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Dissertation zugänglich unter
Geological, geochemical and mineralogical studies of shallow submarine epithermal mineralization in an emergent volcanic edifice at Milos Island (Western Side), Greece
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38.37 , 38.30 , 38.32
Arikas, Kyriakos (PD Dr.)
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Kurzfassung auf Englisch:
Calc-alkaline volcanic rocks in western Milos (pyroclastics and shallow subvolcanic intrusives/lava domes) were emplaced during the Upper Pliocene to Pleistocene times under transitional submarine to locally subaerial conditions, forming a series of volcanic edifices. During the eruptive phase a combination of vent clearing pyroclastic flows and water supported volcaniclastic products created a poorly consolidated pumice pile (pumice cone). Shortly after, it has been intruded by shallowly emplaced rhyolitic domes (cryptodomes). This dominantly acid explosive and intrusive activity has been followed by effusive volcanism that formed numerous dacitic-rhyodacitic-rhyolitic and andesitic domes and lava domes, emplaced shallowly underneath or on the sea-floor, creating a series of hyaloclastitic products intercalated with shallow marine sediments. Locally some of the domes gave origin to bedded pumiceous tuffs (dome-top tuff cones), while other domes and lavas raised above the sea level becoming thus subaerial. All the subvolcanic high-level intrusive and partially extrusive volcanic products represent the equivalents of a deeper magma chamber that fed the volcanic succession, and were the heat source required to drive the hydrothermal convective system responsible for the mineralization. Geochronological data, of several of these intrusions suggest that they are partially coeval with, and partially younger than the pyroclastic succession into which they were emplaced.
Methodological collection of existing information, geological mapping of surface outcrops, geochemistry, X-ray diffraction analysis, drill holes logging, air-photos and satellite imagery structural interpretation, alteration and ore mineralogical studies, were combined in order to better understand and interpret the magmatic-hydrothermal environment of ore deposition at western Milos.
The dominant structural features at western Milos are from one side a series of steep NE striking fault / lineament zones, and from the other side a series of dilational NW trending ones (fault jogs), into which, composite volcanic centers, domes and collapse calderas have been developed. They together have promoted and localized fluid upflow along steep, near vertical feeder conduits, producing in such a way zones of intense silicification, brecciation and veining. With respect to those structures, subsidiary E-W and N-S trending faults were also created and are considered to have enhanced the permeability of the volcanic and volcano-sedimentary sequences, through a further dismembering and dislocation.
Of particular interest can be characterized the quasi E-W trending lineament along the central part of western Milos (Ammoudaraki-Plakota-Rivari lineament), which subdivided it into two sectors, from a geological, hydrothermal and mineralogical point of view, the northern and southern sectors.
The tectonic structures which controlled the principal eruptive centres are the same structures which controlled and localized hydrothermal discharge that formed the various deposits and mineralization.
The alteration patterns observed at western Milos correspond at least to two distinct and partially overlapped environments, namely the volcanic-hydrothermal and geothermal environments, the first corresponding to a high sulfidation (HS) mineralizing system, and the second one to an intermediate sulfidation (IS) system. Hydrothermal alteration at the northern part of western Milos is dominated by a typical acid-sulfate submarine pool, characterized by extreme acid leaching and formation of residual quartz, grading outwards to Na-alunite, dickite/kaolinite ±diaspore± pyrophyllite and then to kaolinite-illite-sericite assemblages.The observed elongated shape of the silica bodies in the northern sector of western Milos suggests that a major structural control was exercised by E-NE striking faults.
In the southern sector the ore bearing quartz-adularia zone is flanked by an illite± smectite alteration assemblage which grades to peripheral alteration dominated by carbonates-chlorite assemblages. The abundance of quartz and adularia increases toward the upflow zone, whereas clay abundance increases toward the margin of the upflow zone with smectite dominating at <150º C and illite dominating at >200º C.
A long-lived zone of hydrothermal flow with an initial submarine to a later subaerial steam heating throughout the southern sector and partially at the northern sector (Kondaros area) occurred.
Any model for the development of the regional alteration in western Milos must account for the extend of the silicification and advanced argillic alterations, the timing of emplacement of the major mineralized veins, and the presence of late advanced argillic alteration (of steam heated nature), that obliterated some of the previous features, by overprinting them.
