Petrography and Mineral Chemistry of Fluorapatite and Monazite from South Nam Xe Carbonatites, Northwest Vietnam

DOI: 10.5176/2251-3353_GEOS14.36

Authors: Nguyen Thi THUY, Le Duy DAT, Hoang Hoa THAM and Hideki WADA

Abstract: This paper presents a study of the petrography and mineral chemistry of fluorapatite and monazite from South Nam Xe calico- and ferrcarbonatite dykes in northwest Vietnam. These minerals occur in both types of the carbonatites and in various modes. Most of the fluorapatites in the calciocarbonatites are oblong to elongated, and have sharp contacts with calcite, aegirine and phlogopite suggesting their primary origin; whereas those in the ferrocarbonatites are commonly present in forms of stubby to perfect hexagonal prismatic crystals, which partly replaced by the monazite. The monazites in the ferrocarbonatites also occur as hundreds micro isolated crystals, which associated with ankerite, feldspar, magnetite and phlogopite.
The fluorapatites contain high F contents (up to 4.26 wt.{6e6090cdd558c53a8bc18225ef4499fead9160abd3419ad4f137e902b483c465}) and high rare earth element (REE) concentrations (up to 5.19 wt.{6e6090cdd558c53a8bc18225ef4499fead9160abd3419ad4f137e902b483c465}) indicating a F- and REE-rich melt for the carbonatite precipitation. The enrichment in light REEs in the fluorapatite is paired with an enrichment in Na, varying from 0.25 wt.{6e6090cdd558c53a8bc18225ef4499fead9160abd3419ad4f137e902b483c465} to 0.89 wt.{6e6090cdd558c53a8bc18225ef4499fead9160abd3419ad4f137e902b483c465}. The replacement of REE and Na for Ca in the fluorapatites, which leads to change in CaO/P2O5 ratios might show an open system where Ca and P are added to areas of the fluorapatite depleted in light REE (LREE). REE contents in the monazite show a very limited variation, and are mainly composed of La, Ce and Nd (La2O3 = 21.09-22.72 wt.{6e6090cdd558c53a8bc18225ef4499fead9160abd3419ad4f137e902b483c465}, Ce2O3 = 32.82-33.35 wt.{6e6090cdd558c53a8bc18225ef4499fead9160abd3419ad4f137e902b483c465}, Nd2O3 = 6.96-8.20 wt.{6e6090cdd558c53a8bc18225ef4499fead9160abd3419ad4f137e902b483c465} in calciocarbonatite; La2O3 = 16.61-17.39 wt.{6e6090cdd558c53a8bc18225ef4499fead9160abd3419ad4f137e902b483c465}, Ce2O3 = 32.07-33.52 wt.{6e6090cdd558c53a8bc18225ef4499fead9160abd3419ad4f137e902b483c465}, Nd2O3 = 8.92-10.94 wt.{6e6090cdd558c53a8bc18225ef4499fead9160abd3419ad4f137e902b483c465} in ferrocarbonatite).

Keywords: fluorapatite, monazite, South Nam Xe, carbonatite