The abundance and distribution of radioelements on bulk and microscopic scales were investigated in residual granitic-derived soil at a facility for investigating the movement of radon into structures. In bulk soil samples, Ra concentrations range from 0.6 to 1.3 pCi/g, and variations in Ra, Th, and K appear to be controlled mainly by heterogeneities inherited from the parent granitic rock, which contains abundant dikes and inclusions. U in soil and parent rock is concentrated in primary minerals (mainly zircon and sphene), and in secondary sites that are of greater importance for Rn emanation. The main U-bearing secondary sites are weathered sphene, grain boundary coatings, weathered biotite and plagioclase, as well as dense Fe-rich coatings and a REE-phosphate mineral present in near-vertical fracture zones in saprolite underlying shallow loam. Elevated U in these sites generally correlates with high Ti, Al, Fe, and/or P. Preferential distribution of U and Ra on grain boundaries and porous weathered minerals is reflected in relatively high Rn emanation rates in the soil. Highest emanation occurs between 1.3 and 2.3 m depth, where fine pedogenic phasesgibbsite and amorphous silica and Fe-OOH—are most abundant; it is related to fixation of Ra by these phases, which precipitate close to the surface and accumulate at these depths by illuviation. Separation of Ra from U may occur locally, given remobilization of U-series elements from secondary sites, and large differences between Ra and U sorption capabilities of several phases present in the soil. Concentration of U along permeable fracture zones in saprolite suggests that contribution of soil-gas Rn from depth (> 2 m) could be significant to Rn availability near the surface. 相似文献
To provide guides for exploration of porphyry copper mineralization at a district scale, we examine the spatial association between known porphyry copper deposits and geologic features in Benguet, Philippines. The spatial associations between the porphyry copper deposits and strike-slip fault discontinuities, batholithic pluton margins and porphyry plutons are quantified using weights of evidence modeling. In the training and testing district, the porphyry copper occurrences are associated spatially with strike-slip fault discontinuities, batholithic pluton margins and contacts of porphyry plutons within distances of 3 km, 2.25 km, and 1 km, respectively. In addition, the porphyry plutons are associated spatially with strike-slip fault discontinuities and contacts of batholithic plutons within a distance of 2.25 km and 3 km, respectively. Based on these significant spatial associations, predictive maps are generated to delineate zones favorable for porphyry copper mineralization and zones favorable for emplacement of porphyry plutons in Benguet province, Philippines. Validations of the predictive models demonstrate their efficacy in pointing to zones for subsequent follow-up exploration work. 相似文献
Feldspar grain-size reduction occurred due to the fracturing of plagioclase and K-feldspar, myrmekite formation and neocrystallization of albitic plagioclase along shear fractures of K-feldspar porphyroclasts in the leucocratic granitic rocks from the Yecheon shear zone of South Korea that was deformed under a middle greenschist-facies condition. The neocrystallization of albitic plagioclase was induced by strain energy adjacent to the shear fractures and by chemical free energy due to the compositional disequilibrium between infiltrating Na-rich fluid and host K-feldspar. With increasing deformation from protomylonite to mylonite, alternating layers of feldspar, quartz and muscovite developed. The fine-grained feldspar-rich layers were deformed dominantly by granular flow, while quartz ribbons were deformed by dislocation creep. With layer development and a more distributed strain in the mylonite, lower stresses in the quartz-rich layers resulted in a larger size of dynamically recrystallized quartz grains than that of the protomylonite. 相似文献