The Study Of Hydrothermal Plume Ore-prospecting Criteria On The Mid-ocean Ridges

Seafloor hydrothermal polymetallic sulfide is a new type of resource, with great potential economic value and good development prospect. With the development of deep-sea survey technology and depletion of mineral resources on land, exploration and development of seabed mineral resources has become a research objective for countries around the world. Currently, the investigation of seafloor hydrothermal systems is still in the scientific research stage, while the exploration of seafloor polymetallic sulfide has been into exploratory stage.Most hydrothermal fields are found on mid-ocean ridges. The type, size and shape of polymetallic sulfide deposits on the mid-ocean ridge are controlled by complex geological factors, such as tectonics, magmatism, crustal permeability and local magma supply. The formation of seafloor polymetallic sulfide deposits is associated with hydrothermal activity. Thus, detection of active hydrothermal vents is a direct and effective step for prospecting. At the present stage, seafloor polymetallic sulfide prospection and exploration begins with active hydrothermal plume detection,then search for inactive sulfide fields around active hydrothermal areas, to narrow down the range of hydrothermal vents.To date, little systematic research into ore-prospecting criteria for seafloor polymetallic sulfides has occurred. Especially in the aspect of hydrothermal plume ore-prospecting criteria, there is almost no research precedent. With the target of hydrothermal plume ore-prospecting criteria on the mid-ocean ridge, this paper tentatively developed the work in the following aspects.Hydrothermal plumes, formed by the turbulent mixing of hot vent fluids and ambient seawater, are rapidly diluted with ambient seawater, yet display physical and chemical signatures distinctly different from background seawater. When hydrothermal plume rising, the metal-rich sulfide and oxide particles are deposited near the vent, forming polymetallic sulfide deposits under a certain geological process.Thus by detecting hydrothermal plume, it helps us locate the hydrothermal vents more quickly and effectively, and guide the prospecting work.Based on the survey data, combined with the detection method of hydrothermal plume, we analyzed the characteristics of the turbidity data and the factors affecting the quality of data, and put forward the corresponding correction methods for each influence factor, finally constructing a data processing method based on deep-sea drag technology to detect hydrothermal plume, which provides technical support for analyzing the distribution of hydrothermal plume.The hydrothermal plume ore-prospecting criteria for seafloor polymetallic sulfide deposits include hydrothermal plume characteristics that can be used as clues during prospecting. We summarized the distribution characteristics of hydrothermal plume in the typical field on the mid-ocean ridge, and put forward the hydrothermal plume ore-prospecting criteria, including the direct hydrothermal plume ore-prospecting criteria and the indirect hydrothermal plume ore-prospecting criteria.The direct hydrothermal plume ore-prospecting criteria mainly refer to physical and chemical features, gas content features and water chemical characteristics. The indirect ones contain the morphological characteristics, element combination index and the characteristics of particulate matter.According to the basic ideas and methods for mineral prospecting and different scales of ore-prospecting criteria, we put forward two-stage hydrothermal plume ore-prospecting pattern. They are regional hydrothermal plume ore-prospecting pattern and local hydrothermal plume ore-prospecting pattern. On the basis of those hydrothermal plume ore-prospecting patterns, we discussed the regional hydrothermal plume ore-prospecting pattern on the East Pacific Rise from 1°N to 7°S segment and South Mid-Atlantic Ridge 13°S~14°S. We discussed the local hydrothermal plume ore-prospecting pattern on the Niaochao South hydrothermal field on EPR, TAG field on MAR and Longqi field on SWIR.Based on the above analysis, the two corresponding stages of hydrothermal plume prospecting model are refined. In regional hydrothermal plume prospecting model, combined with the depth, terrain distribution characteristics, regional tectonic characteristics of the mid-ocean ridge, the water survey can be carried out in three methods, which are deep-tow survey lines, CTD station and AUV detection in first and second phase. The following ore-prospecting criteria can be used: temperature,turbidity, methane, hydrogen sulfide, REDOX potential, pH, total dissolved manganese and iron, etc. As to the local hydrothermal plume prospecting model, it is corresponding to the hydrothermal area over a spatial scale of 100 s of meters to 1000 s of meters. By analyzing the vertical profiles of ore-prospecting criteria within the scope of about 250 m above the seafloor, such as temperature, salinity and turbidity,we can fully recognise the characteristics of rising hydrothermal plume.This paper carried out the study of correlation between the spatial distributions characteristics of hydrothermal plume and polymetallic sulfide ore-prospecting criteria innovatively, and proposed hydrothermal plume ore-prospecting model.Studies show that the proposed hydrothermal plume ore-prospecting pattern for polymetallic sulfide on mid-ocean ridges is feasible, which provides technical support for resource evaluation work in the SWIR exploration area to some content.