Metamorphic rocks, high grade

Like other fluorite deposits, the Albigeois ores are notable for their high grade. In veins of the Le Bure deposit, for example, fluorite comprises 90% of the ore volume (Deloule, 1982). Accessory minerals include quartz (SiCh), siderite (FeCCL), chalcopyrite (CuFeSg), and small amounts of arsenopyrite (AsFeS). The deposits occur in a tectonically complex terrain dominated by metamorphic, plutonic, and volcanic rocks and sediments. 

EUer JM, VaUey JW, Graham CM, Baumgartner LP (1995) Ion microprobe evidence for the mechanisms of stable isotope retrogression in high-grade metamorphic rocks. Contr Miner Petrol 118 365-378... 

The upper limit for tetrahedral Fe3+ I.S. in silicates is shown to be 0.25 mm/sec., whereas the lower limit for octahedral Fe3+ is 0.29 mm/sec. The correlations point to inconsistencies in Mossbauer spectral parameters and cation site occupancy assignments for clintonite, yoderite and sapphirine. New Mossbauer spectral data obtained for these minerals demonstrate that clintonites from skarn deposits contain tetrahedral Fe3+ and octahedral Fe3+ and Fe2+, with relative enrichment of Fe3+ in tetrahedral sites only octahedral Fe2+ and Fe3+ occur in sapphirines from granulite facies rocks and five-coordinated Fe3+ predominates over octahedral Fe3+ ions in yoderites from high grade metamorphic rocks. 

Dostal J. and Capedri S. (1979) Rare earth elements in high-grade metamorphic rocks from the western Alps. Lithos 12, 41-49. 

Another approach is to follow a genetic model for continental crust formation, for example in island arcs, oceanic plateaus or Archean granite-greenstone terrains. For example, the full vertical crustal sequence of a mature island-arc has been reconstructed in the Hidaka belt in Hokkaido, Japan over a total thickness of 30 km. This arc contains high-grade metamorphic rocks representative of lower crustal material and its average rate of crustal heat generation is 0.83 p.W m (Furukawa and Uyeda, 1989 Furukawa and Shinjoe, 1997). 

Contact metamorphism produces homfels facies from clay-rich parent rocks such as shale. If the parent rocks are impure limestones, skarn (low- to high-grade metamorphism) is produced. Skarn is a calcium-rich, silicate rock containing a variety of minerals, including garnet. Relatively pure sandstones and limestones do not typically form new minerals due to contact metamorphism. 

Moorbath, S., Welke, H. Gale, N. H. 1969. The significance of lead isotope studies in ancient, high-grade metamorphic basement complexes, as exemplified by the Lewisian rocks of Northwest Scotland. Earth and Planetary Science Letters, 6, 245-256. 

Biotite (Pe2+. bearing micas) Phyllosilicates Granitic and high-grade metamorphic rocks R Stable in only the youngest or least weathered soils precursor of other 2 1 soil clay minerals and Fe-oxides source of K... 

Muscovite Phyllosilicates Granitic and high-grade metamorphic rocks C Vermiculite, smectite, and interstratified 2 1 precursor, K source... 

Garnets Nesosilicates High-grade metamorphic/acid igneous rocks R Source of Fe, Mg, Ca, Al, and/or Mn precursor of Fe-oxides... 

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