Abstract

The nepheline syenite pegmatite in the Saima alkaline complex in northeastern China is characterized by REE mineralization, mainly rinkite-(Ce) and associated alteration minerals. As the most abundant REE-bearing mineral in the pegmatite, rinkite-(Ce) closely coexists with microcline, nepheline, natrolite, and calcite. Some rinkite-(Ce) grains show compositional sector-zonation, in which the inner core displays relatively high Ti, Ca, and Sr concentrations, but low Zr, REE, and Na contents. Primary rinkite-(Ce) has undergone multiple episodes of fluid interactions, and accordingly, from weak to strong, three different mineral assemblages of hydrothermal alteration can be summarized: (1) rinkite-(Ce) + secondary natrolite ± K-feldspar ± minor fluorbritholite-(Ce); (2) rinkite-(Ce) relics + secondary natrolite + K-feldspar + fluorbritholite-(Ce) + unidentified Ca-Ti silicate mineral + fluorite and calcite; and (3) pseudomorphs after rinkite-(Ce). The pseudomorphs can be divided into two groups characterized by distinct mineral associations: (1) Ca-bearing strontianite + fluorbritholite-(Ce) + natrolite + fluorite + calcite coexisting with silicate minerals; and (2) calcite + fluorite + fluorbritholite-(Ce) + rinkite-(Ce) relics ± Ca-bearing strontianite ± ancylite-(Ce) associated with a calcite matrix. These alteration mineral assemblages are evidence of magmatic-derived alkali metasomatism due to an alkali-CO2-F-rich fluid and Ca-metasomatism due to a different, externally derived Sr- and Ca-rich fluid. The metasomatic events acted as the potential driving force for the rinkite-(Ce) dissolution and pseudomorph-forming process. The high concentration of rinkite-(Ce) in the nepheline syenite pegmatite results from the fractional crystallization of the Saima CO2-rich alkaline silicate magma, and the successive alterations of rinkite-(Ce) attest to the important role played by hydrothermal fluids in controlling the remobilization of REE and the crystallization of secondary rare earth minerals.

This content is PDF only. Please click on the PDF icon to access.
You do not currently have access to this article.