Petrography and sulfur isotopic compositions of SEDEX ores in the early Cambrian Nanhua Basin, South China

Abstract

Widespread Ni-Mo sulfide mineralization in South China during the early Cambrian may have affected the seawater chemistry and biodiversity of this region, but its underlying causes are not well-understood. To better understand the formation of sulfide ores in early Cambrian marine systems, this study evaluated the paragenesis and sulfur isotopic composition (δ34S) of sediment-hosted Ni-Mo sulfide ores in the lower Cambrian Niutitang Formation of the Nanhua Basin of South China. Petrographic analysis revealed three types of sulfide aggregates: (1) ubiquitous framboidal and euhedral pyrite (Py-1), (2) a MoSC phase (i.e., Mo mineralization) that was cogenetic with larger pyrite cubes or nodules (Py-2), and (3) intergrown millerite (NiS), sphalerite (ZnS) and veined pyrite (Py-3) (i.e., Ni-Zn mineralization) characterized by laminated and veined textures. Alternating precipitation of Mo and Ni-Zn sulfides was caused by intermittent emission of Mo-Fe or Ni-Zn-rich hydrothermal fluids into euxinic mid-depth waters of the Nanhua Basin, representing a sedimentary exhalative (SEDEX) process. The different sedimentary ore textures are each associated with a characteristic δ34S distribution. At all study sites, the syngenetic Py-1 phase yields δ34S of −15.7 to + 1.9‰, consistent with a seawater sulfate source that was variably fractionated through microbial sulfate reduction (MSR). In the Nayong and Zunyi areas, δ34S values of −11.8 to +4.4‰ are associated with the MoSC phase, and −20.6 to −6.8‰ with the Py-2 phase. In the Zhangjiajie area, Py-2 yields similar values (−25.1 to −10.8‰) but MoSC higher and less variable values (+8.6 to +18.7‰). These phases are thought to contain a mixture of sulfur sourced from seawater sulfate (via MSR) and hydrothermal fluids in varying proportions. A mode of Ni-Zn sulfides δ34S values between −9.1 and +8.0‰ is likely to represent the isotopic composition of hydrothermal sulfur inputs to the Ni-Zn mineralization. As a consequence, both hydrothermal emissions and biogenic production of H2S had a significant influence on seawater chemistry, with consequences for contemporaneous evolution of early marine animal life (e.g., the small shelly fauna and Chengjiang Biota) in the early Cambrian Nanhua Basin.