Ss. Abramov et Av. Rasskazov, THE MECHANISM OF THE FORMATION OF ORE-BEARING HIGH-FLUORINE MAGMAS AND QUARTZ OSCILLATION CRYSTALLIZATION, Geology of ore deposits, 39(3), 1997, pp. 237-246
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.