The alteration acme occurs on top of a buried normal fault, where abundant degassing is still active, and fades away in 2-3 km.
In this area, sulfate and sulfide mineralizations and strongly altered ignimbrites are exposed. A field and laboratory multidisciplinary approach is used in the fossil (late Pleistocene) portion of an active hydrothermal system (Colli Albani volcano, Rome, Italy).
The main issue addressed in this work is the process leading to fluid subsurface entrapment and pressure increase up to hydrofracturing and, possibly, to paroxysm in a hydro thermal setting, in order to envisage such processes and mitigate their effects in the volcanically active study area and elsewhere. We speculate that the widespread occurrence of boiling and the shallow depth of the precious- and base-metal emplacement prevented the formation of seafloor massive sulfides. Base and precious metal mineralization formed from intermediate- to high-sulfidation state fluids and mostly under boiling conditions as indicated by the widespread occurrence of adularia associated with metallic mineralization. The mineralization represents seafloor/sub-seafloor precipitation of sulfides that formed in stockwork zones. The enrichment of Te, Mo, W, and Bi in the deposits is a strong indication of a direct magmatic contribution of these metals.At western Milos, precious and base-metal vein mineralization was deposited during episodic injection of magmatic volatiles and dilution of the hydrothermal fluids by seawater. The metal signatures of the Triades–Galana and Agathia–Kondaros–Katsimouti–Vani (Mo–Bi–W–As–Hg–Ag–Au) occurrences compared to Profitis Ilias (Te–Au–Ag) reflect different sources of magma (dacite–rhyodacite for Profitis Ilias, andesite–dacite for Triades–Galana, and dacite for Kondaros–Katsimoutis). The differences in precious and base metal abundances may be related to the depths at which the deposits are exposed, and/or different sources of magma. Arsenic is absent from the southern sector but shows elevated concentrations together with molybdenum, bismuth and tungsten at the northern sector (Triades–Galana, Vani deposits). Gold is enriched at Profitis Ilias–Chondro Vouno deposits and to a lesser extent at Triades–Galana. microchimney structures, copper sulfides, widespread occurrence of barite, aragonite) suggests that precious metal mineralization in western Milos mineralization formed in a submarine setting.We present information on the surface distribution of Au, Ag, Cu, Pb, Zn, As, Sb, Hg, Mo, Bi, W and Cd at western Milos.
Gold at Profitis Ilias, both as native gold and silver-gold tellurides, is present in base-metal precipitates within multicomponent blebs, which recrystallized to form hessite, petzite, altaite, coloradoite, and native gold. The high silver and gold content of the western Milos deposits is derived from Ag-bearing sulfosalts (polybasite, pearceite, pyrargyrite, freibergite) and tellurides. Locally, at Triades–Galana and Kondaros–Agathia, high-sulfidation conditions prevailed as suggested by the presence of coexisting enargite and covellite. Mineralogical data indicate common features among the Profitis Ilias–Chondro Vouno, Kondaros–Katsimoutis–Vani and Triades–Galana mineralized centers, all of which are characterized by the presence of galena, Fe-poor sphalerite, and chalcopyrite as well as abundant barite, adularia, sericite and, to a lesser extent, calcite, which are typical of intermediate-sulfidation epithermal type deposits. In the northern sector (Triades–Galana–Agathia–Kondaros), initial magma degassing derived from andesitic–dacitic intrusives along NE–SW to E–W trending faults resulted in the development of pre-ore hypogene advanced argillic alteration (dickite, alunite, ± diaspore, pyrophyllite, halite, and pyrite) in a submarine environment. Hydrothermal alteration of the volcanic rocks includes advanced argillic- (both hypogene and steam-heated), argillic, phyllic, adularia-sericite and propylitic types. 2.2 to 1.5 Ma (Triades–Galana–Kondaros–Katsimouti–Vani). Ma (Profitis Ilias–Chondro Vouno), submarine to subaerial andesite to dacite domes at ca. These deposits are hosted in calc-alkaline volcanic rocks emplaced as a result of three successive magma pulses in an emergent volcanic edifice: submarine rhyolitic to rhyodacitic cryptodomes at ca. Milos Island contains several epithermal deposits (e.g., Profitis Ilias–Chondro Vouno Pb–Zn–Ag–Au–Te–Cu, Triades–Galana–Agathia–Kondaros Pb–Zn–Ag–Bi–W–Mo ± Cu–Au, and Katsimoutis–Kondaros–Vani Pb–Zn–Ag–Mn) of Late Pliocene to Early Pleistocene age.