Clay Mineralogy: A Signature of Granitic Geothermal Reservoirs of the Central Upper Rhine Graben

Clay minerals are the signature of hydrothermal alterations related to fluid circulation in volcanic and crystalline rocks. In the French part of the Upper Rhine Graben, in the deep-seated granites, illitic minerals (illite and I/S mixed layers (ml)) are typical products of the structurally-controlled argillic alteration in the Paleozoic granitic basement. In the new Illkirch geothermal well, GIL-1, drill-cuttings were studied with various petrographic methods to determine the characteristics of illite in paleo- and present-permeable zones, and to compare the alteration mineralogy with that of geothermal Soultz-sous-Forêts and Rittershoffen sites. Alteration petrography, crystal structure as well as the chemical composition of the illitic minerals and the altered bulk rocks were performed all along the well. This complete characterization, combined with geophysical logs and structural results, highlighted that the illitic minerals at Illkirch, Soultz-sous-Forêts, and Rittershoffen are composed of illite and illite-rich illite-smectite mixed layers (I/S ml) (<10% smectite). Two mineralogical assemblages were distinguished: chlorite + illite resulting from the propylitic alteration after the emplacement of the granitic basement under temperatures higher than 350 °C, and illite + I/S ml + carbonates + quartz resulting from the argillic alteration due to fluid circulation in the fractures at temperatures between 130 and 160 °C. Fracture zones are characterized by the occurrence of illitic minerals (illite and I/S ml), and specifically, by higher quantities of I/S ml in present-day permeable zones than in paleo-permeable zones. A conceptual model of the fracture zones at the interface between the overlying sedimentary rocks and the granitic basement is proposed. The present-day permeability distribution is controlled by the fault and fracture network, which consists of sealed zones and unsealed zones. Fluid convection in the URG implies paleo and present fluids circulating in both fractured sedimentary and crystalline reservoirs. Such circulations develop illitic minerals that could be considered as exploration guides for future geothermal sites in the URG. At Illkirch, the repartition of the present-permeable fracture zones (KFZs) in the GIL-1 well indicates that the moderately argillically altered granite distally situated from the Eschau fault is more permeable than the intensely argillically altered granite close to the Eschau fault.

New data on minerals of the hidalgoite-philipsbornite series from the Guatomo mine near Tham Thalu, Yala Province (Thailand)

The chemical composition of three samples of minerals of the hidalgoite-philipsbornite series from the central part of the Guatomo mine pit was studied in detail by EMPA-WDS. They form yellowish-green, light green to pale blue microcrystalline crusts or hollow pseudomorphs after prismatic hexagonal crystals of mimetite in fractures of strongly altered granite or quartz. All studied samples show relatively strong, irregular to oscillatory chemical zoning in BSE, caused by S versus As substitution on T-site. Most of the zones are corresponding to hidalgoite, with only minor, outer zones represented by S-rich philipsbornite. Besides of dominant contents of Pb, Al, As and S also minor amounts of K (up to 0.11 apfu), Na (up to 0.05 apfu), Cu (up to 0.22 apfu), Fe (up to 0.09 apfu) and P (up to 0.04 apfu) were detected in studied samples.

Petrographical, petrochemical characteristics of Ta Kou massif granitoids, Ham Thuan Nam, Binh Thuan

Ta Kou is an isometric shape with an area of ​​about 15 km2. Petrographical composition is mainly biotite-hornblende granodiorite; minors are altered light-colored fine-grained biotite granite. Mineral composition is mainly plagioclase 45–50, quartz 25, potassium feldspar 15–20, biotite 5–10, hornblende (5–7); secondary is pyroxen; Accessory minerals include zircon, apatite, orthit, muscovite and rare ore. In some places, near fault or high arch of massif, rocks have been altered by other magma, especially near faults due to post-magma activity including fine-grained biotite granite which caused strongly by alkalization such as increasing the content of potassium felspar (35–50%) and quartz 30; reducing plagioclase content (30–25%); amphibole -colored minerals is rare, and pyroxene is absent. Chemical composition of medium acid granodiorite SiO2 61.08–62.14 (61.85); total alkalinity (K2O+Na2O) 5,99–6,04 (6.00); ratio of alkaline K2O/Na2O: 0.74-0.77 (0.75 <1). Characterized trace elements content of granodiorite: Rb and Ba are low but Sr is quite high; ratios Rb/Sr: 0.24; Ba/Sr 1.40, Ba/Rb: 5.85; K/Rb: 245.39; La/Yb: 10.33; Ce/Yb: 22.11; normal Eu anomalies. When influencing the post-magmatic activity, some granite is more acidific, the chemical composition of altered granite SiO2 72.27–74.07 (73.17); total alkalinity (K2O+Na2O): 7.48–7.96 (7.72); the ratio of alkaline K2O/Na2O: 1.60–1.69 (1.64>1). Characterized trace elements content of altered granite: Ba and Sr are low but high Rb; ratios of Rb/Sr: 1.43; Ba/Sr: 3.06, Ba/Rb: 2.79; K/Rb: 218.05; La/Yb: 8.60 and Ce/Yb: 15.74; strong Eu anomalies.Ta Kou granitoids belong to the medium to high aluminum series, medium to high potassium alkaline series, negative Eu anomalies are from normal to strong, type of I- granite. Granitoid characterized subduction-related formation and altered, which may be due to the effects of later phase magmatic activity. Compared with granitoid formations in South Vietnam, Ta Kou massif granitoids belong to phase 2 of Định Quán complex.

Damage to the Microstructure and Strength of Altered Granite under Wet–Dry Cycles

This paper presents an analytical method for surrounding rocks in symmetrically shaped tunnels or roadways, with the symmetrical rise and fall of groundwater over a certain period. The influence of reservoir water level on wet–dry cycles were studied. The changes in the microstructure and strength of altered granite and its evolution were explored using mechanical tests and scanning electron microscopy (SEM). The results showed that: (1) the wet–dry cycles weakened the strength of altered granite. Furthermore, the uniaxial compressive strength, elastic modulus, cohesion, and internal friction angle decreased with the increase of the number of cycles, while the maximum reduction in these parameters reached 50.22%, 63.84%, 93.76%, and 53.90%, respectively. (2) The wet–dry cycles damaged the microstructure of altered granite. The SEM analysis showed that, under wet–dry cycles, the structure of altered granite changed from a smooth and integrated internal structure to the initiation, development, and expansion of pores and cracks. The porosity and fractal dimension of rock were determined using the SEM results. The degree of damage to altered granite under wet–dry cycles was quantitatively analyzed. (3) According to the rock mechanics strength tests and SEM and X-ray diffraction analyses, the damage mechanism of altered granite subjected to wet–dry cycles was discussed. The results provide the basis for a stability analysis of symmetrically shaped tunnels, especially symmetrical tunnels constructed in water-rich areas such as symmetric circular tunnels and symmetric horseshoe tunnels.