Skam-type deposits

Figure 1.181. Temperature-log /s diagram. Skam-type deposits (solid squares) are considered to be formed under lower /s, condition than vein-type deposits (open squares). Abbreviations are the same as in Fig. 1.178 (Shimizu and Shikazono, 1985).

Heated meteoric waters are a major constituent of ore-forming fluids in many ore deposits and may become dominant during the latest stages of ore deposition. The latter has been documented for many porphyry skam-type deposits. The isotopic variations observed for several Tertiary North American deposits vary systematic with latitude and, hence, palaeo-meteoric water composition (Sheppard et al. 1971). The ore-forming fluid has commonly been shifted in 0-isotope composition from its meteoric 5 0-value to higher 0 contents through water-rock interaction. Meteoric waters may become dominant in epithermal gold deposits and other vein and replacement deposits. 

Plate 7 Pyroxene Crystal. 7a 2010 Augite-La Panchita Mine, Mun de La Pe, Oaxaca, Mexico by Rock Currier. 7b 2010 Enstatite-83152 by Rob Lavinsky. 7c 2011 Diopside Aosle by Didier Descouens. Pyroxene is Ca-Mg-Fe silicate ((Ca, Mg, Fe) Si03) and classified as augite (Ca, Mg, Fe) Si20s (Plate 2A), enstatite (MgSiOs) (Plate 2B), diopside (CaMgSi20s, (Plate 2C). It occurs in volcanic rocks (basalt, andesite) and skam-type deposits (see Chap. 1)... 

Important metallic ore deposits include Besshi (Kieslager)-type (strata-bound cupriferous pyritic deposits), strata-bound Mn-Fe-type, skam-type, Kuroko-type and vein-type. Dominant non-metallic deposits are limestone, clay, native sulfur, zeolite, silica and gypsum deposits. The deposits are divisible into three groups, based on their ages of formation Carboniferous-Jurassic, Cretaceous-Paleogene and Tertiary-present. 

Figure 1.16. Chemical composition of tetrahedrite-tennantite (Shikazono and Kouda, 1979). A Au-Ag vein-type deposits, B Kuroko deposits, C Taishu-Shigekuma Pb-Zn vein-type deposits, D Skam deposits (Kamioka).

The relationship between the iron content of stannite in equilibrium with sphalerite and pyrite or with sphalerite and pyrrhotite was derived based on thermochemical data by Scott and Barnes (1971), Barton and Skinner (1979) and Nakamura and Shima (1982). These types of deposits are skam-type polymetallic (Sn, W, Cu, Zn, Pb, Au, Ag) vein-type and Sn-W vein-type deposits. As shown in Fig. 1.181, the /s -temperature range for each type of deposits is different at a given temperature, /sj increases from Sn-W vein-type through skam-type to polymetallic vein-type deposits. It is interesting to note... 

As mentioned already, Shimizu and Shikazono (1985) have estimated the /s2 temperature range for stannite-bearing assemblages from Japanese vein-type and skam-type tin deposits. This estimated /sj-temperature region is also shown in Fig. 1.183. The /s2-temperature range for the formation of these two types of tin sulfides is different. 

This group of deposits is closely associated in space and time with magmatic intrusions that were emplaced at relatively shallow depths. They have been developed in hydrothermal systems driven by the cooling of magma (e.g., porphyry-type deposits and skams). From 8D- and 8 0-measurements, it has been concluded that porphyry copper deposits show the clearest affinity of a magmatic water imprint (Taylor 1974) with variable involvement of meteoric water generally at late stages of ore formation. 

Therefore the formation of magnetite in that way could hardly be of essential importance in the metamorphism of iron-formations, and martitiza-tion is still less hkely. However, in deposits of other genetic types, for instance skam deposits, oxidation of iron silicates to magnetite at the contact with large masses of carbonate rocks (dolomite, magnesite) can be considered an ore-forming process. The last conclusion is still feasible because the carbon dioxide released in the dissociation of carbonates probably had an undisturbed CO O2 ratio. 

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