西南印度洋脊龙旂热液区蚀变岩岩石学特征及对热液流体循环的指示

慢速-超慢速扩张洋脊的海底热液活动区多出露类型多样的蚀变岩石,记录了地壳深部的流体与围岩的相互作用,为研究深部热液流体特征以及循环过程提供了样本。本研究选取了中国大洋第30、34和40航次在超慢速扩张西南印度洋脊龙旂热液区(A区、B区和C区)利用电视抓斗采集的蚀变玄武岩、蚀变辉长岩、蚀变辉石岩和蛇纹岩等蚀变岩样品,利用光学显微镜、电子探针开展了岩相学和矿物化学分析。岩相学结果表明,龙旂热液区蚀变岩石样品约95%发生了地壳浅部的脆性变形作用,靠近龙旂1号热液区(A区)约有5%的蚀变岩石混合发育了脆性变形及脆性-塑性变形特征。研究区岩石蚀变属于中-低温变质作用,变质相近似绿片岩相,变质矿物组合为绿泥石-绿帘石-钠长石-阳起石-榍石。其中,A区的蚀变岩中的绿泥石形成温度(201～341℃)以及蛇纹石、阳起石、绿泥石等蚀变矿物的Fe元素含量(17.5%～27.5%)都高于龙旂3号热液区(B区和C区)的绿泥石形成温度(239～303℃)和Fe元素含量(16.8%～26.5%),这也与在该区观测到高温的热液喷口相符合。本研究认为龙旂热液区所在洋脊段发育的拆离断层为热液流体的向上运移提供了通道,洋壳...     

Mineralogy and Chemistry of Sulfides from the Longqi and Duanqiao Hydrothermal Fields in the Southwest Indian Ridge

Recent investigations found that hydrothermal activity and sulfide mineralization occurs along the Southwest Indian Ridge (SWIR). The Longqi and Duanqiao hydrothermal fields between 49° E and 53° E of the SWIR are two prospective mineralization areas discovered by Chinese scientists. With the aim to determine the mineralogical and chemical characteristics of sulfide minerals, we have conducted detailed studies for samples from the two areas using an optical microscope, X-ray diffractometer, scanning electron microscope, and electron microprobe. The mineralization processes in the Longqi area are divided into three main stages: (1) the low-medium-temperature stage: colloform pyrite (Py I) + marcasite → euhedral pyrite (Py II), (2) the high-temperature stage: isocubanite (±exsolved chalcopyrite) + pyrrhotite → coarse-grained chalcopyrite (Ccp I), and (3) the medium–low-temperature stage: sphalerite + fine-grained chalcopyrite inclusions (Ccp II) → aggregates of anhedral pyrite (Py III) ± marcasite → Fe-oxide (-hydroxide) + amorphous silica. The mineralization processes in the Duanqiao area are divided into two main stages: (1) the medium–high-temperature stage: subhedral and euhedral pyrite (Py I′) → coarse-grained chalcopyrite (Ccp I′) and (2) the medium–low-temperature stage: sphalerite → fine-grained chalcopyrite (Ccp II′) + chalcopyrite inclusions (Ccp II′) → silica-cemented pyrite (Py II′) + marcasite → Fe-oxide + amorphous silica. We suggest that the fine-grained chalcopyrite inclusions in sphalerite from Longqi and Duanqiao were formed by co-precipitation and replacement mechanisms, respectively. Primary sphalerites from both fields are enriched in Fe (avg. 5.84 wt% for the Longqi field vs. avg. 3.69 wt% for the Duanqiao field), Co (avg. 185.56 ppm for the Longqi field vs. 160.53 ppm for the Duanqiao field), and Cd (avg. 1950 ppm for the Longqi field vs. avg. 525.26 ppm for the Duanqiao field). Cu contents in pyrite from the Duanqiao field (Py I′: avg. 849.23 ppm and Py II′: avg. 1191.11 ppm) tend to be higher than those from the Longqi field (Py I: avg. 26.67 ppm, Py II: avg. 445 ppm, and Py III: avg. 179.29 ppm). Chalcopyrite from both fields is enriched in Zn (Ccp I: avg. 3226.67 ppm, Ccp II: avg. 9280 ppm, Ccp I′: avg. 848 ppm, Ccp II′ (inclusions): avg. 1098 ppm, and Ccp II′ (fine-grained): avg. 1795 ppm). The varying contents of Zn in the different pyrite and chalcopyrite generations may result from the zone refining process. An integrated study of the mineralogy and mineralogical chemistry suggests that the hydrothermal fluids of the Longqi area are likely conditioned with higher temperatures and relatively lower fO2 and fS2 than those of the Duanqiao area, but in contrast to the former, the latter is much affected by the compositions of the surrounding rocks.