THE MODEL OF FLUID-FORMING TIN-BEARING GREISENS

The equilibrium state of the system ''tin-bearing granitoid rocks-wate r solution'' at temperatures of 600-350 degrees C and pressures of 3-0 .5 kbar was modeled both experimentally and by computer simulation. It was shown that the chemical composition of the water phase of the sys tem corresponds to that of the fluid separated from the ongonite melt. The granitoid rock, it was determined, is a buffer for the water flui d filtering at all natural ratios between the masses of the solid phas es (R) and water (W) in reaction with one another, even if the integra l ratio R/Sigma W becomes lower than unity, as after a long period of rock ''elutriating.'' The fluids equilibrated with granitoid rocks at these T and P parameters, have a pH close to neutral, and an Eh approx imating the value of the QFM buffer. The mobilization of tin into flui d, which is in equilibrium with ongonite or ongonite melt at the solid us temperature, favors an increased content of Cl and, in particular, F in this rock (source). The transfer of tin is mostly favored by the filtration of water fluid through the low-porous granitoid rocks (when R/W much greater than 10 is assured). The precipitation of tin from t he ore-bearing fluid is most efficient in free fractures (or fractured rocks), in which R/W much less than 10.