Several small syngenetic-exhalative Fe-Mn deposits metamorphosed under blueschist-to greenschist-facies conditions occur in Triassic marbles of the Middle Penninic nappes in Val Ferrera (eastern Swiss Alps). Pink muscovite - aegirine - hematite - albite schists associated with the ores in one of these deposits (Starlera mine) contain an unusual assemblage of As-, Sb-, Be-, and REE-rich minerals that developed after the main deformation (D1). In these pink schists, As is present in significant amounts in both titanite (to 3.46 wt% As2O5) and fluorapatite (to 13.8 wt% As2O5), and is a major component of bergslagite, CaBeAsO4(OH). Antimony is an essential constituent of rutile (to 30.63 wt% Sb2O5), titanite (to 9.55 wt% Sb2O5), and pyrophanite (to 1.91 wt% Sb2O5), in which it is accommodated via the exchange vector Sb5+Fe3+Ti4+ -2. Romeite, (Ca,Na,)2(Sb,Ti)2O6(F,OH,O), is also an important host for Sb, but in addition it incorporates major amounts of REE (to 23.4 wt% Ce2O3). The pink schists at Starlera are the second reported occurrence of natural Sb-rich rutile and Sb-rich titanite (after Praborna, western Italian Alps), and the third of As-bearing titanite (Praborna and Wanniglestscher, Binntal, central Swiss Alps). The Starlera deposit is further characterized by the presence of thick veins of romeite-tilasite (CaMgAsO4F). Most of these late (post-D1) veins are closely associated with intensely foliated pink schists, indicating that they formed from fluids that passed through the schists. These fluids must have carried As, Sb, and Ti, and probably were similar to those responsible for growth of the post-D1 As, Sb, Be, Ti, and REE minerals in the pink schists. The metal content of the fluids was most likely derived from the surrounding ores. The unusual assemblages in both veins and pink schists at Starlera document mobility of As, Sb, Be, Ti, and REE under lower greenschist-facies conditions and in a relatively oxidizing environment.
|Number of pages||16|
|Publication status||Published - 1 Feb 1999|
- Fe-Mn deposit
- Swiss Alps
- Val Ferrera