Sulphide mineral evolution and metal mobility during alteration of the oceanic crust: Insights from ODP Hole 1256D
Sulphide mineral evolution and metal mobility during alteration of the oceanic crust: Insights from ODP Hole 1256D
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DOI：
10.1016/j.gca.2016.08.009
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发表时间：
2016-11
影响因子：
5
通讯作者：
C. Patten;I. Pitcairn;D. Teagle;M. Harris
中科院分区：
文献类型：
--
作者：
C. Patten;I. Pitcairn;D. Teagle;M. Harris
大洋地壳热液蚀变过程中的金属通量具有深远的影响，包括缓冲海洋和岩石圈的成分，支持微生物生命和硫化物矿床的形成。在海洋地壳演化过程中，金属活动的机制是复杂的，既没有完全的限制，也没有量化。对硫化物浸出等释放金属的矿物反应进行调查，将有助于更好地了解对海洋地壳中金属流动性的控制。我们调查了硫化物和氧化物矿物共生，以及这些矿物控制大洋钻探计划（ODP）孔1256 D样品中金属收支的程度。ODP孔1256 D岩心提供了一个独特的样品套件，代表了从火山岩段到深成杂岩的快速扩张洋壳的完整剖面。根据矿物组合、岩性位置和结构特征，将1256 D孔硫化物群划分为岩浆硫化物群、交代硫化物群、高温热液硫化物群、低温硫化物群和斑片状硫化物群。在氧化条件下，中温（250-350 °C）热液流体向下流动引发热液蚀变，导致席状岩墙和深成杂岩中岩浆硫化物的交代作用。随后，在还原条件下，温度>350 °C时，热液蚀变程度的增加导致交代硫化物被上升的热液流体浸出。质量平衡计算表明，Cu、Se和Au的迁移是通过高温热液蚀变过程中的硫化物淋滤实现的，Zn、As、Sb和Pb的迁移受硅酸盐控制，而不是硫化物蚀变。1256 D孔的硫化物浸出不完全，更高级的蚀变将动员更多的金属。氧化物矿物的蚀变不会向1256 D孔的热液流体中释放大量的金属。无论是在岩墙上部还是在过渡带，上升的高温流体与低温流体的混合，都触发了局部高温热液硫化物沉淀和Co、Ni、Cu、Zn、As、Ag、Sb、Se、Te、Au、Hg和Pb的捕获。在火山岩段，低温流体循环（<150 °C）导致低温硫化物沉淀，其形式为黄铁矿前缘，由于从循环流体中吸收，具有高As浓度。在席状岩墙和深成杂岩中的深部晚期低温环流导致局部片状硫化物沉淀和局部金属再活化。硫化物对Au、Se和Cu的控制贯穿洋中地壳快速扩张的整个历史，这意味着富含这些金属的热液流体的产生，最终可以形成VMS矿床，强烈地受到硫化物浸出的控制。
Fluxes of metals during the hydrothermal alteration of the oceanic crust have far reaching effects including buffering of the compositions of the ocean and lithosphere, supporting microbial life and the formation of sulphide ore deposits. The mechanisms responsible for metal mobilisation during the evolution of the oceanic crust are complex and are neither fully constrained nor quantified. Investigations into the mineral reactions that release metals, such as sulphide leaching, would generate better understanding of the controls on metal mobility in the oceanic crust. We investigate the sulphide and oxide mineral paragenesis and the extent to which these minerals control the metal budget in samples from Ocean Drilling Program (ODP) Hole 1256D. The ODP Hole 1256D drill core provides a unique sample suite representative of a complete section of a fast-spreading oceanic crust from the volcanic section down to the plutonic complex. The sulphide population at Hole 1256D is divided into five groups based on mineralogical assemblage, lithological location and texture: the magmatic, metasomatised, high temperature hydrothermal, low temperature and patchy sulphides. The initiation of hydrothermal alteration by downward flow of moderate temperature (250–350 °C) hydrothermal fluids under oxidising conditions leads to metasomatism of the magmatic sulphides in the sheeted dyke and plutonic complexes. Subsequent increase in the degree of hydrothermal alteration at temperatures >350 °C under reducing conditions then leads to the leaching of the metasomatised sulphides by rising hydrothermal fluids. Mass balance calculations show that the mobility of Cu, Se and Au occurs through sulphide leaching during high temperature hydrothermal alteration and that the mobility of Zn, As, Sb and Pb is controlled by silicate rather than sulphide alteration. Sulphide leaching is not complete at Hole 1256D and more advanced alteration would mobilise greater masses of metals. Alteration of oxide minerals does not release significant quantities of metal into the hydrothermal fluid at Hole 1256D. Mixing of rising high temperature fluids with low temperature fluids, either in the upper sheeted dyke section or in the transitional zone, triggers local high temperature hydrothermal sulphide precipitation and trapping of Co, Ni, Cu, Zn, As, Ag, Sb, Se, Te, Au, Hg and Pb. In the volcanic section, low temperature fluid circulation (<150 °C) leads to low temperature sulphide precipitation in the form of pyrite fronts that have high As concentrations due to uptake from the circulating fluids. Deep late low temperature circulation in the sheeted dyke and the plutonic complexes results in local precipitation of patchy sulphides and local metal remobilisation. Control of sulphides over Au, Se and Cu throughout fast-spreading mid-oceanic crust history implies that the generation of hydrothermal fluids enriched in these metals, which can eventually form VMS deposits, is strongly controlled by sulphide leaching.