Redistribution of uranium through lamprophyre intrusions in the Permo-Carboniferous coal of the northern Datong Coalfield, China

Magmatic intrusion is a key factor affecting the uranium concentration in coal. It has long been reasoned that magmatic intrusions generally enrich uranium in low-uranium coal. However, less attention has been paid to the redistribution of uranium in uranium-rich coal caused by magmatic intrusions. Thus, a total of 558 uranium data points obtained by spectrophotometric determination were collected from 57 boreholes in the northern Datong Coalfield, rich in lamprophyre intrusions, to determine their effect on the concentration and mode of occurrence of uranium in this Permo-Carboniferous coal. Coal in this area can be classified as either intrusion-affected or unaffected. The uranium concentration was relatively high in both unaffected coal and highly thermally affected coal, while relatively low in slightly thermally affected coal, indicating that lamprophyre intrusions redistributed the uranium in coal. The carbonate content within coal increased markedly with increasing degree of coalification caused by the intrusion. Uranium in unaffected coal was mainly associated with organic compounds, whereas that in intrusion-affected coal occurred mainly in secondary minerals (e.g., carbonates). The uranium concentrations in fresh lamprophyre and carbonatite veins, interspersed therein, were 0.23-0.86 & mu;g/g and 0.97-2.34 & mu;g/g, respectively, suggesting that higher concentrations of uranium in intrusion-affected coal may not have been directly derived from intrusions, but rather from hydrothermal processes at intrusion-coal contacts. The lamprophyre intrusions can reactivate, relocate, or enrich uranium initially complexed with organic matter. With an increase in the degree of coalification, active functional groups responsible for complexation are progressively broken down, resulting in decreased uranium concentrations. However, the uranium concentration was positively correlated with MgO and CaO contents in intrusion-affected coal, indicating that uranyl ions shedding from organic matter may be complexed and subsequently coprecipitated with secondary carbonate minerals.