James Richard

Mass transfer controlled by fracturing in micritic carbonate rocks

Abstract The fractured Coniacian chalk from the Omey area (Paris Basin, France) displays strong evidence of modifications controlled by brittle deformation. Fracturing is associated with important changes in pore space (decrease in total porosity and pore interconnection, change in distribution of pore access diameters and capillary characteristics), nannofacies (gradual evolution from a point-contact fabric to a welded, interlocked or coalescent fabric) and chemical composition (Sr concentration decrease). These modifications result from fluid–rock interaction that control significant mass transfer (percentage of secondary calcite >50%). Sr is a remarkable indicator of these mass transfers. Sr analyses allowed us to prove that the deformed zone (26.7 m) is wider than the fractured zone (11.3 m). They also indicate that the footwall block is less affected than the hanging wall block. A physicochemical model of the deformation mechanism is proposed. It shows that a cyclic process of fracturing controls the temporal evolution of the fluid saturation and fluid pressure and, consequently, the mass transfer.

L'hydrofracturation: une déformation tectonique à haut potentiel diagénétique. Exemple des craies hydrofracturées de la région d'Omey (Bassin de Paris, France)

Resume Letude des craies hydrofracturees de la region dOmey (Bassin de Paris, France) revele que lhydrofracturation est une deformation tectonique a haut potentiel diagenetique. Dans ce secteur geographique, lhydrofracturation declenchee par la mise en traction omnidirectionnelle du milieu a induit dimportantes modifications du reseau poreux, du nannofacies et de la teneur en Sr de la craie. Dimportants transferts de matiere sont associes a ces transformations diagenetiques, qui resultent dune deformation par dissolution-cristallisation. Ce fort potentiel diagenetique est lie a la presence de fluides interstitiels jouant le role de vecteur et dactivateur des transferts de matiere.

Characterization and origin of low-T willemite (Zn2SiO4) mineralization: the case of the Bou Arhous deposit (High Atlas, Morocco)

Willemite (Zn2SiO4) usually reported in hypogene non-sulfide deposits is described as the main ore mineral in the carbonate-hosted Bou Arhous zinc deposit. This deposit is located in the High Atlas intracontinental range that formed during the Tertiary. Based on a set of microscopic observations, it was possible to establish that willemite replaces primary sphalerite. On the basis of cathodoluminescence imaging, three successive generations of willemite are distinguished, with evidence of dissolution–reprecipitation processes. Willemite is also variably enriched in Ge (up to 1000xa0ppm), while Ge contents lower than 100xa0ppm are reported in the primary sulfide minerals. Depending on the willemite generation, this substitution was positively or negatively correlated to the Zn-Pb substitution. According to the nature of zoning (sector versus oscillatory), the incorporation of Ge was either controlled by crystallographic factors or by the nature of the mineralizing fluids. Willemite is associated with other oxidation-related mineral species, like cerussite (PbCO3) but is not in isotopic equilibrium and therefore not considered to be cogenetic. Oxygen isotope compositions support the formation of willemite at temperatures below 130xa0°C, from mixed meteoric and deeper, hydrothermal fluids. The formation of the High Atlas Belt during the Tertiary has contributed to the exhumation of the sulfide minerals and the development of vertical conduits for percolation of meteoric water and ascending hydrothermal fluids. In addition to a local contribution of silicate minerals of the host limestone, hydrothermal fluids probably transported Si and Ge that are incorporated in willemite.