Publication Date:
2020-07-24
Description:
Hydrothermal deposits occur a[ a water depth of 2460 m in Middle Valley, a sediment-fllled railed rift at 48'n' N htitude, on the nordrern Juan de Fuca Ridge. Sites of focused discharge are concentrated above a deep-seated system of fractures paxallel to the regional north-south trend of rift-propagating faults. The Area of Active Venting (AA$ and Bent Hill area are hosts to moderate-temperature (〈276oC) vents. Active chimneys are sulfate-rich and sulfide-poor, consisting of a thick outer wall of anhydrile and a thin clay-rich inner wall with very fine (20 pm) disseminated sulfides. The inner wall contains saponite, pyrrhotite, chalcopyrite, isocubanite, high-iron sphalerite (35-46 mole 7o FeS), pydte, marcasite, galena and arsenopyrite. The active chimneys have low concentrations ofthe base metals but ahigh CulZn value. The fluids discharging through these chimneys have characteristically low a(O2) and a(S2), and locally have deposited significant amounts of hydrocarbon. Hydrothermal mounds associated with active venting are produced by the collapse of chimneys and by mineral precipitation within the pile of sulfide-sulfate talus (i.e., by inflation). The active mounds are composed of anhydrite, amorphous silic4 saponite, serpentine, and minor chalcopyrite, low-iron sphalerite (〈15 mole 7o FeS), lepidocrocite, pyrite, marcasite, galena and barite. Inactive chimneys on or adjacent o the mounds are predominantly barite-rich, with lesser amorphous silic4 hydmcarbons, clay and marcasite. In contrast to the active chimneys, the Bent Hill deposits are sulfide-rich and have higher ZnlCu values. Two older massive sulfide deposits in the Bent Hill area contain pyrite, lepidocrocite, marcasite, moderately iron-rich sphalerite (24-33 mole 7o FeS), chalcopyrite, covellite, galena, silica and barite. Pb-As-Sb sulfosalts occur locally within lowtemperature, silica-rich crusts on the massive sulfides. The mineralogy and geochemistry of the active chimneys suggest hat the present stage of hydrothermal venting at Midclle Valley is botl metaland sulfur-depleted and is likely the product of the moderate-temperature interaction of hydrothermally modified seawater with local sediments. The massive sulfides at Bent Hill are apparently a product of an earlier hydrothermal event dominated by metaland sulfur-enriched fluids, possibly derived from the high+emperature interaction of circulating seawater with basaltic crust in the basement. Recent drilling by the ODP has identified a large pyrite pyrrhotite magnetite deposit beneath the sulfide outcrop at Bent Hill. The present-day mineralogy of this older massive sulfide deposit may be the product of extensive reaction with the cool, sulfur-depleted fluids that are curently venting at Middle Valley.
Type:
Article
,
PeerReviewed
Format:
text
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