| Abstrakt: |
A highly prosperous district for epithermal mineralization hosted in Oligocene-Miocene volcanic rocks is the Rhodope metallogenic province in Greece, located in the broader Western Tethyan metallogenic belt (Melfos and Voudouris 2017; Voudouris et al., 2019). The Evros (Pefka-Loutros) ore district located within the Rhodopes comprises numerous high-sulfidation (HS) and intermediate-sulfidation (IS) epithermal systems in hydrothermally altered volcanic rocks. Among others, the Loutros IS epithermal mineralization is an important prospect that is hosted in a silicified, sericite and zeolite-altered rhyolitic dome (Melfos and Voudouris, 2017). This felsic intrusion of high-K calc-alkaline affinity has been dated at 19.53 ± 0.75 Ma (K-Ar at whole-rock; Christofides et al., 2004). The objective of this study is to present preliminary data on the mineralogy, the ore styles, and the temperatures of formation of the mineralization of the Loutros IS system. Seven samples were studied under polarized and/or reflected light and three samples were imaged and analyzed with scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDS) at the School of Sciences, of the Aristotle University of Thessaloniki (AUTh). Three samples have been analyzed for whole rock trace element analyses at Bureau Veritas Commodities Canada, Vancouver, Canada. Six doubly-polished wafers of barite and quartz veins, were used for fluid inclusion microthermometry at the Department of Mineralogy-Petrology-Economic Geology, AUTh. The host rock of the Loutros IS mineralization is an altered rhyolite (Fig. 1a) that displays porphyritic texture. The ore minerals form veins, breccias and disseminations, and consist of pyrite, marcasite, galena, and minor sphalerite and chalcopyrite. Two types of veins have been identified: (i) early northwest-trending massive pyrite-marcasite veins with straight borders (Fig. 1a,b), and (ii) late-stage barite-galena veins that either crosscut the rhyolite (Fig. 1c) or occur as late-stage ore deposition in pyrite-marcasite veins (Fig. 1b). Minor sphalerite (Fig. 1d) and chalcopyrite may be present in intergrowths with galena and barite (Melfos and Voudouris, 2017). Framboidal pyrite (Fig. 1e,f) is found in disseminated aggregates. Combined with the widespread barite, this indicates a possible shallow submarine condition during deposition of at least part of the mineralization (e.g. epithermal mineralizations at Milos; Alfieris et al., 2013). Other structures include non-mineralized chalcedony veins that are found in proximity to the pyrite-marcasite vein, and hydrothermal breccias that crosscut the rhyolite. Bulk ore trace element analyses showed that the pyrite-marcasite mineralization contains traces of As (579 ppm), Mo (9 ppm), and Co (2 ppm). The galena mineralization contains trace amounts of Sb (≤305 ppm), Ag (≤32 ppm), and Cu (≤62 ppm) (Voudouris et al., 2019). The pyrite-marcasite vein is depleted in Sb (0.4 ppm), Ag (2 ppm) and Cu (11 ppm), compared with the galena mineralization, while the latter is depleted in As (≤137 ppm), and Mo (4 ppm) related to the pyrite-marcasite vein. Barite- and quartz-hosted fluid inclusions are concentrated in clusters and are considered to be primary, based on the criteria proposed by Roedder (1984) and Goldstein and Reynolds (1994). Only two-phase aqueous liquid-vapor inclusions, containing a vapor bubble occupying 10-20 vol.%, were observed. These inclusions homogenized into the liquid phase upon heating. Post-entrapment modifications, such as necking down or leakage, have affected many of these fluid inclusions. In order to eliminate the effects of the post-entrapment modifications, only inclusions with constant liquid-to-vapor ratios were selected for the microthermometry. The microthermometric measurements of fluid inclusions in quartz, which is synchronous with the pyrite-marcasite mineralization, showed a homogenization temperature range of 259° to 329°C (Fig. 2a), and a salinity between 2.8 and 3.9 wt.% NaCl equiv. (Fig. 2b), indicating that the fluids had a meteoric and/or seawater origin, with a minor magmatic contribution. The microthermometric measurements on barite- and quartz-hosted fluid inclusions showed that the stage of the galena mineralization may have taken place between 201° and 255°C (Fig. 2a) from a less saline fluid (1.0 - 1.8 wt.% NaCl equiv.; Fig. 2b), indicating an increased contribution of meteoric and/or seawater fluids in the system. In summary, the Loutros IS epithermal mineralization is hosted in altered rhyolites and is characterized by quartz-pyrite-marcasite veins and disseminations, and barite-galena veins. Textural relationships show that galena postdates the pyrite-marcasite mineralization. This is confirmed by the microthermometric data that show that the pyrite-marcasite event was formed at a temperature range of 259° to 329°C, while the galena stage was formed at lower temperatures (201°-255°C) from a low-saline hydrothermal fluid. Trace element analyses of whole rock samples imply that the galena-associated ore is enriched in Ag (≤32 ppm). Further analysis (whole-rock, fluid inclusions microthermometry, LA-ICP-MS and Raman) and sulfur isotopes are in progress to discriminate the two ore stages and to examine the physicochemical conditions of the ore-forming events. [ABSTRACT FROM AUTHOR] |
