Extreme enrichment of polymetallic Ni–Mo–PGE–Au in Lower Cambrian black shales of South China: An Os isotope and PGE geochemical investigation

Abstract

Extreme metal enrichments occur in the Lower Cambrian Niutitang Formation, a thick black shale sequence deposited on the Yangtze Platform of South China, and form a regionally distributed conformable polymetallic Ni–Mo–PGE–Au sulfide horizon. The origin for the extreme enrichment of metals in these black shales is discussed based on a comprehensive geochemical investigation of Re–Os isotopes and platinum-group elements. The black shales of the Lower Cambrian Niutitang Formation in Guizhou province yielded an Re–Os isochron age of 535 ± 11 Ma with an initial 187Os/ 188Os ratio of 0.80 ± 0.04, which is similar to the initial 187Os/ 188Os ratio (0.78 ± 0.19) of the Ni–Mo sulfide ores, but slightly lower than the initial 187Os/ 188Os ratio (1.18 ± 0.02) of Lower Cambrian black shales from the Lesser Himalaya, India. A comparison of Os isotope data among the Chinese Ni–Mo sulfide ores, the seafloor massive sulfide deposits and contemporaneous seawater suggests a possible similar hydrothermal forming mechanism for these deposits. The PGE concentrations, Pt anomaly (Pt/Pt ⁎), Pt/Pd, Ir/Pd, Au/Pd ratios of the Chinese Ni–Mo sulfide ores are similar to those metal-rich black shales of submarine volcanogenic hydrothermal origin from Czech Republic, Namibia and Canada, but are different from those data of the ocean floor Fe–Mn crusts that have scavenged their PGE and other metals directly from seawater. It is therefore suggested that the Ni–Mo sulfide ores from the Niutitang black shale sequence in South China had a submarine-hydrothermal origin. The anoxic environments and abundant organic matter in the ocean basin also played a key role for the extreme metal enrichments. The discharge of hydrothermal fluids into the Cambrian Ocean may have had a great effect on life during the Cambrian Explosion.