Distribution of gold in altered basalts of DSDP hole 504B

Nesbitt, Bruce E.; Louis, Robert M. St.; Muehlenbachs, Karlis

doi:10.1139/e87-022

Cited by 21 publications

(13 citation statements)

References 20 publications

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“…Strontium data from Table 1 (whole rocks = solid circles; secondary minerals are labeled) and various sources Friedrichsen, 1982, 1987;Barrett, 1983;Hart and Mottl, 1983;Friedrichsen, 1985;Staudigel and Hart, 1985;Kawahata et al, 1987;Kusakabe et al, 1989;Shimizu et al, 1989). Oxygen data from Table 1 (solid circles), Alt et al (1986bAlt et al ( , 1989bAlt et al ( , 1995, and Nesbitt et al (1987). S data from Alt et al (1989aAlt et al ( , 1995, Zuleger et al (1995), and Bach et al (this volume).…”

Section: Methodsmentioning

confidence: 99%

Stable and Strontium Isotopic Profiles through Hydrothermally Altered Upper Oceanic Crust, Hole 504B

Alt¹,

Teagle²,

Bach³

et al. 1996

Proceedings of the Ocean Drilling Program, 148 Scientific Results

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Leg 148 penetrated 111m farther into Hole 504B, to a total depth of 2111 meters below seafloor (mbsf), and recovered diabase dikes similar to the immediately overlying rocks. Secondary mineralogy and whole-rock oxygen and strontium isotopic compositions were determined for samples from Leg 148. In addition, strontium isotopic analyses were performed on previously recovered samples of the lower dikes (1550-2000 mbsf). Trends observed in the lower dikes (1550-2000 mbsf) continue in the Leg 148 section (2000-2111 mbsf). These include the local presence of magnesiohornblende and secondary calcic plagioclase and high Ti contents of amphiboles. Cu, Zn, and S contents of the more altered rocks are low, and whole-rock δ 18 θ values of 33%350°C) and greater extents of recrystallization than in the upper dikes. All of the data for the new Leg 148 section are consistent with the rocks being a continuation of a subsurface reaction zone, where metals and sulfur are leached from the crust by hydrothermal fluids and transported to form metal sulfide mineralizations on or within the crust or vent into seawater.Sr contents of the lower sheeted dikes (1550-2111 mbsf) are generally 48-62 ppm, and whole-rock 87 Sr/ 86 Sr ratios range from 0.70265 to 0.70304, only slightly elevated relative to fresh MORB values. The rocks were altered by seawater-derived hydrothermal fluids, with little change in the Sr concentrations of the rocks. Although amphibole is the most abundant secondary phase, the Sr isotopic composition of the rocks is probably dominated by the abundance of secondary plagioclase. The 87 Sr/ 86 Sr ratios of hydrothermal fluids were higher than measured in variably recrystallized (10%-80%) whole rocks. The seawater component of hydrothermal fluids increased through time, as documented by 87 Sr/ 86 Sr ratios of 0.7028-0.7034 for amphibole and chlorite, to 0.7035-0.7038 for vein epidotes. The latter values record the composition of upwelling black smoker-type endmember hydrothermal fluids at Site 504.The low 87 Sr/ 86 Sr ratios for the lower dikes differ from the uniformly high ratios of the thoroughly recrystallized sheeted dikes from the Troodos ophiolite. The oceanic dikes interacted with much smaller volumes of seawater than the ophiolitic rocks and apparently to a lesser extent. These differences are in some way related to the contrasting tectonic setting, mineralogy, and chemical composition of ophiolites compared to in situ ocean crust.

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Section: Methodsmentioning

confidence: 99%

Stable and Strontium Isotopic Profiles through Hydrothermally Altered Upper Oceanic Crust, Hole 504B

Alt¹,

Teagle²,

Bach³

et al. 1996

Proceedings of the Ocean Drilling Program, 148 Scientific Results

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“…Errors are smaller than plotted points, except for δ"B, where bars indicate where errors are larger. Data from Barrett (1983), Barrett and Friedrichsen (1983), Friedrichsen (1985), Alt et al (1986bAlt et al ( , 1989aAlt et al ( , 1989bAlt et al ( , 1995, Kawahata et al (1987), Nesbitt et al (1987), Shimizu et al (1989), Mengel and Hoefs (1990), Ishikawa and Nakamura (1992), Hart et al (1994), and Agrinier et al (1995). Vertical lines and shaded bars indicate MORB and seawater compositions.…”

Section: Transition Zone and Upper Dike Sectionmentioning

confidence: 99%

Hydrothermal Alteration of a Section of Upper Oceanic Crust in the Eastern Equatorial Pacific: A Synthesis of Results from Site 504 (DSDP Legs 69, 70, and 83, and ODP Legs 111, 137, 140, and 148)

