Fluid metamorphic

Talc of metasedimentary origin is formed by hydrothermal alteration of a dolomitic host rock by a silica-containing fluid. This type of deposit is typical of Montana and AustraUa. It is usually quite pure with talc content of 90 to 98% and often very white as well. Dolomite [17069-72-6], CaMg(C02)2, is the most common accessory mineral. The fourth type is of metamorphic origin, where a siUcaceous dolostone is first converted to tremolite [14567-73-8] or actinohte [13768-00-8] and then partially converted to talc. The Balmat, New York, and Death Valley, California, deposits are of this type. Tremolite, dolomite, and serpentine are common accessory minerals. This type of talc deposit has a variable talc content (30—80%), but is usually white and often commercially exploited because of the properties of its accessory minerals rather than the talc. 

Kase and Horiuchi (1996) obtained a large number of analytical data on sphalerites from sixteen Besshi-type deposits, mainly at Besshi and its vicinity, Hitachi, and Shimokawa. They revealed that (1) the Mn/Zn and Co/Zn ratios of sphalerite may have markedly increased during contact metamorphism, while the Cd/Zn ratios remained unchanged (2) the Emco/ lwzn (2/n total dissolved concentration in ore fluids) and Emco/S/wzn ratios in the initial ore solutions responsible for the mineralizations at Besshi which was calculated ba.sed on the equilibrium fractionation model between hydrothermal solution and sphalerite and analytical data on sphalerites are quite similar to the ratios of hydrothermal solutions at EPR 21 °N (3) however, these ratios for the Hitachi solutions are very low and different from those of the Besshi-subtype solution. 

Nesbitt, B.E., Mendoza, C.A. and Kerrick, D.M. (1995) Surface fluid convection during Cordillera extension and the generation of metamorphic CO2 contributions to Cenozoic atmospheres. Geology, 23, 99-101. 

Lasaga, A. C. and D. M. Rye, 1993, Fluid flow and chemical reaction kinetics in metamorphic systems. American Journal of Science 293, 361 104. 

Concerning estimation of the activity of CO2 in the fluid phase, besides carbonate equilibria (discussed in some detail in section 8.10), equilibria in the CaO-Si02-C02 system are also important. In the medium-low metamorphism of limestones, the equilibrium... 

The importance of aqueous solutions in geochemistry can be appreciated if we recall that two-thirds of the surface of our planet is covered by water and that aqueous fluids of various salinities are determinant in the development of volcanic and metamorphic processes in the earth s upper mantle and crust. 

Figure 11.14 (A) Internal Rb-Sr isochron for a system composed of three crystalline phases of initial compositions Aq, Bq, and Q formed at time t = 0 and thereafter closed to isotopic exchanges up to time of measurement t, when they acquired compositions A, and C. (B) Effects of geochronological resetting resulting from metamorphism or interaction with fluids. X, X2, and X3 bulk isotopic compositions of the three rock assemblages. In cases of short-range isotopic reequilibration, the three assemblages define crystallization age and original ( Sr/ Sr)o of the system the three internal isochrons (concordant in this example) define resetting age.

Eugster H. P. and Baumgartner L. (1987). Mineral solubilities and speciation in supercritical metamorphic fluids. In Reviews in Mineralogy, vol. 17, P. H. Ribbe (series ed.), Mineralogical Society of America. 

Ni-Cu-PGE mineralization at Sudbury is zoned with respect to Cu and PGE as a result of fractional crystallization from a base metal sulfide liquid, but metamorphism, fluids and deformation also affected these ore deposits (e.g., Farrow Lightfoot 2002 and authors therein). The two main styles of mineralization are 1) Fe-Ni-rich pyrrhotite ores (with minor pentlandite and chalcopyrite) in embayments along the base of the complex in noritic/gabbronoritic (contact ores) and 2) Cu-Pt-Pd-Au-rich chalcopyrite ores (with minor pentlandite, pyrrhotite and PGM) forming veins and stockworks in the country rock below the complex (footwall ores). Creighton Mine, on the southern margin of the complex, is one of the largest contact-style deposits. 

These textural observations suggest that the bulk of the mineralization at Morila was precipitated following low-P contact metamorphism and under relatively low fluid pressure conditions (i.e., no significant hydrofracturing). 

Retrograde metamorphic processes, linked to hydrothermal fluid circulation, finally produced a new mineral assemblage constituted mainly by tremolite-actinolite> epidote + chlorite + quartz + sericite + titanite + hematite. The amphibole appears as green fibrous crystals over clinopyroxene and other anhydrous minerals. 

During metamorphism, partial melting of metasediments, and possibly of the basement led to the formation of U-Th-rich anatectic melts and fluids, which have been trapped in skarns s.l. forming the second type of U-Th mineralization. During this event (1790 10Ma), local redistribution of the primary mineralization may also occur. 

In an attempt to establish the fluid and metal sources and to help constrain a genetic model, stable isotope studies (O, Pb, S) were conducted on selected occurrences. Oxygen isotope analysis of quartz vein material from eight separate occurrences yielded values of 10.9-16.0%o. Assuming equilibrium between quartz and the hydrothermal fluid, the values calculated for the mineralizing fluids, at temperatures between 300-400°C, range between 4.0 and 11.9%o. This overlaps accepted values for both magmatic and metamorphic fluids. 

It is generally assumed that metamorphism reduces the isotopic variations in a sulfide ore deposit. RecrystaUization, liberation of sulfur from fluid and vapor phases, such as the breakdown of pyrite into pyrrhotite and sulfur, and diffusion at elevated temperatures should tend to reduce initial isotopic heterogeneities. 

The infiltration of externally derived fluids is a controversial idea, but has gained much support in recent years. Many studies have convincingly demonstrated that a fluid phase plays a far more active role than was previously envisaged, although it is often not clear that the isotopic shifts observed are metamorphic rather than diagenetic (see also Kohn and Valley 1994). 

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