冲绳海槽中、南部热液活动区金属硫化物的地球化学特征

对新近(2014年)采自冲绳海槽南部Yonaguni Knoll IV热液活动区和中部伊平屋热液活动区的金属硫化物样品分别进行了X射线荧光光谱(XRF)和等离子体质谱(ICP-MS)分析,旨在通过对比分析两个热液活动区金属硫化物的元素地球化学特征,探讨两个不同热液活动区成矿元素的富集规律、热液成矿的作用过程及其区域差异。研究结果表明:在冲绳海槽中部和南部热液区分布有两种金属硫化物,分别为富Fe-Zn型硫化物和Ba-Zn-Pb型硫化物(+硫酸盐);相较于大洋中脊和其它弧后盆地的热液硫化物,研究区硫化物中Pb的含量明显较高,而Cu和Fe的含量相对较低,这与冲绳海槽存在较厚的沉积盖层有关;无论是常量元素、微量元素、还是稀土元素(REE),其含量和相关的特征值都变化较大,表明研究区热液硫化物形成于不同的热液成矿阶段;富Fe-Zn型硫化物是在高温热液活动阶段形成的,而Ba-Zn-Pb型硫化物(+硫酸盐)则形成于热液喷出海底与海水混合的过程中,成矿温度较低;即使在同一热液活动区,成矿环境也有较大变化,主要表现在成矿温度的波动上;研究区热液硫化物中REE的配分模式特征表明硫化物中REE的物源相同,但经历的过程(REE的迁移和富集)有所不同,反映了热液硫化物形成于不同的热液成矿阶段;热液成矿作用存在区域性差异,海槽南部的成矿温度较中部低,成矿物质来源方面,南部热液区硫化物的Pb含量较中部高,表明其成矿物质可能部分来源于沉积物。

Geochemical characteristics of hydrothermal sulfide from the middle and southern hydrothermal fields in the Okinawa Trough

Previous studies of the hydrothermal fields in the Okinawa Trough mainly focused on the middle of the Okinawa Trough. In this paper, new hydrothermal sulfide samples collected from the Yonaguni Knoll IV and the Iheya hydrothermal field were analyzed based on their x-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). By comparing the geochemical characteristics of the hydrothermal sulfides in two hydrothermal fields, this paper discusses the enrichment regularity of metallogenic elements, the process of hydrothermal mineralization, and the differences in hydrothermal processes and mineralization among the different hydrothermal fields. The results showed that there are two kinds of hydrothermal sulfides in the middle and southern Okinawa Trough, namely sulfide enriched in Fe and Zn, and sulfide (+sulfates) enriched in Ba, Zn, and Pb. Compared with the sulfides in the mid-ocean ridge and other back-arc basins, the contents of Pb in the samples were obviously higher, while the contents of Cu and Fe were relatively low, which is related to the thick sedimentary cover in the Okinawa Trough. Additionally, the content and related characteristic values of major elements, trace elements, and rare earth elements in the samples change greatly, indicating that the hydrothermal sulfides were formed in different hydrothermal mineralization stages. The sulfide enriched in Fe and Zn was formed at the high temperature hydrothermal stage, while the sulfide (+ sulfate) enriched in Ba, Zn, and Pb was formed during the mixing of hydrothermal fluid and seawater, which implies a low-temperature environment. Even with in the same hydrothermal fields, the mineralization environments also changed greatly, mainly in the fluctuation of mineralization temperature. The rare earth element patterns in the hydrothermal sulfides of the study area showed that the source of REE in the samples is the same, but the migration and enrichment of REE were different. This reflects that the hydrothermal sulfides formed in different hydrothermal mineralization stages. The hydrothermal mineralization in the different fields were different. The ore-forming temperature in the south of the trough was lower than that in the middle. In terms of mineral source, the mineral source in the southern trough may be partly from the sediment, as its content of Pb in the sulfide is higher than that in the middle part.