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.

The deep structure of the Duanqiao hydrothermal field at the Southwest Indian Ridge

Polymetalic sulfide is the main product of sea-floor hydrothermal venting, and has become an important sea-floor mineral resources for its rich in many kinds of precious metal elements. Since 2007, a number of investigations have been carried out by the China Ocean Mineral Resources Research and Development Association（COMRA）cruises（CCCs） along the Southwest Indian Ridge（SWIR）. In 2011, the COMRA signed an exploration contract of sea-floor polymetallic sulfides of 10 000 km2 on the SWIR with the International Seabed Authority. Based on the multibeam data and shipborne gravity data obtained in 2010 by the R/V Dayang Yihao during the leg 6 of CCCs21, together with the global satellite surveys, the characteristics of gravity anomalies are analyzed in the Duanqiao hydrothermal field（37°39′S, 50°24′E）. The ＂subarea calibration＂ terrain-correcting method is employed to calculate the Bouguer gravity anomaly, and the ocean bottom seismometer（OBS） profile is used to constrain the two-dimensional gravity anomaly simulation. The absent Moho in a previous seismic model is also calculated.The results show that the crustal thickness varies between 3 and 10 km along the profile, and the maximum crustal thickness reaches up to 10 km in the Duanqiao hydrothermal field with an average of 7.5 km. It is by far the most thicker crust discovered along the SWIR. The calculated crust thickness at the Longqi hydrothermal field is approximately 3 km, 1 km less than that indicated by seismic models, possibly due to the outcome of an oceanic core complex（OCC）.

Ga isotopic fractionation in sulfides from the Yuhuang and Duanqiao hydrothermal fields on the Southwest Indian Ridge

This study examines the Ga isotopic compositions of sulfides in the Yuhuang and Duanqiao hydrothermal fields on the Southwest Indian Ridge,mid-ocean ridge basalts(MORB),and calcareous sediments around the hydrothermal fields.Theδ^(71/69)Ga_(NIST-994) values of theMORB samples vary little(+1.20‰to+1.23‰,with an average of+1.22‰)and are consistent with the δ^(71/69)Ga_(NIST-994) values of two standard basalt samples(BCR-2 and BHVO-2),indicating that Ga isotopes may either not fractionate or fractionate only slightly under high-temperature geological processes;therefore,the δ^(71/69)Ga_(NIST-994) value of oceanic crust may be+1.22‰.The sediments(+1.28‰to+1.47‰,with an average of+1.38‰)are rich in heavier Ga isotopes than the basalts,and the Ga present in the sediments may have originated from soluble Ga present in the seawater that was adsorbed by(Mn,Fe)oxides/hydroxides.The Ga contribution of basaltic debris to the sedimentswas almost negligible.Thus,wespeculate that theδ^(71/69)Ga_(NIST-994) value of seawater in the study area fell within a range from+1.92‰to+2.36‰.Theδ^(71/69)Ga_(NIST-994) values of the sulfides in the Yuhuang hydrothermal field range from+0.99‰to+1.57‰,with an average of+1.25‰,and theδ^(71/69)Ga_(NIST-994) values of the sulfides in the Duanqiao hydrothermal field range from+0.93‰to+1.55‰,with an average of+1.19‰.Theδ^(71/69)Ga_(NIST-994) ranges of the sulfides in the Yuhuang and Duanqiao hydrothermal fields are similar,with the Ga isotopic fractionation reaching 0.58‰and 0.62‰,respectively.The averageδ^(71/69)Ga_(NIST-994) values in the sulfides are close to those in the MORBs.This suggests that Ga within the sulfides in the Yuhuang and Duanqiao hydrothermal fields mainly originated from MORBs,with seawater and sediments making only small contributions.The Ga isotopic fractionation in the sulfides may be related to processes associated with the formation of sulfides,such as rapid precipitation or the admixture of different stages of sulfide.This study is of great significance for understanding the global distribution of Ga isotopes and the Ga cycle in submarine hydrothermal systems.

西南印度洋中脊龙旂热液区(49°39′E)的地震波各向异性研究进展与展望

龙旂热液区(49°39′E)是超慢速扩张西南印度洋中脊(SWIR)上发现的首个活动热液喷口,也是我国开展海底多金属硫化物资源勘探和走向深海大洋的重点研究区域。前期三维层析成像研究成功地揭示了该区的深部结构特征,但仅仅是提供了静态信息,相比之下,地震波各向异性研究是揭示其深部动力学机制的有效手段之一。本文简要介绍了龙旂热液区开展的主动源和被动源海底地震仪(OBS)探测实验,并初步分析了前期三维层析成像的走时残差数据,发现其与方位角之间存在显著的余弦函数关系,表明该区速度结构存在各向异性,但其各向异性来源尚不明确。探索各向异性来源将有望揭示该区的热液循环机制,为认识超慢速扩张SWIR热液区形成的动力学演化过程提供科学依据。因此,本文拟在前期研究基础上,利用主动源和被动源OBS地震数据,通过纵波方位各向异性和横波分裂联合分析的方法,重点研究地壳和地幔各向异性特征,并结合三维速度模型与区域地质背景资料,对获得的快波方向和快、慢波到时差参数进行深入分析,阐明地壳内裂隙分布、应力场变化和地幔流动特征,从而揭示龙旂热液区的热液循环机制、岩石圈形变和深部动力学过程等科学问题。