The development of a well framed vein system mainly at the southern sector and sub- ordinately at the northern one, together with a well localized breccias field at the northern sector, are the principal surface manifestations of the mineralization. In a particular way precious ± base metals mineralization seems to have taken place where the N-S to NW- SE trending structures met NE-SW to ENE-WSW tectonic lineaments, thus creating feeder structures. In such a fashion it can also be considered that both veins and breccias/domes at western Milos were constrained between deep-slip and strike-slip structures. Indeed all structural elements (big lineaments, faults, fractures, joints) at western Milos display forms of the pre-, syn- and post-mineralization history (activity) as a response to the different stress regime that took place. As a whole, it is considered that strike-slip movement provided syn-mineralization mechanisms for dilatacy, which permitted to the various kinds of subvolcanic bodies to be emplaced, as well as resulted in post mineralization offsets of the various vein systems.
The Mavrovouni-Profitis Ilias-Chondro Vouno-Koumaria (southern sector) and Triades-Galana-Kondaros-Katsimoutis-Agathia-Vani (northern sector) areas, represent two large (16 Km2, and 5 Km2 respectively) zoned magmatic-hydrothermal systems, related to the development of the several explosive (rhyolitic/rhyodacitic) volcanic centres and associated with a postulated caldera - dome/flow dome volcanic setting. The submarine magmatic-hydrothermal systems at W. Milos produced deposits with intermediate and high sulfidation variants. The precious metals were initially introduced by hydrothermal fluids of intermediate sulfidation nature (Profitis Ilias-Chondro Vouno), and in a successive stage by intermediate/high-sulfidation states (Triades-Galana) and still later by intermediate sulfidation/high-sulfidation states (Katsimoutis-Kondaros-Vani). All the above mineralized centres are characterized by the occurrence of galena + Fe-poor sphalerite + chalcopyrite and abundance of barite, adularia, sericite and partly calcite. High sulfidation conditions prevailed at Triades-Galana-Agathia-Kondaros as indicated by the presence of enargite-tennantite-chalcopyrite-covellite in the ore assemblage. At Profitis Ilias area the bulk of gold mineralization has been deposited at the upper topographic levels, whereas base-metal and gold-silver telluride mineralization have been deposited at deeper levels, due probably to dilution of injected brines by sea water and/or local boiling processes, as the presence of second generation copper sulfides/galena and adularia respectively suggested. Based on thermodynamic calculations the values of logfS2, logfO2 and logfTe2 of the mineralizing fluids were estimated.
Evidences for the W. Milos mineralization to be formed in a submarine (seafloor/sub-seafloor) environment are the abundance of framboidal pyrite, of micro-chimneys structures composed of either pyrite or various sulfide combinations including the skeletal intergrowths between sphalerite, pyrite and galena, the extreme abundance of barite and finally the seafloor deposited stratiform Mn±baryte mineralization at Vani. In this last it is also included the formation of white smokers along a NE-SW tectonic structure.
Minor and trace elements geochemical maps (As, Sb, Hg, W, Bi, Pb, Zn, Ag, Au, Mo) showed several areas of anomalous geochemical behaviour suggesting potential for epithermal precious metal mineralization. Of major importance are the elevated contents in Mo, W and Bi at the northern sector suggesting a magmatic contribution from porphyry style mineralization buried at depth. In addition magmatic-hydrothermal fluids may have played an important role in the evolution of the mineralizing system(s) at Milos, as indicated by the presence of Au-Ag tellurides (Profitis Ilias) and the high activity of sulphur evidenced by the mineralogical assemblages present. Under this aspect the close spatial relationship between the three successively emplaced intrusive/extrusive domes (Rhyda, Anda, Dado) with the three main mineralizing centers (Profitis Ilias-Chondro Vouno, Triades-Galana, Katsimoutis-Vani) respectively, probably indicates also a genetic relationship between them.
Although a number of geological, geochemical and mineralogical characteristics at western Milos mineralization show features commonly observed in submarine massive sulfide deposits, several other features are typical of subaerial epithermal ores, which suggest that the high grade intermediate/high sulfidation epithermal silver-gold mineralization took place in a shallow submarine to transitional/subaerial volcanic setting.