ABSTRACT

Hydrothermal alteration is characterized by a change in the mineral composition (mode) of rocks in water-rock systems. The mode of rocks is changed by reaction between minerals and solution, with the composition of the solution also changing as a result of the reaction. The composition of the splution within a hydrothermal system under set conditions is given by simultaneous equations which are based on formulae of each mineral, and written in a matrix form. On the basis of mass balance between the minerals and the solution, the amounts of minerals dissolved and precipitated due to changing conditions are given by simultaneous equations which are also written in a matrix form. The latter matrix is transposed from that of the former. The change in the mode due to changes in the physicochemical condition is obtained by combining the two calculations given above. The calculations are applied to hydrothermal alteration in a system in which there is a geothermal gradient and circulation of hydrothermal fluid. For a system involving a solution and rock consisting of quartz, feldspars, muscovite and others, a decreasing temperature results in an increase of quartz and muscovite and a decrease in the amount of feldspars. In systems where the number of mineral phases is more than that of metal elements, stable mineral assemblages in the solution are examined in a model in which the solution interchanges among domains in a rock. One example is the Al-Si system involving solution, quartz, gibbsite and kaolinite. The results indicate that the stable mineral assemblages under set conditions depend on the original mode. Formation of veins accompanied by wall rock alteration is calculated in the system described above. The result indicates the formation of quartz veins with temperature decrease; the general feature of wall rock alteration can also be predicted.