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 Penninic 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 sediments deposited around submarine springs, similar to some modern seafloor metalliferous sediments. Circulation of the hydrothermal fluid within the granite-bearing basement underlying the sedimentary rocks is inferred as the most likely mechanism for the derivation of the Be and W concentrated in the ores. The orebodies studied shared a similar Alpine metamorphic evolution, culminating 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 main 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 accessory minerals growing in the main schistosity. These syn-DI minerals represent important sinks of trace elements released by recrystallization or breakdown 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 various mineral parageneses that either overgrow the S-1 schistosity or occur in several types of discordant veins. In addition to the chemical and mineralogical controls, the structural position of the deposit influenced the remobilization of trace elements. Element mobility during the Alpine greenschist-facies metamorphism within the Fe-Mn ores involved large-ion lithophile 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 the remobilization observed during subduction-related metamorphism and during metasomatism in the mantle.