Modern barite occurrences

Barium is a relatively abundant element in the Earth’s crust and occurs at concentrations of 200 to 500 ppm in most rocks. However, economically significant barite and lead-zinc-barite deposits are confined to marine strata, owing to the need for a large reservoir of dissolved sulfate (SO₄^2-), which seawater supplies.  The barium ion (Ba²⁺) forms an extremely insoluble sulfate, so that, even at the low barium content of seawater, the oceans are close to saturation with respect to the mineral barite. Therefore it takes only a slight increase in Ba²⁺ concentration  to induce barite precipitation.

Paytan et al. (2002) identified four different modes of formation of barite in modern sediments:

• Authigenic  -- crystallizes in the water column, makes small crystals 0.5 to 5 µm.
• Diagenetic, diffusive – upward diffusive flux from anoxic sediments where sulfate reduction has released barium to pore water
• Diagenetic, advective – barium carried by movement of water expelled to the seafloor along faults or in seeps
• Hydrothermal – advection of hot water to the seafloor

Note that the first mechanism constitutes a “from above” source of barium to the seafloor, whereas the other three have a “from below” source.

Significant modern occurrences of barite mineralization
Location	Form	Tectonic setting	Reference
Guaymas Basin	Barite chimneys, barite interbedded with diatomite	Marine rift on mid-ocean ridge	Koski et al. 1985
Escanaba trough	Barite chimneys, barite crusts on massive sulfides	Marine rift on mid-ocean ridge	Koski et al. 1994
Lau Basin	Barite-sphalerite chimneys	Marine rift on back-arc ridge	Fouquet et al. 1993
Marianas back-arc	Barite-sphalerite-galena chimneys	Marine rift on back-arc ridge	Kusakabe et al. 1990
Gulf of Mexico	Barite cement and vug fillings of carbonates around methane vents	Passive continental margin	Canet et al 2006; Feng and Roberts 2011
Peru continental margin	Massive barite at sediment-water interface	Accretionary prism of continent-ocean subduction zone	Aquilina et al. 1997
Sea of Okhotsk	Barite chimneys associated with methane vents	Transpressional horst in back-arc basin	Greinert et al. 2002
Monterey Canyon	Barite mounds associated with methane vents	Transtensional basin	Naehr et al. 2000
San Clemente  Basin	Barite blocks on seafloor	Transtensional basin	Torres et al. 2002
Southern California offshore	Barite blocks on seafloor	Transtensional basin	Hein et al. 2007

The from-above enrichment involves the incorporation of Ba²⁺ into sinking organic matter, and is marked by a strong correlation between organic carbon (Corg ) and Ba that has led to the wide-spread use of Ba contents of deep-sea sediments as a paleo-productivity indicator. The from-below enrichment involves dissolution of barite at depth in the sediment pile via reduction of the sulfate component to sulfide, followed by the transport of dissolved Ba²⁺ in sulfate-free water to the sediment-water interface where it reacts with seawater sulfate to redeposit barite. The transport mechanism can simply be diffusion through pore waters, in which case a “barite front” is formed within the shallow part of the sediment. A layer of barite nodules is the common product. For large barite accumulations, fluid movement is required. Advective transport of Ba²⁺ can be in cold water, producing “cold-seep barites” or in hot water, producing “hydrothermal barite”.

Isotope Chemistry

Barite carries three isotopic signatures: S, O, and Sr. Considerable inquiry has been made into the use of isotope measurements to help distinguish tectonic settings, depositional environments, and mineralization processes (see table of isotope values). For S isotopes, there is considerable overlap among the various categories, indicating that δ³⁴S is controlled by the environment of deposition.

 

 

Strontium isotopes, however, show some relationship to tectonic setting. First, the pelagic samples show a very small variance and have average strontium and sulfur values close to seawater. Thus pelagic barite is confirmed as a good recorder of ocean chemistry, if it can be differentiated from the other types. Hydrothermal barites in oceanic rifts have distinctive strontium isotopic values, much less radiogenic than seawater. The Guaymas samples are nearly the same, but show more continental influence, and so should perhaps be considered as belonging to a transitional category. The other groups have a high degree of overlap, and it appears unlikely that strontium can be used alone as an effective discriminator. Calcium isotopes show some promise, but so far the database is too small to make an assessment (see Isotopes in Modern Barite Deposits).

 

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