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A review of the current knowledge of lithostratigraphy, geochronology, mineralogy, alteration, fluid chemistry and structural data is provided in order to discuss the main controls on mineralization in the Crixás gold deposit and the existing structural framework and evolution. Gold mineralization at Crixás represents orogenic upper mesothermal to lower hypothermal types developed within the overturned Paleoproterozoic (Rhyacian) stratigraphic sequence. The structural data indicates that upright folding of the stratigraphy and formation of a distinct S1 foliation characterizes D1 deformation. This controls the formation of laminated quartz veins, precipitation of Au-rich sulfides, and the development of NW trending orebodies. Localized F2 folding of D1 structures and strong L2 stretching lineations characterizes the D2 event. This localized F2 folding created oreshoots trending W-WNW within distinct orebodies. The D2 deformation is associated with the precipitation of native Au. The ‘bulk plunge’ of these orebodies is a combination of both plunges defined by D1 and D2 deformation events. A critical assessment of lithostratigraphic and structural data reveal two possible tectonic scenarios operating during the Paleoproterozoic. The first possible tectonic scenario considers sagduction and the development of a regional-scale synclinorium, followed by tilting and localized folding of D1 structures. The second scenario, which is similar to the current model, considers accretion producing a synclinorium, followed by thrusting. Based on the current knowledge, both models are considered to be permissible and a series of focused research studies are proposed to test both the current and the new tectonic models.

The supergiant Obuasi gold deposit is the largest deposit in the Paleoproterozoic Birimian terranes of West Africa with 62 Moz of gold (past production + resources). The deposit is hosted in the Paleoproterozoic Kumasi Group sedimentary rocks composed of carbonaceous phyllites, slates, psammites, and volcaniclastic rocks intruded by different generations of felsic dikes and granites. A three-stage deformation history is defined for the district. The D1Ob stage is weakly recorded in the sedimentary rocks as a layer-parallel fabric and indicates that bedding parallel shearing occurred during the early stage of deformation at Obuasi. The D2Ob is the main deformation stage affecting the Obuasi district and corresponds to a NW-SE shortening. Tight to isoclinal folding, as well as intense subhorizontal stretching, occurred during D2Ob, parallel with the plane of a pervasive NE-striking subvertical foliation (S2Ob). Finally, a N-S shortening event (D3Ob) refolded previously formed structures and formed a distinct ENE-striking, variably dipping S3Ob cleavage that is domainal in nature throughout the deposit. Two economic styles of mineralization occur at Obuasi and contribute equally to the gold budget. These are (1) gold-bearing sulfides, dominantly arsenopyrite, mainly disseminated in metasedimentary rocks and (2) native gold hosted in quartz veins that are as much as 25 m wide. Microstructural evidence, such as strain shadows surrounding gold-bearing arsenopyrite parallel with S2Ob, but folded by S3Ob, indicates that the sulfides were formed during D2Ob. Concentrations of as much as 700 ppm Au are present in the epitaxial growth zones of the arsenopyrite grains. Although the large mineralized quartz veins are boudinaged and refolded (indicating their formation during D2Ob), field and microanalytical observations demonstrate that the gold in the veins is hosted in microcracks controlled by D3Ob, where the S3Ob cleavage crosscuts the quartz veins in the main ore zones. Thus, these observations constitute the first evidence for multiple stages of gold deposition at the Obuasi deposit. Futhermore, three-dimensional modeling of stratigraphy, structure, and gold orebodies highlights three major controls on oreshoot location, which are (1) contacts between volcaniclastic units and pre-D1 felsic dikes, (2) fault intersections, and (3) F3Ob fold hinges. The maximum age for the older disseminated gold event is given by the age of the granites at 2105 ± 2 Ma, which is within error of hydrothermal rutile in the granites of 2098 ± 7 Ma; the absolute age of the younger gold event is not known.

