Enrichment of Platinum Group Elements in Lower Cambrian Polymetallic Black Shale, SE Yangtze Block, China

Yong Fu,Zhen Yang,Chao Li,Peng Xia

doi:10.3389/feart.2021.651948

Abstract

Platinum group elements (PGEs) occur mainly in basic–ultrabasic igneous rocks and are concentrated by the differentiation and crystallization of magma. Thin polymetallic layers including Ni, Mo, V, PGEs, and rare-earth elements are widely developed in lower Cambrian black shales in southern China. The PGE contents in such layers may not be economically significant but are still unusually enriched. PGE enrichment mechanisms have not been widely studied, but here the PGE compositions of polymetallic layers in the Shuidong (Nayong) and Niuchang (Weng’an) sections of the SE Yangtze block in China were determined, and results compared with published data for the region. Results indicate that PGEs are enriched in the polymetallic layers, whereas the surrounding country rocks are barren. The ΣPGE contents in the Shuidong Ni–Mo deposits are generally lower than in the Niuchang V deposits. PGE distribution patterns in the polymetallic layers are similar to those in basic–ultrabasic rocks, indicating that such rocks played a role in the PGE enrichment. Oceanic hypoxia during the Sinian–Cambrian transition resulted in the production of large amounts of organic matter and H2S in the ocean. When high-salinity brine reached the ocean bottom, rapid changes in Eh–pH conditions caused enrichment of metals at the sediment–seawater interface, and this enrichment was later enhanced during diagenesis.

Highlights

Lower Cambrian black shales contain important ore-bearing strata worldwide, including large and super-large deposits of Ni–Mo–V–Platinum group elements (PGEs), rare-earth elements (REEs), barite, P, and stone coal (Fan et al, 1984, Fan et al, 1987; Mao et al, 2002; Xu et al, 2012; Paava et al, 2019)
Study of the formation mechanisms and metallogenic characteristics of polymetallic mineralization in lower Cambrian black shales is important to meet the strategic needs of various countries
This proposal is difficult to reconcile with the above mechanisms of PGE enrichment in black shale, with the PGEs likely being derived from multiple sources (Jiang et al, 2003)

Summary

Introduction

Lower Cambrian black shales contain important ore-bearing strata worldwide, including large and super-large deposits of Ni–Mo–V–PGE, rare-earth elements (REEs), barite, P, and stone coal (Fan et al, 1984, Fan et al, 1987; Mao et al, 2002; Xu et al, 2012; Paava et al, 2019). Several formation mechanisms have been proposed for lower Cambrian black-shale polymetallic deposits, including biogenesis (Jewell, 2000; Lu et al, 2004), a Ba-rich coldspring origin (Paytan et al, 2002; Griffith and Paytan, 2012; Zhou et al, 2016), and hydrothermal origins (Wang and Li, 1991; Wu et al, 1999; Clark et al, 2004; Yang et al, 2007; Wu et al, 2009; Gao et al, 2018). Ni–Mo sulfide ores in black shale of the Niutitang Formation, South China, have a submarine hydrothermal origin, and the extreme metal enrichment is likely to have occurred in an anoxic environment with abundant organic matter in an ocean basin (Jiang et al, 2007). This study aimed to improve our understanding of the genetic types of PGE deposit and to constrain the genesis of black-shale polymetallic deposits

Objectives

Methods

Discussion

Conclusion