Relationships among zirconosilicates: examination by cathodoluminescence and transmission electron microscopy .

Abstract

The origin, whether primary (magmatic) or secondary (Ca-metasomatic) after elpidite, of the Ca-rich zirconosilicates armstrongite and gittinsite in the subsolvus granite of the Strange Lake peralkaline complex (Quebec-Labrador border, Canada) has been a well debated topic. These zirconosilicates, along with zircon and amphibole, account for most of the Zr in the associated Zr-, REE-, Y-, Nb-, and Be-rich ore deposit. In this study, cathodoluminescence (CL) examination and transmission electron microscopy (TEM) have been used to shed light on the relationships among and origin of these zirconosilicates. Optical and CL observations indicate that elpidite undergoes epitaxial replacement by armstrongite and, subsequently, by gittinsite. This gittinsite is generally associated with quartz and variable amounts of hematite. TEM results suggest the bulk replacement of elpidite by poorly crystalline armstrongite, followed by gittinsite and quartz, or zircon and quartz. The Ca-rich compositions of elpidite and its structural similarity with armstrongite imply the existence of a solid solution with armstrongite. Moreover, the enrichment of gittinsite in Mn and Fe (and in some cases Y), and armstrongite and zircon in Y, as determined by analytical electron microscopy (AEM), correlates with the presence of these elements in the Ca-metasomatic fluid. These structural and compositional relationships indicate that the primary, magmatic elpidite is replaced by armstrongite and, subsequently, by gittinsite and quartz during Ca metasomatism of the complex.