Molybdenum isotopic composition as a tracer for low-medium temperature hydrothermal ore-forming systems: A case study on the Dajiangping pyrite deposit, western Guangdong Province, China

Abstract

Little is known about the range or controls on the molybdenum isotopic composition of low-medium temperature hydrothermal ore-forming systems. We present molybdenum isotope data from 12 hydrothermal syndepositional silicalite and carbonaceous slate samples from the Dajiangping pyrite deposit in western Guangdong Province, South China. The δ 97/95 Mo values from Orebody III range from −0.02‰ to 0.29‰, with an average of 0.18‰. In contrast, the composition values from Orebody IV display a larger variation from −0.70‰ to 0.62‰. However, the five samples from the main ore bed all show strong negative values. This indicates that the significant variation in molybdenum isotopic composition supports different hydrothermal ore-forming metallogenesis and a metallogenic environment between the two orebodies. Orebody III is likely to have been deposited from submarine exhalative hydrothermal fluids under a relatively strong reducing environment and Orebody IV may have also been influenced by hydrothermal superimposition in a more oxidized disequilibrium condition. In addition, the δ 97/95 Mo values of Orebody IV are clearly negative, together with the values increasing stratigraphically upward in the ore beds, suggesting that the metallogenic environment of Orebody IV is different from the open oceanic systems. There might be dynamic fractionation in this restricted environment. Therefore, the molybdenum isotope can be used as an effective tracer for the ore-forming fluid and metallogenic environment. molybdenum isotope, metallogenic environment, Dajiangping pyrite deposit Citation:Song S M, Hu K, Wen H J, et al. Molybdenum isotopic composition as a tracer for low-medium temperature hydrothermal ore-forming systems: A case study on the Dajiangping pyrite deposit, western Guangdong Province, China.

16 Good Golly, Why Moly? THE STABLE ISOTOPE GEOCHEMISTRY OF MOLYBDENUM

“…Molybdenum isotope studies point to the importance of redox reactions on the δ 98 Mo of Mobearing mineral phases in low-temperature systems (Ryb et al, 2009;Greber et al, 2011;Song et al, 2011). In a Pliocene low-temperature system (100-160°C) in Switzerland, molybdate may have been transported by oxidizing surface waters into brecciated rocks (Grimsel breccia) where it was reduced, leading to precipitation of Mo-bearing sulfide phases (Greber et al, 2011; the mineralogy could not be identified by the authors).…”

“…The larger Mo isotope variation of ~3‰ in the brecciated rocks compared with individual high-temperature systems may reflect a combination of lower temperature crystallization, reduction of MoO4 2-(an uncommon species in high-temperature systems), and multiple stages of re-dissolution and re-precipitation of Mo (Greber et al, 2011). Variable redox conditions and depositional environments (open marine versus restricted) were invoked to explain the range of Mo isotope compositions in the different orebodies of the Dajiangping pyrite deposit in China (Song et al, 2011).…”

Uranium sorption characteristics onto synthesized pyrite

“…To assess uranium resources in a rational and comprehensive way, the clarification of oreforming mechanism of uranium is an important topics that requires deeply investigation [8]. Ore-forming process is generally linked to crystal fractionation, sedimentation, metamorphism, weathering and adsorption etc [9][10][11]. However, many aspects of the mineralization processes of uranium in China remain unresolved, and the effect of adsorption is one of the crucial problems pressing for solution.…”