Mineral assemblages

Diamonds were first discovered in ancient times in India and Borneo and later in Brazil in 1670 in alluvial deposits where water had sorted minerals on the basis of density and toughness. This type of tumbling often concentrates the better quality crystals such as those found in the ocean off the west coast of Africa. Exploration can be done by stream panning or drilling in conjunction with a search for the heavy mineral assemblages that accompany diamond. Alluvial deposits account for about 40% of the diamond found in primary sources. 

The Y, C and B sub-types roughly correspond to types 1, 2 and 3 as defined by Urabe (1974a), who classified Kuroko deposits based on hydrothermal alteration and ore mineral assemblages type 1, kaolinite-pyrophyllite-diaspore-type type 2, sericite-chlorite-type type 3, sericite—chlorite-carbonate-type. Hydrothermal alterations in the Kuroko mine area are described in section 1.3.2. 

If sulfur isotopic equilibrium between coexisting sulfates and sulfides was attained, using average values of sulfates and sulfides, -i-22%c and +5%c, respectively, we could estimate temperature using the equation by Ohmoto and Rye (1979). This temperature seems too high compared with temperature estimated from fluid inclusions and mineral assemblages (section 1.3.3). That means that sulfates and sulfides precipitated under the condition far from equilibrium. 

Among the epithermal vein-type deposits in Japan, four major types of hydrothermal alteration ean be diseriminated. They are (1) propylitie alteration, (2) potassic alteration, (3) intermediate argillic alteration, and (4) advaneed argillic alteration. The definitions of these types of alteration are mainly based on Meyer and Hemley (1967) and Rose and Burt (1979) who elassified the hydrothermal alteration in terms of alteration mineral assemblages. 

Based on the hydrothermal alteration mineral assemblages and the fluid inclusion, the probable range of gas fugacities (/s2, /o2 /H2S) and temperature can be seen in Figs. 1.81 and 1.82 these estimated fugaeities are quite different from those of the propylitic alteration. 

The relation between zinc content in carbonate and many physicochemical variables such as /02, temperature, pH, and so on was derived on the basis of the equilibrium between coexisting carbonate and sphalerite (Figs. 1.95 and 1.96). It is predicted theoretically that zinc content increa.ses with increasing /oj in the oxidized sulfur species and oxidized carbon species region (Fig. 1.95). This estimate is consistent with mineral assemblage containing the siderite. 

Carbon dioxide fugacity (fc02h The /CO2 values can be estimated from (1) gangue mineral assemblages including carbonates and (2) fluid inclusion analyses. 

Figure 1.100. Typical /coj-temperature ranges for Au-Ag-rich, Pb-Zn-Mn-rich, and Cu-Pb-Zn-rich vein-type deposits estimated from gangue mineral assemblages, homogenization temperatures of fluid inclusions, and thermochemical calculations (Shikazono, 1985b).

In the siliceous body, electrum and Cu minerals (enargite, luzonite, covelline), and native sulfur occur. The Ag content of electrum is lower (0.0-5.3 wt%) than that from epithermal Au-Ag vein-type deposits (Fig. 1.194) (Shikazono and Shimizu, 1987). Low Ag content of electrum and sulfide mineral assemblage (enargite, native sulfur, covellite, pyrite) indicate high fs2 condition (Fig. 1.194). 

Bird, D.K. and Helgeson, H.C. (1981) Chemical interaction of aqueous solution with epidote-feldspar mineral assemblages in geologic systems II, Equilibrium constraints in metamorphic/geothermal processes. Am. J. Set, 281, 576-614. 

Moh, G.H. (1975) Tin-containing mineral systems. Part II. Phase relations and mineral assemblages in the Cu-Fe-Zn-Sn-S system. Chem. Erde, 34, 1-61. 

Motomura, T. (1986) Chemical composition of electrum and sphalerite from the Inakuraishi-type Mn deposits. In Effect of Ore-Forming Environment on the Ore Texture, Mineral Features and Mineral Assemblage. Symposium held at Ito City, Japan 1986 (in Japanese). 

Chemical compositions of geothermal waters controlled by hydrothermal alteration mineral assemblage... 

Previous studies clearly indicated that the chemical compositions of geothermal waters are intimately related both to the hydrothermal alteration mineral assemblages of country rocks and to temperature. Shikazono (1976, 1978a) used a logarithmie cation-Cl concentration diagram to interpret the concentrations of alkali and alkaline earth elements and pH of geothermal waters based on thermochemical equilibrium between hydrothermal solution and alteration minerals. 

Fig. 2.14. The variation of concentration of with concentration of CP in aqueous solution in equilibrium with a given mineral assemblage at 250°C. I Equilibrium curve based on albite-sericite-Na-montmorillonite-quartz-aqueous solution equilibrium and Na-K-Ca relationship obtained by Fournier and Truesdell (1973). 2 Equilibrium curve based on albite-K-feldspar-aqueous solution equilibrium and Na-K-Ca relationship obtained by Fournier and Truesdell (1973). 3 Wairakite-albite-sericite-K-feldspar-quartz. 4 Calcite-albite-sericite-K-feldspar-quartz (/jjhjCO, = 10 ). 5 Calcite-albite-sericite-Na-montmorillonite-quartz (mH2C03 = 10 ). 6 Ca-montmorillonite-albite-sericite-Na-montmorillonite-quartz. 7 Calcite-albite-sericite-K-feld-spar-quartz (mnjCOj = 10 ). 8 Calcite-albite-sericite-Na-montmorillonite-quartz (mHjCOj = 10 ). 9 Ca-montmorillonite-albite-sericite-K-feldspar-quartz. 10 Anhydrite = 10 ). (Shikazono, 1976)...

