西南印度洋超慢速扩张脊海底热液硫化物中金银矿物的富集特征及富集机制研究

The Enrichment Characteristic and Mechanism of Gold-Silver Minerals in Submarine Hydrothermal Sulfides from the Ultra-Slow-Spreading SWIR

通过电感耦合等离子体光谱仪(ICP-AES)以及扫描电镜等手段对超慢速扩张的西南印度洋中脊热液区的3个硫化物样品的金银含量及其赋存状态进行了研究。通过ICP-AES测定金银含量,结果发现3个样品均有Au和Ag富集的现象。通过能谱-扫描电镜联用(SEM/EDS),在三个样品中发现了大量的金银矿物。其中S27-4中,主要以不规则粒状的银金矿和自然金的形式赋存于硫化物中或者晶粒之间,而S35-22中,则发现了大量的银矿物和部分银金矿。能谱分析表明银矿物中常伴随有一定量的卤族元素,推测以卤化银的形式存在,银金矿除了以粒间金赋存还可以呈乳滴状被吸附在黄铁矿表面或者晶棱上,推测与黄铁矿表面缺陷有关。S35-17样品中银金矿是唯一被发现的矿物相,以包体金、吸附金和间隙金形式赋存。由于不同样品中,银金矿的摩尔比有所不同,指示形成环境有所不同。研究结果表明,S27-4的成矿流体中,Ag主要以AgCl-2络合物迁移,Au经历了以AuCl-2络合物到AuHS0络合物为主的转变,指示了其温度从中高温到中低温的变化过程,说明了传导冷却在这个变化过程中有重要作用。S35-22中金银矿物的形成也具有类似的富集机制,但S35-22中卤化银的形成显示了早期高温高氯度的流体环境。而黑烟囱样品S35-17中银金矿的形成则与热液流体与海水的混合作用有密切关系。

In the present study,content and occurrence of Au,Ag in three submarine hydrothermal sulfide samples from the ultra-slow-spreading Southwest Indian Ridge（SWIR）were studied by using inductively coupled plasma-atomic emission spectrometry（ICP-AES）,scanning electron microscopy（SEM）and energy dispersive X-ray spectroscopy（EDS）.The results of ICPAES show that all of the samples have signs of Au-Ag enrichment.By SEM/EDS,we discovered a mass of gold-silver minerals in the samples.In S27-4,gold occurs as irregular-shaped native gold and electrum grains in sulfides or between crystal particles.However,we discovered lots of Au-independent silver minerals except parts of electrum in S35-22.EDS results of silver minerals indicate that silver minerals closely related with halogen element,inferring that silver minerals may be silver halides.Electrum in S35-22 can be absorbed at the surface or crystal edge of pyrite besides occurring in or between sulfides as S27-4,supposed to be related to surface defect in pyrite.Electrum is the only Au-Ag mineral discovered in S35-17.These electrum gains occur as inclusion gold,absorbed gold or fissure gold.In addition,there are different Au-Ag mole ratios of electrum in three samples,indicating distinct hydrothermal conditions.In the base of research before,we consider that AgCl-2is the dominant complex of silver in ore-forming fluid of S27-4,however,gold is transported as AuCl-2transforming to AuHS0,indicating that hydrothermal fluids decreased from high-moderate to moderate-low temperature and conductive cooling played an important role in this process.Similar enrichment mechanism happened in S35-22,but silver halides discovered in S35-22 suggest a higher temperature and chloride in the early stage.However,The enrichment of electrums in black smoke sample（S35-17）relates to mixing of hydrothermal fluids and seawater.