Meta-Exhalites as Exploration Guides to Ore*
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.
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The types of mainly metallic mineralization found in metamorphic terranes are reviewed and an attempt is made to define the genetic relations between the mineralization and the metamorphic events.The terms metamorphosed, metamorphic, and metamorphogenic as applied to ores are also considered.The development of thought and the history of investigations on ores in metamorphic terranes aretraced from the early work in the second half of the nineteenth century onward. Early conceptions ofmetamorphism as an ore-forming process (metamorphogenesis) were seemingly not followed up by theiroriginators, contemporaries, or immediate successors and were neglected until comparatively recentyears. The idea of metamorphism as a modifier of preexisting, mainly sulfidic, but also oxidic, mineralizationwon more immediate and general acceptance in the early decades of the present century. InNorth America, emphasis seems to have been mainly on the deformational aspects of the metamorphism,whereas elsewhere, especially in Europe, the textural and mineralogical results of the metamorphic recrystallizationalso received considerable attention and metamorphism as an ore-forming process hadwon a certain degree of acceptance. This difference in emphasis may perhaps be referred to the differentviews held regarding the initial genesis of the ores in the two regions.The late 1940s and the 1950s witnessed a considerable revision of ideas on ore genesis, especially regardingstrata-bound massive sulfide ores. A parallel revival of interest in the role of metamorphism,probably not unrelated to the foregoing, began in the early 1950s, to begin with concerning metamorphosedores. However, new thoughts concerning metamorphogenesis related to granitization or ultrametamorphismas an ore-forming process began to be published.The following decades witnessed an almost explosive increase in the number of publications dealingwith the effects of metamorphism on ore mineralization of practically all types, but with a definite emphasison sulfide ores of the strata-bound type. One of the most significant breakthroughs in this respectconcerned the world-famous Broken Hill deposit, New South Wales, although the metamorphosed natureof ores in the Scandinavian Caledonides, the North American Appalachians, the Lachlan fold beltof eastern Australia, the Sanbagawa terrane of Japan, the Urals, and Proterozoic fold belts in southernAfrica, have all been thoroughly documented.In recent years, however, the interpretation of many massive sulfidic ores in metamorphic terranes asmetamorphosed has been increasingly questioned, and syntectonic, metamorphogenic, origins havebeen advocated. There is obviously a great need to be able to distinguish more