SULFIDE, SEDIMENT, AND BIOLOGICAL ZONATION 

 AT ASHES VENT FIELD, AXIAL VOLCANO 



Alexander Malahoff, Anne M. Arquit, and Gary M. McMurtry 



Hawaii Institute of Geophysics 



University of Hawaii, Honolulu, HI 96822 



ABSTRACT 



A young, high-temperature hydrothermal vent field consisting 

 of two polymetallic sulfide chimneys 5 m high was sampled along 

 a fault complex associated with the southwestern wall of the 

 summit caldera of Axial Volcano during four Alvin dives in 1984. 

 Temperatures up to 270°C were measured within the orifice of one 

 of the chimneys, which was then broken off and recovered for 

 analysis. Both black and white "smoke" were emanating from the 

 chimney that was sampled. The chimney has formed directly on top 

 of lobate lava flows that occur about 200 m east of the caldera 

 wall at 1,550 m water depth, making this vent one of the 

 shallowest active high-temperature vents studied to date. The 

 vent is surrounded by clumps of vestimentif eran worms and is 

 surrounded on all sides by an elongate field of yellowish iron 

 smectite sediment and chimney structures. This low-temperature 

 field of hydrothermal precipitates is 150 m wide and extends 

 about 1,300 m along the base of the caldera wall. 



Chemical analyses of the hydrothermal deposits from the 

 metal sulfides of the chimney to the outer zones of the low- 

 temperature field show a range of elemental assemblages that can 

 be explained by thermo-chemical gradients in the near-vent 

 region. Chemical zonation within the high- temperature chimney is 

 indicated by mineralogical banding and is confirmed by chemical 

 gradients such as the observed increase of gold from 750 parts 

 per billion ( ppb ) in the center to 2,800 ppb at the outer edge of 

 the chimney. The sulfides sampled on the floor of the caldera 

 are relatively small volume but have a high gold content compared 

 to other deposits precipitated along mid-ocean rift valleys. 

 These findings support the hypothesis that there are larger 

 volume, lava-hosted sulfide deposits at depth that are subject to 

 dissolution by upward-moving hydrothermal fluid along the 

 fissures associated with the normal fault forming the caldera 

 wall. This suggested concentration mechanism could repeat over 

 several cycles, following each successive burial of sulfides by 

 fresh lava flows. 



INTRODUCTION TO ZONATION STUDIES 



Axial Volcano straddles the Juan de Fuca Ridge at 45°56'N 

 (Fig. 1). Active hydrothermal venting was first discovered in 

 the summit caldera of the volcano during the Canadian-American 



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