The importance of supersaturated silica deposition for base-metal Au–Ag mineralisation in Western Turkey.

In the Arapuçandere LS-IS epithermal deposit in western Turkey, either well crystallised euhedral quartz or milky amorphous silica occurrences are associated with the precious and base-metal mineralisation. This study has determined the relative importance of the processes precipitating both of these in relation to the deposition of the ore. Analysis by LA-ICPMS of euhedral quartz and amorphous silica show that the latter is associated with much higher metal concentrations and suggests that the process responsible is more important for deposition of ore in the deposit. Mineralisation at Kumarlar and Koru may be similar. Slow cooling precipitates euhedral quartz and some ore but flashing of hydrothermal fluids causes drastic decreases in both T and P, sufficient to quantitatively deposit base metals and Au. In amorphous silica bands base metals reach concentrations of 1000’s of ppm compared with a few 10’s ppm in euhedral quartz; Au can reach over 1 ppm compared with a few 10’s ppb in quartz. The drastic pressure change from lithostatic to sub-hydrostatic could reasonably be caused by opening of faults and fractures by seismic events.