Gangue minerals

Quartz is the most abundant gangue mineral. It occurs commonly in Au-Ag and Pb-Zn deposits but is scarce in Cu deposits. Chalcedonic quartz coexisting with Au-Ag minerals occurs abundantly in Au-Ag deposits. Amethyst is generally rare and occurs as a late-stage mineral in Au-Ag and Pb-Zn deposits. 

Magnetite is common in Pb-Zn-Mn and Cu deposits but has not been reported in Au-Ag deposits. It commonly coexists with other iron minerals such as hematite, pyrite, pyrrhotite, siderite, and chlorite and also occurs in both the main stage of sulfide mineralization and in the late stage of mineralization. 

The occurrence of other Mn-Ca silicates such as johannsenite, bustamite, rhodonite, pyroxmangite, tephroite, and penwithite has been reported from Au-Ag and Pb-Zn deposits, but these minerals are not common. They have not been reported from Cu deposits. 

Hydrated calcium silicate minerals such as xonotlite, truscottite, and gyrolite are rare but have been reported from several Au-Ag deposits. They do not coexist with Au-Ag minerals but instead are found with quartz, carbonates, and johannsenite. However, in the Keisen No. 3-2 vein in the Hishikari Au-Ag deposits, a close association of electrum with truscottite, smectite and calcite is observed (Imai and Uto, 2001). 

Prehnite is found with Au-Ag minerals in the Au-Ag vein (Kanisawa vein) of the Yatani Pb-Zn-Au-Ag deposit but is not found in the Pb-Zn vein (Yatani-Honpi vein). 

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Although the size separation/classification methods are adequate in some cases to produce a final saleable mineral product, in a vast majority of cases these produce Httle separation of valuable minerals from gangue. Minerals can be separated from one another based on both physical and chemical properties (Fig. 8). Physical properties utilized in concentration include specific gravity, magnetic susceptibility, electrical conductivity, color, surface reflectance, and radioactivity level. Among the chemical properties, those of particle surfaces have been exploited in physico-chemical concentration methods such as flotation and flocculation. The main objective of concentration is to separate the valuable minerals into a small, concentrated mass which can be treated further to produce final mineral products. In some cases, these methods also produce a saleable product, especially in the case of industrial minerals. 

Smelting. The term copper smelting designates the operations of melting the concentrate and extracting the copper by heat, flux, and the addition of oxygen. Copper concentrate is a mixture of the sulfides of copper, copper—iron, and iron with smaller amounts of gangue minerals. It normally contains 25—35% copper. 

Recently, epithermal gold deposits were divided into several types based on gangue minerals, and physicochemical environment of ore deposition (pH, H2S concentration of ore fluids). They are hot spring-type (Silberman, 1982 Berger, 1983a Berger and Eimon,... 

Dominant gangue minerals in Kuroko deposits are quartz, barite, anhydrite, gypsum, chlorite, sericite, and sericite/smectite. Morphology of quartz changes from euhedral in the centre to the irregular in the margin of the deposits (Urabe, 1978). No amorphous silica and cristobalite have been found. 

Precipitation of barite and quartz. Barite and quartz are the most common gangue minerals in the submarine hydrothermal ore deposits such as Kuroko deposits and back-arc basin deposits (e.g., Okinawa, Mariana deposits) (Halbach et al., 1989 Shikazono, 1994 Shikazono and Kusakabe, 1999). These minerals are also common in midoceanic ridge deposits. 

Calcium silicates such as wairakite, epidote, prehnite, laumontite, and stilbite are common in the wall rocks of some Au-Ag deposits in the Izu peninsula. Epidote occurs as a gangue mineral coexisting with sulfides and quartz in some Cu deposits, but none of the other above-mentioned Ca and Mn silicates have been reported from these deposits. Laumontite is a common mineral in propylite, which is the host rock for Au-Ag deposits. Other zeolites such as mordenite and dachiardite are not generally common, but they are the main gangue minerals associated with Au-Ag minerals in the Ohnoyama and Awagano Au-Ag deposits. 

