BACKGROUND The micro/nano silicon-carbon deposit in Fengcheng City, Jiangxi Province, is the first sedimentary deposit mineralized by phytoliths in the world. According to the research carried out here, micro/nano silicon-carbon ore may have been formed by the long-term accumulation and consolidation of phytoliths. In terms of distribution range, genetic type and resource type, the micro/nano silicon-carbon deposit has many new features. Therefore, considering the particularity of the deposit, it is of great significance to carry out research, especially chronological research. Moreover, Re-Os isotope system of organic-enriched sedimentary rocks has been applied to directly dating deposition ages or stratigraphic boundary age, which has made many achievements in chronological research. However, most of the studies focus on marine sediments samples, while lacustrine sediments samples are rarely affected by many factors, such as provenance, geological processes, and terrigenous clastic materials.

OBJECTIVES To accurately determine the mineralization age of the natural micro/nano silicon-carbon deposit.

METHODS Re-Os isotope testing was used to date the phytolith rock samples (lacustrine sediment samples) in the lower section of the Shilukeng Formation in the mining area. Isotope ratios were determined by NTIMS, and GBW04477 (JCBY) was used as the reference material.

RESULTS The Re-Os isotope age of the samples is 43.1±3.7Ma (n=7, MSWD=6.2) which directly defines the diagenetic mineralization age of phytolith rock of the deposit. The high ¹⁸⁷Re/¹⁸⁸Os value of the samples is closely related to the sedimentary environment and the origin of biological sedimentation. The high initial ¹⁸⁷Os/¹⁸⁸Os ratio may be related to tectonic movement events, high Os content and migration rate of adjacent strata. The study shows that Re and Os isotopes are adsorbed and sealed in phytolith with organic carbon in the process of enrichment. Owing to the phytolith’s stable silica structure, high temperature resistance and corrosion resistance, the Re-Os isotope system has a good sealing property in it. Protected by the special structure, the organic carbon in phytolith mainly exists in the tiny cavity of phytolith, and the organic carbon has not been exchanged with the outside world due to its storage, which provides favorable conditions for dating. Moreover, the large amount of phytolith in the samples also provides favorable conditions for the enrichment of Re and Os isotopes.

CONCLUSIONS The results of this study provide for the determination of sedimentary age about lacustrine sediments samples by the Re-Os isotope analysis. It is important to strengthen research about the study of Re-Os enrichment mechanism by micro/nano silicon-carbon ore samples.