Magnetite–hematite, oxygen fugacity, adakite and porphyry copper deposits: Reply to Richards
Magnetite–hematite, oxygen fugacity, adakite and porphyry copper deposits: Reply to Richards
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DOI：
10.1016/j.gca.2013.07.037
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发表时间：
2014-02
影响因子：
5
通讯作者：
Weidong Sun;Ruifang Huang;Huaying Liang;M. Ling;Cong-ying Li;Xing Ding;Hong Zhang;Xiaoyong Yang
中科院分区：
文献类型：
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作者：
Weidong Sun;Ruifang Huang;Huaying Liang;M. Ling;Cong-ying Li;Xing Ding;Hong Zhang;Xiaoyong Yang
1. Sun et al.（2013）是基于硫酸盐是母岩浆中主要的硫物种，但硫化物在斑岩矿床中占主导地位的观察。理查兹声称，硫化物占主导地位的斑岩矿床比它们的硫酸盐占主导地位的母岩浆更氧化，这是没有道理的。S6+/S总比率对氧逸度敏感（马修斯等人，1994; Metrich和Clocchiatti，1996; Jugo等人，2005年）。斑岩铜系统（包括母岩浆和矿床）富含硫，因此其氧逸度通常受硫形态控制。硫酸盐是岩浆中的主要硫物种，但在硫化物矿石中非常罕见，几乎没有孤立的硬石膏包裹体。我们认为这是母岩浆比矿体氧化程度高得多的明显迹象。我们还想指出，磁铁矿-黄铁矿边界位于SO2-H2S（不是硫酸盐）边界之上，温度高于650 ℃，这比成矿温度高得多。更重要的是，磁铁矿和黄铁矿都是固态，而SO2和H2S不是固态，因此SO2-H2S边界假定两种物质的浓度为1个单位。在讨论斑岩成矿机制时，我们涉及三件事，母岩浆，矿体和它们之间的联系（成矿过程）。一般来说，Fe和S的氧化还原反应是斑岩Cu矿床的关键控制因素（Sun等人，2004，2013; Liang等人，2009年）。母体岩浆的最佳氧逸度高于DFMQ+ 2（Mungall，2002），因此岩浆中有足够的硫酸盐以获得高的初始Cu含量，并且低于HM缓冲液以维持亚铁，用于在矿化过程中还原硫酸盐（Sun等人，2013年）。如Sun et al.（2013），在矿化过程中，硫酸盐的氧化电位随着pH的降低而增加，最终达到HM缓冲液（Sun等人，2013年）。我们还想指出的是，赤铁矿-磁铁矿组合在斑岩系统中非常常见，这也被许多其他人报道过，包括（Sillitoe，2010）。然而，在消耗H+的变化过程中，pH值增加，因此，硫酸盐的氧化电位降低。斑岩矿体（富硫化物）中非常低的S6+/S总比值表明氧逸度远低于FMQ缓冲区。然而，reac-
1. The title of Sun et al.(2013) is based on the observation that sulfate is the dominant S species in the parental magma, but sulfides are dominant in porphyry deposits. Richards claims that sulfidedominant porphyry deposits are more oxidized than their sulfate-dominant parental magmas, which does not make sense. The S6+/Stotal ratio is sensitive to oxygen fugacity (Matthews et al., 1994; Metrich and Clocchiatti, 1996; Jugo et al., 2005). The porphyry Cu system (both parental magmas and deposits) is rich in sulfur, such that its oxygen fugacity is usually controlled by sulfur speciation. Sulfate is the dominant sulfur species in magmas but is very rare in sulfide ores, with scarce isolated anhydrite inclusions. We take this as a clear sign that parental magmas are much more oxidized than the ore bodies. We also would like to point out that the magnetite–pyrite boundary lies above the SO2–H2S (not sulfate) boundary at temperatures higher than 650 C, which is much higher than the mineralization temperature. More importantly, both magnetite, pyrite are solid states, whereas SO2 and H2S are not, such that the SO2–H2S boundary assumes 1 unit concentrations of both species.When talking about the mechanism of porphyry mineralization, we are dealing with three things, parental magmas, ore bodies and the linkage between them (mineralization process). In general, the redox reactions of Fe and S are the key controlling factors of porphyry Cu deposits (Sun et al., 2004, 2013; Liang et al., 2009). The most optimal oxygen fugacity of parental magmas is higher than DFMQ+ 2 (Mungall, 2002), such that there is sufficient sulfate in the magmas to obtain high initial Cu contents, and lower than the HM buffer to maintain ferrous iron for the reduction of sulfate during the mineralization process (Sun et al., 2013). As discussed in detail in Sun et al.(2013), the oxidation potential of sulfate increases with decreasing pH during the mineralization process, which eventually reaches the HM buffer (Sun et al., 2013). We would like to point out also that the hematite-magnetite assemblage is quite common in porphyry systems, which has also been reported by many others, including (Sillitoe, 2010). Nevertheless, the pH values increase during alterations that consume H+, consequently, the oxidation potential of sulfate decreases. The very low S6+/Stotal ratio in porphyry ore bodies (sulfide rich) indicates oxygen fugacity much lower than the FMQ buffer. Nevertheless, the reac-