Adularia is abundant in Au-Ag deposits, where it is commonly found with Au-Ag minerals only rarely does it occur in Pb-Zn and Cu deposits. Albite is very rare and is reported only from the Nebazawa Au-Ag deposits. Barite is a common gangue constituent in Pb-Zn-Mn deposits, especially those in the southwestern part of Hokkaido and the northern part of Honshu, where it is usually a late-stage mineral coexisting with carbonate and quartz but rarely with sulfide minerals. Other rare gangue minerals include fluorite, apatite, gypsum, bementite, rutile, and sphene, but they have not been studied. 

Main gangue minerals of the Se-type deposits comprise quartz, adularia, illite/ smectite interstratified mixed layer clay mineral, chlorite/smectite interstratified mixed layer clay mineral, smectite, calcite, Mn-carbonates, manganoan caleite, rhodoehrosite, Mn-silicates (inesite, johannsenite) and Ca-silicates (xonotlite, truscottite). 

Principal gangue minerals in base-metal vein-type deposits are quartz, chlorite, Mn-carbonates, calcite, siderite and sericite (Shikazono, 1985b). Barite is sometimes found. K-feldspar, Mn-silicates, interstratified mixed layer clay minerals (chlorite/smectite, sericite/smectite) are absent. Vuggy, comb, cockade, banding and brecciated textures are commonly observed in these veins. 

The predominant gangue minerals vary with different types of ore deposits quartz, chalcedonic quartz, adularia, calcite, smectite, interstratified mica/smectite, interstratified chlorite/smectite, sericite, zeolites and kaolinite in Au-Ag rich deposits chlorite, quartz, sericite, carbonates (calcite, rhodoehrosite, siderite), and rare magnetite in Pb-Zn rich deposits chlorite, serieite, siderite, hematite, magnetite and rare epidote in Cu-rich deposits (Sudo, 1954 Nagasawa et al., 1976 Shikazono, 1985b). 

The vein is composed of rhythmic banding of quartz layers and fine-grained sulfides such as argentite, acanthite, sphalerite, galena, pyrite and chalcopyrite, and elec-trum. The principal gangue minerals are quartz, calcite, adularia and interstratified chlorite/smectite. Minor minerals are inesite, johansenite, xonotlite and sericite. These gangue minerals except for quartz, adularia, calcite and sericite are not found in the wall rocks. 

Gangue minerals and salinity give constraints on the pH range. The thermochemical stability field of adularia, sericite and kaolinite depends on temperature, ionic strength, pH and potassium ion concentration of the aqueous phase. The potassium ion concentration is estimated from the empirical relation of Na+/K+ obtained from analyses of geothermal waters (White, 1965 Ellis, 1969 Fournier and Truesdell, 1973), experimental data on rock-water interactions (e.g., Mottl and Holland, 1978) and assuming that salinity of inclusion fluids is equal to ffZNa+ -h m + in which m is molal concentration. From these data potassium ion concentration was assumed to be 0.1 and 0.2 mol/kg H2O for 200°C and 250°C. 

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

Shikazono (1985b) summarized the assemblage and mode of occurrence of common gangue minerals from more than 70 Neogene epithermal vein-type deposits in Japan. 

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).

Some characteristic features (S S, S C, Ag/Au total production ratio, metals produced, gangue minerals) of epithermal Au-Ag vein-type deposits of the Green tuff-type and the Non-Green tuff-type in Japan (after Shikazono, 1996)... 

Occurrence of gangue minerals in both types of deposits is different. For example, Mn minerals (Mn carbonates, Mn silicates) occur abundantly in the Rendaiji, Yugashima, Yatani, and Todoroki epithermal Au-Ag vein-type deposits in the Green tuff region but not in the Non-Green tuff-type. values of barite from these deposits are high (-1-18%o... 

Gangue minerals Quartz (fine-large grained), adularia, illite/smectite, chlorite/smectite, calcite, rhodochrosite Quartz (very fine-grained), barite, illite, kaolinite, adularia... 

Dominant opaque and gangue minerals from the Se-type epiihennal gold deposits (Shikazono et aJ., 1990)... 

Deposit Se-bearing minerals Opaque minerals Gangue minerals... 

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