The H2S concentration of hydrothermal solution is plotted in Fig. 2.33. Based on these data, we can estimate the temperature of hydrothermal solution buffered by alteration mineral assemblages such as anhydrite-pyrite-calcite-magnetite and pyrite-pyrrhotite-magnetite for Okinawa fluids. 

This difference indicates that primary texture and mineral assemblages in the Kuroko deposits were modified after the formation of ore deposits. 

Chemical composition of selected mineral assemblages from midocean ridge sulfide deposits (Hannington et al.. 1995). N number of analyses... 

Although several differences are observed in both basalts from Kuroko mine area and midoceanic ridges, the hydrothermal alteration mineral assemblages as a function of... 

Fig. 2.47. Model predicting mineral assemblages and proportions produced when basalt reacts with seawater in different water/rock mass ratios. The model is based on experimental data but is close to actual observed assemblages in recovered greenschist facies metabasalts (Mottl, 1983).

In the other areas, generally, the host rocks weakly suffered regional and contact metamorphisms but suffered ocean-floor hydrothermal alteration. For example, hydrothermal alteration mineral assemblages in the Minamidani mine district in the Maizuru range from prehnite-pumpellyite facies to a transition state from green schist to amphibolite... 

It is noteworthy that bornite, chalcocite and tetrahedrite-tennantite which are common minerals in Kuroko deposits occur in gold bearing Besshi-type deposits. Although these minerals are considered to be secondary minerals, depositional environments of these minerals are characterized by higher /s, and foj conditions. It is also noteworthy that these deposits are rich in pyrite rather than pyrrhotite. Probably, Besshi-subtype deposits in Shikoku formed under the higher fo and /sj conditions than the deposits characterized by pyrrhotite (Maizuru, Hidaka, Kii, east Sanbagawa). Such typical Besshi-type deposits (Besshi-subtype deposits in Shikoku) are characterized by simple sulfide mineral assemblage (chalcopyrite, pyrite, small amounts of sphalerite). Inclusion of bornite in pyrite is also common in these deposits. 

Kase, K. (1972) Metamorphism and mineral assemblages of ores from cupriferous iron sulfide deposit of the Besshi mine, central Shikoku, Japan. J. Fac. Set U. Tokyo, Sec. 2, 18, 301-323. 

Nedachi, M., Veno, H., Ossaka, J., Nogami, H., Hashimoto, J., Fujikura, K. and Miura, T. (1992) Hydrothermal ore deposits on the Minami-Ensei Knoll of the Okinawa Trough - Mineral assemblages. In Proc. JAMSTEC Symp. Deep Sea Res., 8, 95-106 (in Japanese). 

If the above argument is correct and the /CO2 of hydrothermal solutions from back-arc basins is in equilibrium with alteration mineral assemblage including dolomite and calcite, t[ g2+ lmQ + of fluids can be estimated to be 0.03-0.055 from dolomite-calcite-... 

Because there is no natural mechanism for producing isotopically pure Th, any measurable Th in a sample for °Th/U dating implies the presence of at least some °Th when the sample (mineral or mineral assemblage) was formed. This initial °Th must then be subtracted from the total measured °Th before a valid °Th/U age can be calculated. In addition, a correction for initial U isotopes is required when the material being dated is a mechanical mixture of an isotopically homogeneous detrital material with an initially thorium-free mineral such as chemically precipitated carbonate. In these cases we are usually interested only in the time of formation of the Th-free material, and so must also subtract the detrital and before calculating the °ThAJ age. 

The alkali process uses sodium hydroxide and is well known as Bayer s process. It involves relatively simple inorganic and physical chemistry and the entire flowsheet can be divided into caustic digestion, clarification, precipitation and calcination. Although mineral assemblage in bauxites is extensive, processing conditions are primarily influenced by the relative proportions of alumina minerals (gibbsite and boehmite), the iron minerals (goethite and hematite), and the silica minerals (quartz and clays-usually as kaolinite). 

Rasmussen C, Tom MS, Southard RJ (2005) Mineral assemblage and aggregates control carbon dynamics in a California conifer forest. Soil Sci Soc Am J 69 1711-1721... 

Sometimes the calculation predicts that the fluid as initially constrained is supersaturated with respect to one or more minerals, and hence, is in a metastable equilibrium. If the supersaturated minerals are not suppressed, the model proceeds to calculate the equilibrium state, which it needs to find if it is to follow a reaction path. By allowing supersaturated minerals to precipitate, accounting for any minerals that dissolve as others precipitate, the model determines the stable mineral assemblage and corresponding fluid composition. The model output contains the calculated results for the supersaturated system as well as those for the system at equilibrium. 

A calculation procedure could, in theory, predict at once the distribution of mass within a system and the equilibrium mineral assemblage. Brown and Skinner (1974) undertook such a calculation for petrologic systems. For an -component system, they calculated the shape of the free energy surface for each possible solid solution in a rock. They then raised an n -dimensional hyperplane upward, allowing it to rotate against the free energy surfaces. The hyperplane s resting position identified the stable minerals and their equilibrium compositions. Inevitably, the technique became known as the crane plane method. 

Such a method has seldom been used with systems containing an aqueous fluid, probably because the complexity of the solution s free energy surface and the wide range in aqueous solubilities of the elements complicate the numerics of the calculation (e.g., Harvie el al., 1987). Instead, most models employ a procedure of elimination. If the calculation described fails to predict a system at equilibrium, the mineral assemblage is changed to swap undersaturated minerals out of the basis or supersaturated minerals into it, following the steps in the previous chapter the calculation is then repeated. 

A calculated solution may have just one supersaturated or undersaturated mineral, in which case calculating the solution for the new mineral assemblage will give the equilibrium state. Not uncommonly, however, more than one mineral appears... 

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