[SMP43-P11] Formation of mineralized quartz veins induced by flash evaporation from liquid to vapor under sub and supercritical conditions
Keywords:Flash vaporization, Supercritical fluid, Quartz vein
In the experiments of silica precipitation by flashing, the input solution (Si = 370 ppm) was prepared by dissolution of composites of quartz and granite sand. The rapid decompression experiment was carried out by flashing from 30 MPa under liquid (200, 250, 300, 350°C) and supercritical conditions (400, 450°C).The pressure of the input solution was instantaneously decrease to the vapor conditions within a few seconds. In contrast to our previous studies of silica precipitation by flow-through apparatus (Okamoto et al. 2010), where phase transition of water occurred by increasing temperature, silica precipitates did not include quartz but occurred as spherical particles of amorphous silica with size of 1-3 micron, probably due to a lack of duration of transformation into more stable phases. The Si concentration of the output solution ranged from 10 to 60 ppm, which is lower than the solubility of silica on the saturated vapor pressure curve along the P-T path. These occurrences of silica indicate that metastable amorphous silica particles were formed by rapid decompression and transported from sub- to supercritical fluids, which leads to the formation of silica scales and clogs of fractures.
Next, we prepared the initial fluids (pH = 2, HCl added) with metal components, Cu (170 ppm) and Mo (30ppm), as well as silica (Si = 400 ppm). The input solution was kept four hours at 350°C, 30 MPa, then was decompressed in two ways; first one is slow decompression by natural cooling, and the other was near adiabatic decompression by flash vaporization. As the result of slow decompression, the composition of the solution after the experiment does not change significantly (Si = 360 ppm, Cu = 130 ppm, Mo = 12 ppm). In contrast, in the rapid decompression experiment, each concentration decreased drastically to Si = 13 ppm, Cu = 4 ppm, Mo = 0.2 ppm, respectively. Our results indicate that mechanism and rate of the formation of mineralized quartz veins was highly depends of P-T-t path of the fluids, and that flashing from sub to supercritical conditions (phase transition of water) potentially plays roles of mineralizeation.
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