Re-Os Dating of Polymetallic Ni-Mo-PGE-Au Mineralization in Lower Cambrian Black Shales of South China and Its Geologic Significance

Abstract

Black shales of the basal Lower Cambrian Niutitang Formation, southeast China, host a regionally distributed concordant, several centimeter-thick, sulfide layer which displays extreme metal enrichment, i.e., Mo-Ni-Se-Re-Os-As-Hg-Sb >1,000 times enriched and Ag-Au-Pt-Pd >100 times enriched over bulk continental crust. Mineable portions have about 5.5 wt percent Mo, 3.5 wt percent Ni, and 1 g/t PGE + Au. A six-point 187 Os/ 188 Os versus 187 Re/ 188 Os isochron on molybdenum-nickel ore samples defines an age of 541 ± 16 Ma (2σ) with an initial 187 Os/ 188 Os ratio of 0.78 ± 0.19. This age is in agreement with the depositional age of the black shale host; the initial ratio is close to present-day seawater. The sulfide layer/average seawater metal ratio is on the order of 10 6 to 10 8 , about 10 to 100 times higher than that for the black shale host and for average metalliferous black shale. Synsedimentary metal enrichment from seawater under anoxic (sulfate-reducing) conditions appears likely but requires an unusually low sedimentation rate and/or high replenishment rate of fresh seawater to the marine basin. The paleogeographic setting of the Lower Cambrian continental margin of the Yangtze craton indicates local basins controlled by synsedimentary rifting. Stagnant water episodically replenished by upwelling oxidized seawater is thought to be responsible for the formation of the polymetallic sulfide layer and of phosphorite, barite, and sapropelic "stone coal" (combustible black shale) beds.

SHRIMP U-Pb zircon age of tuff at the bottom of the Lower Cambrian Niutitang Formation, Zunyi, South China

“…Li et al [41] dated the Ni-MO-PGE ore and reported a Re-Os isochron of 542 ± 11 Ma, which they interpreted as the age for the successions of the Niutitang Formation. In addition, Mao et al [44] and Jiang et al [43] reported Re-Os isochrons of 541±16 (2σ) and 537 ± 10 Ma (MSWD = 11.9) for the Ni-MO-PGE ore in Songlin area, Zunyi. They also considered these ages as representing the depositional age of the Niutitang Formation.…”

“…The ore-forming mechanism for the Ni-Mo-PGE deposit hosted in the black shales at the basal Niutitang Formation is highly controversial [25,31,[41][42][43][44][45] . Mao et al [45] suggested that both elements leached from seafloor mafic-ultramafic rocks by hydrothermal fluids and deep Mo-Re-(Cu) chambers, with the latter serving as both a source of heat and a source for the metal materials, contributed to the mineralization.…”

“…Mao et al [45] suggested that both elements leached from seafloor mafic-ultramafic rocks by hydrothermal fluids and deep Mo-Re-(Cu) chambers, with the latter serving as both a source of heat and a source for the metal materials, contributed to the mineralization. In a more recent paper, Mao et al [44] argued for a synsedimentary ore genesis. A hydrothermal origin of this deposit, however, is still favored by most workers [25,[41][42][43] , who at the same time emphasize the roles that the hydrothermal fluids played on the mafic-ultramafic rocks and that some of the metal elements were mantle-derived [41][42][43] .…”

Discussion on the PGE anomalies and source materials of K-bentonite (Bed 5) in the Lower Cambrian Meishucun section, Yunnan

“…This is in good agreement with previous K-bentonites analyzed from the same layer Pašava et al (2010); B1 and B2 represent continental tholeiites and alkali basalts respectively (Barnes and Maier 1999); CA represents acid rocks in East China (Chi and Yan 2006); CFV represents felsic volcanics in central East China (Gao et al 1998); NAV represents the average of 14 acid volcanics in Namibia (Borg et al 1988); ZY represents a black shale bearing polymetallic Ni-Mo-PGE in Zunyi, Guizhou (Luo et al 2003); HJW-1 and HJW-2 represent polymetallic NiMo-PGE layer and Ni-Mo ore, respectively, of the Wangjiawan mine in Zunyi, Guizhou (Mao et al 2002) of the same section (Zhang et al 1997) and the same layer of a different section . A La anomaly can interfere with the correct identification of a Ce anomaly (Bau and Dulski 1996); thus, a Ce/ Ce* versus Pr/Pr* diagram is used to check the Ce anomaly in most of the relevant works.…”

Multiple Proxies on the Paleoenvironment of the Early Cambrian Marine Black Rock Series in the Tarim Basin, NW China: Molybdenum Isotope and Trace Element Evidence

“…Trace element and Mo isotope data suggest that interbedded chert-phosphorite assembles were scavenged from seawater under restricted conditions. The inferred scavenging process could occurred under replenishment of nutrition to the restricted basins by upwelling oxidize seawater [25] [72]. High current or wave energy in combination with oxic seawater did not favor the preservation of organic matter, thus , the TOC contents of the basal Yurtus Formation are quiet low (0.05% -0.94%) in cherts-phosphorite assemblages layer.…”