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The meillers Autunian hydrothermal chalcedony : first evidence of a similar to 295 Ma auriferous epithermal sinter in the Frech Massif Central.

Abstract : The Meillers "quartzite" deposit represents a major hydrothermal siliceous sinter, some 50 m thick and covering an area of some 30 ha within Autunian sandstone of the northern Massif Central. This siliceous sinter comprises three facies of microcrystalline quartz: (i) a dark-colored facies (black chalcedony), locally rich in pyrite, at the base, (ii) a gray-brown fairly massive facies (gray-brown chalcedony) in the middle; and (iii) a white, finely banded, facies (white chalcedony) at the top. Orientation measurements of the banding have revealed the paleo-flow channels of the silica-saturated fluids. The geochemistry of the deposits shows modest metal values, in particular for gold (average of 0.58 g/t Au, giving a gold metal content of some 20 t). The hydrothermal event is stratigraphically well correlated with the basal Autunian (around 295 Ma); an age that has been confirmed through radiochronological determination (300±21 Ma; SHRIMP on hydrothermal zircon). The lead isotopic composition is not very radiogenic (206Pb/204Pb=18.20) and is similar to that of the Late Hercynian gold lodes in the Massif Central. The mineralized sinter appears to have derived from a geyser-type hot springs. This hydrothermal activity coincides with the auriferous metallogenic peak that occurred in the Hercynides around 300 Ma. This is the first known epithermal-type surface manifestation described for the Hercynian gold event.
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Submitted on : Friday, May 19, 2006 - 9:16:39 AM
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Eric Marcoux, P. Le Berre, Alain Cocherie. The meillers Autunian hydrothermal chalcedony : first evidence of a similar to 295 Ma auriferous epithermal sinter in the Frech Massif Central.. Ore Geology Reviews, Elsevier, 2004, 25, pp.1-2 69-87. ⟨10.1016/j.oregeorev.2003.10.001⟩. ⟨hal-00068297⟩

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