Platinum group elements as useful genetic tracers for the origin of polymetallic Ni-Mo-PGE-Au sulfide ores in Lower Cambrian black shales, Yangtze Platform, South China

Abstract

The polymetallic Ni-Mo-PGE-Au sulfide ores occur in the Lowermost Cambrian Niutitang Formation of a thick black shale sequence in Yangtze Platform of South China. The origin for this special type of sulfide ore has been hotly debated. Three genetic models have been proposed: (1) extraterrestrial impact origin (Fan et al. 1984); (2) submarine hydrothermal exhalative origin (Coveney et al., 1992; Lott et al. 1999; Steiner et al. 2001); (3) seawater scavenging origin (Mao et al. 2002). In this paper, we discuss the origin for this special type of sulfide ore based on platinum group element geochemistry. We suggest that the chondrite-normalized PGE patterns of the sulfide ores show some distinctive difference from the seawater pattern and may indicate a submarine hydrothermal input for the metal enrichments in the sulfide ore. We defined a new parameter, namely Pt anomaly (Pt/Pt*), and found that the Pt anomaly in the polymetallic Ni-Mo-PGE-Au sulfide ores is similar to those ancient and modern sea-floor massive sulfide deposits. Other platinum group element pairs, such as Pt/Pd, Ir/Pd, and Au/P ratios of the Ni-Mo sulfide ores also share some similarity with those of ancient and modern submarine massive sulfide deposits. In conclusion, we support the submarine hydrothermal exhalative origin for the sulfide ores, and suggest that this is a new type of sediment-hosted submarine hydrothermal exhalative sulfide deposits.