The geological inventory of the Variscan Bohemian Massif can be summarized as a result of Early Devonian subduction of the Saxothuringian ocean of unknown size underneath the eastern continental plate represented by the present-day Tepla-Barrandian and Moldanubian domains. During mid-Devonian, the Saxothuringian passive margin sequences and relics of Ordovician oceanic crust have been obducted over the Saxothuringian basement in conjunction with extrusion of the Tepla-Barrandian middle crust along the so-called Tepla suture zone. This event was connected with the development of the magmatic arc further east, together with a fore-arc basin on the Tepla-Barrandian crust. The back-arc region – the future Moldanubian zone – was affected by lithospheric thinning which marginally affected also the eastern Brunia continental crust. The subduction stage was followed by a collisional event caused by the arrival of the Saxothuringian continental crust that was associated with crustal thickening and the development of the orogenic root system in the magmatic arc and back-arc region of the orogen. The thickening was associated with depression of the Moho and the flux of the Saxothuringian felsic crust into the root area. Originally subhorizontal anisotropy in the root zone was subsequently folded by crustal-scale cusp folds in front of the Brunia backstop. During the Visean, the Brunia continent indented the thickened crustal root, resulting in the root's massive shortening causing vertical extrusion of the orogenic lower crust, which changed to a horizontal viscous channel flow of extruded lower crustal material in the mid- to supra-crustal levels. Hot orogenic lower crustal rocks were extruded: (1) in a narrow channel parallel to the former Tepla suture surface; (2) in the central part of the root zone in the form of large scale antiformal structure; and (3) in form of hot fold nappe over the Brunia promontory, where it produced Barrovian metamorphism and subsequent imbrications of its upper part. The extruded deeper parts of the orogenic root reached the surface, which soon thereafter resulted in the sedimentation of lower-crustal rocks pebbles in the thick foreland Culm basin on the stable part of the Brunia continent. Finally, during the Westfalian, the foreland Culm wedge was involved into imbricated nappe stack together with basement and orogenic channel flow nappes.

A detailed field study reveals a gradual transition from high-grade solid-state banded orthogneiss via stromatic migmatite and schlieren migmatite to irregular, foliation-parallel bodies of nebulitic migmatite within the eastern part of the Gfohl Unit (Moldanubian domain, Bohemian Massif). The orthogneiss to nebulitic migmatite sequence is characterized by progressive destruction of well-equilibrated banded microstructure by crystallization of new interstitial phases (Kfs, Pl and Qtz) along feldspar boundaries and by resorption of relict feldspar and biotite. The grain size of all felsic phases decreases continuously, whereas the population density of new phases increases. The new phases preferentially nucleate along high-energy like–like boundaries causing the development of a regular distribution of individual phases. This evolutionary trend is accompanied by a decrease in grain shape preferred orientation of all felsic phases. To explain these data, a new petrogenetic model is proposed for the origin of felsic migmatites by melt infiltration from an external source into banded orthogneiss during deformation. In this model, infiltrating melt passes pervasively along grain boundaries through the whole-rock volume and changes completely its macro- and microscopic appearance. It is suggested that the individual migmatite types represent different degrees of equilibration between the host rock and migrating melt during exhumation. The melt topology mimicked by feldspar in banded orthogneiss forms elongate pockets oriented at a high angle to the compositional banding, indicating that the melt distribution was controlled by the deformation of the solid framework. The microstructure exhibits features compatible with a combination of dislocation creep and grain boundary sliding deformation mechanisms. The migmatite microstructures developed by granular flow accompanied by melt-enhanced diffusion and/or melt flow. However, an AMS study and quartz microfabrics suggest that the amount of melt present did not exceed a critical threshold during the deformation to allow free movements of grains.

This work contributes to the experimental investigations of the origin and 3-D orientation of micropores in low porosity crystalline rocks. The origin and spatial orientation of microporosity in two eclogites with different microstructures were studied by 1) quantitative and qualitative microstructural analysis of grains and grain boundaries, 2) measurement of lattice preferred orientation using the SEM-EBSD method and 3) experimental measurement of velocity of elastic P-waves in spherical samples in 132 directions under confining pressures up to 400 MPa. Results show good correlation between the elastic properties and the orientation of grain boundaries and cleavage planes in clinopyroxene. The magnitude and anisotropy of velocity change with pressure shows that microporosity in the fine-grained sample is relatively large and strongly preferentially oriented, whereas it is significantly lower and less preferentially oriented in the coarse-grained sample. Seeing that the lattice preferred orientation of clinopyroxene is similar in both samples we can deduce from velocity changes that the grain size of the rock forming minerals controls the amount of microporosity. Also, the orientation of microporosity depends mostly on preferred orientation of grain boundaries and somewhat less on the orientation of cleavage planes. Grain boundaries are therefore the most important contributors to the bulk microporosity in the studied rocks.

