THE MECHANISM OF THE FORMATION OF ORE-BEARING HIGH-FLUORINE MAGMAS AND QUARTZ OSCILLATION CRYSTALLIZATION

A model of the origin of ore-bearing hi,oh-fluorine reduced granites b y the filtration of fluids through apical parts of large magmatic cham bers of acid composition is proposed. Within the framework of this mod el, certain conditions, including the oscillation crystallization of m inerals in ore-bearing magmas, are discussed. Magma crystallization le ads to a separation of fluids migrating to upper partly crystallized l evels of plutons. As a result, fluid filtration through magmas of apic al parts occurs under the condition of a temperature gradient. With th e fluid ascent against the background of a temperature drop, the HF so lubility in the fluid decreases. Therefore, the HF concentrations in t he magma, which serves as a permeable matrix, increase. Due to fluorin e accumulation in the magma, the concentrations of fluid components as sociated with fluorine change, whereas the oxygen activity decreases. The following dynamic parameters of the model are calculated: (1) the vertical rate of the fluid movement through the magma, and (2) the tim e span for establishing chemical and thermal equilibrium. In a substan ce exchange system magma-fluid, interrelated counterflows of SiO2 and HF may result in quartz crystallization from a cooled fluid. In turn, this will enhance the fluorine flow from the fluid to the melt and lea d to a counterflow of silica from the magma to the fluid, and the quar tz crystallization from the fluid will be repeated. Thus, rhythmic qua rtz crystallization from the fluid moving through the magma is respons ible for the formation of the rhythmic-zonal granites.