It is generally considered that a significant change in oceanic redox conditions occurred during the Ediacaran–Cambrian transition. However, there are currently two major conflicting views on the degree of oxygenation of deep water(oxic vs. ferruginous) during this interval. To date, the oxygenation conditions of the Early Cambrian ocean have not been well constrained. The oxygenation magnitude and mechanism of the Early Cambrian ocean could be critical to the significant biological evolution of the "Cambrian Explosion". To constrain the Early Cambrian oceanic redox environment, we conducted an integrated study on iron and sulfur isotopes and redox-sensitive elements(Mo, U, and V) of Lower Cambrian phosphorite deposits from two shallow sections(Meishucun and Gezhongwu) and a deeper water section(Zunyi) from the Yangtze Platform, South China. The near zero δ^(56)Fe values from the two shallow sections studied here reflect oxic conditions in the lower phosphorite deposition. An obvious positive shift in δ^(56)Fe and redox-sensitive element content was observed in the middle parts of the two shallow water sections, which might reflect loss of light iron by dissimilatory iron reduction during early diagenesis under suboxic shallow water in the platform. However, the highly positive δ^(56)Fe values in the deep section could reflect a lower oxidation degree of dissolved Fe(II) under anoxic deep water. The data suggest redox-stratified oceanic conditions during the Early Cambrian, in which completely oxygenated shallow water(platform) coexisted with anoxic deep water(slope). We propose that prolonged upwelling of dissolved organic carbon(DOC)-, Fe(II)- and phosphorus-rich anoxic deep water in a redox-stratified ocean could have increased exchange with the open ocean, resulting in major phosphorite deposition in oxic-suboxic conditions. The progressive oxygenation of the ocean may have facilitated the Early Cambrian biotic diversification.
The source and evolution of ore-forming fluids is important to understand the genesis of Carlin-type gold deposit.Constraints on the source and evolution of ore fluid components by the con-ventional geochemical methods have long been a challenge due to the very fine-grained nature and complex textures of hydrothermal minerals in these deposits.In this study,we present the crush-leach analyzed solute data of fluid inclusion extracts within quartz,calcite,realgar,and fluorite from the Shuiyindong,Nibao,and Yata gold deposits in the Youjiang Basin,providing new insights into the source and evolution of ore-forming fluids.The results show that the high molar Cl/Br ratios up to 2508 in fluid inclusion extracts are indicative of a contribution of magmatic hydrothermal fluids.Flu-ids mixing between basinal and magmatic-hydrothermal fluids are evident on the plots of Cl/Br versus Na/K ratios,showing that ore-stage milky quartz near the magmatic-hydrothermal fluids reflects magma origin of the ore-forming fluids,whereas late ore-stage drusy quartz and realgar near the de-fined basinal fluids suggest the later input of basinal fluids in late-ore stage.Although the predominate-ly host rocks in Shuiyindong,Nibao and Yata gold deposit are bioclastic limestone,sedimentary tuff,and calcareous siltstone,respectively,the solute data of fluid inclusion extracts records they underwent the similar fluid-rocks reactions between the Na-rich magmatic hydrothermal fluids and the Ca-and Mg-rich host rocks.This study highlights the solute data of fluid inclusion extracts obtained by crush-leach analyses have the potential to fingerprint the source and evolution of ore-forming fluids of the Carlin-type gold deposit.
The Youjiang Basin is characterized by a wide distribution of Au and Sb deposits.These deposits are mainly hosted by sedimentary rocks from Cambrian to Triassic and are structurally controlled by faults and folds.Three types of Sb mineralization can be distinguished based on geologic characteristics,economic metals,and mineral associations.The first type is dominated by Sb mineralization but contains minor or little Au,similar to the large Qinglong deposit.The second type has a spatial association with the gold deposit but formed independent Sb mineralization,reminiscent of the Badu deposit.In the third type,Sb generally formed as an accompanying element in the Carlin-type gold deposit,and stibnite occurred as euhedral crystals filling the open space and faults in the late stage of gold mineralization,analogous to the Yata deposit.Trace element concentrations and sulfur isotopic ratio of stibnite,and oxygen isotope of stibnite bearing quartz were analyzed to infer the ore source(s)for Sb mineralization and genesis.To distinguish the various types of stibnite mineralization between the deposits,Cu,Pb,and As have recognized most diagnostic,with an elevated concentration in Au and Au-Sb deposits and depleted in Sb deposit.Theδ34S isotopic composition of stibnite samples from three deposits show a wide variation,ranging from-6.6%to+17.45%.Such isotopic values may indicate the sedimentary sulfur source,introduced by fluid–rock interaction.On the other hand,fluid mixing of several end members cannot be excluded.The calculatedδ18O isotopic data of Sb-bearing quartz show the initial ore fluid in Au and Au-Sb deposits most likely have a magmatic or metamorphic origin that enriched during fluid–rock interaction,and Sb deposit characterized by initial meteoric water.From these data,we proposed that different lithologies,fluid–rock interaction,fluid pathways,and different ore fluids controlled the compositional evolution of fluids,which might be the main reason for the diversity of Au or Sb mineralization.
