A review and comparison of fracture mineral investigations and their application to radioactive waste disposal

摘要

A compilation and comparison of fracture mineral studies from the Canadian and Fennoscandian Shields and the French Massif Central shows many similarities indicating larger external control over fracture mineral deposition, with different rock types exerting local controls. The sites investigated represent a wide range of geological settings, and host rock types ranging from felsic intrusive and extrusives to ultramafic intrusives and volcanics that span an age range from 2.5 to 0.36 Ga. Typical fracture minerals found at Canadian Shield sites include calcite, quartz, chlorite and clays, and these do not appear to be dependant on age, erosional depth or geological environment. The Fennoscandian Shield has a much larger variety of fracture filling minerals with epidote, zeolites, prehnite, fluorite, pyrrhotite, Fe oxides, serpentine, graphite, magnesite and barite in addition to the minerals typically found at Canadian Shield sites. The major control on fracture mineral type is most likely variations in rock type, and fluid chemistry and temperature. The C and O isotopic range of calcite is very similar among sites. Late-stage hydrothermal calcite, with strongly depleted delta O-18 values, is common at many sites. All of the sites have calcite with delta O-18 isotopic values in the range of -5 to -20% PDB, indicative of formation from meteoric water or basinal brines that have undergone varying degrees of water/rock interaction. One Canadian and a few Swedish sites have calcite in the shallower portion of the rock that shows isotopic evidence of dissolution and re-precipitation in equilibrium with the present-day waters. There are some striking similarities in fluid inclusion data among sites. Most sites have an elevated temperature (100-300 degrees C), low salinity group of fluid inclusions within the NaCl-H2O system, and a lower temperature (50-150 degrees C), higher salinity group of fluid inclusions within the NaCl-CaCl2-H2O System. Fluid inclusion density plots show some evidence of simple cooling, but most sites show two or more fluids were responsible for calcite formation. The origin of most of these fluids was magmatic/hydrothermal or meteoric water that had undergone varying degrees of water/rock interaction, but basinal brines and seawater were also possible sources. Several techniques and methods have been used to further characterize calcites. Strontium isotopes and rare earth elements can be useful to recognize different families of calcite. Uranium-Th dating has found many old calcites beyond the useful range of the technique, but also some relatively young calcites that may be related to interglacial periods. Where fluid inclusion data exists, formation temperatures were not consistent with a glacial water origin. Crush and leach experiments (with ion and gas chromatography and thermal ion mass spectrometry) have characterized inclusion fluids, but special care must be used to ensure only one generation is sampled at a time. Cathodoluminescence and scanning electron microscopy with energy-dispersive spectrometry has been useful in identifying multiple fluid generations within single calcite samples. Laser ablation and Raman spectrometry are additional techniques that are useful in determining individual fluid inclusion chemistry and isotopes.