Academic literature on the topic 'Phyllosilicate rich mylonites'

AbstractPalladium-bearing minerals from the Cauê iron mine, Itabira District, Minas Gerais, Brazil, are found in gold-rich jacutinga, a hydrothermally-altered Lake Superior-type carbonate-bearing oxide facies iron-formation. Palladium occurs as: native palladium with trace contents of Au, Fe and Cu; palladseite ((Pd,Cu,Hg)17Se15), which was found in the core of a grain of palladium; palladium–copper oxide ((Pd,Cu)O); and arsenopalladinite (Pd8(As,Sb)3), with inclusions of palladium–copper oxide. The palladium and palladium–copper oxide grains are coated with films of gold and commonly do not exceed 100 µm in width. These palladium minerals occur in hematite bands and in boudinaged bands of quartz and white phyllosilicate parallel to the S1 mylonitic foliation. Palladium-copper oxide also occurs as inclusions in gold grains which are strongly to weakly stretched parallel to S1.Palladium mineralization is interpreted as synchronous with intense D1 shearing and contemporaneous with the peak of thermal metamorphism. At high oxygen fugacities and high temperatures (up to 600°C), Pd may have been transported as chloride complexes and deposited following changes in pH caused by mineralizing fluids reacting with jacutinga. Deposition may also have been prompted by the formation of insoluble selenide and arsenide–antimonide minerals and by the dilution of C1 concentrations in the mineralizing fluid. Textural studies, and the zonation observed in palladium and other hydrothermal minerals, suggest that oscillations in the physico-chemical conditions of hydrothermal fluids occurred during the mineralizing event.

Abstract Brittle reactivation of plastic shear zones is frequently observed in geologically old terranes. To better understand such deformation zones, we have studied the >700 Ma long structural history of the Himdalen–Ørje Deformation Zone (HØDZ) in SE Norway by K–Ar and 40Ar–39Ar geochronology, and structural characterization. Several generations of mylonites make up the ductile part of HØDZ, the Ørje Shear Zone. A 40Ar–39Ar white mica plateau age of 908.6 ± 7.0 Ma constrains the timing of extensional reactivation of the Ørje mylonite. The mylonite is extensively reworked during brittle deformation events by the Himdalen Fault. 40Ar–39Ar plateau ages of 375.0 ± 22.7 Ma and 351.7 ± 4.4 Ma from pseudotachylite veins and K–Ar ages of authigenic illite in fault gouge at c. 380 Ma are interpreted to date initial brittle deformation, possibly associated with the Variscan orogeny. Major brittle deformation during the Early–Mid Permian Oslo Rift is documented by a 40Ar–39Ar pseudotachylite plateau age of 294.6 ± 5.2 Ma and a K–Ar fault gouge age of c. 270 Ma. The last datable faulting event is constrained by the finest size fraction in three separate gouges at c. 200 Ma. The study demonstrates that multiple geologically significant K–Ar ages can be constrained from fault gouges within the same fault core by combining careful field sampling, structural characterization, detailed mineralogy and illite crystallinity analysis. We suggest that initial localization of brittle strain along plastic shear zones is controlled by mechanical anisotropy of parallel-oriented, throughgoing phyllosilicate-rich foliation planes within the mylonitic fabric.

Phyllosilicates, even in relatively small quantities, dramatically influence the mechanical behavior of rocks. In laboratory triaxial tests on foliated rocks, for content in phyllosilicates greater than 20-25%, a relevant mechanical anisotropy appears, as the internal friction coefficient (tangential stress/normal stress at failure) varies between 0.3 and 0.7 with orientation of the sample with respect to the maximum compressive stress. This reflects different fracture modes: when the foliation is favorably oriented, fractures develop along it and the rocks are weak, whilst when fractures cut the foliation at a high angle, rocks are stronger. This kind of mechanical anisotropy is one possible explanation for the relative and absolute fault weakness shown by non-Andersonian misoriented faults (i.e. faults with an orientation, with respect to the regional stress field, not fulfilling Anderson’s theory of faulting). Examples of misoriented faults are low angle normal faults (LANFs), high angle reverse faults and strike slip faults developed at a high angle with the most compressive regional stress axis. In this thesis I have considered two field examples of misoriented faults represented by the Simplon Line Fault Zone (SFZ), in the Swiss Alps, and a zone of (ultra)cataclastic bands in the Grandes Rousses Massif of the French Alps (GRM). These structures have been characterized from the regional scale (paleostress), to the meso-scale (fault zone architecture), and micro-scale (optical microscope, SEM and micro-CT). Moreover, I have characterized the petrophysical properties and mechanical anisotropy of the GRM rocks with density, porosity, uniaxial (UCS) and triaxial (TXT) lab tests performed at the Environmental Science Centre of the British Geological Survey in Keyworth (Nottingham,UK). Both fault zones have been involved in Alpine brittle deformations, which acted on rocks characterized by a pre-existing schistosity. The SFZ is a LANF developed in mylonitic phyllosilicate-rich paragneiss and orthogneiss, characterized by the alternation of anastomosing phyllosilicate films and quartz and feldspar lithons/layers. The schistosity is misoriented for brittle reactivation (high angle with respect to σ1), and its weakness is demonstrated by its brittle activation with the progressive development of cataclasites, which nucleate in the phyllosilicate levels. Brittle deformation in the GRM micaschists, around the glacial plain of the Saint Sorlin Lakes, shows two different failure modes, depending on the schistosity orientation. For a σ1/foliation ẞ angle of ca. 80°, we observe the development of Andersonian conjugated fractures, with a stair-stepping failure path controlled by either [001] mica planes or brittle fractures crosscutting the more competent quartz and feldspar layers. With a ẞ angle of ca. 72° we observe no Andersonian fractures and the complete brittle activation of the schistosity, along which penetrative ultracataclastic seams are developed. The mechanical characterization of GRM’s samples with UCS and TXT highlights the relationships between elastic moduli, peak strength, and failure modes, as a function of the ẞ angle. Peak strength follows a continuously varying anisotropy model, since we observe a progressive and continuous strength decrease from ẞ = 0° to ẞ = 45°, and a similarly smooth and continuous increase up to ẞ = 90°. If we consider the different failure modes, we can see a progressive transition from an Andersonian behavior (ẞ = 0° or 90°), a hybrid failure mode dominated by stair-stepping fractures, and a failure mode dominated by slip along phyllosilicate films (20° < ẞ < 70°). Field and lab results evidence the primary role of phyllosilicate rich foliation in nucleation and development of fracturing and faulting.