wt%) [6]. As a comparison, the total Sr cation number in Lovozero and Murun Sr-rich apatites from the Russian Kola peninsula can reach 4.66 apfu [2,6]. The basicities of these syenites are much higher than that in the Dangzi Mountain.…”

“…The highest Sr apfu was 1.51, which indicates that basicity of the system gradually increased during the formation of the Sr-rich apatites. The Sr-rich apatites in the Pilansberg nephelite syenite possessed a Sr apfu of 4.24 (SrO = 62 wt%) [6]. As a comparison, the total Sr cation number in Lovozero and Murun Sr-rich apatites from the Russian Kola peninsula can reach 4.66 apfu [2,6].…”

“…Liferovich et al [6] studied the Pilansberg nephelite syenite (South Africa), which was shown to have experienced post-magmatic subsolidus equilibration derived from the same-sourced Cl and Na fluids, as indicated by the alteration in primary aluminosilicates (rinkite, eudialite) and Frich apatite. Secondary mineral assemblages better reflected the Na/Cl ratio in the Na-rich and Cl-rich fluids, as well as the pH value of the post-magma interval.…”

“…Sr-rich apatite is widely found in alkaline rocks, kimberlites, pegmatites and lamprophyre. Studies of Sr-rich apatite have mainly focused on its mechanism of genesis in different geological settings; its crystal-structural modification as a result of Sr substitution in the Ca site; sources of Sr-rich fluids; and influence of Sr-rich fluid evolution on melt basicity [1][2][3][4][5][6]. In China, while there are many large *Corresponding author (email: zhangwl@nju.edu.cn) alkaline complexes (e.g.…”