Origin and distribution of some trace elements in metamorphosed Fe-Mn deposits, Val Ferrera, eastern Swiss Alps

Numerous small Fe-Mn deposits occur in Triassic marbles of the Middle Penni nic Suretta, Starlera, and Schams nappes in Val Ferrera, eastern Swiss Alps . These deposits are characterized by high contents of Ba, Sb, As, V, Be, W , and rare-earth elements (REE), and most likely represent chemical sedimen ts deposited around submarine springs, similar to some modern seafloor meta lliferous sediments. Circulation of the hydrothermal fluid within the grani te-bearing basement underlying the sedimentary rocks is inferred as the mos t likely mechanism for the derivation of the Be and W concentrated in the o res. The orebodies studied shared a similar Alpine metamorphic evolution, c ulminating with blueschist- to greenschist-facies conditions. The chemical and mineralogical composition of the ores strongly influenced the behavior of the minor constituents Ba, Sr, Sb, As, V, Be, W, and REE. During the mai n deformation (D-1), As and V, for example, were incorporated into hematite in Fe-rich ores, whereas in Mn-rich ores they were concentrated into acces sory minerals growing in the main schistosity. These syn-DI minerals repres ent important sinks of trace elements released by recrystallization or brea kdown of their primary hosts as a result of prograde metamorphism. Post-D-1 mobility of Ba, Sb, As, V, Be, REE and, in some cases, W is recorded by va rious mineral parageneses that either overgrow the S-1 schistosity or occur in several types of discordant veins. In addition to the chemical and mine ralogical controls, the structural position of the deposit influenced the r emobilization of trace elements. Element mobility during the Alpine greensc hist-facies metamorphism within the Fe-Mn ores involved large-ion lithophil e elements (Be, Sr, Ba), high-field-strength elements (Mo, W, Sb, As), and the light REE. The geochemical signature is analogous in many respects to t he remobilization observed during subduction-related metamorphism and durin g metasomatism in the mantle.