OCCURRENCE OF CONTRASTING SKARN FORMATIONS IN DOLOMITES OF THE TRAVERSELLA DEPOSIT (IVREA, ITALY)

Metasomatic columns hosted in dolomitic marbles in the thermal aureole of the Traversella monzodiorite (Ivrea, Italy) differ by their minera logy and/or mineral composition. Three groups have been distinguished. In group A, the first zone always contains forsterite-calcite and the second zone contains clinopyroxene. The last zone is made of wollasto nite (A 1), andradite-rich garnet (A 2) or grossular-rich garnet (A 3) . In group B, tremolite instead of forsterite occurs in the first two zones. In group C columns, there are only two zones and clinohumite or chondrodite characterize the first zone. Field, petrographic and chem ical data demonstrate that the occurrence of contrasted mineral zonati ons in the same protolith has not been induced by local heterogeneitie s in the dolomitic marbles. The presence of tremolite in group B inste ad of forsterite is due to the lower temperature prevailing in the ext ernal part of the contact aureole. Chemical data as well as mu - mu di agrams suggest that the columns in groups A and C were formed through the interaction of the dolomitic marbles with fluids with different mu (Fe2O3), mu(Al2O3) and mu(F). A fluid with low mu(Al2O3) and mu(F2O3) is assumed for the metasomatic column with wollastonite (A1) and diffe rent mu(Al2O3)/mu(Fe2O3) values in the fluid are responsible for the f ormation of columns A2 and A3. The stability of clinohumite or chondro dite in group C columns has been induced by a higher mu(F) of the flui d than in the other groups. The presence during the prograde metasomat ic stage of distinct fluids differing by their mu(Al2O3)/mu(Fe2O3) (Co lumns A2 and A3) is likely to have been induced by variations in the s ource composition. Moreover, the low Al and Fe content in column A1 co uld be due to the reaction of the dolomitic marble with a fluid previo usly depleted in these components during percolation and reaction with the country rocks. As shown by isocon diagrams and Gressen's equation , skarn formation has locally induced both mass and volume change. The volume decreased in the wollastonite zone of A1 (19%) and in the Fo - Cc zone of A2 (17%). Mass is always increased except in the wollaston ite of A1 where it is decreased. Mass balance of major elements has sh own that Ca is likely to be a perfectly inert component and that Si is always strongly increased. Moreover, Fe is increased in A2 and A3 whe reas Al is only increased in A3. Mg is leached in the internal part of column A1. Quantification of chemical potential diagrams using differ ent values of mu(Al2O3), mu(Fe2O3), X(F) and T suggests that the succe ssive zones observed in all types of columns can be obtained along an increase of mu(SiO2) towards the inner zones. A simultaneous decrease in mu(MgO) is inferred for group A columns.