Application of apatite U-Pb and fission-track double dating to determine the preservation potential of magnetite-apatite deposits in the Luzong and Ningwu volcanic basins, eastern China

摘要

The Cretaceous Luzong and Ningwu volcanic basins in eastern China contain numerous magnetite-apatite deposits with similar geological characteristics and mineralization ages (-130 Ma). These deposits, however, occur at distinctly different burial depths. To explain this difference, LA-ICP-MS apatite U-Pb and fission track double dating of five samples were carried out to establish the thermal histories from crystallization to the exhumation of four representative deposits: the deeply buried Nihe (665-1065 m underground) and Luohe deposits (425-856 m underground) from the Luzong basin; the shallow Meishan deposit (40-530 m underground) and the exposed Dongshan deposit from the Ningwu basin. The cooling histories of the four deposits could be divided into rapid cooling for the Dongshan deposit and slow continuous cooling for the Nihe, Luohe, and Meishan deposits. Combined with geological evidence, it can be determined that the Dongshan and Meishan deposits formed at shallow depths with the Dongshan deposit emplaced at a very high level while the Meishan deposit was emplaced relatively deeper. The Nihe and Luohe deposits were emplaced deeper than the Dongshan and Meishan deposits and the burying of the Shuangmiao volcanic cycle after mineralization increased the burial depth. However, this burial event did not occur in the Ningwu basin. All of these resulted in the different burial of the magnetite-apatite deposits between the two basins prior to 120 Ma while the exhumation after 120 Ma among these deposits was similarly slow and continuous. The metallogenic conditions of both basins were similar but the smaller number and gross reserve of magnetite-apatite deposits in the Luzong basin, we believe that the Luzong basin has better preservation potential for the magnetite-apatite deposits.