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We analyse the fluid flow regime within sediments on the Eastern levee of the modern Mississippi Canyon using 3D seismic data and downhole logging data acquired at Sites U1322 and U1324 during the 2005 Integrated Ocean Drilling Program (IODP) Expedition 308 in the Gulf of Mexico. Sulphate and methane concentrations in pore water show that sulphate–methane transition zone, at 74 and 94 m below seafloor, are amongst the deepest ever found in a sedimentary basin. This is in part due to a basinward fluid flow in a buried turbiditic channel (Blue Unit, 1000 mbsf), which separates sedimentary compartments located below and above this unit, preventing normal upward methane flux to the seafloor. Overpressure in the lower compartment leads to episodic and focused fluid migration through deep conduits that bypass the upper compartment, forming mud volcanoes at the seabed. This may also favour seawater circulation and we interpret the deep sulphate–methane transition zones as a result of high downward sulphate fluxes coming from seawater that are about 5–10 times above those measured in other basins. The results show that geochemical reactions within shallow sediments are dominated by seawater downwelling in the Mars-Ursa basin, compared to other basins in which the upward fluid flux is controlling methane-related reactions. This has implications for the occurrence of gas hydrates in the subsurface and is evidence of the active connection between buried sediments and the water column.

In the eastern Sierras Pampeanas, tourmalinites are found in close association with stratabound scheelite deposits. Chemical and boron isotopic compositions of the tourmalinites from four localities are reported. They are variable in composition, but generally belong to the dravite-schorl tourmaline series with Fe:Fe+Mg ratios ranging from 0.33 to 0.85, Al:Al+Fe+Mg ratios of 0.65-0.75 and significant foitite components. It is suggested that the rocks are submarine hydrothermal-exhalative in origin and their boron sources were probably derived from footwall clastic sediments and volcanics. Non-marine evaporites are also a possible boron source, but geological evidence to support this is lacking., In the eastern Sierras Pampeanas, tourmalinites are found in close association with stratabound scheelite deposits. Chemical and boron isotopic compositions of the tourmalinites from four localities are reported. They are variable in composition, but generally belong to the dravite-schorl tourmaline series with Fe:Fe+Mg ratios ranging from 0.33 to 0.85, Al:Al+Fe+Mg ratios of 0.65-0.75 and significant foitite components. It is suggested that the rocks are submarine hydrothermal-exhalative in origin and their boron sources were probably derived from footwall clastic sediments and volcanics. Non-marine evaporites are also a possible boron source, but geological evidence to support this is lacking.

The clastic sedimentary rocks of the Lower Aldridge Formation that host the orebody have delta 30Si values of 0-0.1 per mil, similar to Si isotope ratios for other clastic sediments. The Si isotope compositions of tourmalinites from the Sullivan area support the genetic model for their origin, that tourmalinites from deep in the feeder zone formed by the replacement of clastic sediments and stratiform and garnet-rich tourmalinites in the immediate footwall of the massive sulphide orebody formed mainly by hydrothermal exhalative processes., The clastic sedimentary rocks of the Lower Aldridge Formation that host the orebody have delta 30Si values of 0-0.1 per mil, similar to Si isotope ratios for other clastic sediments. The Si isotope compositions of tourmalinites from the Sullivan area support the genetic model for their origin, that tourmalinites from deep in the feeder zone formed by the replacement of clastic sediments and stratiform and garnet-rich tourmalinites in the immediate footwall of the massive sulphide orebody formed mainly by hydrothermal exhalative processes.