西南印度洋中脊龙旂热液区中硫化物的结构和微量元素特征对金赋存形式和沉淀机制的启示

相比于快速和中速扩张洋中脊，慢速和超慢速扩张洋中脊热液区通常含有丰富的金属硫化物资源。近年来的研究表明大洋中脊的扩张速率与矿石中金的品位呈明显的负相关，即超慢速扩张洋中脊热液区矿石中金的含量高。前人对龙旂热液区的构造环境以及硫化物组合进行了详细研究，但是对龙旂热液区硫化物中贵金属金的赋存形式和沉淀机制研究较少。本文对西南印度洋龙旂热液区中的硫化物进行了精细的矿物结构和微量元素分析，并探讨了金的赋存形式和沉淀机制。龙旂热液区的硫化物主要以黄铁矿为主，其次是黄铜矿和闪锌矿，黄铜矿普遍出溶等轴古巴矿，此外还观察到了少量的针钠铁矾和自然金等矿物。根据矿物结构和形态，黄铁矿明显被划分为两期，一期黄铁矿（Py1）自形度低，呈细粒状或胶状，内部多孔洞；二期黄铁矿（Py2）自形度高，呈自形−半自形，且粒径较大。Py1往往存在于Py2内部或以包体的形式被Py2所包裹，Py2则与自形−半自形黄铜矿和闪锌矿等矿物共生。自然金主要存在于Py1的内部孔洞之中，少量存在于Py2以及Py2与其他硫化物之间。相比于Py2，Py1含有更高的Ni、Zn、Pb、Ba、Mn、V、Mg、U、Au、Ag、Cd元素含量，更低的Co、Se、As、Sb元素含量。在龙旂热液区的物理化学条件下，热液流体中金的主要存在形式为Au(HS)，HS⁻浓度的降低和pH值升高均会促进金的沉淀。龙旂热液区早期热液流体与海水的混合造成热液流体pH值升高，而混合作用导致的热液流体温度降低会促使黄铁矿（Py1）的结晶，从而促使热液流体中HS⁻浓度的降低，热液流体pH值升高和黄铁矿结晶（Py1）引起的硫逸度降低是龙旂热液区自然金沉淀的主要机制。

Textural and trace elemental characteristics of sulfide from the Longqi hydrothermal field,Southwest Indian RidgeImplication for the occurrence and precipitation mechanism of gold

Compared with the fast and intermediate spreading mid-ocean ridges, the hydrothermal fields forming at slow and ultra-slow mid-ocean ridges usually contain abundant metal sulfide resources. Previously statistical results suggested the gold concentrations in massive sulfide deposits decreases with the increase of spreading rate, and the hydrothermal fields, located in ultra-slow mid-ocean ridges, have the highest gold concentrations. Previous studies carried out detailed research on the tectonic environment and sulfide assemblage of the Longqi hydrothermal field, but the occurrence and precipitation mechanism of gold in the Longqi hydrothermal field still need further research. In this paper, the texture and trace element concentration of sulfides in the Longqi hydrothermal field are analyzed in order to investigate the occurrence and precipitation mechanism of gold. The sulfides in the Longqi hydrothermal field are mainly pyrite, along with chalcopyrite (isocubanite) and sphalerite. Minerals such as ferronatrite and native gold have also been observed. According to the mineral texture and morphology, pyrite is divided into two types the first type (Py1) is fine-grained or colloidal, while the other type (Py2) shows subhedral-euhedral with coarse grain. Py1 usually exists in Py2 or is surrounded by Py2 as inclusions, and Py2 coexists with euhedral-subhedral chalcopyrite and sphalerite. Native gold mainly exists in the internal pores of Py1, and minor grains exist between Py2 and other sulfides. Compared with Py2, Py1 contains higher trace element contents of Ni, Zn, Pb, Ba, Mn, V, Mg, U, Au, Ag, Cd, and lower contents of Co, Se, As, Sb. Under the physico-chemical conditions of the Longqi hydrothermal field, Au(HS) is the main existing form of Au. The decrease of HS− concentration and the increase of pH value will promote the precipitation of gold. The mixing of hydrothermal fluids with seawater in the Longqi hydrothermal field results in an increase in pH values and a decrease in temperature, which can lead to pyrite crystallization and consequently decrease of sulfur fugacity of hydrothermal fluid. An increase in pH and decrease of sulfur fugacity both contributed to the precipitation of gold.