How fluid-mediated rock transformations can mimic hydro-fracturing patterns in hydrothermal dolomite

Abstract

Numerous studies of hydrothermal dolomite bodies describe brecciated rocks characterized by dark angular fragments of dolomicrite bounded together by saddle dolomite crystals forming veins. Often associated with ore deposits, these hydrothermal dolomite breccia (HDB) patterns are usually interpreted as related to hydro-fracturing and so are considered as univocal markers of fluid overpressure. In this contribution we report an occurrence of typical HDB pattern in a dolomitized sedimentary breccia related to Mesozoic hyper-extension located in the Northern Pyrenees (France). We propose a model that accounts for the textural and crystallographic patterns of a HDB, without any fluid overpressure. The following observations lead us to propose that this HDB results from texturally controlled, hydrothermal fluid-initiated recrystallization of the initial sedimentary breccia by grain boundary migration: (1) the occurrence of rough contacts between the dolomite crystals and an initial dolomitized sedimentary breccia structure; (2) the crystallographic pattern of the hydrothermal dolomite; (3) the different grain size distribution in HDB texture and initial breccia structure; (4) the spatial distribution of oxide particles. Our alternate interpretation was further tested by 2D numerical simulations of the replacement process using the ‘ELLE’ modelling toolbox. Simulations confirm that advection of hot fluids along porous zones can locally trigger recrystallization by grain boundary migration, a replacement of the initial breccia texture and a preservation of larger, less permeable and cold fragments. We propose key features in textural evidence to assess whether HDB texture is pinpointing towards episodes of fluid overpressure and fracturing in carbonates, or if it rather is related to fluid-mediated rock recrystallization.