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Abstract:

Shallow hydrothermal venting is a widespread phenomenon associated with submarine arc volcanism. In August 2007, shallow drilling was conducted for the first time at two localities in the Aeolian island arc to study the third dimension of hydrothermal systems precipitating polymetallic sulfides at shallow water depth. A lander-type drilling device (Rockdrill I of the British Geological Survey) was deployed from the German R/V Meteor permitting drilling to a maximum penetration of 5 m. Eleven holes were successfully drilled into the sediment-covered sulfide mineralization at the Palinuro volcanic complex ( approximately 650 m water depth). The mineralized zone remained open in all directions and at depth because all drill holes ended in sulfide mineralization. Up to 4.8 m of massive sulfides/sulfates were recovered from a single drill hole. Metal enrichment at the top of the mineralization was observed with polymetallic (Zn, Pb, Ag) sulfides overlying massive and dense pyritic sulfides. The massive sulfide mineralization at Palinuro contains a number of unusual sulfide minerals including enargite, tennantite, luzonite, covellite, and Ag-sulfosalts. Native sulfur, forming late veins, was an abundant component in all drill holes. In analogy to epithermal deposits forming on land, the occurrence of these unusual minerals suggests a high-sulfidation state of the hydrothermal fluids. The acidic and oxidizing character of the mineralizing fluids is probably related to the presence of magmatic volatiles, presumably derived from a degassing magma chamber at depth. This is also supported by preliminary sulfur isotope data on native sulfur and sulfide separates. The isotope ratios show substantial variation at the surface (-26 to -50/00), but a narrow range between -5 to 00/00 at depth. These variations could be explained by the mixing of magmatic sulfur with a biogenic component derived from the host sediments. Extensive drilling was also conducted at the submerged portion of the Panarea volcanic complex ( approximately 60-90 m water depth) in areas where circular depressions (<50 m diameter) have previously been reported. Video seafloor mapping suggests that these depressions formed in response to submarine gas explosions and/or collapse following intense gas venting. Drilling yielded up to 3 m of massive anhydrite+ or -gypsum with minor sulfides. The massive sulfates are interpreted to form a seal at the interface between cold seawater and upwelling magmatic-hydrothermal fluids.

Notes:

GeoRef, Copyright 2018, American Geological Institute.<br/>2009-027459