Ree Fractionation Between Scheelite and Apatite in Hydrothermal Conditions

The geochemical analysis of hydrothermal apatite and scheelite pairs from various types of W ore deposits (skarns, disseminated scheelite, and quartz veins) provides an insight into REE partitioning between the two minerals. Among the 18 analyzed pairs, only five appear to have grown in equilibrium conditions. Ten other pairs show more or less important departures from equilibrium. The remaining apatite and scheelite pairs have quite different REE patterns, indicating crystallization from different fluids. Both minerals concentrate REE. The relative behavior of HREE and LREE is quite similar in the two minerals. Scheelite is only slightly more enriched in HREE relative to LREE than apatite, with K(ap-sch)La-Yb = 0.86 +/- 0.22. Beside these regularities, some dispersion in the lanthanide content ratios of apatite and scheelite, ranging from 0.6 to 5, may be related to fluid composition. The behavior of Eu can be related to redox conditions, which appear to be more oxidizing in vein associations than in skarn environments. Determination of REE in coexisting scheelite and apatite seems an efficient tool for identification of successive ore-bearing fluids

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 σ18O 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 σ18O decreases more gradually from 5.0 per mil to 3.0 per mil. The ISr of the early skarn silicates ranges from the values observed in the dolomitic marbles (0.70874 to 0.70971) to the ISr of the intrusion (0.70947 to 0.71064). During the secondary stage, there is a progressive increase of the minerals ISr 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 (σ18O) and radiogenic (87Sr/86Sr) isotopes. © 1991 Springer-Verlag.SCOPUS: ar.jinfo:eu-repo/semantics/publishe