Mineralogical and geological significance of hydrothermal products: A case from the Chihu hydrothermal field, South Mid-Atlantic Ridge
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摘要: 赤狐热液区是2019年中国在南大西洋中脊23.7°S发现的一个位于拆离断层上盘的热液区。本文利用光学显微镜、电子探针、TerraSpec Halo矿物鉴别仪和XRD粉晶衍射仪，对该区热液产物进行了矿物学和矿物成分研究。结果表明，赤狐热液区热液产物可分为硅质硫化物、硅质矿化角砾岩和碳酸盐质矿化角砾岩。这些产物均由不等量蚀变岩石碎屑、团块状硫化物角砾、细粒浸染状硫化物以及硅质和碳酸盐质胶结基质组成。团块状硫化物与细粒浸染状硫化物之间在矿物组合、结构构造和矿物成分上的差异指示该热液区可能经历了多个喷发周期，蚀变岩石碎屑的复杂性和多样性指示该热液区除了基性源岩外，还可能受到超基性源岩影响。Abstract: The Chihu hydrothermal field is a mineral occurrence related to a detachment fault structure. It was discovered in 2019 at 23.7°S of the southern mid-Atlantic ridge. The detachment fault is believed having important influences on the hydrothermal activity and mineral deposition of the field. In this paper, mineralogy and mineralogy chemistry studies are conducted for the hydrothermal products by means of transmission-reflection microscope, Terraspec Halo mineral discriminator and X-ray diffraction (XRD). The results suggest that the hydrothermal products are mainly composed of siliceous sulfides, siliceous mineralized and carbonate mineralized breccias consisting of altered rock fragments, massive sulfide breccia, fine-grained disseminated sulfide, and siliceous and carbonate cemented matrix. The difference in mineral composition, structure and mineral composition between massive sulfide and fine-grained disseminated sulfide indicates that the hydrothermal area may have experienced multiple times of eruption cycles. The complexity and diversity of altered rock fragments suggest that in addition to basic source rocks, this hydrothermal field may also be affected by ultrabasic source rocks.
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图 2 赤狐热液区样品照片
a. 硅质硫化物（13-2），可见流体通道构造；b. 硅质硫化物（13-5），硫化物矿物呈浸染状分布其中；c. 碳酸盐质矿化角砾岩（13-3），表面覆盖大量红色Fe氢氧化物以及白色碳酸盐矿物，中部出露有黄铁矿；d.硅质矿化角砾岩（13-7），质地松散细腻，表面分布有细小的黄铁矿颗粒；e. 硅质矿化角砾岩（13-1），可见团块状硫化物碎块和蚀变岩屑不均匀分布其中。
Figure 2. Typical sulfide images from ChiHu hydrothermal field
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图 3 硅质硫化物的矿物特征
a. 粗粒黄铜矿（cpy）边部交代小的粒状黄铁矿（py），反射单偏光；b. 铜蓝（cv）包围黄铜矿（cpy），反射单偏光；c. 闪锌矿（sp）交代有黄铜矿（cpy），反射单偏光；d. 闪锌矿（sp）表面有呈“乳滴”状黄铜矿（cpy），即“黄铜矿病”，反射单偏光； e. 自形黄铁矿（py）被非晶质硅胶结（asi），反射单偏光；f. 黄铜矿（cpy）表面析出有呈固溶体出溶结构浅粉色等轴古巴矿（iso），并有交代的粒状黄铁矿（py），反射单偏光；g. 皂石由于Fe含量的不同显示出不同级别干涉色，透射正交偏光；h. 呈鳞片状滑石，透射正交偏光；i. 皂石中充填有细小的黄铁矿颗粒，且晚于皂石形成，应为后期富含金属的流体经过皂石在其表面通过卸载作用形成（图g同视域反射单偏光下视图）。
Figure 3. Mineralogical characteristics of siliceous sulfides
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图 4 硅质硫化物中粗粒团块黄铜矿与细粒浸染状黄铜矿中Cu-Fe含量变化图
