The genetic association between magnetite–hematite and porphyry copper deposits: Reply to Pokrovski
The genetic association between magnetite–hematite and porphyry copper deposits: Reply to Pokrovski
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磁铁矿-赤铁矿与斑岩铜矿床之间的成因关联：回复 Pokrovski
DOI：
10.1016/j.gca.2013.07.038
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发表时间：
2014-02
影响因子：
5
通讯作者：
Wei-ming Fan
中科院分区：
文献类型：
--
作者：
Hong Zhang;Xiao-yong Yang;Trevor Irel;Wei-ming Fan
斑岩成矿是一个非常复杂的过程，有数百种不同的可能反应。Sun等人（2013）为了使论文更直接，只提出了几个组合的重要反应方程。我们对Pokrovski的评论的回应为我们提供了一个更好地解释斑岩成矿作用的控制因素和反应的机会。认识斑岩成矿作用最重要的是岩浆和流体都参与了斑岩成矿作用。温度在550-350 C之间的热液过程对最终的斑岩铜矿化最为重要（Sillitoe，2010）。大多数反应发生在流体中和/或流体/岩石相互作用期间，Pokrovski（2013）和许多其他人在讨论斑岩矿化反应时没有充分考虑到这一点（Zajacz等人，2011年、2012年）。必须始终考虑建议的反应可能无法模拟地质观测。赤铁矿-磁铁矿共生体在基本上所有大型斑岩Cu-Au矿床中都很常见（Sillitoe，2010; Sun等人，2013年）。这一点曾一度困扰我们，因为岩浆的亚铁/铁比值通常远高于磁铁矿，因此在相同的物理化学条件下，磁铁矿的结晶会进一步提高亚铁/铁比值，并相应地降低母岩浆的氧化逸度（Sun et al.，2004; Liang等人，2009;詹纳等人，2010年）。在pH值变化不大的岩浆作用中，这是正确的。经过一段时间的努力，我们认识到磁铁矿的形成和进一步氧化是pH控制的硫酸盐还原的原因，这是最终成矿的关键过程。长期以来，人们一直认为高氧逸度的岩浆有利于斑状铜矿化（Ballard等人，2002; Mungall，2002; Liang等人，2006，2009; Sillitoe，1997，2010; Sun等人，2010，2011，2012 a，B; Zhang等人，2013年）。以不同氧化态存在并以足够丰度存在以影响斑岩成矿系统的氧化还原状态的元素，例如C、H和S，在标准条件下均建立低于HM缓冲液的氧逸度（图1）（Mungall，2002）。与FMQ相比，HM氧缓冲剂（图1，Eqs.（3和4）），硫相关的氧化还原反应对pH和所涉及的物质的浓度敏感（Eqs. (1)和（5）），即硫酸盐的氧化电位随着pH值和H2S/H2SO 4（方程式5）的降低而增加。① ①）。为了赶上硫研究的进展，我们使用Pokrovski和Dubrovinsky（2011）的硫形态的logfO 2与pH图来解释磁铁矿-赤铁矿共生对斑岩矿化的影响及其背后的机制（Sun等人，2013年）。尽管声称三硫S3-离子已被证明在250-450 ℃之间的温度、5-35 kbar之间的压力和1-7的pH下是主要的硫物质，并且在更高的温度下更稳定（Pokrovski和Dubrovinsky，2011），
Porphyry mineralization is a very complicated process with hundreds of different possible reactions. Sun et al.(2013) presented only several combined important reaction equations in order to make the paper more straightforward. Our response to Pokrovski’s comment provides us with an opportunity to better explain the controlling factors and reactions of porphyry mineralization. The most important thing for understanding porphyry mineralization is that both magmas and fluids are involved in porphyry mineralization processes. Hydrothermal processes at temperatures between 550–350 C are the most important for the final porphyry Cu mineralization (Sillitoe, 2010). Most of the reactions occurred in fluids and/or during fluid/rock interactions, which is not fully considered by Pokrovski (2013) and many others when discussing reactions responsible for porphyry mineralization (Zajacz et al., 2011, 2012). It must always be considered that reactions proposed may not mimic geological observations. Hematite–magnetite intergrowths are common in essentially all large porphyry Cu–Au deposits (Sillitoe, 2010; Sun et al., 2013). This has puzzled us for a while, because the ferrous/ferric ratio of magmas are usually much higher than that of magnetite, therefore the crystallization of magnetite would further increase the ferrous/ferric ratio and correspondingly tend to lower the oxidation fugacity of the parental magmas, under the same physical–chemical conditions (Sun et al., 2004; Liang et al., 2009; Jenner et al., 2010). This is true for magmatic process when the pH value does not change much. After struggling with this problem for some time, we realized that the formation and further oxidation of magnetite are responsible for sulfate reduction controlled by pH, which is the key process for the final mineralization.It has long been recognized that magmas with high oxygen fugacity favors porphyry Cu mineralization (Ballard et al., 2002; Mungall, 2002; Liang et al., 2006, 2009; Sillitoe, 1997, 2010; Sun et al., 2010, 2011, 2012a, b; Zhang et al., 2013). Elements that exist in variable oxidation states and present in sufficient abundances to affect the redox state of the porphyry mineralization system, eg, C, H and S, all establish oxygen fugacity lower than the HM buffer under standard conditions (Fig. 1)(Mungall, 2002). In contrast to FMQ, HM oxygen buffers (Fig. 1, Eqs.(3 and 4)), sulfur-related redox reactions are sensitive to pH and concentrations of species involved (Eqs.(1) and (5)), ie, the oxidation potential of sulfate increases with decreasing pH values and H2S/H2SO4 (Eq.(1)). To catch up with progress in sulfur studies, we used the logfO2 versus pH diagram for sulfur speciation of Pokrovski and Dubrovinsky (2011) to explain the effects of magnetite–hematite intergrowth on porphyry mineralization and the mechanism behind it (Sun et al., 2013). Although it is claimed that the trisulfur S3 Ą ion has been demonstrated to be the dominant sulfur species at temperatures between 250–450 C, pressures between 5–35 kbar and pH of 1–7, and is more stable at higher temperatures (Pokrovski and Dubrovinsky, 2011),
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DOI：
10.5382/sp.10.15
发表时间：
2003
期刊：
--
影响因子：
--
作者：
M. Einaudi;J. W. Hedenquist
通讯作者：
M. Einaudi;J. W. Hedenquist
影响因子：
5
作者：
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通讯作者：
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影响因子：
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作者：
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通讯作者：
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影响因子：
2.7
作者：
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通讯作者：
S. Matthews;A. Jones;A. Beard
影响因子：
2.6
作者：
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通讯作者：
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