Effects of magmatic volatile influx in mafic VMS hydrothermal systems: Evidence from the Troodos ophiolite, Cyprus
Effects of magmatic volatile influx in mafic VMS hydrothermal systems: Evidence from the Troodos ophiolite, Cyprus
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DOI：
10.1016/j.chemgeo.2019.119325
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发表时间：
2020-01
期刊：
影响因子：
3.9
通讯作者：
Andrew J. Martin;M. Keith;Daniel B. Parvaz;I. McDonald;A. Boyce;K. McFall;Gawen R. T. Jenkin;H. Strauss;C. MacLeod
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文献类型：
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作者：
Andrew J. Martin;M. Keith;Daniel B. Parvaz;I. McDonald;A. Boyce;K. McFall;Gawen R. T. Jenkin;H. Strauss;C. MacLeod
摘要塞浦路斯特罗多斯蛇绿岩是镁铁质火山成因块状硫化物（VMS）矿床的主要陆上类似物。这项研究首次在区域尺度上提供了来自VMS矿床和Troodos蛇绿岩中其他矿化带的硫同位素（δ 34 S）数据。结合不同热液硫化物的δ 34 S、Se/S比值和微量元素（Se、Cu和Au）化学特征，探讨了古热液系统中岩浆挥发分的变化及金属和硫的来源。19个矿床的硫同位素分析（n= 180）表明Troodos VMS热液系统中硫的来源是可变的，这反过来又允许推断金属的来源是可变的。黄铁矿δ 34 S变化范围为− 5.5‰ ~+ 13.2‰，平均值为+ 4.6‰（n= 160）。这些δ 34 S的变化不能仅用海水硫酸盐还原（δ 34 S+ 18 ~+ 19‰）和岩浆硫淋滤（δ 34 S 0-1‰）的比例变化来解释。因此，提出了两个过程，解释了微量金属和δ 34 S的变化在Troodos成矿系统，包括，i）一个可变的金属来源的席状岩墙复杂和ii）添加的岩浆挥发物丰富的阶段VMS热液系统。两个不同的熔岩单位存在于特罗多斯地层，即上部和下部枕状熔岩（UPL和LPL）。相对于富集在LPL中的As、Sb和Zn，更原始的UPL富集Au、Se和Cu。一些VMS矿床的形成时间早于UPL（如Agrokipia A），表明Se、Cu和Au已贫化。因此，VMS矿床的地层位置和UPL：LPL亲和元素的比值（如As+ Zn+ Sb vs. Cu+ Se+ Au）暗示了微量元素分布与地层深度之间的系统关系;这与金属源区内UPL和LPL亲和熔岩的相对比例有关。在某些VMS矿床中记录的δ 34 S值< 0‰，低于特罗多斯岩浆平均值0-1‰，可能与流体上升过程中硬石膏的缓冲作用、微生物硫酸盐还原作用或岩浆来源的硫通过挥发性出溶作用从下伏岩浆房直接贡献给热液系统有关。黄铁矿中负的δ 34 S值和Se/S× 10 6> 50 0的比值表明岩浆挥发组分（如Apliki和Skouriotissa）的贡献。我们证明，在特罗多斯VMS热液系统中的金属和硫的来源是受区域尺度的过程相关的i）可变源岩性，ii）的贡献的岩浆挥发相的一些特罗多斯VMS热液系统。
Abstract The Troodos ophiolite, Cyprus is the principal on-land analogue for mafic-hosted volcanogenic massive sulfide (VMS) deposits. This study, for the first time, presents sulfur isotope (δ 34 S) data on a regional scale from VMS deposits and other mineralised zones across the Troodos ophiolite. In combination δ 34 S, Se/S ratios and trace element chemistry (eg, Se, Cu and Au) of different hydrothermal sulfides are used to assess variations in magmatic volatile influx and the source of metals and sulfur in ancient hydrothermal systems. Sulfur isotope analyses (n= 180) across 19 mineral deposits indicate a variable source of sulfur in the Troodos VMS hydrothermal system, this in turn allows a variable source of metals to be inferred. Pyrite δ 34 S range from− 5.5‰ to+ 13.2‰ with an average of+ 4.6‰(n= 160) for all deposits investigated. These δ 34 S variations cannot only be explained by variable proportions of thermochemical seawater sulfate reduction (δ 34 S+ 18 to+ 19‰) and leaching of primary magmatic sulfur from igneous rocks (δ 34 S 0-1‰). Consequently, two processes are proposed, explaining the trace metal and δ 34 S variation across the Troodos ore-forming systems including, i) a variable source of metals in the sheeted dyke complex and ii) the addition of a magmatic volatile-rich phase to the VMS hydrothermal systems. Two distinct lava units exist in the Troodos stratigraphy, namely the upper and lower pillow lavas (UPL and LPL). The more primitive UPL are enriched in Au, Se and Cu relative to As, Sb and Zn that are concentrated in the LPL. Some VMS deposits pre-date the formation of the UPL (eg, Agrokipia A) suggesting Se, Cu and Au depleted source rocks. Hence, the stratigraphic position of VMS deposits and the ratio of UPL: LPL affinity elements (eg, As+ Zn+ Sb vs. Cu+ Se+ Au) imply a systematic relationship between trace element distribution and stratigraphic depth; this relates to the relative proportion of UPL and LPL affinity lavas within the metal source region. δ 34 S values< 0‰ recorded in some VMS deposits that are less than the Troodos magmatic mean of 0-1‰ may be related to anhydrite buffering during fluid ascent, microbial sulfate reduction or the direct contribution of magmatically derived sulfur, to the hydrothermal system from an underlying magma chamber via volatile exsolution. We propose that negative δ 34 S values combined with Se/S× 10 6 ratios> 500 in pyrite suggest the contribution of a magmatic volatile component (eg, Apliki and Skouriotissa). We demonstrate that the source of metals and sulfur in the Troodos VMS hydrothermal system is affected by regional scale processes related to i) variable source lithologies and, ii) the contribution of a magmatic volatile phase to some Troodos VMS hydrothermal systems.