Figure 4. Cu-Fe variation of chalcopyrite in siliceous sulfide
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图 5 矿化角砾岩中蚀变岩屑的矿物特征
a.细腻鳞片状滑石，似蚀变辉石形成，透射正交偏光；b.皂石中夹杂黄铁矿，反射单偏光；c.皂石中夹杂黄铁矿，透射正交偏光；d.夹杂在无定形硅（白色）中的橙玄玻璃（橙黄色），透射单偏光；e.短柱状阳起石，部分可见多色性，整体呈浅黄色，透射正交偏光；f.红色伊丁石，呈橄榄石假象，下部可见蚀变的他形橄榄石，透射正交偏光；g.菱镁矿呈细粒集合体，发育菱形解理，透射单偏光；h.粒状白云石（dol），解理发育，透射正交偏光；i. 白云石和菱镁矿被红色Fe氢氧化物覆盖，透射正交偏光。
Figure 5. Mineral characteristics of altered debris in mineralized breccia
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图 6 矿化角砾岩中金属硫化物的矿物特征
a.硅质碎屑角砾硫化物中蚀变非金属矿物分布，透射正交偏光；b.碳酸盐质碎屑角砾硫化物中蚀变非金属矿物分布，透射正交偏光；c.闪锌矿（sp）、黄铜矿（cpy）、黄铁矿（py）三者共生，被非晶质硅（asi）胶结，反射光；d.铜蓝（cv）与黄铜矿共生，反射单偏光；e.共生的黄铁矿（py）和白铁矿（mar），交代有黄铜矿（cpy），反射单偏光；f.浅橘色矿物Phase A出溶在黄铜矿（cpy）中，表面可见被Cu硫化物切割，反射单偏光；g.自形-半自形黄铜矿（cpy）和黄铁矿（py）松散分布在非晶质二氧化硅中，反射单偏光；h.不规则黄铁矿（py）赋存在碳酸盐矿物中，反射光；i. 黄铁矿（py）、黄铜矿（cpy）、斑铜矿（bor）三者共生，赋存在碳酸盐矿物中，反射单偏光。
Figure 6. Mineralogical characteristics of metallic sulfides in mineralized breccia
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图 7 硅质矿化角砾岩中粗粒与细粒黄铁矿的Fe-Cu含量变化图
Figure 7. Fe-Cu variation of pyrite in siliceous mineralized breccia
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图 8 矿化角砾岩XRD衍射图谱
Talc-滑石，Cpy-黄铜矿，Py-黄铁矿，Liz-利蛇纹石，Doi-白云石，Mag-菱镁矿，Arg-银镍黄铁矿。 a. 硅质矿化角砾岩（岩屑角砾），b. 碳酸盐质矿化角砾岩（全岩）。
Figure 8. X-ray (XRD) diffraction pattern of mineralized breccia
表 1 赤狐热液区样品特征
Table 1 Characteristics of Chihu hydrothermal samples
样品编号 样品种类 样品特征 矿物种类 13-2 硅质硫化物 整体呈黑色，致密结构，表面存在黄色的硫单质。可见流体通道构造。 黄铁矿、黄铜矿、铜蓝、皂石、滑石、针铁矿 13-5 硅质硫化物 整体呈黑色，表面可见呈颗粒状的细小黄铁矿，可见通道构造，硫化物呈浸染状出现在二氧化硅基质中。 黄铁矿、黄铜矿、闪锌矿、铜蓝、等轴古巴矿、皂石、滑石、针铁矿 13-3 硅质矿化角砾岩 表面被大量红色Fe氢氧化物覆盖，内部可见硫化物团块，岩屑呈大小不等、未见磨圆的角砾分布于硅质和碳酸钙质基质中。 黄铁矿、黄铜矿、斑铜矿、针铁矿、白云石、菱镁矿、蛇纹石、滑石 13-7 硅质矿化角砾岩 呈长条状，质地松散细腻，易碎，表面存在细小的黄铁矿颗粒，可见粒度不等的棱角状碎屑。 黄铁矿、黄铜矿、闪锌矿、皂石、滑石、蛇纹石、针铁矿 13-1 碳酸盐质矿化角砾岩 可见粒度不等的棱角状蚀变岩屑和硫化物角砾不均匀分布于硅质基质中。 黄铁矿、白铁矿、黄铜矿、闪锌矿、Phase A、橙玄玻璃、皂石、滑石、伊丁石、阳起石 
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表 2 矿物种类及分布
Table 2 Minerals and their distribution
样品种类 矿物种类 化学式 硅质硫化物 黄铁矿（Pyrite） FeS2 闪锌矿（Sphalerite） ZnS 黄铜矿（Chalcopyrite） CuFeS2 等轴古巴矿（Isocubanite） CuFe2S3 铜蓝（Covellite） CuS 皂石（Saponite） Nax(H2O)4{Mg3[AlxSi4-xO10](OH)2} 滑石（Talc） Mg3[Si4O10](OH)2 针铁矿（Goethite） FeOOH 矿化角砾岩 黄铁矿（Pyrite） FeS2 白铁矿（Marcasite） FeS2 闪锌矿（Sphalerite） ZnS 黄铜矿（Chalcopyrite） CuFeS2 Phase A Cu10Fe3S11 斑铜矿（Bornite） Cu5FeS4 皂石（Saponite） Nax(H2O)4{Mg3[AlxSi4-xO10](OH)2} 滑石（Talc） Mg3[Si4O10](OH)2 针铁矿（Goethite） FeOOH 伊丁石（Iddingsite） H4MgFe2[Si3O12]·2H2O 橙玄玻璃 蛇纹石 Mg6[Si4O10](OH)8 白云石 CaMg[CO3]2 菱镁矿 Mg[CO3] 阳起石 Ca2(Mg,Fe2+)5[Si8O22](OH)2
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表 5 闪锌矿电子探针分析结果
Table 5 Chemical composition of sphalerite
