The sources of geogenic arsenic in aquifers at Datong basin, northern China: Constraints from isotopic and geochemical data

Mineralogical, geochemical and zircon U-Pb dating studies were carried out to identify the sources of arsenic in the shallow aquifers of Datong Basin in northern China. A sediment sample from 18 m depth containing 10.3 mg/kg arsenic showed a Zircon U-Pb concordant age of 2528 +/- 20 to 271 +/- 4 Ma that can be divided into two groups (2528 +/- 20 to 1628 +/- 21 Ma and 327 +/- 4 to 271 +/- 4 Ma) and is comparable to that of the sedimentary rocks of Taiyuan (upper Carboniferous) and Shanxi Formation (lower Permian) outcropping to the west of Datong Basin. In contrast, a sediment sample from 22.5 m depth containing 5.7 mg/kg arsenic displayed a Zircon U-Pb concordant age ranging from 2561 +/- 21 to 1824 +/- 26 Ma that is comparable to that of the Hengshan Complex (Ne-Archaean Precambrian) outcropping to the east of. Electron microscope analysis of the rock samples of Taiyuan and Shanxi Formations revealed that iron oxides and sulfides occurred unevenly in a matrix of clay and organic matter. Arsenic was elevated in the iron oxides with a mean of 660 mg/kg (n = 11) and 970 mg/kg (n = 4) for Shanxi and Taiyuan Formations, respectively. Except one pyrite grain, arsenic contents were lower than the detection limit (1 mg/kg) in other pyrite grains. Arsenic in iron oxides from Shanxi and Taiyuan Formations contributed to high arsenic in aquifer sediment of Datong Basin. The low arsenic sediment of Datong Basin is likely to be derived from weathering of the Hengshan Complex with low arsenic content (ranging from 0.37 to 4.14 mg/kg). (C) 2011 Elsevier B.V. All rights reserved.