Andesite

Mercury ore deposits occur in faulted and fractured rocks, such as limestone, calcareous shales, sandstones, serpentine, chert, andesite, basalt, and rhyolite. Deposits are mostiy epithermal in character, ie, minerals were deposited by rising warm solutions at comparatively shallow depths from 1—1000 m (6). 

Fig. 9-4 Photograph of landslides (soil avalanches) that occurred following earthquakes in Panama on July 17,1976, near Jaque. In the background is a bay of the Pacific Ocean. The effects of this earthquake are described by Garwood et al. (1979), who estimated that about 42 km (about 10%) of the region near the epicenter of the earthquake was devegetated. The bedrock is mostly island-arc basalts and andesites. (Photography by N. C. Garwood.)...

In Southwest Japan, two styles of vein-type mineralization (Hg and Sb) formed from middle Miocene to the present. Many Hg and Sb deposits are present along the Median Tectonic Line, associated with the Setouchi andesites and ilmenite-series granitic rocks (Fig. 1.3). 

These different sites of hydrothermal and ore-forming activity may have resulted from the mode of subduction of the Pacific Plate. Mariana-type subduction (characterized by a steep angle of subduction and back-arc basin formation Uyeda and Kanamori, 1979) during middle Miocene caused WNW-ESE extension, submarine hydrothermal activity, thick accumulation of bimodal (basaltic and dacitic) volcanic activity (Green tuff) and Kuroko-type formation (Shikazono and Shimizu, 1993). Plio-Pleistocene Chilean-type subduction (shallow-dipping subduction zone, E-W compression Uyeda and Kanamori, 1979) and oblique subduction of the Pacific Plate beneath the North American Plate led to uplift and expansion of land area, subaerial hydrothermal activity accompanied by meteoric water circulation, subaerial andesitic volcanic activity and formation of vein-type deposits. 

Menaichizawa, Sasahata), is composed mostly of brecciated andesite lavas and andesitic hyaloclastics. This formation is conformably overlain by the formation (Hotakizawa, Sunakobuchi) composed of thick sequence of basaltic lavas and tuff breccias with minor intercalations of mudstone and felsic tuff. 

Andesite volcanism and subsidence from near-shore to shallow-sea environments (30( )-ca. 17 m.y. ago). 

Differential uplifts, broad folding, andesitic volcanism and formation of smaller basins (5 m.y. ago-present). 

Q> C o o o % Q. Danitna Odoi andesite lava andesite lava +500m +500nn... 

Koshimoda Andesite andesite lava volcanic breccia +500tn... 

Koshimoda Volcano (andesite lava, volcanic breccia)... 

The Seigoshi and Toi deposits occur in the andesitic pyroclastic rocks of the upper horizon of the Yugashima Group and basic intrusive rocks. Distributions of the wallrock alteration minerals from underground in the Seigoshi mine and on the surface near the... 

Figure 1.79. Geology and alteration zoning in the Ugusu silica mine (modified from Iwao, 1949, 1962). Toi F = Toi Formation Koshimoda = Koshimoda andesite Hakko = Hakko orebody Shibayama = Shibayama orebody. Numbers indicate metres above sea level. Alteration zoning in the section of A -B is shown in Fig. 1.80 (Shikazono, 1985a)...

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. 

The coupled precipitation kinetics-fluid flow model was applied to the distribution of Si02 content and K2O content of the hydrothermally altered andesite in the Hishikari Au-Ag mine area, south Kyushu, Japan by Shikazono et al. (2002). This will be described in section 1.4.6. 

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

The district is composed of sedimentary rocks of the pre-Paleogene Shimanto Supergroup (dominantly shale and sandstone) and Quaternary andesitic and dacitic volcanic rocks. The Shimanto Supergroup is comprised of shale, sandstone and their alternations. Although no fossil data are available, the age of sedimentation is thought to be middle to upper Cretaceous age from its lithology (Izawa et al., 1990). The Shimanto... 

Quaternary volcanic rocks unconformably overlie the Shimanto Supergroup. Quaternary volcanic rocks consist of Hishikari Lower Andesites (0.98-1.62 Ma), Hishikari Middle Andesites (0.78-0.79 Ma), Kurozonsan Dacites (0.95-1.56 Ma), Shishimano Dacites (0.66-1.6 Ma), and Hannyaji Welded Tuff (0.56-0.731 Ma) (Izawa et ah, 1990, 1993). 

The Hishikari Upper Andesites overlying the Hishikari Lower Andesites and the Shishimano Dacites consist of hypersthene-augite andesite lava flows and their pyroclastic rocks. 

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. 

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