Applications of Natural Radiation-Induced Paramagnetic Defects in Quartz to Exploration in Sedimentary Basins

摘要

Quartz grains in contact with uranium-bearing minerals or fluids are characterized by natural radiation-induced paramagnetic defects (e.g., oxygen vacancy centers, silicon vacancy centers, and peroxy radicals), which are amenable to study by electron paramagnetic resonance (EPR) spectroscopy. These natural radiation-induced paramagnetic defects, except for the oxygen vacancy centers, in quartz are concentrated in narrow bands penetrated by α particles: (1) in halos around U-and Th-bearing mineral inclusions and (2) in outer rims or along fractures. The second type of occurrence provides information about uranium mineralization or remobilization (i.e., sources of uranium, timing of mineralization or remobilization, pathways of uranium-bearing fluids). It can also be used to evaluate sedimentary basins for potential of uranium mineralization. In particular, the peroxy radicals are stable up to 800 ℃ and, therefore, are useful for evaluating metasedimentary rocks (e.g., Paleoproterozoic metasedimentary sequences in the central zone of the North China craton). EPR study of the Changcheng Series can focus on quartz from the sediment-basement unconformity and faults to determine the presence and types of natural radiation-induced paramagnetic defects, with which to identify and prioritize uranium anomalies. Other potential applications of natural radiation-induced paramagnetic defects in quartz include uranium- bearing hydrocarbon deposits in sedimentary basins. For example, the Junggar, Ordos, and Tarim basins in northwestern China all contain important oil and natural gas fields and are well known for elevated uranium concentrations, including economic sandstone-hosted uranium deposits. Therefore, systematic studies on the distribution of natural radiation-induced paramagnetic defects in quartz from host sedimentary sequences are expected to provide information about the migration of oil and natural gas in those basins.