Extreme enrichment of polymetallic Ni-Mo-PGE-Au in lower Cambrian black shales of South China: An Os isotope and PGE geochernical investigation

摘要

Extreme metal enrichments occur in the Lower Cambrian Nintitang 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 Os-187/Os-188 ratio of 0.80 +/- 0.04, which is similar to the initial Os-187/Os-188 ratio (0.78 +/- 0.19) of the Ni-Mo sulfide ores, but slightly lower than the initial Os-187/Os-188 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. (c) 2007 Elsevier B.V. All rights reserved.