慢速扩张卡尔斯伯格脊远轴热液成矿作用的发现和意义

Discovery and Significance of the Distal Axis Hydrothermal Mineralization on the Slow-spreading Carlsberg Ridge

慢速扩张洋中脊是海底多金属硫化物勘查的重要地区,然而目前对离轴的大洋核杂岩(OCC)热液成矿作用的调查及认识非常有限。本文对卡尔斯伯格脊2.54°N附近一处离轴20 km处OCC采集的样品进行了详细的矿物学和元素地球化学研究。结果表明,蚀变玄武岩和石英角砾岩的矿物共生组合以及化学特征指示了高温热液蚀变与矿化过程,以及低温海底风化过程。从热液蚀变至矿化阶段,绿泥石化和黄铁矿化逐渐增强,部分亲石(Ca、Sr)、亲铜(Zn)和稀土元素迁出,Cu逐渐迁入。通过典型矿床黄铁矿对比分析发现,研究区蚀变玄武岩和石英角砾岩中的黄铁矿总体具有高Ni、Co和Se含量,以及高Ni/As值的特征,反映了高温和强还原的成矿流体性质,并受到深部超镁铁质围岩的影响。相比蚀变玄武岩中黄铁矿(PyⅠ),石英角砾岩黄铁矿(PyⅡ)更富集Co和Se,反映流体温度升高的趋势。此外,受到热液流体与海水混合作用等过程影响,在成矿后期Ag进一步在蚀变玄武岩黄铁矿中富集。本次研究证实了在远离脊轴的OCC区域,拆离断层的构造作用有利于深部热液矿化线索抬升至海床面,为慢速扩张洋中脊的硫化物资源分布规律与热液成矿机理等研究提供了新视角。

Slow-spreading mid-ocean ridges are critical sites for the exploration of seafloor massive sulfides,however,the investigation and understanding of hydrothermal mineralization on off-axis oceanic core complex(OCC)is limited.In this paper,a detailed mineralogical and elemental geochemical study of altered basalts and quartz breccia samples at an off-axis distance of about 20 km near the Carlsberg Ridge 2.54°N has been carried out.The results show that mineral paragenetic assemblage and chemical characteristics of samples indicate high-temperature hydrothermal alteration and mineralization processes,as well as low-temperature seafloor weathering processes.From hydrothermal alteration stage to mineralization stage,the intensity of chloritization and pyritization are gradually enhanced,some lithophile elements(Ca,Sr),chalcophile elements(Zn)and rare earth elements migrate out,whereas Cu gradually migrate in.Through the comparative analysis of pyrite in typical seafloor hydrothermal deposits,it is found that the pyrite occurred in the altered basalts and quartz breccia in the study area is characterized by high Ni,Co and Se content,as well as high Ni/As value.This reflects pyrite precipitates from high temperature and reduced ore-forming fluid,and the fluid is affected by the ultramafic rock in the deep.Compared with pyrite in altered basalts(PyⅠ),Co and Se are more enriched in pyrite in quartz breccia(PyⅡ),which reflects the trend of increasing fluid temperature.In addition,in the late stage of mineralization,affected by the mixing of hydrothermal fluid and seawater,Ag is further enriched in the pyrite from the altered basalt.These findings confirm that in the OCCs region far from the spreading axis,the tectonic activities of the detachment fault are conducive to the uplift of the hydrothermal mineralization clues in the deep to the seabed,which provides a new perspective for the study on the sulfide resource prospecting and hydrothermal mineralization mechanism along the slow-spreading ridge.