Coupling of the redox history and enrichment of Ni-Mo in black shale during the early Cambrian: Constraints from S-Fe isotopes and trace elements of pyrite, South China

Abstract

Black shales were widely distributed in the world during the early Cambrian. Ni and Mo are enriched in black shale and can be mined only in Guizhou and Hunan, South China. The enrichment and mineralization of Ni and Mo have always been considered. However, the genesis of the deposits still has many interpretations. The migration and precipitation of elements were controlled by redox. It is conducive to explore the genesis of Ni-Mo enrichment that clarifying the relationship between the marine redox structure and metal precipitation. Pyrite nodules were selected along the profile for Fe isotope analysis, and nodules, colloidal and granular pyrite were analyzed in situ for S isotope and trace elements in the Xiaozhuliushui (XZLS) Ni-Mo deposit, Guizhou. Results show that the Fe isotopes have a slight upward trend from the lower black shale profile to the Ni-Mo layer. The maximum values (δ56Femax = 0.63; δ57Femax = 0.94) are reached at the polymetallic layer and then decrease rapidly. The δ34S values of nodules and granular pyrite decrease first and then increase rapidly from the lower part of the black shale, through the polymetallic Ni-Mo layer to the upper part of the black shale. The Co/Ni values of nodules, granular and colloidal pyrite < 0.01, in totally. Ni, Mo, As and Se are abnormally enriched in the Ni-Mo layer. These results indicate that all pyrite were formed in sedimentary and that the formation of a polymetallic Ni-Mo layer involves hydrothermal fluid. A large volume of clastic particles and sulfate were supplied due to enhanced continental weathering. It gradually becomes relatively oxidized in the depths of the ocean, which allows the metal elements to easily migrate. Then with the addition of submarine hydrothermal fluid, which disturbs the redox balance of the system, metal precipitation and enrichment occur. Means, the enrichment of polymetallic Ni-Mo in black shale is due to the coupling of continental weathering, sedimentation and hydrothermal action.