Geochemistry Of Basalts And Hydrothermal Sulfide Mineralization From Juan De Fuca Ridge

The basalts and sulfides we study are from the Main Endeavour, Endeavour field, Juan de Fuca Ridge. We use the mineralogy, petrology and geochemical methods and theories to get the trace elements, isotopic composition, mineral structures and assembled characteristics by high performance ICP-MS, Electronic Probe Microanalyzer and Scanning Electronic Microscope. We get the chemical characteristics of the special minerals from which we can get the information about the hydrothermal activities and mineralization to discuss hydrothermal mineralization, forcast the ore-forming potential of Juan de Fuca Ridge and get the theory for exploring the submarine hydrothermal deposits.The basalt is pillow, filemot of the rock surface by weathering. The rupture is fresh. The fresh surface is grey with white flech and yellow eroding round. The glassy shell is on the surface. The thickness is about 4-6 mm. The basalt sample is porphyritic and pore structure. Plagioclase and little olivine constitute the phanerocryst. The substrate is aphanitic. Magnesium number is 63-65. The oxide contents are consistent with MORB. The magnesia is linear with other oxides, positive proportion with SiO2 and negative proportion with CaO, Na2O+K2O and TiO2.The trace element normalized pattern is smooth, with prominency at the Ta and U elements. The trace elements distribution characteristics are consistent with N-MORB, poor in LILE and rich in HFSE. REE patterns are left leaned and are enriched in HREE, (La/Yb)N=0.567-0.621, (La/Sm)N=0.608-0.649, (Gb/Yb)N=0.928-0.993. The total REE contents are relatively low (∑REE=40.20-42.61 ppm), with light negative Eu anomaly and positive Ce anomaly (δEu=0.846-0.926,δCe=1.0032-1.03). The Pb isotopic ratios are 206Pb/204Pb=18.0～18.4, 207Pb/204Pb=15.4～15.6 and 208Pb/204Pb=37.6～38.0. Their projective dots are on the right of the Zero Isochrone and in the range of Indian MORB and Pacific and Atlantic MORB above the NHRL. In the N-MORB normalized pattern, the ratio of samples/N-MORB is near to 1. The Endeavour field is spreading at an intermediate rate.Hydrothermal sulfide is dust-colour and is mainly composed of pyrite and marcasite. The percentage of pyrite is 65% and 35% is marcasite, with little sphalerite and barite. The microstructure and chemical composition indicate there are two stages in the ore-forming of this area, barite-pyrite-marcasite (Ⅰ) and pyrite-sphalerite (Ⅱ) stages. The difference of pyrite in different stage is mainly in the contents of Cu and Zn. The contents of Cu and Zn in the stageⅡare much higher than stageⅠ. The Fe content in sphalerite produced in this area is high. The sphalerite is riched in Fe content (>10%), indicating the temperature is high during mineralization.We do research on the source of ore-forming material, the characteristic of hydrothermal fluid, the ore-forming geological background and ore-forming model of Juan de Fuca ridge. ODP Leg 139 and Leg 169 drillings show that Middle Valley deposit of Juan de Fuca Ridge has about 8-9 Mt sulfide. There are only 12 fields of the 200 found at present which have the size and quality to explore. Before exploring, we have to evaluate the environmental effect. Only when it has little influence on the environment, can we explore the submarine hydrothermal massive sulfide deposits.