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Geological and Atmospheric Sciences

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Metamorphic and Metamorphogenic Ore Deposits

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Meta-exhalites consist of a variety of rock types including iron formation, coticule (garnet-quartz rock), tourmalinite, quartz-gahnite rock, apatite-rich rock, zincian staurolite-bearing rock, and barite-rich rock. Such lithologies may be spatially associated with a diversity of ore deposits, but they are particularly linked to sea floor base metal sulfides that formed in rift settings. Meta-exhalites generally form layers less than 2 m thick, above, below, in, and along strike from stratiform or exhalative ore deposits. Geochemical diagrams for iron formations, coticules, and tourmalinites (chondrite-normalized rare earth element (REE), ternary Al-Fe-Mn, Al/(Al + Fe + Mn) vs. Fe/Ti, TiO2 vs. Al2O3) suggest variable contributions of detrital material and hydrothermal components. The detrital component, terrigenous or clastic, is generally less than 30 wt percent for iron formations, whereas for coticules and tourmalinites it is generally 30 to 70 wt percent and greater than 70 wt percent, respectively. Of the major constituents of meta-exhalites, Fe, Mn, B, P, and Zn generally have a hydrothermal source, whereas Al and Ti are from detrital clastic material. Silica can have hydrothermal and/or detrital sources. Hydrogenous contributions are generally small.

The variable setting, mineralogy, primary sedimentary structures, geochemistry, and lithological variants of exhalites show that precursor constituents formed under a variety of physicochemical conditions (e.g., T, ⁠, pH, ionic strength, ⁠, ⁠) and were derived from different sources (clastic and volcanic). Iron formations, coticules, and tourmalinites form by the replacement of permeable aluminous sediments and by exhalation into submarine brine pools. Hydrothermal fluids responsible for the formation of precursors to meta-exhalites range in temperature from approximately 100° to 400°C. Layering in meta-exhalites reflects rapid fluctuations in Eh-pH conditions, metal contents, ⁠, ⁠, and detrital input. Fractionation of Fe and Mn in the hydrothermal fluids is due to gradual increases in pH or Eh during mixing of ambient seawater with the fluids and may account for differences in proximity of iron formations and coticules to sulfide deposits. The amount of hydrothermal input via venting, fluid/rock ratio, bottom current drift, and the degree of basin isolation from clastic sedimentation also dictate the chemical composition and mineralogy of meta-exhalites.

The presence of a meta-exhalite is indicative of a fossil zone of sea floor hydrothermal activity and, as such, can be utilized as a field guide in the exploration for ore deposits, particularly base metal sulfides. Relative abundance of certain minerals (e.g., iron carbonates, apatite, gahnite, zincian staurolite), bulk compositional variations that record increased ratios of hydrothermal components to detrital material, characteristic elements and elemental ratios, variations in the compositions of mineral phases (e.g., Zn to Fe ratio of staurolite, gahnite, and högbomite as well as the Mg to Fe ratio of ferromagnesian silicates due to metamorphic sulfide-silicate reactions), and stable isotope data (S, C, O, and B) provide vectors that are useful in exploration.


This chapter is published as Paul G. Spry, Jan M. Peter, John F. Slack, 1998. "Meta-Exhalites as Exploration Guides to Ore*", Metamorphic and Metamorphogenic Ore Deposits, Reviews in Economic Geology, v. 11, p. 163-201.


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