Sulfide mounds

Deposits which are forming are frequentiy characterized by venting streams of hot (300°C) mineralized fluid known as smokers. These result in the local formation of metalliferous mud, rock chimneys, or mounds rich in sulfides. In the upper fractured zone or deep in the rock mass beneath the vents, vein or massive sulfide deposits may be formed by the ckculating fluids and preserved as the cmstal plates move across the oceans. These off-axis deposits are potentially the most significant resources of hydrothermal deposits, even though none has yet been located. 

Stainless steels attacked by sulfate reducers show well-defined pits containing relatively little deposit and corrosion product. On freshly corroded surfaces, however, black metal sulfides are present within pits. Rust stains may surround pits or form streaks running in the direction of gravity or flow from attack sites. Carbon steel pits are usually capped with voluminous, brown friable rust mounds, sometimes containing black iron sulfide plugs fFig. 6.10). 

Back-arc spreading center 1 North Fiji Basin, Station 4 (16°59 S. 173°55 E) 1980 Axial graben at topographic high of north-central segment near triple junction. Sheet lava floor. Active (r = 290°C) anhydrite chimneys standing on dead sulfide mound. Forest of dead sulfide chimneys. Anhydrite, amorphous silica in dead chimneys pyrite, marcasite, chalcopyrite, sphalerite, wurtzite, goethite. 

Mozgova, N.N., Borodaev, Yu.S., Gablina, I.F. et al. (2005) Mineral assemblages as indicators of the maturity of oceanic hydrothermal sulfide mounds. Lithology and Mineral Resources, 40(4), 293-319. 

Figure 15 Plots of particulate copper, vanadium, and neodymium concentrations versus particulate iron for suspended particulate material filtered in situ from the TAG hydrothermal mound, MAR, 26° N (data from German et al., 1990, 1991b). Note generally positive correlations with particulate Fe concentration for all three tracers but with additional negative (Cu) or positive (Nd) departure for sulfide-forming and scavenged elements, respectively.

Similar ponds of metalliferous sediment are observed close to other inactive sulfide stmctures throughout the TAG area (Rona et al., 1993). Metz et al. (1988) characterized the metalliferous sediment in a core raised from a sediment pond close to one such deposit, 2 km NNE of the active TAG mound. That core consisted of alternating dark red-brown layers of weathered sulfide debris and fighter calcareous ooze. Traces of pyrite, chalcopyrite, and sphalerite, together with elevated transition-metal concentrations were found in the dark red-brown layers. 

Cragg, B.A., Summit, M. and Parkes, RJ. (2000) Bacterial profiles in a sulfide mound (Site 1035) and an area of active fluid venting (Site 1036) in hot hydrothermal sediments from Middle Valley (Northeast Pacific), in Proceedings of the Ocean Drilling Program, Initial Reports, vol. 169 (eds R.A. Zierenberg, Y. Fouquet, DJ. Miller and W.R. Normark), pp. 1—18 [online]. 

Drilling has delineated two major massive sulfide lenses in the Bent Hill area (Fig. 28). The Bent Hill Massive Sulfide HMS) deposit (Zierenberg et al. 1998) consists (from top to bottom) of 93 m of massive sulfide, 100 m of sulfide feeder or stringer veins in highly altered sediments, a stratiform deep copper zone of chalcopyrite and pyrrhotite in a totally altered sandstone, and basaltic lava flows and sills at about 500 mbsf An additional sulfide deposit, or possibly an extension of the BHMS deposit, occurs 400 m to the south at the ODP mound (Fig. 28). The ODP mound has an active 264°C hydrothermal vent on top (Butterfield et al. 1994a), three separate massive... 

Figure 28. Schematic cross-section of the Bent Hill Massive Sulfide and the ODP Mound, Middle Valley. Modified from Fouquet et al. (1998), ODP Sci Results 169 used with permission of the Ocean Drilling Project.

Manus Basin is a back-arc basin located north of the New Britain island arc that consists of NE-SW trending ridge segments offset by transform faults. On the Manus Spreading Center in central Manus Basin the Vienna Woods hydrothermal field consists of a 300 m-diameter sulfide mound with an extensive foresf of active and inactive sphalerite-barite-rich, sulfide-sulfate-oxide chimneys up to 15 m-long atop it. Vent fluids from one of the active chimneys reach temperatures as high as 302°C. In the eastern Manus Basin, two important hydrothermal fields are known. The PACMANUS field includes discontinuous sulfide occurrences over a 3 by 0.8 km area and active venting of unknown nature. Volcanic rocks are andesitic to rhyodacitic. 

Knott R, Fouquet Y, Hoimorez JJ, Petersen S, Bohn M (1998) Petrology of hydrothermal mineralization a vertical section through the TAG mound. Proc ODP, Sci Results 158 5-26 Koski RA, Shanks WC 111, Bohrson WA, Oscarson RL (1988) The composition of massive sulfide deposits from the sediment-covered floor of Escanaba Trough, Gorda Ridge Implications for depositional processes. Can Mineral 26 655-673... 

Due to its buoyancy at high temperatures, the hydrothermal fluid rises rapidly from the deep-seated reaction zone to the seafloor along major faults and fractures within the rift valley or close to the flanks of the rift. In particular the intersections of faults running parallel and perpendicular to the ridge axis are the loci of high-velocity discharge, black smokers and massive sulfide mounds. The sulfide precipitation within... 

Growth of Black Smokers and Massive Sulfide Mounds... 

Emhley, R.W., Jonasson, I.R., Perfit, M.R., Franklin, J.M., Tivey, M.A., Malahoff, A., Smith, M.F. and Francis, T.J.G., 1988. Suhmersihle investigation of an extinct hydrothermal system on the Galapagos Ridge Sulfide mounds, stockwork zone, and differentiated lavas. Canadian Mineralogist, 26 517-539. 

---