I have studied syntectonic veins from shales and coarse calcareous sands of the Ordovician Womble Shale, Benton uplift, Arkansas. All veins are composed of calcite with minor quartz and trace feldspar and dolomite or high-Mg calcite in the coarser veins. All host lithologies have a pressure-solution cleavage, more closely spaced in the fine-grained shale beds. The vein internal fabrics are coarsely to finely fibered, with a strong host-rock grain size control on fiber width. The finest fibers are in veins with shale host and the coarsest in the coarse-grained calcareous sandstone. Fiber aspect ratio is inversely proportional to host grain size; more equant vein grains are found in the veins hosted in the coarse host fraction. Within one outcrop, the δ13C and δ18O compositions of the host lithologies range from 1.5 to -3.0 per mil and 7.5 to -14.0 per mil (VPDB), respectively. By contrast, the δ18O composition of the veins is remarkably constant (-13.5 per mil) among veins of starkly different fabrics. This composition is identical to that of the coarse calcareous sandstone lithology in the outcrop. No cathodoluminescence or stable isotope zoning was observed in the veins. In addition, there were no gradients in Ca or Si in the vicinity of the veins, suggesting either that the host did not contribute these elements or that diffusion was not the rate-limiting step to vein formation. In any case, the wide variety of veins was probably formed from meter-scale migration of fluid derived from local calcite-rich layers in calcareous sandstone.

I have studied syntectonic veins from shales and coarse calcareous sands of the Ordovician Womble Shale, Benton uplift, Arkansas. All veins are composed of calcite with minor quartz and trace feldspar and dolomite or high-Mg calcite in the coarser veins. All host lithologies have a pressure-solution cleavage, more closely spaced in the fine-grained shale beds. The vein internal fabrics are coarsely to finely fibered, with a strong host-rock grain size control on fiber width. The finest fibers are in veins with shale host and the coarsest in the coarse-grained calcareous sandstone. Fiber aspect ratio is inversely proportional to host grain size; more equant vein grains are found in the veins hosted in the coarse host fraction. Within one outcrop, the δ13C and δ18O compositions of the host lithologies range from 1.5 to -3.0 per mil and 7.5 to -14.0 per mil (VPDB), respectively. By contrast, the δ18O composition of the veins is remarkably constant (-13.5 per mil) among veins of starkly different fabrics. This composition is identical to that of the coarse calcareous sandstone lithology in the outcrop. No cathodoluminescence or stable isotope zoning was observed in the veins. In addition, there were no gradients in Ca or Si in the vicinity of the veins, suggesting either that the host did not contribute these elements or that diffusion was not the rate-limiting step to vein formation. In any case, the wide variety of veins was probably formed from meter-scale migration of fluid derived from local calcite-rich layers in calcareous sandstone.

The Uitkomst Complex is located within the Great Escarpment area close to the town of Badplaas, approximately 300 km due east of Pretoria, in the Mpumalanga Province, South Africa. This complex is believed to represent a layered conduit system related to the 2.06 Ga Bushveld Complex. The succession from the bottom up comprises the Basal Gabbro- (BGAB), Lower Harzburgite- (LHZBG) and Chromitiferous Harzburgite (PCR) Units, collectively referred to as the Basal Units, followed by the Main Harzburgite- (MHZBG), Upper Pyroxenite-(PXT) and Gabbronorite (GN) Units, collectively referred to as the Main Units. The Basal Unit is largely hosted by the Malmani Dolomite Formation, in the Pretoria Group of the Transvaal Supergroup sediments. The Lower Harzburgite Unit contains numerous calc-silicate xenoliths derived from the Malmani Dolomite. The Basal Units host the economically important nickel-bearing sulphide and chromite deposits exploited by the Nkomati Mine. An area of extensive localized talc-chlorite alteration is found in the area delineated for large scale open cast mining. This phenomenon has bearing on the nature and distribution of the sulphide minerals in the Chromitiferous Harzburgite and to a lesser extent the Lower Harzburgite Units. The Basal Unit is comprised of both near pristine areas of mafic minerals and areas of extensive secondary replacement minerals. Of the olivine minerals, only fosterite of magmatic origin is found, the fosterite suffered hydrothermal alteration resulting in replacement of it by serpentine and secondary magnetite. Three different types of diopside are found, the first is a primary magamatic phase, the second is a hybrid “transitional” phase and the third, a skarn phase. Hydrothermal alteration of the matrix diopside led to the formation of actinolite-tremolite pseudomorphs. This secondary tremolite is intergrown with the nickeliferous sulphide grains. Chromite grains are rimmed or replaced by secondary magnetite. Pyrrhotite grains is also rimmed or replaced by secondary magnetite. Talc and chlorite is concentrated in the highly altered rocks, dominating the PCR unit. Primary plagioclase and calcite do not appear to have suffered alteration to the same extent as the other precursor mafic magmatic and hydrothermal minerals. It is suggested that the PCR was the first unit to be emplaced near the contact of the dolomite and shale host rock. The more primitive mafic mineral composition and presence of chromitite attest to this interpretation. The LHZBG and MHZBG units may have been emplaced simultaneously, the LHZBG below and the MHZBG above. Interaction and partial assimilation of the dolomitic country rock led to a disruption of the primary mafic mineralogy, resulting in the preferential formation of diopside at the expense of orthopyroxene and plagioclase. Addition of country rock sulphur resulted in sulphur saturation of the magma and resulted in the observed mineralization. The downward stoping of the LHZBG magma, in a more “passive” pulse-like manner led to the formation of the calc-silicate xenolith lower third of this unit. It is proposed that the interaction with, and assimilation of the dolomitic host rock by the intruding ultramafic magmas of the Basal Units are responsible, firstly, for the segregation of the nickeliferous sulphides from the magma, and secondly for the formation of a carbonate-rich deuteric fluid that affected the primary magmatic mineralogy of the Basal Unit rocks. The fluids released during the assimilation and recrystallization of the dolomites also led to the serpentinization of the xenoliths themselves and probably the surrounding hybrid and mafic- ultramafic host rocks. The CO2-rich fluids migrated up and outward, while the H2O-rich fluids remained confined to the area around the xenoliths and LHZBG unit. The H2O-rich fluid is thought to be responsible for the retrograde metamorphism of the precursor magmatic and metamorphic minerals in the Lower Harzburgite Unit. The formation of an exoscarn within the dolomitic country rocks and a selvage of endoskarn on the contact form an effective solidification front that prevented further contamination of the magma. It is also suggested that these solidification fronts constrained the lateral extent of the conduit. The CO2-enriched deutric fluid was able to migrate up to the PCR unit. Here the fluid was not removed as effectively as in the underlying parts of the developing conduit. This resulted in higher CO2-partial pressures in the PCR unit, and the stabilization of talccarbonate assemblages that extensively replaced the precursor magmatic mineralogy. Intrusion of the magma into the shales, which may have been more susceptible to assimilation and greater stoping, led to a broadening in the lateral extent of the Complex, in the Main units above the trough-like feature occupied by the Basal Units. Late-stage, hydrous dominated fluid migration is inferred to have been constrained to the central part of the conduit. This is demonstrated by the dominance of chlorite in the central part of the Uitkomst Complex in the study area. The Uitkomst Complex was further deformed by later intrusions of dolerite dykes. Weathering of the escarpment led to exposure of the conduit as a valley and oxidation of the surficial exposed rocks. / Thesis (PhD)--University of Pretoria, 2012. / Geology / unrestricted

