Boron Enrichment in Salinized Lacustrine Organic-Rich Shale of the Paleogene Biyang Depression, East China: Occurrence and Geological Controlling Factors

Although boron (B) is widely applied as a paleosalinity indicator for ancient lakes, the occurrence and geological controls of B enrichment in salinized lacustrine organic-rich shale (SLORS) are poorly understood. This study addresses this issue by comparing the mineral and element compositions of high-boron shale (HBS) and low-boron shale (LBS) from the Paleogene Biyang Depression, using integrated XRD, XRF, and ICP-MS analyses. The mineral composition of HBS is dominated by illite, whereas LBS primarily consists of albite; both are of detrital origin. Compared to the element composition of UCC, HBS is extremely enriched in Mo and W, whereas LBS is extremely enriched in W and U. Boron is positively correlated with Al2O3 and negatively correlated with Na2O, suggesting that B primarily occurs in illite. An enhanced extent of chemical weathering prevailed during the deposition of HBS, providing a greater supply of illite to the basin. Higher pH levels and greater reduction during HBS deposition encouraged illite absorption of B, ultimately leading to B enrichment in shale. Our findings suggest that pH and redox conditions, as well as the mineral compositions of shale, should be fully considered during the application of B and related ratios as paleosalinity indicators.