超基性岩热液硫化物矿床磁性物质来源和磁异常特征
通讯作者: 吴招才, 男, 1980年生, 博士, 副研究员, 主要从事海洋地球物理和海底构造研究.E-mail: [email protected]
Magnetic source and magnetic anomaly characteristics of sulfide deposits in ultramafic hydrothermal system
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摘要:
围岩为超基性岩的热液硫化物矿床是一种重要的海底热液硫化物矿床类型，相比围岩为玄武岩的热液硫化物矿床，超基性岩热液硫化物矿床研究相对较少，磁性特征也更为复杂.超基性岩热液硫化物矿床在热液蚀变作用下产生磁铁矿和磁黄铁矿等磁性矿物，从而呈现出与玄武岩型热液矿床相反的强烈的正磁异常.在超基性岩热液硫化物矿床发育的初始阶段，高温热液流体与周围超基性岩作用发生蛇纹石化，开始产生磁铁矿等磁性矿物，呈现出强烈的正磁异常；随着热液系统持续活跃，超基性岩蛇纹石化程度逐渐增加，产生的磁性矿物含量也逐渐增加，呈现的正磁异常达到最强；随着热液活动逐渐停止，热液蚀变伴随的蛇纹石化反应逐渐停止，热液系统温度逐渐降低，无法继续维持还原环境，之前产生的磁黄铁矿和磁铁矿会被氧化成非磁性硫化物或氧化物导致其磁性大大降低形成弱的正磁异常.然而，如Lost City这样的低温活跃的超基性岩热液系统也表现为较弱的正磁异常，原因可能为低温环境下超基性岩的热液蚀变与高温流体不同，低温环境下的蛇纹石化过程中铁元素会转移到所生成的非磁性水镁石等岩石中，导致也形成弱的正磁异常.
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关键词:
- 超基性岩热液系统 /
- 硫化物矿床 /
- 磁异常
Abstract:Ultramafic hydrothermal sulfide deposits are an important type of submarine hydrothermal sulfide deposits. Compared with the basalt hydrothermal sulfide deposits, the study of hydrothermal sulfide deposits in ultramafic rocks is relatively Less, the magnetic characteristics are more complex. Ultrabasic hydrothermal sulfide deposits under hydrothermal alteration to produce magnetic minerals such as magnetite and pyrrhotite, which presents a strong positive magnetic anomaly opposite to basalt hydrothermal deposits. At the initial stage of ultramafic hydrothermal sulfide deposit development, serpentinization occurs when the high-temperature hydrothermal fluid reacts with the surrounding ultramafic rocks, and magnetic minerals such as magnetite are produced, presenting strong positive magnetic anomalies. As the hydrothermal system continues to be active, the serpentinization degree of ultramacfic rocks gradually increases, and the content of magnetic minerals also gradually increases, presenting the strongest positive magnetic anomaly. As the hydrothermal activity gradually ceases, the serpentinization reaction accompanying the hydrothermal alteration gradually ceases, and the temperature of the hydrothermal system gradually decreases, and the reducing environment can not be maintained. The magnetite and pyrrhotite previously produced will be oxidized into non-magnetic sulfide or oxide, lead to the magnetization of sulfide deposits is reduced rapidly, forming weak positive magnetic anomaly. However, ultramafic-hosted hydrothermal system with low fluid temperature such as the Lost City is also characterized by weak positive magnetic anomaly, the reason may be that the ultramafic rock hydrothermal alteration in low temperature fluid and high temperature fluid exists different reaction process. Serpentinization in low temperature environment, the iron will be transferred to the generated weak magnetic brucite, also resulting in the formation of weak positive magnetic anomaly.
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Key words:
- Ultramafic-hosted hydrothermal system /
- Sulfide deposits /
- Magnetic anomaly
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图 1
全球热液系统分布图
Figure 1.
Global distribution of hydrothermal systems
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图 2
Rainbow热液区测深图(左：三维测深图)以及等效磁化强度图(右)(据Szitkar et al., 2014b)
Figure 2.
3D bathymetric map of the Rainbow hydrothermal area (left) and equivalent magnetization map (right) (from Szitkar et al., 2014b)
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图 3
Rainbow热液区采集样品的磁性性质
Figure 3.
