Trace Elements Distribution In Sphalerite From Epithermal-Style Veins At The Pagoni Rachi And Konos Hill Porphyry/Epithermal Prospects, NE Greece

The Sapes and Kirki prospects in NE Greece belong to the Rhodope metallogenic district and host significant porphyry and epithermal type mineralization, which are enriched in several precious and critical metals. Host rocks are mostly magmatic and/or sedimentary rocks of Middle-Upper Eocene to Oligocene age, which intrude or unconformably overlie the metamorphic basement of the Rhodope and Circum-Rhodope units. Porphyry-style Mo-Re-Cu-Au mineralization, which occurs at Konos Hill (broad Sapes area) and Pagoni Rachi (Kirki area) is overprinted by quartz-calcite veins with epithermal affinities (“E-type” veins, Fig. 1a-d). These veins are associated with lateral sericitic and/or argillic alteration of the granodiorite host rock and contain various amounts of base metal sulfides (pyrite, galena, sphalerite, chalcopyrite) along with minor sulfosalts (e.g., tetrahedrite/tennantite group minerals) and Ag-Au alloy. Sphalerite is abundant in E-type veins at both prospects, forming coarse-grained (up to 0.5 cm), euhedral to subhedral crystals associated with pyrite, galena, chalcopyrite, and tetrahedrite/tennantite group minerals (Fig. 1e, f). EPMA and preliminary LA-ICP-MS analyses (Table 1, Figure 2) reveal differences in the chemistry of sphalerite between the two prospects: At Konos Hill, sphalerite contains high Cd values (up to 39434 ppm) exhibiting a positive trend with Fe. In contrast, at Pagoni Rachi the Cd content of sphalerite is much lower (up to 4577 ppm) and correlates negatively with Fe. The Fe content is slightly variable in the two prospects (up to 2.75 wt% corresponding to 4.81 mol % FeS at Konos Hill, and up to 3.39 wt% corresponding to 5.93 mol % FeS at Pagoni Rachi, respectively) and is (along with Cd) responsible for the zoned nature of sphalerite. The Mn content is much higher in sphalerite from Pagoni Rachi than in Konos Hill (up to 2620 ppm and 222 ppm, respectively), while a positive relationship occurs between Mn-Fe and Mn-Cd at Konos Hill. Copper has a higher concentration in sphalerite from Pagoni Rachi (up to 817 ppm) than Konos Hill/ Sapes (up to 78 ppm) and exhibits a positive trend with Sn at both prospects. Indium and Cu correlate negatively in sphalerite from Konos Hill/Sapes but show a slightly positive correlation in sphalerite from Pagoni Rachi. Gallium is enriched in sphalerite Konos Hill and Pagoni Rachi, reaching up to 1852 ppm and 2043 ppm, respectively. Indium is present in sphalerite from Konos Hill (average 540 ppm). A single spot analysis yielded 2049 ppm, implying the presence of a submicroscopic In-bearing inclusion. Germanium is characterized by low concentrations at both prospects, reaching up to 51 ppm in sphalerite from Pagoni Rachi. Other trace elements (e.g., Ag, Au, W, Ni, etc.) are present in very small concentrations (e.g., <10 ppm) or are below the detection limit. The trace element budget of the sphalerite reflects a series of substitution mechanisms and reveals the existence of submicroscopic inclusions that carry significant amounts of critical metals like Mn, Cd, In, and Sn. The presence of bivalent elements like Fe, Mn, and Cd, suggests a simple cation exchange mechanism (Zn2+↔ M2+) that is also supported by the smooth ablation profiles, thus implying a homogeneous distribution. The positive trend between Sn and Cu observed in both prospect is attributed to submicroscopic inclusions of Sn-bearing minerals, rather than a 3Zn2+ ↔ 2Cu++Sn4+ substitution mechanism, since Cu/Sn ratios are ≫2 (Cook et al., 2009). At Pagoni Rachi, In is incorporated within sphalerite via the coupled substitution Cu++In3+ ↔ Zn2++Fe2+. However, such a substitution mechanism cannot be supported for sphalerite from Konos Hill/Sapes, where submicroscopic In-bearing inclusions occur.