Alt¹,

Laverne²,

Vanko³

et al. 1996

Proceedings of the Ocean Drilling Program

151

255

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Mineralogical, chemical, and isotopic results from seven drilling legs that visited DSDP/ODP Hole 504B over 14 years are compiled here to present an integrated view of hydrothermal alteration of oceanic crust at Site 504. Hole 504B reaches to 2111 mbsf, through 274.5 m sediment, 571.5 m of volcanic rocks, a 209 m transition zone, and 1050 m into a sheeted dike complex. The volcanic section was altered through a series of processes involving interaction with seawater at low temperatures, with the effects of cold, oxidizing seawater decreasing downward. These processes and their effects on the volcanic section are generally similar to those in other oceanic upper crustal sections.The transition zone and upper dikes were altered in a subsurface mixing zone, where hydrothermal fluids upwelling through the dikes mixed with cooler seawater circulating in the overlying more permeable volcanic rocks. Alteration of the transition zone and upper dikes (down to 1500 mbsf) occurred in a series of stages, reflecting the thermal and chemical evolution of the hydrothermal system from (1) early chlorite, actinolite, albite-oligoclase, and titanite, to (2) quartz, epidote and sulfides, to (3) anhydrite, and finally to (4) zeolites and local calcite. The maximum temperature estimated for the first two stages is 350°-380°C, and the inferred mineral assemblages for these early stages are typical of the greenschist facies.The lower dikes (1500-2111 mbsf) underwent an early, high-temperature (>400°C) alteration stage, resulting in the formation of hornblende and calcic secondary plagioclase, consistent with reactions inferred to occur in deep subsurface reaction zones, where hydrothermal vent fluids acquire their final compositions. Much of the subsequent reactions produced greenschist assemblages at ~300°-400°C. The lower dikes have lost metals and sulfur and are a source of these elements to hydrothermal vent fluids and seafloor sulfide deposits. The lower dikes underwent subsequent alteration stages similar to the upper dikes, with rare epidote + quartz veins recording the presence of upwelling hydrothermal fluids, and limited late off-axis effects (zeolites and prehnite). Anhydrites in the lower dikes indicate more reacted fluid compositions than in the upper dikes.Alteration of the sheeted dikes from Hole 504B is heterogeneous, with recrystallization controlled by fracturing and access of fluids. Defining the position of the seismic Layer 2/3 transition depends upon the scale of observation, but the change at Site 504 occurs within the sheeted dikes and is correlated with progressive changes in porosity and hydrothermal alteration. However, we still do not know the nature of the transition from sheeted dikes to gabbros in in situ ocean crust, or the nature of the inferred fault at the base of Hole 504B and its role in fluid flow and alteration.

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“…Temperatures inferred for this stage are less than 250°C, and reactions are interpreted to have occurred during continued circulation of more evolved (Mg-depleted, Caenriched) off-axis hydrothermal fluids (Alt, Honnorez, Laverne, and Emmermann, 1986). Alt et al (1985); Alt, Honnorez, Laverne, and Emmermann (1986); Alt, Muehlenbachs, and Honnorez (1986); Alt, Anderson, Bonnell, and Muehlenbachs, (1989); and Nesbitt et al (1987). Vertical lines in (A) indicate average S contents for different zones in the core.…”

Section: Sequence and Conditions Of Alteration Of The Lower Sheeted Dmentioning

confidence: 99%

Mineralogy and Stable Isotopic Compositions of the Hydrothermally Altered Lower Sheeted Dike Complex, Hole 504B, Leg 140

Alt¹,

Evelyn²,

Erzinger³

1995

Proceedings of the Ocean Drilling Program, 137/140 Scientific Results

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Drilling during Legs 137 and 140 of the Ocean Drilling Program deepened Hole 504B, the only hole to penetrate through the volcanic section and into the underlying hydrothermally altered sheeted dike complex, by 438.1 m to a total depth of 2000.4 meters below seafloor. This paper presents the secondary mineralogy, bulk-rock sulfur contents, and stable isotopic (O, S) compositions, plus oxygen isotopic compositions of secondary minerals from the lower sheeted dike complex drilled during Legs 137 and 140. Various evidence indicates higher temperatures of hydrothermal alteration in the lower dikes than in the upper dikes, including: the local presence of secondary clinopyroxene in the lower dikes; secondary anorthite and hornblende in the lower dikes vs. mainly actinolite and albite-oligoclase in the upper dikes; generally increasing Al and Ti contents of amphibole downward in the dike section; and greater 18 O depletions of the lower dikes (δ 18 θ = 3.6-5.0‰) compared with the upper dikes. Early high-temperature alteration stages (T = 350°-500°C) resulted in 18 O depletions and losses of metals (Cu, Zn) and sulfur from the rocks. Local incorporation of reduced seawater sulfate led to elevated δ 34 S values of sulfide in the rocks (up to 2.5‰). Quartz + epidote formed in crosscutting veins at temperatures of 310°-320°C from more evolved fluids (δ 18 θ = l‰). Late-stage lower-temperature (~250°C) reactions producing albite, prehnite, and zeolites in the rocks caused slight 18 O enrichments, but these were insufficient to offset the 18 O depletions caused by earlier higher-temperature reactions. Addition of anhydrite to the rocks during seawater recharge led to increased S contents of rocks that had previously lost S during axial hydrothermal alteration, and to further increases in δ 34 S values of total S in the rocks (up to 12‰). Despite the evidence for seawater recharge to near the base of the sheeted dike complex, the paucity of late zeolites in the lower dikes suggests that late-stage, off-axis circulation was mainly restricted to the volcanics and shallowest dikes, or to localized high-permeability zones (faults) at depth.

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Distribution of gold in altered basalts of DSDP hole 504B

Cited by 21 publications

References 20 publications

Stable and Strontium Isotopic Profiles through Hydrothermally Altered Upper Oceanic Crust, Hole 504B

Stable and Strontium Isotopic Profiles through Hydrothermally Altered Upper Oceanic Crust, Hole 504B

Hydrothermal Alteration of a Section of Upper Oceanic Crust in the Eastern Equatorial Pacific: A Synthesis of Results from Site 504 (DSDP Legs 69, 70, and 83, and ODP Legs 111, 137, 140, and 148)

Mineralogy and Stable Isotopic Compositions of the Hydrothermally Altered Lower Sheeted Dike Complex, Hole 504B, Leg 140

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