engleski

Evolution of ore-forming fluids at the carbonate- hosted Pb-Zb Mežica deposit, Slovenia

INTRODUCTION Numerous Pb-Zn occurrences are known in Triassic sediments of the Eastern and Southern Alps as well as along the Periadriatic Lineament (e.g. Topla, Bleiberg, Mežica) but the origin of these deposits is still somewhat controversial. They have been explained as products of either syngenetic (Štrucl, 1984) or diagenetic/epigenetic processes (Grafenauer, 1958). The Mežica Pb-Zn ore deposit (19 Mt of ore at 5.3 wt % Pb and 2.7 wt % Zn) represents a classic example for this type of mineralization (Spangenberg & Herlec, 2006). The deposit is located in the Northern Karavanke geotectonic unit of the Eastern Alps in Slovenia. The mineralization is hosted by Triassic platform carbonates of the Ladinian to early Carnian stage. The deposit was formed in Late Triassic to Early Jurassic by thermally driven fluid flow during the crustal extension (Kuhlemann et al., 2001). Galena and sphalerite are the main ore minerals while calcite and dolomite are the most common gangue minerals. A mixture od anglesite, smithsonite, cerussite, wulfenite and calcite represents a supergene alteration assemblage. FLUID INCLUSIONS STUDY Fluid inclusions were studied in sphalerite, wulfenite and syn-ore, post-ore and supergene carbonates. All measured inclusions were two- phase (liquid + vapor). The recorded eutectic temperatures around -52°C suggest that mineralizing fluids were NaCl-CaCl2 aqueous solutions. Fluid inclusions hosted by sphalerite and syn- ore carbonates have overlapping salinities in the range between 16.4 and 23.1 wt% NaCl equ. Homogenization into the liquid phase was recorded in temperature intervals between 101° and 181°C for syn-ore carbonates and between 102° and 150°C for sphalerite. Post-ore carbonates occur in the form of veins that cross-cut syn-ore carbonates and the sulphide mineralization. They host fluid inclusions of variable salinities (11.1 - 22.4 wt% NaCl equ.) and homogenization temperatures (Th) between 108° - 205°C. Obtained fluid inclusion data (Figure 1) suggest that the main mechanism for formation of the carbonate-hosted Pb-Zn Mežica deposit is mixing of metal-bearing brines with deluted fluids of presuamably meteoric origin. The decreased salinity destabilized metal-chloride complexes and reduced the fluid capacity to transport base metals: PbCl2 (aq) + H2S (aq) ↔ PbS + 2H+ + 2Cl- ZnCl2 (aq) + H2S (aq) ↔ ZnS + 2H+ + 2Cl- According to previously published studies (Spangenberg & Herlec, 2006 ; Herlec et al., 2010) the main source of sulfur is bacterially reduced Middle to Upper Triassic seawater sulfate. Post-ore carbonates formed under an influence of CO2 originating from oxidation of organic matter (Spangenberg & Herlec, 2006) indicating that organic matter from host dolomite might have contributed to reductive conditions in the system. The supergene alteration processes controlled by infiltration of oxygen-rich meteoric water mobilized Mo from organic-rich country rocks. Descending oxidizing fluids rich in MoO42- partly replaced galena with wulfenite: PbS + MO42- + 4H2O ↔ PbMoO4 + SO42- + 8H+ Fluid inclusions data obtained from wulfenite and supergene carbonates revealed that the descending fluids were deluted hot water (~0 wt.% NaCl equ. ; Th = 61-250°C).

Pb-Zn ore deposit, Mežica, sphalerite, carbonate, fluid inclusions

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