Host rock

Isolation of radioactive wastes for long periods to allow adequate decay is sought by the use of multiple barriers. These include the waste form itself, the primary containers made of resistant materials, overpacks as secondary layers, buffer materials, concrete vaults, and finally the host rock or sod. Barriers limit water access to the waste and minimize contamination of water suppHes. The length of time wastes must remain secure is dependent on the regulatory limit of the maximum radiation exposure of individuals in the vicinity of the disposal site. 

Regulations include guidelines on geologic conditions. Of special interest is the stabiUty of the geology against faulting, volcanic action, and earthquakes. The repository is to be located in an arid region, where the water table is quite low. The host rock is to have a suitable porosity and a low hydrauhc conductivity. 

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. 

Fig. 1. Asbestos fibers (chrysotile, crocidoHte, and amosite) as separated from host rock and their massive varieties (antigorite, riebeckite,...

The fiber extraction (milling) process must be chosen so as to optimize recovery of the fibers in the ore, while minimizing reduction of fiber length. Since the asbestos fibers have a chemical composition similar to that of the host rock, the separation processes must rely on differences in the physical properties between the fibers and the host rock rather than on differences in their chemical properties (33). 

The analysis of geochemical liquids such as crude oils, hydrothermal bitumens, extracts of coals, and host rocks containing dispersed organic matter or pyrolysates... 

Several sub-classification.s of epithermal preciou.s-metal deposits have been proposed mineralogy, host-rock composition and elemental association (Lindgren, 1928), gold-silver ratios of metal weights (Ferguson, 1929 Nolan, 1933), mineral paragenesis (Nishiwaki et al., 1971), and production ratios of metals (Heald-Wetlaufer et al., 1983). 

Many Cu-Pb-Zn vein-type deposits are hosted by organic sedimentary rocks such as shale and mud.stone but almost all Au-Ag deposits occur in altered volcanic rocks. This difference in the host rocks affects the chemical features of ore fluids (/02. /s2> /CO2) (section 1.4.4). 

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. 

Chlorite compositions from areas (Toyoha Pb—Zn vein, Kuroko deposits) deviate significantly from a line of 1 1 slope. This deviation implies that the Fe +/Mg value of chlorite from these areas is controlled not only by the FeO/MgO value of the Ifesh host rocks, but also by factors such as the ratio of Fe " to Mg " " in the fluid phase. 

Few data on the chemical compositions of feldspars (albite, K-feldspar) are available. Fuji (1976) indicated that K-feldspar and albite in the propylite of west Izu Peninsula, middle Honshu are of nearly end member composition. Nagayama (1992) showed that K-feldspars in the Hishikari Au-Ag vein and in the host andesitic rock have different composition Na/K ratio of K-feldspars from the vein is lower than that from the host rocks. 

Sedimentary rocks often occur as host rocks, footwall rocks and basement rocks in the Non-Green tuff mine area. For example, in southern Kyushu, the Shimanto Supergroup shale is dominant as basement and a host rock for epithermal Au-Ag vein-type deposits (e.g., Hishikari). 

Tonnages of gold, silver and the other associated metal, silver/gold ratio, K-Ar ages and host rocks for the Te-type and Se-type epithermal gold deposits (Shikazono et al., 1990)... 

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). 

Host rocks Sedimentary rocks (dominantly shale), volcanic rocks (dacite and andesite) Volcanic rocks (dacite and andesite)... 

Host rocks for the Se-type and Te-type epithermal An—Ag deposits are summarized in Table 1.17. In general, the dominant host rocks for both deposits types are intermediate and felsic volcanic rocks. Sedimentary rocks (usually shale) sometimes host the Se-type (e.g., Sanru, Kohryu, Takadama, Ohmidani, Hishikari), but never host the Te-type deposits. 

D and 5 0 data on fluid inclusions and minerals, 8 C of carbonates, salinity of inclusion fluids together with the kind of host rocks indicate that the interaction of meteoric water and evolved seawater with volcanic and sedimentary rocks are important causes for the formation of ore fluids responsible for the base-metal vein-type deposits. High salinity-hydrothermal solution tends to leach hard cations (base metals, Fe, Mn) from the country rocks. Boiling may be also the cause of high salinity of base-metal ore fluids. However, this alone cannot cause very high salinity. Probably the other processes such as ion filtration by clay minerals and dissolution of halite have to be considered, but no detailed studies on these processes have been carried out. 

Hg deposits are distributed along the MTL (Median-Tectonic Line) in Southwest Japan and in Northeast Hokkaido (Fig. 1.184). The deposits are vein or disseminated in form. The deposits are hosted by sedimentary and igneous rocks. No K-Ar age data on the deposits are available. However, from the age of host rocks, the age of igneous activities along MTL and the studies on the movement of MTL and K-Ar ages of Au-veins associated with Hg mineralization in Northeast Hokkaido (e.g., Khonomai), it is likely that these deposits formed at middle Miocene age. However, mercury mineralization in Kitami Province (north Hokkaido) occurred at approximately the same age as the epithermal gold-silver mineralization in the same district (4.5-5.3 Ma) (Maeda, 1997). 

The veins are composed mostly of quartz and a small amount of sulfide minerals (pyrite, pyrrhotite, arsenopyrite, chalcopyrite, sphalerite, and galena), carbonate minerals (calcite, dolomite) and gold, and include breccias of the host rocks with carbonaceous matters. Layering by carbonaceous matters has been occasionally observed in the veins. Banding structure, wall rock alteration and an evidence of boiling of fluids that are commonly observed in epithermal veins have not been usually found. 

In the Yamizo Mountains, the correlation between A Ag and a kind of the host rocks is found A Ag of gold from gold-quartz veins in shale is lower than that in sand.stone. 

Factors in controlling chemical compositions of gold in equilibrium with the ore fluids are temperature, pH, concentration of aqueous H2S and Cl in the ore fluids, concentration ratio of Au and Ag species in the ore fluids, activity coefficient of Au and Ag components in gold, and so on (Shikazono, 1981). In the Yamizo Mountains, as a result, Ag/Au ratios of gold are correlated with a kind of the host rocks and sulfur isotopic compositions of the deposits. This correlation could be used to interpret Ag/Au ratios of gold. 

It is concluded that, in either case, in situ interaction of the ore fluids with the host rocks controls the chemical compositions of gold in the Yamizo Mountains. 

Kawahata and Shikazono (1988) summarized S S of sulfides from midoceanic ridge deposits and hydrothermally altered rocks (Fig. 2.42). They calculated the variations in 5 " S of H2S and sulfur content of hydrothermally altered basalt as a function of water/rock ratio (in wt. ratio) due to seawater-basalt interaction at hydrothermal condition (Fig. 2.43) and showed that these variations can be explained by water/rock ratio. The geologic environments such as country and host rocks may affect S S variation of sulfides. For example, it is cited that a significant component of the sulfide sulfur could... 

Host rocks Rhyolite, basalt, shale, sandstone Basalt, chert, shale Basalt, shale (chert absent... 

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