Kinetics of the Formation and Transformation of Iron Oxides in Hydrothermal Systems

世界上大部分的铁供应来自：(A)前寒武纪条带状铁建造(BIF)，它由海底热液环境中聚集的各种含铁矿物(赤铁矿、磁铁矿、菱铁矿和黄铁矿)组成；以及(B)由BIF中的磁铁矿、赤铁矿、菱铁矿和黄铁矿转化而成的富含赤铁矿的次生铁矿。拟议研究的目标有三个方面：(1)了解海底热液系统(例如，生物多样性框架和火山成因块状硫化物矿床)中氧化铁(和其他主要含铁矿物)形成和转化所涉及的地球化学环境和过程的系统性；(2)发展矿物学和地球化学方法，以确定含铁矿物的沉积环境；(3)开发新的铁矿勘探模式。这些目标将通过两个系列的实验室研究来实现，探索：(I)含铁和含硅的热液流体与不同组成的海水混合时，含铁矿物和二氧化硅的形成动力学，以及(Ii)赤铁矿磁铁矿在低于250℃的温度下的非氧化还原转化动力学。一些重要的假设将在这项研究中得到验证，包括：(1)当地海水中溶解O2和HCO3-的浓度在确定BIF和其他海底热液矿床的铁矿物学方面起主要作用；(2)富赤铁矿BIF是太古代海洋和大气氧化的良好证据；(3)富菱铁矿BIF表明它们形成于缺氧盆地；(4)赤铁矿向磁铁矿或磁铁矿向赤铁矿的转变，在许多类型的矿床中，这种转变不是由氧化还原反应引起的，而是由于简单地向赤铁矿中添加Fe2+或从磁铁矿中浸出Fe2+引起的：Fe2O3(Hm)+Fe2++H2O=Fe3O4(Mt)+2H+；(4)富赤铁矿形成于地壳深部和陆地表面。

Most of the world's Fe supply comes from: (a) Precambrian banded iron formations (BIFs), which were composed of a variety of iron-bearing minerals (hematite, magnetite, siderite, and pyrite) that accumulated in submarine hydrothermal environments; and (b) hematite-rich secondary iron ores that were transformed from the magnetite, hematite, siderite, and pyrite of BIFs. The goals of the proposed research are three-fold: (i) to understand the systematics of the geochemical environments and processes involved in the formation and transformation of iron oxides (and other major Fe-bearing minerals) in submarine hydrothermal systems (e.g., BIFs and VMS -volcanogenic massive sulfide deposits); (ii) to develop mineralogical and geochemical methods to identify the depositional environments of Fe-bearing minerals; and (iii) to develop a new exploration model for iron ore deposits. These goals will be achieved through two series of laboratory investigations, exploring: (I) the kinetics of the formation of iron-bearing minerals and silica during the mixing of Fe2+- and silica-bearing hydrothermal fluids with seawater of various compositions, and (II) the kinetics of the non-redox transformation of hematite magnetite at temperatures below 250 C. Some of the important hypotheses will be tested in this research, including: (1) the concentrations of dissolved O2 and HCO3- in the local seawater played major roles in determining the iron mineralogy of BIFs and other hydrothermal ore deposits formed on the ocean floor; (2) hematite-rich BIFs are good evidence for the oxygenated oceans and atmosphere during the Archean era; (3) siderite-rich BIFs suggest they formed in anoxic basins; (4) the transformation of hematite to magnetite, or magnetite to hematite, which in many types of ore deposits was not caused by redox reactions, but caused by simple additions of Fe2+ to hematite, or leaching of Fe2+ from magnetite: Fe2O3 (hm) + Fe2+ + H2O = Fe3O4 (mt) + 2H+; (4) hematite-rich iron ores formed in deep crust as well as on the land surface.