Στοιχεία επιβλεπόντων καθηγητών:
Στέφανος Κίλιας, Καθηγητής, Τμήμα Γεωλογίας και Γεωπεριβάλλοντος, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
Αριάδνη Αργυράκη, Αναπληρώτρια Καθηγήτρια, Τμήμα Γεωλογίας και Γεωπεριβάλλοντος, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
Αθανάσιος Γκοντελίτσας,Καθηγητής, Τμήμα Γεωλογίας και Γεωπεριβάλλοντος, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
Περίληψη:
The active hydrothermal vent field on the floor of the Kolumbo shallow-submarine arc-volcano, near Santorini, Hellenic Volcanic Arc, features the only known modern sea-floor massive sulphide (SMS) deposits associated with thinned continental margin volcanism. Optical microscopy, SEM-EDS and LA-ICP-MS analyses of SMS from Kolumbo, show a distinct enrichment in trace elements such as Au,Cu, As, Sb, Pb, Hg, Ag, Tl and Zn. Optical microscopy and ESEM-EDS were used for the study of the mineralogy and major element chemistry respectively of the main minerals present, which were pyrite, marcasite, minor chalcopyrite, galena, minor anglesite, sphalerite, Pb-Sb-sulphosalts, As-Pb-Sb-sulphosalts and As-sulphide. Barite is the main sulphate mineral. Emphasis was given in the study of pyrite textures, which amounted to three: (1) colloform pyrite (type 1), (2) porous pyrite (type 2) and sub- to euhedral pyrite (type 3). LA-ICP-MS was used for trace element chemistry of the aforementioned sulphide minerals, with emphasis given to pyrite types 1 and 3. Pyrite 1 contains up to 58 ppm Au, 2 wt.% Cu and 9071 ppm As, whereas pyrite type 2 and 3 are characterized by lower concentrations (up to 37 ppm Au, 1.5 wt.% Cu and 9071 ppm As for pyrite 2 and up to 24 ppm Au, 1.3 wt.% Cu and 4297 ppm As for pyrite 3). Gold correlates well with Cu in pyrite 3 (R2 = 0.705).The study of numerous time resolved spectra showed that concentrations of Au and Cu are mainly related to structurally bound Au and Cu in colloform pyrite (type 1), and submicron-sized Au-bearing and Cu-sulphide particles in sub- to euhedral pyrite (type 3), whereas As is in solid solution in both. It can be suggested that Au was firstly incorporated within colloform pyrite, mainly in solid solution, owing to the presence of As and local supersaturation possibly led the limited presence of submicron-sized Au-bearing particles. Increasing fluid temperature and recrystallization of colloform pyrite to porous (type 2) and euhedral pyrite (type 3), led to decreasing solubility of Au and As in pyrite, resulting in partial expulsion of As from pyrite and concomitantly: (1) an increase in the nucleation of Au sub-micron particles and Cu-sulphide sub-micron particles in sub- to euhedral pyrite (positive correlation between Cu and Au) and (2) expulsion of Au from pyrite, and incorporation in other sulphide phases (galena contains up to 60 ppm Au, Pb-Sb-sulfosals contain up to 87 ppm Au and As-sulphide contains up to 171 ppm Au) and late chalcopyrite,respectively. Degassing of a MORB-like mantle through lithospheric faults resulted in outgassing of mantle-derived volatiles rich in Au, along with the other studied elements, such as Cu, As, Sb, Pb, Hg, Ag, Tl and Zn. The magmatic contribution is supported by the sulphur isotope composition from Kolumbo pyrite, which points to a magmatic source of S. Gold was most likely transported to the surface as a Au(HS)-2 complex and themixing of hydrothermal fluids was a key process that led to not only the enrichment of the hydrothermal fluids in Au and the other elements, but also to their precipitation within the Kolumbo sulphides. Average and maximum Au contents , analysed by LA-ICP-MS in pyrite from Kolumbo SMS, the only known SMS deposits associated with continental margin volcanism present the highest reported values in modern sea-floor hydrothermal systems.
