Continental-scale environmental and geochemical distal effects of the Rochechouart impact at the lower Hettangian
Résumé
We give arguments showing that the Pb Ag Ba F Tl anomalies on the 200 Ma paleo-(sub)surface in western Europe likely have an impact-driven epithermal origin. We propose that they derive from the dust cloud induced by a bolide fall at Rochechouart (NW French Massif Central, France), then removal by rain and percolation along reactivated crustal lineaments. Coeval alterite formation resulted from a greenhouse effect related, and favoured Ba and Pb release from feldspars. I Introduction Impact events are now well recognized as important markers of terrestrial geological history, as well as objects of prime economic potential. World class ore deposits are directly or indirectly related to giant (>130 km-wide) meteorite craters, e.g., the Sudbury Cu-Ni-PGM ores or the Witwatersrand Au-U mineralizations in Paleoproterozoic cratons (Pirajno 2009). At the scale of North America, the current worth of impact-related valuable substances has been estimated to $5 billion per year (Grieve and Masaitis 1994). In hypervelocity impact craters, a huge amount of kinetic energy is converted to heat. Target rocks are partly molten and vaporized, they are brecciated and comminuted as a compression wave propagates through them. Hot and permeable fall back and fall out ejecta as well as impact-induced fractures below and around the crater are thus favourable settings for post-impact hydrothermal circulations. Based on mineralogy, geochemistry and fluid modelling, the concentrations of Au, U, Pb-Zn sulfides to be found in crater rocks are shown to result from long duration fluid flows (10 3 s to 10 6 years), which extend at the crater-scale and deeper than 1 km below it (e.g., Komor et al. 1988; Naumov 2005). It may seem paradoxical that, whereas terrestrial meteorite impacts may cause global environmental catastrophes (Pierazzo and Artemieva 2012), the economic potential of an impact has seldom been documented to extend at a continental scale so far. In France, the economic importance of the Hettangian paleosurface has long been recognized (e.g., Samama 1980; Lhégu and Touray 1980), but no single unified metallogenic model was ever proposed to account for the complex interplay of hydrothermal and continental weathering processes on this paleosurface. Recently, Schmieder et al. (2010) suggested that some Hettangian Pb-Zn-U-F-Ba mineralizations could be related to the Rochechouart crater (RC), based on their new dating of the impact at 201±2 Ma. Figure 1a shows the crater, ≈ 20km in diameter, located on the NW margin of the French Massif Central (FMC) part of the Variscan belt and on the NE edge of the Aquitaine basin. It affected a mixed crystalline target primarily composed of granitic, metamorphic and intrusive igneous rocks of the Variscan orogeny (Lambert 1977, 2010; Chévremont et al. 1996). In this context, we review and discuss some metallogenic and high-energy environmental effects recorded in the lower Hettangian (LH) of western Europe (WE), in the scope of the Rochechouart impact.
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