Major and trace element migration and metallogenic processes of the Xinshuijing U-Th deposit in the Longshoushan metallogenic belt, Gansu Province

The Xinshuijing U-Th deposit in the Longshoushan metallogenic belt of Gansu Province is a typical alkali metasomatic U-Th deposit hosted in albitite. The ore-forming processes can be divided into three major mineralization stages, i.e., Nametasomatism, U-Th mineralization and post-ore stage. In this paper, the authors systematically analyzed major and trace elements in less-altered granite, albitite and ore of the Xinshuijing deposit, and discussed the element transportation using the isocon diagram proposed by Grant. During the Na-metasomatism stage, Na, Ca, Sc, V, Cr, Co, Ni, U, Th and CO₂, H₂O were enriched, while large ion lithophile elements and some of the light rare earth elements were depleted. The ore-forming fluids of the U-Th mineralization stage were rich in heavy rare earth elements, U, Th, PO₄^3-, with volatile components (CO₂, H₂O, F, etc) abundantly escaped. Combined with former studies, the authors hold that the ore-forming fluid was the mixture between the mantle fluid and meteoric water. Vapor escape and the oxygen fugacity decrease seem to have been the major mineralization mechanism, which induced coprecipitation of U-Th minerals and apatite. Th transported in the form of compound phosphate was further precipitated after apatite formation.