Magnetic properties of samples collected from Rainbow hydrothermal area
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图 4
Lost City热液区测深图(据Szitkar et al., 2017)
Figure 4.
3D bathymetric map of the Lost City hydrothermal area (from Szitkar et al., 2017)
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图 5
Lost City热液区等效磁化强度图(a)与磁异常图(b)(据Szitkar et al., 2017)
Figure 5.
Equivalent magnetization (a) and RTP magnetic anomaly (b) of the Lost City hydrothermal area (form Szitkar et al., 2017)
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图 6
(a) Ashadze热液区多波束测深图，图中黑色实线为调查船测线，黑色三角形代表Ashadze1、Ashadze2热液喷口区；(b)地形剖面图；(c)磁异常图(Ondréas et al., 2012)
Figure 6.
(a)Multi-beam bathymetry. The black solid line represents the survey line, and the black triangle represents the Ashadze1 and Ashadze2 hydrothermal sites; (b)Topographic profile; (c)Magnetic anomaly map in the Ashadze hydrothermal area (Ondréas et al., 2012)
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图 7
Ashadze 1热液区地形与地壳磁化强度三维图(据Szitkar et al., 2014b)
Figure 7.
3D topography and crustal magnetization of the Ashadze 1 hydrothermal area (from Szitkar et al., 2014b)
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图 8
(a) YHF热液矿床区测深图.其中采样样品为玄武岩(红色)、辉绿岩(棕色)、辉长岩(黄色)、蛇纹石化橄榄岩(绿色)和硫化物(紫色).CIR=印度洋洋中脊，NTD=非转换断层，OCC=海洋核杂岩，KHF=Kairei热液区；(b)YHF热液区磁化强度图(据Fujii et al., 2016)
Figure 8.
(a) Bathymetric map of the YHF hydrothermal deposit area. The samples were basalt (red), basalt (brown), gabbro (yellow), serpentine peridotite (green) and sulfide (purple). CIR=Central Indian Ridge, NTD=non-transform discontinuity, OCC=oceanic core complex, KHF=Kairei hydrothermal field; (b)Magnetization of the YHF hydrothermal area (from Fujii et al., 2016)
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图 9
不同岩石样品的(a)磁化率K(SI)和自然剩余磁化强度(NRM)的关系和(b)各岩石样品的磁化强度
Figure 9.
The relationship between magnetic susceptibility K (SI) (a) and Natural Remnant
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图 10
不同蛇纹石化程度橄榄岩的磁化强度(据Fujii et al., 2016)
Figure 10.
Magnetization of peridotites with different degrees of serpentinization (from Fujii et al., 2016)
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图 11
超基性岩热液系统磁性矿物形成的3阶段模型.第一阶段: 在还原条件下，蛇纹石化反应和硫化物矿化的积累形成部分磁铁矿和磁黄铁矿，第二阶段: 大量磁铁矿和磁黄铁矿在还原条件下通过蛇纹石化反应和硫化物矿化积累，第三阶段: 超基性岩完全蛇纹石化后，氧化条件下只有磁铁矿作为主要磁源(据Fujii et al., 2016)
Figure 11.
Three-stage model of magnetic mineral formation in ultramafic rock hydrothermal system. The first stage: under reduction conditions, serpentine reaction and sulfide mineralization accumulation form partial magnetized magnetite and pyrrhotite. The second stage: a large number of magnetized magnetite and pyrrhotite accumulate through serpentinization and sulfide mineralization under reduction conditions. The third stage: after the ultramafic rock is completely serpentinized, only magnetite is the main magnetic source under oxidation conditions (from Fujii et al., 2016)
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表 1
不同温度和反应阶段下热液区的磁化强度对比
Table 1.
Basic information of the hydrothermal areas at different temperature
热液区名称 探测手段 温度/℃ 磁化强度/(A/m) 所处反应阶段 Ashadze 1 ROV 300 4 第一阶段 Rainbow ROV 365 30 第二阶段 Yokoniwa 深拖、AUV 5 10 第三阶段 Lost City ROV 116 2
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