Kaolinization is typically the result of surface agencies, except, perhaps, where hydrothermal solutions were acid in character. This process may be produced in some gold deposits and consists in the decomposition, and solution of feldspars and other minerals with the formation of kaolin as the residual product. A thorough kaolinization destroys the original character of the rocks attacked, and diverse rocks upon kaolinization yield products so similar that their original nature is distinguishable with difficulty, if at all.
This process of alteration appears to depend upon the acid solutions set free by the oxidation of pyritic sulphides. The presence of any considerable amount of kaolin is likely to indicate that sulphides have existed in the kaolinized area and that they have been removed in solution, perhaps with the formation of enriched sulphides in depth; kaolinization, therefore, often constitutes a valuable guide in a search for secondary gold deposits. Theoretically, kaolinization is attended by shrinkage in volume of from 10 to 14%, which is partly offset by increased porosity. Kaolinization readily attacks feldspars, of which the sodium-calcium varieties appear to be the most susceptible, and microcline the most resistant; sericite also is readily decomposed, and dark silicates are bleached through the removal of iron. The typical product of complete kaolinization is a white, soft, earthy mass, in which no minerals may be distinguished by the unaided eye except kaolin, quartz and limonite, and in which the structure of the original rock is completely destroyed.
It has been established that a rock that exhibits the outlines of the feldspar phenocrysts cannot be considered completely kaolinized, and such incomplete alteration rarely overlies important bodies of enriched sulphides that contain gold. Kaolinization rarely extends into the wall rocks of gold veins, except where these rocks themselves originally carried a sulphide mineralization. Sericitization is a hydrothermal metamorphism that results in an almost complete loss of sodium and a large gain in potassium, silica, and commonly also of pyrite, with occasionally lesser quantities of carbonic acid and fluorine. The typical result of complete sericitization is a finely granular aggregate of sericite, quartz, pyrite and calcite; after thorough sericitization it is difficult to distinguish between rocks that were originally quite dissimilar. Sericitization is a common form of hydrothermal metamorphism, and is an intimate accompaniment of the mineralization of most ore-deposits in igneous rocks; the alteration is commonly most intense immediately along the veins, and decreases with distance from them, sometimes passing into a propylitic alteration, which latter transition is likely to take place within a short distance of the veins in basic rocks; sericitization not infrequently persists over large areas in rocks of intermediate or acid composition.