<p> Groundwater samples from seven boreholes at the Atikokan research area, northwestern Ontario, have been analysed to determine their Sr content and isotopic composition. Whole rock and mineral separates have also been analysed. The groundwaters can be broadly divided into two groups. The 'shallow' waters have low Sr content and high and variable isotopic ratios (.705 - .728) while the 'deep' Sr-rich saline groundwaters have low and constant 87Sr/86Sr ratios (.706 - .707). The saline waters are in isotopic equilibrium with plagioclase and the role of other major rock forming minerals in controlling the isotopic and chemical composition of these waters is negligible. Gypsum is in isotopic equilibrium with the saline waters and appears to be 'young'.</p> <p> The degree of water-plagioclase interaction appears to have been extensive implying low water/rock ratios (ie., 'closed' system. The chemistry of the Atikokan groundwaters is similar to several Shield mine waters. If seawater was a precursor of these waters, there is no evidence for it today. Some degree of mixing with surface waters is indicated in all samples. </p> <p> The granites of the Eye-Dashwa lakes pluton are very Sr-rich. This is reflected in the low isotopic ratios of plagioclase and other minerals and in turn in the low ratios for the saline groundwaters, which are some of the lowest values yet reported.</p> / Thesis / Master of Science (MSc)

Quantifying the influence of pressure on the effective elastic rock properties is important for applications in rock physics and reservoir characterization. Here I investigate the relationship between effective pressure and seismic velocities by performing inversion on the laboratory-measured data from a suite of clastic, carbonate and igneous rocks, using different analytic and discrete inversion schemes. I explore the utility of a physical model that models a natural fracture as supported by asperities of varying heights, when an effective pressure deforms the tallest asperities, bringing the shorter ones into contact while increasing the overall fracture stiffness. Thus, the model is known as the ?asperity-deformation? (ADM) or ?bed-of-nails? (BNM) model. Existing analytic solutions include one that assumes the host rock is infinitely more rigid than the fractures, and one that takes the host-rock compliance into account. Inversion results indicate that although both solutions can fit the data to within first-order approximation, some systematic misfits exist as a result of using the rigid-host solution, whereas compliant-host inversion returns smaller and random misfits, yet out-of-range parameter estimates. These problems indicate the effects of nonlinear elastic deformation whose degree varies from rock to rock. Consequently, I extend the model to allow for the pressure dependence of the host rock, thereby physically interpreting the nonlinear behaviors of deformation. Furthermore, I apply a discrete grid-search inversion scheme that generalizes the distribution of asperity heights, thus accurately reproduces velocity profiles, significantly improves the fit and helps to visualize the distribution of asperities. I compare the analytic and numerical asperity-deformation models with the existing physical model of elliptical ?pennyshape? cracks with a pore-aspect-ratio (PAR) spectrum in terms of physical meaning and data-fitting ability. The comparison results provide a link and demonstrate the consistency between the use of the two physical models, making a better understanding of the microstructure as well as the contact mechanism and physical behaviors of rocks under pressure. ADM-based solutions, therefore, have the potential to facilitate modeling and interpretation of applications such as time-lapse seismic investigations of fractured reservoirs.