ISSN:
1432-1866
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract The sulfide deposits of Chizeuil (Saone et Loire) occur in a Late Devonian volcano-sedimentary sequence within an acid volcanic unit and close to a Carboniferous granite batholith. Sulfide bodies, mainly pyrite, are enclosed in andalusite-bearing siliceous rocks lacking primary textures and recrystallized by the granitic thermal metamorphism. Several genetic interpretations were proposed for these siliceous rocks and the associated mineralization, i.e., as either being related to the granite intrusion or of volcanogenic derivation. Detailed studies led to their identification as hydrothermal alterites. A petrographic study of these siliceous alterites reveals that the main mineral phases are only constituted of silica and alumina: quartz, andalusite, kaolinite and minor contents of muscovite, diaspore and corundum. Neither K-feldspar or biotite are present with andalusite. This implies that thermal metamorphism occurred on an already alkali-, calcium- and magnesium-depleted rock. These siliceous alterites show less mobile-element (Al, Ti, V, Zr, Nb) concentration ranges similar to those of acid volcanic host rocks. A metasomatic model is computed from chemical data on surrounding soda dacites, assuming that acid hydrolysis was the only phenomenon involved, and that Al was stable in this process. Although altered rock types grading to soda dacites do not crop out, their existence may be deduced from surficial bedrock multielement geochemical data. The zoned distribution of elements agrees with that deduced from reactions and experimental phase diagrams. The pyrite bodies are surrounded by two distinct concentric alteration zones; the inner one is advanced argillic and the outer one is sericitic. Such a pattern is unusual for volcanogenic sulfide deposits but commonly associated with porphyry deposits. It may be related to the strong acidity (pH≦3) of hydrothermal solutions. These siliceous rocks were produced by an in-situ alteration of brecciated dacitic lavas, inside which a stockwork-type pyritic mineralization was deposited.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00204275
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