Clay mineral formation and fluid-rock interaction in fractured crystalline rocks of the Rhine rift system : case studies from the Soultz-sous-Forêts granite (France) and the Schauenburg Fault (Germany)

Abstract

The following study deals with the fluid-alteration history of fractured and faulted crystalline granitic basement in the Rhine rift system, based on the analytical investigation of clay mineral assemblages. It evaluates the argillite mineralizations along faults and fracture zones at key tectonic positions in relation to the upper Rhine graben rift structure and assesses the timing and episodic nature of clay mineral formation that occurred within both the buried basement rocks and similar exhumed units exposed in the uplifted rift shoulders. The rock alteration history is established largely on the basis of combined X-ray diffraction, X-ray textural goniometry, electron microscopy (SEM, HRTEM) and isotopic (K-Ar) techniques. One chapter presents results from the granitic basement rocks of the 2230m deep EPS-1 drilling site at Soultz-sous-Forets, situated in the western part of the Upper Rhine graben basin (France). Pure illite without any sign of mixed-layerings could be distinguished in the different hydrothermal altered and fractured granite and argillite veins. The morphology of these illites varies clearly in the different rock types, with pseudo-hexagonal, platy 2M1 illite polytypes dominating the veins and thin and fibrous to lath shaped 1M trans-vacant illite polytypes common in pores of the granite matrix. The separated grain size fractions (> 63µm, 10-63µm, 4-10µm, 2-4µm, 1-2µm, 0.4-1µm, 0.2-0.4µm, < 0.2µm) indicate mixtures of both illite shapes and polytypes to characterize all samples, which based on K-Ar isotope results, occurred during three main crystallization episodes spanning over 200 Ma in time. The oldest illite crystallization recognized is of Permian age, which is most evident in the argillite veins (2176m) and samples of hydrothermally altered granite (e.g. 1570m). In contrast, a mixture of Jurassic and Cretaceous (or younger) illite was recorded in fractured rocks at 2167m, but Tertiary K-Ar values were not encounted in the limited amount of sample studied here. Younger Tertiary ages have, however, been reported in other studies. The multiple episodes of local illite crystallizations are suggested to reflect the range of fluid events documented from quartz fluid inclusions. The Permian vein mineralization probably relates to the high temperature CO2-rich fluids and the Mesozoic and Cenozoic illite to the younger, lower temperature saline brines. On the basis of the mineral characteristics presented, it is also suggested that the 2M to 1M transition was favored by lowering of the rock permeability and the related increase in the state of fluid saturation, which occurred during successive episodic crystallization and progressive sealing of the granite. The next chapter presents the results from the uplifted shoulder locality of the Schauenburg Fault, near Heidelberg. These fault rocks occur along a E-W trending dextral oblique-slip fault (displacement ca. 100m), which separates the Variscan Heidelberg granite from younger Permian volcanics. In comparison to the Soultz-granite, the alteration assemblages of the Schauenburg indicate a retrograde sequence of fluid-controlled, low-temperature clay mineral reactions, ranging from Jurassic times to more recent activity during uplift of the rift shoulder. The progressive decrease in fluid temperatures caused a back-reaction of 2M1 illite to 1Md (R3) illite-smectite, as well as smectite and kaolinite. The enhanced rock permeability and fluid movement along the cataclastic fault zone is evident from the illite texture and rock fabric. The last chapter synthesizes and discusses the different alteration histories of both studied localities of the Rhine Graben rift structure and evaluates the nature of clay mineral reactions and the timing and mechanisms of crystal growth in relation to the history of fluid-rock interaction. Despite the petrological similarities of both Variscan granites, there are notable differences in mineral alteration history. The hydrothermal alteration of the buried Soultz-sous-Forets granite shows a number of episodic pure illite crystallization events with repeated transition from 2M to 1M polytypes occuring during Permian, Jurassic and Cretaceous (to younger) time. In contrast, the exhumed Schauenburg Fault rocks document a similar hydrothermal illite crystallization event during the Jurassic, but was subsequently flushed by more dilute meteoric waters attributed to rift shoulder uplift.