Restrictions of Weathering on Phosphorus Migration and Enrichment of the Lower Cambrian Phosphorus-Bearing Rock Series in Eastern Guizhou.

Abstract

Phosphorite deposits in eastern Guizhou belong to a typical type of marine sediments. Phosphorus-bearing layers are distributed at the bottom of the Lower Cambrian Niutitang Formation. Generally, the average content of P2O5 in phosphorite deposits is 13.01%. In this study, significant P enrichment on or near the phosphorite deposit surface was found, and the average content of P2O5 reached 24.76%. However, the migration and enrichment mechanisms of P in the supergene environment still lack in-depth research. Therefore, in this work two representative phosphorite deposits, namely, primary sedimentary and weathered phosphate ores, were analyzed. The deposits were examined by polarized light microscope observation, X-ray diffraction analysis, scanning electron microscope observation, energy spectrum analysis, and NMR analysis. Major, trace, and rare earth elements in the deposits were detected by geochemical tests. The results showed that, compared with the primary phosphate ore, the porosity of the weathered phosphate ore was significantly increased. The NMR T 2 spectrum showed that the number of micropores increased, the pore size was enlarged, and more mesopores and macropores appeared in the weathered phosphate ore. The weathered phosphate ore had a lighter color than the primary phosphate ore. Under the action of aqueous acid, the primary phosphate minerals (collophosphate and apatite) and Al-bearing rocks transformed into a typical weathered mineral, sasaite. Carbonate minerals were dissolved, and pyrite was oxidized to limonite. K, Si, and Al in silicate minerals precipitated to form secondary clay minerals, such as kaolinite. The weathered phosphate ore developed recrystallized, pisolitic, and pseudomorphic textures and nodular, oriented, and metacolloidal structures, features specific to a weathered mineral. In the supergene environment, soluble components such as carbonate minerals in the primary rock were leached out, while residual P2O5 with relatively stable chemical properties was enriched in situ. Phosphorus migration and enrichment were restricted by such factors as primary rock/ore composition, primary rock/ore texture and structure, climatic conditions, topography, geological structure, and hydrogeological conditions. A three-stage enrichment model was established after mechanical crushing of the primary rock/ore, carbonate mineral dissolution, and secondary phosphate mineral formation.