A study of scale deposition: An analogue of meso- to epithermal ore formation in the volcano of Milos, Aegean arc, Greece Original Research Article

The viability of boiling as a mechanism for meso- to epithermal ore formation has been tested in the Milos volcano, Aegean Island Arc, by investigating rates of deposition and composition of scale, and by drawing an analogy between the results and actual field occurrences on the island. Milos offers ideal conditions for such testing: it consists of predominantly felsic volcanic rocks with numerous late-stage hydrovolcanic (phreatic) breccias. A geothermal system is at present active in the area, with numerous terrestrial and submarine hot spring outlets and kaoline and bentonite alteration zones. Au---Ag bearing polymetallic, as well as manganese and barite ore deposits, occur on the island; scaling tests were made possible after the installation of a 2 MW geothermoelectric plant on Milos. The rates of scale deposition were measured during different test times and analyses made on the chemistry and mineralogy of scales from the Milos plant; the results indicate that the rates of scale deposition are higher near the flashing valve, and that the metals Pb, Cu, Zn are deposited as sulphides in the vicinity of the flashing point, particularly PbS, which has the lowest solubility and, to a lesser degree, Cu-sulphide. ZnS is the dominant sulphide downstream. Higher scaling rates are observed in tests which are suspect of a higher incidence of two-phase flow in the pipe. Pressure, temperature and salinity conditions of the hydrothermal fluid are conducive to phase separation in the upper parts of the Milos geothermal system and down to a depth of 1000–1100 m but, as scaling tests indicate, self-sealing of the system could rapidly halt hydrothermal flow and ore formation; however, hydrovolcanic (phreatic) explosions may initiate new cycles of ore-deposition. The metals have mainly precipitated as sulphides, except for Fe, which is found mostly in the siliceous matrix of the scale. The paragenesis of the metals in the Milos scales is similar to those of Au---Ag bearing polymetallic deposits on the island. Domains with signs of multiple episodes and craters of hydraulic fracturing, argillic alteration, siliceous sinters and gossaneous cappings, should be targeted on Milos for exploration for meso- to epithermal polymetallic and precious metal ores.