湖南白云铺金矿含砷黄铁矿和毒砂环带的原位拉曼光谱研究

In Situ Raman Spectra Characterization of Zonal Arsenic-Bearing Pyrite and Arsenopyrite in the Baiyunpu Gold Deposit, Hunan

含砷黄铁矿、毒砂是金矿床中重要的载金矿物,常发育多阶段生长环带结构,可为认识矿床成因和矿床形成过程提供大量信息。前人对含砷黄铁矿、毒砂环带结构的形貌、成分演化等研究较为充分,但未见系统的原位拉曼光谱特征相关报道。同时,不同环带拉曼谱峰偏移规律的研究也可为认识含砷黄铁矿、毒砂内部不可见金的赋存状态提供一定参考。电子探针结果显示,含砷黄铁矿由内核到外环带(Py1→Py2→Py3), S和Fe含量先减少后增加, As先增加后减少(Py2中砷含量可达10.86%)。Au含量与As的变化呈典型正相关,最多可达0.14%。毒砂由内核到环带(Apy1→Apy2), S和Fe含量减少, As相对增加。原位拉曼分析显示,含砷黄铁矿不同环带均主要显示三个拉曼峰,分别对应于黄铁矿的Fe-[S2]^2-变形振动峰(Eg)、 Fe-[S2]^2-伸缩振动峰(Ag)和S-S伸缩振动峰(Tg)。内核Py1的拉曼位移集中在345.8~346.9, 382.0~382.9和434.6~434.8 cm^-1;中间环带Py2为331.9~338.7, 359.2~365.4和404.3~414.2 cm^-1;外环带Py3为343.0~344.9, 375.5~378.3和417.3~431.5 cm^-1。Py2的拉曼位移相对于Py1和Py3显著向低频偏移,偏移量3.1~27.2 cm^-1。分析认为黄铁矿拉曼位移的偏移主要与As、 Au离子的替代作用有关。含砷黄铁矿中不可见金可能以化学结合态形式进入晶格中,引起化学键力常数和折合质量的变化,导致拉曼谱峰振动频率变小,向低频偏移。毒砂主要发育六个拉曼峰,分别集中于136.2~139.8, 174.8~179.4, 198.9~200.7, 307.0~314.2, 338.5~343.9和407.8~410.5 cm^-1,与RUEFF数据库中R050071样品以及相关文献报道的数据相似。此外, Apy2的拉曼位移相对于Apy1略向低频偏移,偏移量0.7~5.4 cm^-1,认为毒砂拉曼位移的变化主要与As离子类质同象置换S离子引起的振动偏移有关。白云铺含砷黄铁矿、毒砂环带结构中原位拉曼光谱特征的研究,为不同成分黄铁矿、毒砂矿物的鉴定提供了丰富的拉曼谱学数据,为揭示不同环带拉曼谱峰的偏移规律,探讨不可见金的赋存形式提供了重要参考。

Arsenic-bearing pyrite and arsenopyrite are important gold-bearing minerals in various types of gold deposits. They often develop multi-stage concentric rings, which provide a lot of information for understanding the genesis and formation process of the deposit. The variation characteristics of morphology and composition of zonal arsenic-bearing pyrite and arsenopyrite grains had been fully studied, but no systematic in-situ Raman spectra data have been reported. Meanwhile, the study of Raman peak migration in layered zones can also provide some reference value for understanding the occurrence forms of invisible gold. Electron microprobe analysis results show that arsenic-bearing pyrite decreases first and then increases in S and Fe content from the core to the outer layer(Py1→Py2→Py3), and opposite in As content(up to 10.86% in Py2). The content of Au is positively correlated with that of As, and it is also relatively enriched in Py2(up to 0.14%). From the core to the outer layer of arsenopyrite(Apy1→Apy2), the S and Fe content decreases, while the As content increases. In situ Raman spectra analysis shows that there are three main Raman peaks in different layered zones of arsenic-bearing pyrite, corresponding to Fe-[S2]^2- deformation vibration peak(Eg), Fe-[S2]^2- stretching vibration peak(Ag) and S-S stretching vibration peak(Tg) of pyrite, respectively. The Raman shifts of inner core Py1 are concentrated in 345.8~346.9, 382.0~382.9 and 434.6~434.8 cm^-1. That of intermediate zone Py2 are concentrated in 331.9~338.7, 359.2~365.4, 404.3~414.2 cm^-1;and that of outer zone Py3 are 343.0~344.9, 375.5~378.3 and 417.3~431.5 cm^-1. The Raman peaks of Py2 shift significantly to low frequency with the offset ranging from 3.1 to 27.2 cm^-1. It is concluded that the Raman shift change of pyrite is mainly related to the substitution of As and Au ions. The invisible gold in arsenic-bearing pyrite may enter the lattice in the form of chemical bonding state, resulting in the change of chemical bonding force constant and reduced mass, and it leads to the decrease of Raman peak vibration frequency and the shift to low frequency. There are six Raman peaks in arsenopyrite of 136.2~139.8, 174.8~179.4, 198.9~200.7, 307.0~314.2, 338.5~343.9 and 407.8~410.5 cm^-1, which is similar to the data of R050071 sample in RUEFF database and some reference value in relevant pieces of literature. In addition, the Raman peaks of Apy2 shift slightly to low frequency related to Apy1, and the offset ranges from 0.7 to 5.4 cm^-1. We thought that the change of Raman shifts in arsenopyrite is mainly related to the vibration migration caused by the isomorphic substitution of S ions by As ions. The study on in-situ Raman spectra characteristics of zonal arsenic-bearing pyrite and arsenopyrite from Baiyunpu samples provide abundant Raman spectra data for the identification of pyrite and arsenopyrite minerals with different compositions, and provide an important reference for revealing the frequency shift of Raman peaks measured in different layered zones and exploring the existence state of invisible gold.