Hydrothermal alteration zoning

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. 

Representative propylitie alteration minerals inelude epidote, albite, earbonates, quartz, chlorite, sericite, and smectite. The less common minerals are mixed-layer elay minerals such as chlorite/smectite and sericite/smectite and zeolite minerals. 

The term propylite is widely used to describe altered volcanic rocks recognized 

Zeolite minerals (wairakite, laumontite etc.), mixed-layer clay minerals and sme-cite occur in the upper part of the propylitically altered rocks (e.g., Seigoshi, Fuke, Kushikino), but they are sometimes poor in amounts. Generally carbonates are more abundant in the mine area as in the Toyoha district. Temporal relationship between the formation of high temperature propylitic alteration minerals (epidote, actinolite, prehnite) and low temperature propylitic alteration minerals) (wairakite, laumontite, chlorite/smectite, smectite) in these areas (Seigoshi, Fuke, Kushikino) is uncertain. 

The area of the potassic alteration is not wide, compared with the propylitically altered area. The width of potassic alteration zone away from the vein is generally within several tens of meters (ca. 50 m) (Shikazono and Aoki, 1981 Imai, 1986). The potassic alteration is usually found in the intermediate vicinity of the vein in the epithermal deposits in Japan. Thus it is evident that this type of alteration occurs genetically related to the ore deposition. 

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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... 

Figure 1.142 shows the dependence of solubility of Si02 minerals (quartz, cristobalite) on temperature. As described already, cristobalite occurs in peripheral and shallower part of hydrothermal alteration zone. Quartz is present in zones occurring in deeper and closer to the gold-quartz veins. Such zoning from quartz to cristobalite is also common in main active geothermal systems (Hayashi, 1973 Takeno et al., 2000). 

Recalculation of whole-rock geochemical data to normative corundum represents a particularly useful geochemical tool that can point to hydrothermal alteration zones. Normative corundum represents alkali imbalance in sub-alkaline rocks which is generally an indicator of Na depletion and K... 

Eberly, P., Janeczek, J. Ewing, R. C. 1995. Precipitation of uraninite in chlorite-bearing veins of the hydrothermal alteration zone (argiles de pile) of the natural nuclear reactor at Bangombe, Republic of Gabon. Proceedings of Material Research Society Symposium, 353, 1195-1202. 

Lovering, T. S., and Shepard, A. 0. Hydrothermal alteration zones caused by halogen acid solutions. East Tintic District, Utah. Amer. J. Sci 258-A, 215-229 (1960). 

Valley of the Geysernaya River is represented as an erosive V-shaped valley depth 300 00 m formed in the eastern part of the Uzon-Geyser volcano-tectonic depression. Sides of the valley have slopes 30-35°. Lacustrine-volcanic deposits forming slopes of the valley are represented interbedded argillites and sandstones with lenses of coarse debris and hydrothermal alteration zones. Strength properties of lacustrine-volcanic formations are dependent on humidification and decrease sharply at water saturation. It should be noted that the formation of the landslide occurred after a heavy snow melt (Dvigailo Melekeszev, 2009). 

The rocks in active and fossil geothermal areas are suffered hydrothermal alterations and the causes for hydrothermal alteration zoning are considered in terms of partial chemical equilibrium model (Giggenbach 1981 Shikazono 1988 Takeno 1989). 

Kaolin minerals (kaolinite, dickite, nacrite), pyrophyllite and mica-rich mica/smec-tite mixed layer mineral occur as envelopes around barite-sulfide ore bodies in the footwall alteration zones of the Minamishiraoi and Inarizawa deposits, northern part of Japan (south Hokkaido) (Marumo, 1989). Marumo (1989) considered from the phase relation in Al203-Si02-H20 system that the hydrothermal alteration minerals in these deposits formed at relatively lower temperature and farther from the heat source than larger sulfide-sulfate deposits in the Hokuroku district. 

Shikazono (1985a) has studied hydrothermal alterations in the epithermal Au-Ag mine district in Izu Penin.sula, middle part of Honshu, and indicated that (1) the propylitic alteration occurs widely in the district (2) at the centre of the district and stratigraphically upper horizon, there exists advanced argillic alteration (3) epithermal Au-Ag vein-type deposits are distributed at marginal zone in the district (Fig. 1.125) ... 

They calculated the change in 8 0 values of hydrothermally altered volcanic rocks as a function of water to rock ratio by weight and temperature, assuming that oxygen isotopic equilibrium is attained in a closed system, and demonstrated that the increase in 8 0 values of altered andesitic rocks from the veins towards peripheral zones can be interpreted as a decrease in temperature from the vein system (Fig. 1.135). In their calculations, the effect of mixing of hydrothermal solution with groundwater was not considered. 

Temperature of each reservoir was estimated from the assemblage of hydrothermal alteration minerals and temperature of alteration zone in active geothermal system (e.g., Hayashi, 1973 Takeno et al., 2000). 

Hydrothermal alteration in the Osorezan area is extensive. At the foot of the lava dome, highly silicified alteration occurs. From this zone towards marginal parts, kaolinite zone and montmorillonite zone exist. This type of alteration was caused by the acid hydrothermal solution. But at present such acid hot solutions are not present in the Osorezan area. The acid solution is considered to be of volcanic origin. It is therefore thought that the water chemistry evolved from extremely acid at the early stage to neutral pH at present (Aoki, 1992a). 

Janecky, D.R. and Shanks, W.C. Ill (1988) Computational modeling of chemical and sulfur isotopic reaction processes in seafloor hydrothermal systems, chimney, massive sulfides, and subjacent alteration zones. Can. Mineral, 26, 805—826. 

Porphyry Cu-Mo deposits and other hydrothermal deposits are commonly associated with characteristic hydrothermai aiteration patterns (Loweii Guiibert 1970) that may be far more extensive than the ore zone, thereby providing a much iarger target for expioration. Consequentiy, identification and mapping of hydrothermal alteration have become important techniques in exploring for porphyry Cu-Mo deposits at both regional and local scales. 

The >7 Moz Morila Au deposit, Mali, is an enigmatic member of a continuum of Au deposits now described across the West African Craton (Fig. 1). In contrast to more traditional shear zone-hosted and stockwork deposits, hydrothermal alteration and brittle structures at Morila are cryptic and there are no apparent lithological controls on mineralization. 

The interpretation of lithogeochemical data from basaltic structural domes is complicated by lithological changes associated with the transition from basalt to overlying siliciclastic rocks, as well as by the polydeformed nature of the host sequence. Ferroan carbonate alteration is well developed, and low-level Au enrichment extends for a considerable distance away from zones of economic interest. Arsenic and Sb/AI anomalies are restricted to within approximately 10 m of mineralized zones. Sericite alteration is indicated by Na depletion and K enrichment in basalt within 20 to 40 m of mineralized zones. A number of other elements, including Mn, P, S, Zn, Mo, Cu, Se and Ba, are variably enriched within the rocks hosting Au mineralization, but it is not clear whether elevated concentrations of these elements are a product of syn-sedimentary exhalative activity or result from later hydrothermal alteration. 

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