Adularia

Sonnen-. of the sun, sun, solar, helio-. -auge, n. adularia. -bahn, /. ecliptic, -belichtung, /. exposure to sunlight, solar irradiation, insolation. 

Most of epithermal precious-metal vein-type deposits in Japan can be classed as adularia-sericite-type, and low sulfidation-type. Very few hot spring-type deposits (quartz-alunite-type, high sulfidation-type) are found in the Japanese Islands. A summary of various characteristic features of adularia-sericite type (low sulfidation-type) is given mainly in section 1.4. 

Shikazono et al. (1990) divided epithermal precious-metal vein-type deposits into Te-bearing and Se-bearing deposits. As will be considered later, Te-bearing deposits are regarded as intermediate-type of adularia-sericite-type and hot spring-type. The distinction between these two types of deposits is discussed in section 1.4. 

The age of formation of epithermal vein-type deposits can be estimated from K-Ar ages of K-bearing minerals (adularia, sericite) in veins and in hydrothermal alteration zones nearby the veins. A large number of K-Ar age data have been accumulated since the work by Yamaoka and Ueda (1974) who reported K-Ar age data on adularia from Seigoshi Au-Ag (3.7 Ma) and Takadama Au-Ag deposits (8.4 Ma). Before their publication on the K-Ar ages of these deposits it was generally accepted that epithermal... 

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). 

In eomparison, the Te-type deposits contain fine-grained quartz, chalcedonic quartz, sericite, barite, adularia, ehlorite/smectite interstratified mixed layer clay mineral and rarely anatase. Carbonates and Mn-minerals are very poor in the Te-type deposits and they do not coexist with Te-minerals. Carbonates are abundant and barite is absent in the Se-type deposits. The grain size of quartz in the Te-type deposits is very fine, while large quartz crystals are common in the Se-type deposits although they formed in a late stage and do not coexist with Au-Ag minerals. 

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. 

Figure 1.86. Variation in chemical compositions (in molal unit) of hydrothermal solution with temperature. Thermochemical data used for the calculations are from Helgeson (1969). Calculation method is given in Shikazono (1978a). Chloride concentration in hydrothermal solution is assumed to be 1 mol/kg H2O. A-B Na concentration in solution in equilibrium with low albite and adularia, C-D K concentration in solution in equilibrium with low albite and adularia, E-F HaSiOa concentration in equilibrium with quartz, G-H Ca + concentration in equilibrium with albite and anorthite (Shikazono, 1978a, 1988b).

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. 

Figure 1.96. Log /oj-pH diagram constructed for temperature = 200°C, ionic strength = 1, ES = 10 m, and EC = 10 m. Solid line represents aqueous sulfur and carbon species boundaries which are loci of equal molalities. Dashed lines represent the stability boundaries for some minerals. Ad adularia. Bn bomite, Cp chalcopyrite, Ht hematite, Ka kaolinite, Mt magnetite, Po pyrrhotite, Py pyrite, Se sericite. Heavy dashed lines (1), (2), and (3) are iso-activity lines for ZnCOs component in carbonate in equilibrium with sphalerite (1) 4 co3=0-1- (2) 4 ,co3=0-01- (3) 4 co3 =0-001 (Shikazono, 1977b).

Morishita (1993) showed based on carbon isotopic composition of carbonates that carbon of carbonates in the gold-bearing quartz vein in southern Kyushu was derived from the Shimanto Supergroup shale. Imai et al. (1998) considered that hydrogen in the ore fluids was derived from the Shimanto Supergroup shale based on 5D (—60%o to — 100%c) of inclusion fluids in quartz and adularia of the Hishikari veins. These isotopic... 

K-Ar ages data on adularia and sericite in the veins and altered host rocks indicate that ages of mineralization vary widely, ranging from 1 Ma to 68 Ma and from 1 Ma to 24 Ma for the Se-type and Te-type, respectively (Tables 1.17 and 1.18). 

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

Sanru aguilarite, naumannite, polybasite, pyrargyrite, stephanite electrum, miargyrite, chalcopyrite, fahore, arsenopyrite, marcasite, pyrite, sphalerite, stibnite cinnabar quartz, adularia, kaolinite, sericite, calcite... 

Koryu aguilarite, pearceite, polybasite, proustite, pyrargyrite electrum, miargyrite, native silver, chalcopyrite, fahore, hematite, magnetite, pyrite, galena, sphalerite quartz, adularia, johannsenite, chlorite, kaolinite, vermiculite, Mn-calcite... 

Hishikari naumannite electrum, chalcopyrite, marcasite, pyrite, galena, sphalerite, stibnite quartz, adularia, montmorillonite... 

Sado hessite eleetrum, polybasite, pyrite quartz, adularia... 

Pb. Their results clearly demonstrate that basement rocks affect vein components in epithermal precious and base-metal quartz-adularia-type deposits. 

The sequence of mineralization has been studied by Nagayama (1993a) and Taka-hashi et al. (1998) (Fig. 1.132). Quartz is the most abundant mineral occurring throughout the vein. Adularia tends to occur at earlier stage. Smectite is the earliest mineral. Electrum tends to occur in early and middle stages. Naumannite occurs after the early-stage... 

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