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TitleComparison of the TAG mound and stockwork complex with Cyprus-type massive sulfide deposits
AuthorHannington, M D; Galley, A G; Herzig, P M; Petersen, S
SourceTAG: Drilling an active hydrothermal system on a sediment-free slow-spreading ridge - Covering Leg 158 of the cruises of the Drilling Vessel JOIDES Resolution, Las Palmas, Gran Canaria, to Las Palmas, Gran Canaria, Site 957, 23 September-22 November 1994; by Herzig, P M (ed.); Humphris, S E (ed.); Miller, D J (ed.); Zierenberg, R A (ed.); Proceedings of the Ocean Drilling Program: Scientific Results vol. 158, 1998 p. 389-415, Open Access logo Open Access
Alt SeriesGeological Survey of Canada, Contribution Series 1996368
PublisherOcean Drilling Program
Mediapaper; CD-ROM; on-line; digital
File formatpdf
ProvinceEastern offshore region
AreaMid-Atlantic Ocean ridge
Lat/Long WENS -44.8333 -44.7667 26.1667 26.1333
Subjectssurficial geology/geomorphology; general geology; mid-ocean ridges; oceanic crust; drilling reports; drilling, offshore; deep drilling; deep sea drilling; drilling, offshore; sulphide deposits; anhydrite; hydrothermal systems; Trans-Atlantic Geotraverse (TAG)
Illustrationstables; location maps; diagrams; plots
ProgramOcean Drilling Program
Released1998 02 01
AbstractDrilling of the active Trans-Atlantic Geotraverse (TAG) deposit during ODP Leg 158 indicated that the size of the mound-stockwork complex is approximately 3.9 million tonnes, including 2.7 million tonnes of massive and semi-massive sulfide (ca. 2% Cu) at the seafloor and 1.2 million tonnes of mineralized breccias (ca. 1% Cu) in a subseafloor stockwork. Quartz-pyrite veining in the stockwork zone extends from about 40 meters below seafloor (mbsf) to a depth of 95 mbsf. Siliceous wallrock breccias in the lower part of the stockwork grade abruptly into chloritized basalt breccias at the margins of the mineralized zone, and massive sulfides at the flanks of the deposit onlap relatively unaltered, partially hematized basalts. The pipe-like dimensions of the stockwork zone do not exceed the diameter of the sulfide mound. Comparisons with samples collected during earlier dive series confirm that the vent complexes at the surface of the mound are not representative of the bulk composition of the deposit. Dramatic vertical zonation of metals within the mound suggests that a long history of hydrothermal reworking has effectively stripped the constituents that are soluble at lower temperatures from the massive sulfides and concentrated them at the top of the deposit through a process of zone refining.
The bulk of the mound is composed of massive pyrite and anhydrite-cemented breccias. The massive anhydrite (~165,000
t) occupies a high-temperature zone, immediately beneath the central Black Smoker Complex and above the quartz-rich stockwork.
Fracturing in the underlying quartz-pyrite stockwork also has resulted in anhydrite veining at considerable depths in the
stockwork zone. Despite the abundance of anhydrite in the mound, the amount of seawater penetrating the region of hightemperature
upflow is small in comparison to the total mass flux of hydrothermal fluid. The anhydrite has been deposited by
conductive heating of a small amount of entrained seawater at the margins of high-temperature conduits, and little or no mixing
has occurred with the end-member fluids. Collapse of the anhydrite-supported portion of the mound following major episodes
of hydrothermal upflow has caused extensive in situ brecciation of the mound and is an important mechanism for the formation
of “breccia ores” in the deposit. Although anhydrite is not well preserved in the geologic record, given its retrograde solubility,
it has likely played an important role in the development of similar ore types in ancient massive sulfides.
The morphology, size, and bulk composition of the TAG mound-stockwork complex is identical to that of some of the largest
Cyprus-type massive sulfide deposits in the Troodos ophiolite. Typical Cyprus-type deposits comprise massive brecciated
pyrite ores, underlain by a vertically extensive quartz-pyrite-chlorite stockwork. Sandy pyrite or conglomeratic ore, similar to
that found in the TAG mound, is characteristic of the upper parts of Cyprus-type deposits. Textures in these ores, previously
attributed to seafloor weathering and erosion, are most likely the result of anhydrite dissolution. Massive, granular pyrite (hard,
compact ore), with abundant vuggy cavities lined by idiomorphic pyrite and quartz, occur below the conglomeratic ores and
closely resemble sections of massive pyrite and pyrite-silica breccias from the TAG mound.
At TAG, seafloor oxidation of the sulfides is currently taking place, even as the deposit is forming. Fe-oxide gossans have
developed at the surface of the mound as a result of weathering of chimney debris. These deposits are modern analogs of the
extensive ochers that typically overlie the massive sulfide deposits in Cyprus. By analogy with TAG, a number of the weathering
features of Cyprus-type deposits (e.g., red clays, leached lavas), previously thought to be products of acid alteration by
meteoric groundwaters, may have formed while the deposits were still on the seafloor. Low-temperature venting through this
material has locally produced distinctive red cherts (silicified Fe oxides). This material is common within the mound and in the
underlying basalts and closely resembles the red jaspers found throughout the pillow lava sections in Cyprus. Silicification in
the upper part of the TAG mound also has produced a cherty, sulfide carapace at the top of the deposit that inhibits further degradation
of the mound by seafloor weathering. This may have important implications for the long-term preservation of the
deposit, although dissection of the mound along active fault scarps may eventually expose its interior to seafloor oxidation.
An estimated growth rate for the TAG deposit, based on a total accumulation of 2.7 million t of massive sulfides and a
cumulative venting history of 5 to 10 k.y., is between 500 and 1,000 t per yr. This is consistent with observed growth rates for
the central Black Smoker Complex and with estimates of mass fluxes from heat and fluid flow at black smoker vents on the
East Pacific Rise. Although TAG is among the largest of the known mid-ocean ridge deposits, grade-tonnage models for
Cyprus-type massive sulfides world-wide suggest that much larger deposits are likely forming elsewhere on the mid-ocean
ridges and at similar, slow-spreading centers in extensional back-arc basins.

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