% 矿石类型 S Zn Fe Ni Pb Co Cu 总量 矿化角砾岩 32.96 57.86 3.66 0 0 0.15 2.71 97.34 33.23 60.22 3.60 0 0.06 0.18 0.40 97.68 33.38 62.97 3.36 0 0.08 0.08 0.29 100.15 33.22 58.19 8.91 0 0 − 0.32 100.63 33.14 57.77 8.35 0 0.05 − 0.48 99.78 32.88 57.65 9.39 0 0.10 − 0.34 100.36 32.17 57.27 9.10 0 0 − 0.27 98.82 硅质硫化物 “干净”闪锌矿 33.76 61.89 2.81 0 0.31 0.09 1.20 100.06 32.02 63.91 2.02 0 0.15 0.06 0.78 99.94 33.16 61.63 1.34 0.06 0.80 0.06 2.70 99.74 32.42 63.24 2.10 0 0.14 0.06 1.13 99.09 32.69 63.74 2.56 0 0.26 0.01 0.89 100.12 “黄铜矿病”
闪锌矿33.41 57.01 8.01 0 0.22 0.08 0.08 98.81 32.95 59.58 6.39 0.02 0.20 0.06 0.19 99.35
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表 3 黄铜矿电子探针分析结果及结晶分子式
Table 3 Chemical composition and crystalline formula of chalcopyrite
% 矿石类型 Cu Fe S Zn Pb 结晶分子式 硅质硫化物 粗粒黄铜矿 32.59 31.40 34.56 0 0.13 Cu0.94Fe1.03S2.00 34.05 31.37 34.88 0 0.14 Cu0.99Fe1.03S2.00 33.58 30.70 34.52 0 0 Cu0.98Fe1.02S2.00 32.76 31.06 34.72 0 0.12 Cu0.96Fe1.03S2.00 32.88 31.24 35.17 0 0.02 Cu0.95Fe1.02S2.00 细粒黄铜矿 33.21 28.90 34.82 0 0.14 Cu0.95Fe0.95S2.00 32.93 29.25 34.78 0 0.19 Cu0.96Fe0.96S2.00 33.51 29.08 34.80 0 0.19 Cu0.97Fe0.95S2.00 矿化角砾岩 碳酸盐质矿化角砾岩 34.24 31.55 35.08 0 0 Cu0.99Fe1.02S2.00 34.63 30.00 35.39 0 0 Cu1.00Fe0.98S2.00 33.75 30.51 35.31 0 0.14 Cu0.96Fe1.00S2.00 34.05 30.24 35.02 0 0.15 Cu0.99Fe0.99S2.00 硅质矿化角砾岩 粗粒团块状黄铜矿 33.30 29.36 34.66 0 0.01 Cu0.97Fe0.98S2.00 33.65 29.87 34.83 0 0.12 Cu0.99Fe0.98S2.00 细粒浸染状黄铜矿 33.57 30.02 34.56 0 0.01 Cu0.98Fe0.99S2.00 34.32 30.09 35.02 0 0.17 Cu0.99Fe0.99S2.00 33.55 30.01 34.50 0 0 Cu0.98Fe1.00S2.00
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表 4 黄铁矿电子探针分析结果
Table 4 Chemical composition of pyrite
% 矿石类型 Fe Ni Zn S Pb Co Cu 总量 硅质硫化物 自形黄铁矿 45.18 0.04 0 53.91 0.34 0.03 0.09 99.58 45.21 0.05 0 54.11 0.12 0 0.72 100.21 45.78 0.04 0.14 52.67 0.31 0.05 0.35 99.33 45.09 0 0.01 53.86 0.02 0.06 0.02 99.06 粒状黄铁矿 45.47 0 0.12 53.85 0.24 0.13 0.44 100.23 45.55 0 0.01 54.65 0.09 0.03 0.07 100.39 45.98 0 0.20 54.23 0.33 0.06 1.26 102.04 45.80 0.01 0.19 53.77 0.18 0.08 0.64 100.67 45.26 0 0 55.14 0.07 0.12 0.57 101.16 矿化角砾岩 碳酸盐质矿化角砾岩 46.08 0.07 0.12 53.92 0.15 0.08 0.17 100.57 46.25 0 0.14 53.25 0.07 0.05 0.22 99.99 45.63 0 0.05 52.68 0.16 0.07 0.23 98.82 硅质矿化角砾岩 细粒黄铁矿 45.80 0.04 0 53.98 0.09 − 0.13 100.04 44.88 0.03 0.69 54.59 0.34 − 0.16 100.69 45.58 0 0 53.68 0.18 − 0.09 99.53 粗粒黄铁矿 47.36 0.04 0 53.97 0.17 − 0 101.53 46.55 0 0 54.03 0 0.08 0.09 100.75 46.24 0 0 53.97 0.10 0.03 0.13 100.47 46.25 0.02 0 54.08 0.08 0.10 0 100.53 46.09 0 0 53.63 0.30 0.05 0.02 100.08 注：“−”表示未检测。
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