Pore space geometry of granitic rocks and its evolution with depth are key factors in large-scale seismics or in projects of enhanced geothermal systems or of deep hazardous waste repositories. In this study, we studied macroscopically anisotropic schlieren-bearing granite by experimental P-wave velocity (V-P) measurements on spherical sample in 132 directions at seven different confining pressures in the range 0.1-400 MPa. In order to discriminate the phenomena affecting the rock elastic properties we analysed the orientation of microcracks and of grain boundaries and we measured the anisotropy of magnetic susceptibility of the rock. Three sets of microcracks were defined, with two of them linked to the massif exfoliation process and one to cooling contraction of the massif. During pressurization the measured mean V-P and V-P anisotropy degree at ambient pressure and at highest confinement (400 MPa) yielded 3.3 km s(-1) and 24 per cent, and 6.2 km s(-1) and 3 per cent, respectively. The associated V-P anisotropy pattern was transversely isotropic and governed by the schlieren, with a minimum V-P direction perpendicular to them and a girdle of high V-P directions parallel to them. The highest change in V-P was observed between 0.1 and 10 MPa, suggesting a significant closure of porosity below depths of 500 m. Change of the V-P anisotropy pattern to orthorhombic together with increase of mean V-P and V-P anisotropy degree during depressurization was attributed to inelastic response of one of the sets of microcracks to the loading-unloading cycle.

[1] The deformation study of midcrustal porphyritic granite reveals exceptionally high strain intensities of feldspar aggregates compared to stronger quartz. Three types of microstructures corresponding to evolutionary stages of deformed granite were recognized: (1) the metagranite marked by viscous flow of plagioclase around strong alkali feldspar and quartz, (2) quartz augen orthogneiss characterized by development of banded mylonitic structure of recrystallized plagioclase and K-feldspar surrounding augens of quartz, and (3) banded mylonite characterized by alternation of quartz ribbons and mixed aggregates of feldspars and quartz. The original weakening of alkali feldspar is achieved by decomposition into albite chains and K-feldspar resulting from a heterogeneous nucleation process. The subsequent collapse of alkaline feldspar and development of monomineralic layering is attributed to the onset of syn-deformational dehydration melting of Mu-Bi layers associated with production of ∼2% melt. The final deformation stage is marked by mixing of feldspars which is explained by higher melt production due to introduction of external water. An already small amount of melt is responsible for extreme weakening of the feldspar because of Melt Connectivity Threshold effect triggering grain boundary sliding deformation mechanisms. The grain boundary sliding controls diffusion creep at small melt fraction and evolves to particulate flow at high melt fractions. Strong quartz shows a dislocation creep deformation mechanism throughout the whole deformation history marked by variations in the activity of the slip systems, which are attributed to variations in stress and strain rate partitioning with regard to changing rheological properties of the deforming feldspars.

[1] High-grade orthogneisses from granulite-bearing lower crustal unit show extreme finite strains of both K-feldspar and plagioclase with respect to weakly deformed quartz aggregates. K-feldspar aggregate in the most intensely deformed sample shows interstitial grains of quartz and albite, which also mark some intragranular fractures within K-feldspar grains. Both interstitial grains and fractures are oriented mostly perpendicular to the sample stretching lineation. Quartz and albite grains within K-feldspar bands are interpreted as crystallized from interstitial melt and the petrology study shows that the melt was produced by a metamorphic reaction in plagioclase-mica bands. Thermodynamic Perple_X modeling shows that melt volume increase was negligible and melt amount was too small to generate considerable melt overpressure for calculated PT conditions. It is therefore suggested that dilation of K-feldspar aggregates and fracturing of its grains represent a final creep failure state, which resulted from the cavitation process accompanying grain boundary sliding controlled diffusion creep. The consequence of cavitation-driven dilation of K-feldspar aggregates is the local underpressure resulting in infiltration of melt from plagioclase bands. Analogy with metallurgy experiments shows that the cavitation process, exclusively developed in cryptoperthitic K-feldspar, can be attributed to its lower purity compared to more pure plagioclase. Contrasting rheological behavior of feldspars with respect to quartz prior to fracturing is attributed to different deformation mechanisms. Feldspars appear weaker due to grain boundary sliding accommodated by coupled melt-enhanced diffusion creep along grain boundaries and dislocation creep within grains, in contrast to quartz deforming via grain boundary migration accommodated dislocation creep.