Deep sea metalliferous sediments: A medium for reconstructing hydrothermal inputs and locating areas of mineralization
|Location||International Geological Congress,oslo 2008|
|Holding Date||23 September 2008|
Fields of deep sea metalliferous and low-metalliferous sediments (MS) are halos of dispersion of submarine hydrothermal mineral-forming systems and carry information on these systems. Cores with MS are records of not only hydrothermal events but also of intensity and temporal variations of hydrothermal activity and associated mineral deposition. In MS accumulation rate of hydrothermal metal-bearing matter (HM) with correction for bottom currents reflects intensity of HM contribution to the ocean and intensity and amount of mineral formation and deposition. These properties of MS enable their use in reconstructing intensity and locations of hydrothermal activity and mineral formation in geological past and also in locating areas of present occurrence of buried mineral accumulations formed within hydrothermal fields.
Hydrothermal Fe (Fe") is the best proxy for HM in MS. I have developed methods of reconstructions with use of Fe". They are based on observations that settling flux of Fe" from hydrothermal plumes and its accumulation rate (ARFe") in distal MS are quantitatively related with intensity of HM supply to the ocean and distance from hydrothermal vent areas and also affected by bottom currents.
ARFe" in MS is a parameter for reconstructing intensity of hydrothermal activity and mineral formation in geological past. Its values in MS collected at coring and deep-sea drilling vary from place to place and along cores. Usually MS in the lower parts of the cores have accumulated earlier and closer to spreading axes than overlying ones. Therefore, to compare records of hydrothermal activity even in one core (and especially in many ones) techniques of correction for effects of plate motion and plate boundary migration are of great importance. ARFe" variations in MS from sections parallel or subparallel to spreading axes (along with mapping: if possible) enable reconstructing history of hydrothermal activity on these axes. Analysis of ARFe" isopleths in spatiotemporal dissection (data for each hole are plotted along its backtracked path) is a very productive approach. If direction of a bottom current is normal to a spreading axis, the method enables reconstructing positions of past and present areas of hydrothermal activity, estimating their relative intensities by calculating near-axial and axial ARFe", and reconstructing current directions. Areas of present locations of buried hydrothermal mineral accumulations can be found with some probability by calculating ARFe" in the lowermost parts of basal MS sequences. For other current directions a combination of ridge-parallel (ridge-subparallel) and 2-D temporal ridge-normal (ridge-subnormal) sections (sample ages are plotted on the Y axis and time since the beginning of sediment stratum formation — on the X axis) can be used.
The first experience of 0-12 Ma reconstructions and locating areas of mineralization in the SE Pacific has been successful. Cenozoic reconstructions for the East Pacific are planned.