伟晶岩型锂矿床地球物理探测及可可托海实例

doi:10.13745/j.esf.sf.2023.5.14

摘要/Abstract

摘要：

锂是一种战略性关键金属,因其高电位特征在新能源时代的战略地位突出,并且需求急剧增长。锂矿床是锂资源的自然载体,勘探开发锂矿床是保障低碳能源金属锂资源需求的重要途径。伟晶岩是硬岩型锂矿床重要的赋矿岩体,但大部分锂矿床赋矿岩体规模小,物性特征不明显,地球物理探测难度极大,并且在相当一段时间地球物理勘探与研究被忽视。随着锂矿勘探需求的持续攀升及仪器与方法技术的进步,地球物理在伟晶岩探测中的作用越来越被重视,在勘探应用中也在不断取得突破。本文在简要回顾锂资源需求现状的基础上,讨论和综述了伟晶岩型锂矿床地球物理探测技术,包括岩石物理特征、星载遥感技术、重磁勘探和电法勘探,最后展示了中国新疆可可托海伟晶岩稀有金属矿区音频大地电磁测深探测实例。伟晶岩与淡色花岗岩具有低磁化率、低密度、低极化率和高波速特征,而伟晶岩的电阻率受亲水矿物的影响,在几个数量级的范围内变化。由于伟晶岩和与其共伴生的花岗岩相对于片岩、片麻岩等围岩具有低密度、低磁化率特征,常用区域重磁勘探圈定这一类的花岗岩体。重磁勘探通常难以判别是否为伟晶岩,但近期有少量实例报道微重力能识别花岗岩中的伟晶岩。电法勘探因其相对较高的分辨率和穿透能力能在隐伏伟晶岩型锂矿勘查中发挥重要作用,在伟晶岩型锂矿勘探中被广泛应用。我们在可可托海开展了音频大地电磁测深。其结果表明:矿区及外围电阻率整体较高,但3号脉以南岩田内发育多处低阻体,据已知岩脉发育特征和地质调查结果,推测部分低阻异常体为隐伏伟晶岩反映。地质分析表明,可可托海伟晶岩均含矿,部分同期淡色花岗岩也有矿化特征,综合地质-地球物理特征,推测可可托海伟晶岩田南部有较好的资源前景。

关键词: 锂矿床, 伟晶岩, 地球物理探测, 电法勘探, 可可托海, 岩石物理

Abstract:

Lithium is a critical metal widely used in Li-ion batteries for energy storage. The demand for lithium resource in a low-carbon economy is immense and rapidly growing, where a jump between 2015 to 2050 is predicted by Science. Currently more than half of lithium production comes from pegmatite lithium deposits. However, due to similar rock property between pegmatites and granite, lithium pegmatite prospecting by geophysical methods has proven difficult. Nevertheless, with the advancements in instrumentation and method developments, geophysical approaches have become increasingly widely used in lithium exploration. In this paper we briefly review the status of lithium resource as strategic raw material, and discuss and summarize the art of geophysical exploration for pegmatite lithium deposits, including rock physics, space remote sensing, gravity and magnetic prospecting, and geoelectrical exploration. A case study of the Koktokay rare-metal pegmatite by audio-frequency magnetotelluric (AMT) method is presented. Pegmatites and leucogranites are characterized by low magnetic susceptibility, low density, low polarizability and high velocity, and the electrical resistivity of pegmatites is affected by hydrophilic minerals and can vary by several orders of magnitude. As the host schist and gneiss in comparison have higher magnetic susceptibility and density, regional gravity and magnetic data are often used to delineate granite bodies. Recent reports show that micro-gravity in some cases can be used to identify pegmatite in host granite. With relatively high resolution and penetration depth, geoelectrical exploration plays an important role in the exploration of concealed pegmatite lithium deposits. In Koktokay, the rock formation and rock mass in the mining district are generally characterized by high resistivity by AMT method, but many low resistivity anomalies are detected. Based on the known ore deposits and geological survey results, the low-resistivity anomalies most likely reflect hidden pegmatites and indicate a good rare-metal resource prospect in the southern Koktokay mining district.

Key words: lithium deposit, pegmatite, geophysical approaches, geo-electical prospecting, Koktkay, rock physics

中图分类号:

何兰芳, 李亮, 申萍, 王斯昊, 李志远, 周楠楠, 陈儒军, 秦克章. 伟晶岩型锂矿床地球物理探测及可可托海实例[J]. 地学前缘, 2023, 30(5): 244-254.

HE Lanfang, LI Liang, SHEN Ping, WANG Sihao, LI Zhiyuan, ZHOU Nannan, CHEN Rujun, QIN Kezhang. Geophysical approaches to the exploration of lithium pegmatites and a case study in Koktohay[J]. Earth Science Frontiers, 2023, 30(5): 244-254.

图/表 7

表1 部分国家典型矿区伟晶岩地球物理探测方法

Table 1 Geophysical approaches typically used for pegmatite exploration in selected countries

数据来源	各类地球物理探测方法的使用情况
数据来源	重力	磁法	遥感	电法	放射性
GreenPeg			○	○
芬兰	○	○	○	○
津巴布韦		○		○
莫桑比克		○			○
尼日利亚		○	○	○	○
阿富汗		○	○
印度	○(微重力)
巴西				○(雷达)
美国	○	○	○	○
中国	○	○	○	○	○

表2 不同地区伟晶岩密度和磁化率变化统计表

Table 2 Statistics of density and magnetic susceptibility values of pegmatites from different regions

数据来源	岩石	密度/(g·cm^-3)		磁化率/(10^-6 SI)
数据来源	岩石	变化范围	均值	变化范围	均值
甲基卡^[26]	锂辉石伟晶岩	2.60~4.25	2.605
	花岗岩	2.623~2.692	2.646	73.99~24.14	47.64
	伟晶岩	2.593~2.699	2.630	50.90~28.60	40.33
	二长花岗岩	2.554~2.772	2.629
喜马拉雅淡色花岗岩	花岗岩、淡色花岗岩			10~280	73.4
欧洲(EuroPeg)	伟晶岩	2.55~3.19	2.67

图1 印度西南Western Dharwar 克拉通萨尔古尔片岩带微重力探测结果(据文献[22]) a—微重力异常图;b—穿过钻孔微重力剖面;c—钻探剖面。

Fig.1 Microgravity survey results on pegmatites in western Dharwar, India. Adapted from [22].

图2 伟晶岩样品含水前后复电阻率时移测量结果样品从含水的测量电极接触海绵吸水,总吸入量为1.46 g, 占比0.47%, 测量时间持续3 d。

Fig.2 Time-lapse complex resistivity of pegmatite samples before and after water absorption

图3 可可托海矿区地质和测线布置(a) 与典型地质剖面图 (b)(据文献[61])

Fig.3 (a) Geological sketch map of the Koktokay mining district and AMT layout and (b) typical geological cross-sections. Adapted from [61].

图4 可可托海东西向AMT-L09线深度-电阻率断面图图中的箭头表示潜在的伟晶岩通道,虚线表示推测的隐伏伟晶岩潜在有利区。

Fig.4 Typical 2D inversion cross-section of AMT data (L09) along W-E direction in Koktohay

图5 可可托海南北向AMT-L01线深度-电阻率断面图图中的箭头表示潜在的伟晶岩通道,虚线表示隐伏伟晶岩推测的板状体顶界面。

Fig.5 Typical 2D inversion cross-section of AMT data (L01) along S-N direction in Koktohay

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