Alteration potassic

It is rather difficult to determine the sequence of each type of alteration in a mine area. However, it is widely accepted that the hydrothermal alteration proceeds as follows propylitic alteration —> potassic alteration and intermediate argillic alteration advanced argillic alteration. The actual sequence alteration might be more complicated and superimposition of each type of alteration could be common. 

Diorite, granodiorite, and QFP can be discriminated using TiOa concentrations and Zr/TiOa mass and Al/Ti molar ratios. Granodiorite with Anio piagioclase was intruded by QFP with up to Anas piagioclase. Granodiorite is intensely phyiiicaiiy altered whereas QFP is commonly less altered. Potassic alteration is not well expressed in this... 

EXPLANA1ION Propylytic alteration Phyltic alteration Potassic alteration Sample site... 

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. 

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. 

In contrast to the hardly investigated lateral zonation around Japanese epithermal vein-type deposits, a few examples of vertical zonation are known. Potassic alteration grades upwards into intermediate argillic alteration in the wall rocks for the Toyoha (Okabe and Bamba, 1976), Ohe (Tsukada and Uno, 1980), Chitose (Hasegawa et al., 1981) and Kushikino (Imai, 1986). 

Advanced arigillic alteration is found at the upper horizon than the sites of potassic and intermediate argillic alterations where the Au-Ag mineralization occurs (e.g., Seigoshi, Yatani, Kushikino, Hishikari). This type of alteration takes blanket-form in upper part and vein-form in lower part (Iwao, 1962 Shikazono, 1985a). The conspicuous zonation from upper to lower horizon is known at the Ugusu silica deposit, namely, silica zone, alunite zone, kaolinite zone and montmorillonite zone (Iwao, 1949, 1958, 1962). 

Usually propylitic alteration precedes the base metal and Au-Ag mineralizations. Potassic and intermediate argillic alterations are nearly contemporaneous with ore deposition. 

Formation of albite which is characteristic mineral of propylitic alteration occurs by heating of rocks and descending fluids at recharge zone in the hydrothermal system (Giggenbach, 1984 Takeno, 1989). Thus, it is considered that the propylitic alteration takes place at recharge zone in the hydrothermal system, while potassic alteration at discharge zone. 

The dependence of concentration of K+, Na+, Ca + and H4Si04 in equilibrium with common alteration minerals (K-feldspar, Na-feldspar, quartz) on temperature is shown in Fig. 1.140 (Shikazono, 1988b). This figure demonstrates that (1) chemical compositions of hydrothermal solution depend on alteration minerals, temperature and chloride concentration, and K" " and HaSiOa concentrations increase and Ca + concentration decrease with increasing of temperature. In this case, it is considered that potassic alteration adjacent to the gold-quartz veins occurs when hydrothermal solution initially in... 

Therefore, it is thought that the mixing of acidic solution with hydrothermal solution occurred and andesite near the gold-quartz veins suffered superimposed potassic and advanced argillic alterations. 

Key indicators of alteration and proximity to ore are increased K2O (particularly in the shale component) near complete loss of Na20 increased FeO (particularly in the siltstone-sandstone component) and increased CO2 in shale. These changes reflect the development of iron carbonate (siderite and ankerite) by carbonate introduction and some alteration of existing calcic carbonate in siltstone-sandstone samples. Destruction of albite, absence of chlorite and increased abundance of muscovite due to potassic alteration, are the other major mineral alteration effects in the altered host rocks. Trace elements enriched in the primary dispersion zone are Zn, Pb, Ag, Sb, As, Rb, and TI. Antimony provides the most consistent and extensive trace element dispersion halo around the deposit and is also preserved in most of... 

Furthermore, a Pearce element ratio (PER) scatterplot testing feldspar compositional control (Fig. 3) illustrates that some hypogene rocks have compositions controlled by feldspar (primary igneous or potassic alteration). 

Fig. 3. PER scatterplot of hypogene data testing feldspar (fresh igneous or potassic alteration) and muscovite (phyllic alteration) compositional controls.

The Williams Brook property has yielded high gold assays. Preliminary work presents evidence suggesting that Au mineralization occurs in two forms 1) as refractory Au in mm scale massive sulfide (dominantly pyrite) and disseminated veins that cut potassically altered rocks, and 2) as vuggy quartz veins as finegrained (< mm) free gold. Trace element correlations indicate that Au may be related to epithermal mineralization as there are good correlations of Au with Ag,... 

Burnham, 1962 Keller and Hanson, 1968 Rose, 1970 Lowell and Guilbert, 1970 Keller, 1963). These studies indicate that kaolinite can be formed by hydrothermal alteration at the surface as well as to depths of several kilometers. Although information is lacking for low temperatures, intermediate conditions of pressure and temperature are known to permit the stability of the potassic mica-beidellite mixed layered composition series which excludes the stable coexistence of K-feldspar and kaolinite. If one accepts the argument that both beidellite-sodic and potassic are... 

Probably the most passionately debated mineral (if one might use this adverb in a discussion of clays) found in argillaceous sediments, rivalling perhaps the enigmatic dolomite and dolomitization in the realm of sedimentary rocks, is the mineral or group name illite. Defined and redefined by its originator, R. E. Crim debated and further redefined, denied a proper existence and reprieved, this species has attracted the attention of clay mineralogists for the past two decades. It represents, in fact, the dominantly potassic, dioctahedral, aluminous, mica-like fraction of clay-size materials. Known as sericite or hydro-mica in studies of hydrothermal alterations, soil mica or illite in soils and illite... 

Enrichment-depletion diagrams are also useful as a way of displaying element mobility this has been used particularly in alteration zones associated with hydrothermal mineralization. For example, Taylor and Fryer (1980) show the relative mobilities of trace and major elements in the zones of potassic and propylitic alteration associated with a porphyry copper deposit. In this case the enrichment/depletion is measured relative to the unaltered country rock. 

As far as sodic or potassic glasses are concerned, either in an aqueous environment or hidden in the ground, it is well known that the initial composition of glass is the essential factor with regard to the alteration, to the evolution the alteration layer (thickness, structure, global chemical composition, etc.), and to the role of certain chemical elements such as Mg, Ca, P, or Si in the development of this layer. 

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