Trace elements (REE) and isotopes (O,C, Sr) to characterize the metasomatic fluid sources: evidence from the skarn deposit (Fe,W, Cu) of Traversella (Ivrea,Italy)

The skarn complex of Traversella was formed at the expense of various rock types (calcic hornfels, gneiss, dolomitic marble) occurring in the contact aureole of the dioritic intrusion of Traversella (30±5 Ma). Application of phase equilibria has fixed the temperature of the primary stage of skarn formation between 550° C to 625° C. Similar applications indicate a larger range of temperature (525° C to 300° C) for the secondary stage. The different types of skarn (primary stage) are enriched in REE relative to the corresponding precursor rock (T.R.=126 ppm (protolith) to 228 ppm (inner zone) for the skarn on gneisses; T.R.=14 ppm to 71 ppm for the skarn on calcic hornfelses; T.R.=12 ppm to 200 ppm for the skarn on dolomitic marbles), but all the inner zones of these different types of skarn show a similar REE distribution with a slight LREE fractionation and no Eu anomaly. It is inferred that the primary metasomatic fluid has a parallel REE pattern. The oxygen isotope composition of water in equilibrium with the early stage of skarn at T=600° C ranges from 8.3 per mil to 8.9 per mil. At the beginning of the first hydroxylation stage (secondary stage), the fluid σ ¹⁸O remains in the range observed in the primary stage but within it, there is a sharp decrease from 8.0 per mil to 5.0 per mil. During the sulphidation stage, the fluid σ ¹⁸O decreases more gradually from 5.0 per mil to 3.0 per mil. The I _Sr of the early skarn silicates ranges from the values observed in the dolomitic marbles (0.70874 to 0.70971) to the I _Sr of the intrusion (0.70947 to 0.71064). During the secondary stage, there is a progressive increase of the minerals I _Sr up to 0.71372. The REE pattern of the primary metasomatic fluid does not put any precise constraint on the primary fluid source. On the other hand, both stable and radiogenic isotopes suggest that the early high-temperature metasomatic fluid was isotopically equilibrated with the dioritic intrusion. This implies that this early fluid is either exsolved from the crystallizing intrusion or a metamorphic water previously equilibrated with the intrusion. During the secondary stage, the replacement of the early anhydrous phases by hydrated parageneses is accompanied by the mixing with meteoric fluid as indicated by stable (σ ¹⁸O) and radiogenic (⁸⁷Sr/⁸⁶Sr) isotopes.