Occurrence of Contrasting skarn formations in dolomites of the Traversella deposit (Ivrea,Italy) |
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| Authors: | J Vander Auwera J Verkaeren |
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| Institution: | (1) Laboratoires associés de Géologie, Pétrologie et Géochimie (B 20), Université de Liège, B-4000 Sart Tilman, Belgium;(2) Laboratoire de Minéralogie et Géologie appliquée, Université Catholique de Louvain, Pl. L. Pasteur, 3, B-1348 Louvain-La-Neuve, Belgium |
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| Abstract: | Metasomatic columns hosted in dolomitic marbles in the thermal aureole of the Traversella monzodiorite (Ivrea, Italy) differ by their mineralogy 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 wollastonite (A1), 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 chemical data demonstrate that the occurrence of contrasted mineral zonations in the same protolith has not been induced by local heterogeneities in the dolomitic marbles. The presence of tremolite in group B instead of forsterite is due to the lower temperature prevailing in the external part of the contact aureole. Chemical data as well as  - diagrams suggest that the columns in groups A and C were formed through the interaction of the dolomitic marbles with fluids with different Fe
2O3, Al
2
O
3 and F. A fluid with low Al
2
O
3 and Fe
2O3 is assumed for the metasomatic column with wollastonite (A 1) and different Al2O3/Fe2O3 values in the fluid are responsible for the formation of columns A 2 and A 3. The stability of clinohumite or chondrodite in group C columns has been induced by a higher F of the fluid than in the other groups. The presence during the prograde metasomatic stage of distinct fluids differing by their Al
2
O
3/Fe
2O3 (columns A 2 and A 3) is likely to have been induced by variations in the source composition. Moreover, the low A1 and Fe content in column A 1 could be due to the reaction of the dolomitic marble with a fluid previously depleted in these components during percolation and reaction with the country rocks.As shown by isocon diagrams and Gresens's equation, skarn formation has locally induced both mass and volume change. The volume decreased in the wollastonite zone of A 1 (19%) and in the Fo-Cc zone of A2 (17%). Mass is always increased except in the wollastonite of A 1 where it is decreased. Mass balance of major elements has shown that Ca is likely to be a perfectly inert component and that Si is always strongly increased. Moreover, Fe is increased in A 2 and A 3 whereas Al is only increased in A 3. Mg is leached in the internal part of column Al.Quantification of chemical potential diagrams using different values of Al
2
O
3 Fe
2
O
3, X
F and T suggests that the successive zones observed in all types of columns can be obtained along an increase of SiO
2 towards the inner zones. A simultaneous decrease in MgO is inferred for group A columns. |
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| Keywords: | Skarns Dolomites Italy Metasomatism |
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