中印度洋海岭Edmond热液区块状硫化物中自然金的发现及其意义

摘要

There are three types of hydrothermal precipitates sampled from the Edmond hydrothermal field, Central Indian Ridge; Fe-rich massive sulfides mainly composed of pyrite and chalcopyrite, silica-rich hydrothermal precipitates, and anhydrite-dominated sulfate ore samples. Irregular-shaped or tabular grains of native gold ( <20μm) mainly associated with anhydrite and sphalerite have been identified in sulfate ore samples and Fe-rich massive sulfides by SEM/EDS observation and XPS analysis. This is the first documented occurrence of native gold grains in seafloor hydrothermal precipitates from the Edmond vent field, CIR. To a lesser extent, gold occurs as submicroscopic particles deposited on the surface of coarse-grained pyrite. EPMA results indicate that high gold contents (with an average of 6700 × 10-6 Au) are associated with late-stage, low-temperature ( Fe-poor) sphalerite, and silver is significantly enriched in sulfosalt minerals (5.0% -6.7% Ag) occurring at the margins of exsolved chalcopyrite grains within sphalerite. In conclusion, the mineralization of precious metals in polymetallic massive sulfides is closely related to late-stage, low to moderate temperature hydrothermal ore-forming processes at the seafloor. Evaluation of possible complexes of gold indicates that AuCl2 - or AuHS° is the dominant complex, which is consistent with the high temperature and chlorinity of acidic end-member hydrothermal fluids in the Edmond vent field. Mixing of hydrothermal fluids and seawater, together with conductive cooling and/or phase separation may be important factors controlling gold precipitation and enrichment.%位于中印度洋中速扩张洋脊的Edmond热液区块状硫化物矿石样品主要分为以黄铁矿-黄铜矿为主的富Fe块状硫化物、热水沉积成因的富含硅质块状矿石和以硬石膏为主的硫酸盐矿石等3种不同类型.通过扫描电镜观察和X射线光电子能谱分析,在硫酸盐矿石和富Fe块状硫化物中首次发现了自然金,最大粒径可达20 μm左右,主要呈不规则粒状或板状与硬石膏、闪锌矿等硫化物颗粒紧密共生,少量以次显微金形式沉淀在自形黄铁矿晶体表面.电子探针分析结果显示,晚期形成于中低温条件下的贫Fe闪锌矿中Au富集程度普遍较高(平均含量约为6700×10-6)；Ag主要以类质同象形式赋存于与闪锌矿、黄铜矿伴生的硫盐矿物中(5.0％～6.7％ Ag),这表明贵金属元素的富集成矿作用与海底热液活动晚期的中低温成矿阶段有关.推断Au在该研究区以高温、酸性和氯度较高为特征的热液流体中主要呈AuCl2-或AuHS0形式迁移.而海水与热液流体混合、喷口流体发生相分离以及传导冷却作用,被认为是导致Au有效沉淀的重要控制因素.