Hydrothermal dolomite breccia: when pre-existing rock heterogeneities control fluid-mediated replacement patterns and mimic tectonic features.

Breccia structure is a ubiquitous feature that is characterized by angular fragment in a matrix composed of smaller grain size, often associated to brittle tectonics or to specific sedimentary environment such as karst collapse. Many different studies across the world describe breccia related to dolomite geobodies, themselves associated to ore deposits occurring during major extensional events (Hungary, Spain, France, Canada, Poland, Canada). The mineralogical textures of these structures, i.e. angular fragments of dark dolomicrite bound by elongated blocky, white, dolomite crystals in veins, are interpreted as univocal markers of fluid overpressure and hydrofracturing, hydrothermal dolomite breccia (HDB) being a precious tool to help to reconstruct pressure history.

This contribution presents a case study that challenge this hydrofracturing origin of HDB, questioning the role of fluid-mediated replacement in the observed crystallographic textures. The Mano Formation located in the Mail Arrouy, an anticline related to Mesozoic hyper-extension of the crust located in the Chaînons Béarnais (Northwest Pyrenees, France), presents classical HDB, i.e. characterised by black dolomite fragment surrounded by a white dolomite-matrix supposedly related to hydrofracturing. Yet, in some places, it is possible to observe this angular black fragment in contact with a brown dolomite matrix. As attested by the presence of dolomitized fossils, the brown dolomite and black fragments constitute an initial sedimentary breccia structure, that is described regionally.

Textural and chemical analyses of the HDB and of the initial sedimentary breccia have been carried out by scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and electron probe microanalyzer (EPMA) across different dolomite-dolomite interfaces. Quantitative data has been obtained by image processing, showing that oxide particles that are randomly distributed in the brown matrix appears pushed at the tips of the white crystals of dolomites, suggesting a cleaning process during growth. Also, the initial breccia comprises small-size around 1830 µm² (surface area) clasts that are absent from the HDB. Moreover, the contact between black, white and brown dolomite show a roughness similar to what is observed in fluid-mediated dissolution/replacement processes. Finally, EBSD results show that white dolomite crystal grew under local stress generated by a competition between grain growth, typical of slow, fluid-limited, grain growth.

This array of results leads us to propose that the HDB results from texturally controlled, fluid initiated hydrothermal recrystallization of initial sedimentary dolomicrite. This model is further tested by 2D numerical simulations of phase separation process using the modelling environment “ELLE” that reproduce the patterns observed in natural samples. Hence, a critical reappraisal of the origin and process behind HDB must be conducted, as we show that, in the case of the Mano Formation in the Mail Arrouy, no fluid overpressure were required to create HDB.