We report evidence for the seawater origin of an extremely metal-enriched sulfide- and organic carbon–rich marker bed in a transgressive Early Cambrian black shale sequence along the passive margin of the Yangtze platform. The element concentration pattern in this marker bed suggests that it formed in a sediment-starved, stratified basin with a euxinic water column below an oxic surface layer. Biological activity was high in the surface layer, which was resupplied by communication with oxic oceans. The extremely low terrigenous input and the sulfate-reducing environment in the deeper part of the basin led to exceptionally high metal enrichments by factors of ∼107 with respect to modern seawater. The composition of the sulfidic rocks reflects the composition of the Early Cambrian oceans. The molybdenum isotope ratio suggests that during this time <35% of marine Mo was deposited in oxic sediments, and that suboxicanoxic marine environments were more widespread during the Early Cambrian than today.
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-NiSe-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.
Cu and Fe skarns are two economically important types of skarn deposit worldwide, but the critical factors controlling the difference in metal associations remain enigmatic. The Edong ore district, China, presents an excellent opportunity to study the differences between Cu–Fe and Fe skarn deposits. We have measured He–Ar isotopes trapped in fluid released by crushing pyrite and chalcopyrite from four well known Cu–Fe and Fe deposits in the Edong district, Eastern China, with the aim of constraining their different fluid source and then discussing the factors controlling their variations between Cu–Fe and Fe skarns. He–Ar isotopic compositions are markedly different between the Cu–Fe and Fe skarn deposits in the Edong district. 3He/4He ratios in the Cu–Fe deposits are 0.75–1.87 Ra and 40Ar/36Ar ratios are 300–472. By contrast, He–Ar isotopic compositions in minerals from the Fe deposits have lower 3He/4He and 40Ar/36Ar ratios of 0.08–0.93 Ra and 299–361, respectively. These results suggest that noble gas of the Cu–Fe and Fe skarn deposits in the Edong district formed by variable degrees of mixing between a magmatic fluid containing a mantle component, and modified air–saturated water (MASW). Importantly, He–Ar isotope data provide compelling evidence that contrasting fluid sources were involved in the formation of the Cu–Fe and Fe deposits, i.e., mineralizing fluids of the Cu–Fe deposits could have a greater contribution from mantle component, and little involvement of MASW than those of the Fe deposits in the Edong district. This conclusion is consistent with obvious differences in the nature of the intrusions related to mineralization, as well as sulfur isotopic compositions of sulfides in the Cu–Fe and Fe deposits. It is most likely that different proportion of mantle-derived noble gases play an essential role in controlling differences between the Cu–Fe and Fe skarn deposits.
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