Original scientific paper

https://doi.org/10.4154/GC.2016.10

Origin and K-Ar age of the phreatomagmatic breccia at the Trepča Pb-Zn-Ag skarn deposit, Kosovo: Implications for ore-forming processes

Sabina Strmić Palinkaš ; University of TromsøFaculty of Science and TechnologyDepartment of GeologyDramsvegen 201, N-9037 TromsøNorway
Ladislav A. Palinkaš ; University of Zagreb, Faculty of Science, Geological Department
Magda Mandić ; Thermo Fisher Scientific
Zvjezdana Roller-Lutz ; University of Rijeka, Medical Faculty, Department of Physics
Zoltan Pécskay ; Hungarian Academy of Sciences, Institute of Nuclear Research
Gani Maliqi ; Prishtina University, Faculty of Mines and Metallurgy
Vladimir Bermanec ; University of Zagreb, Faculty of Science, Geological Department

Abstract

The Trepča Pb-Zn-Ag skarn deposit in Kosovo is spatially and temporarily related to the phreatomagmatic breccia of Oligocene age (~23 Ma). The deposit shows features typical for skarn deposits worldwide, including a stage of isochemical metamorphism, a prograde stage of an anhydrous, low oxygen and low sulfur fugacity character and a retrograde stage characterized by an increase in the water activity as well as by an increase in oxygen and sulfur fugacities. The mineralization is hosted by the recrystallized Upper Triassic limestone. The prograde mineralization consists mainly of Ca-Fe-Mn±Mg pyroxenes. The host recrystallized limestone at the contact with the prograde (skarn) mineralization have the increased content of Fe, Mn, Mo, As, Au, Cs, Ga, REE and Y suggesting their transport by infiltrating magmatic fluids. The decreased  d13C and  d18O values reflect a contribution of magmatic CO₂. The retrograde mineral assemblage comprises ilvaite, magnetite, arsenopyrite, pyrrhotite, marcasite, pyrite, Ca-Fe-Mn±Mg carbonates and quartz. Hydrothermal ore minerals, mostly galena, sphalerite and pyrite, were deposited contemporaneously with the retrograde stage of the skarn development. Syn-ore and post-ore carbonates reflect a diminishing influence of magmatic CO₂. Syn-ore carbonates are enriched in Fe, Mg, Mn, many chalcophile elements, including Ag, As, Bi, Cd, Cu, Pb, Sb and Zn, as well as in Au, Y and REE. The post-ore stage accompanied the precipitation of significant amount of Ca-rich carbonates including the travertine deposits at the deposit surface.

The phreatomagmatic breccia was developed along a NW dipping contact between the ore bearing recrystallized limestone and the overlying schist. It has an inverted cone shape with the vertical extension up to 800 m and diameter up to 150-m. The upper part of the diatreme (an underground segment of the phreatomagmatic breccia) is characterized by the presence of a hydrothermally altered rootless quartz-latite dike surrounded by an unsorted polymict breccia mantle. Despite the alteration processes, the dike has a preserved porphyritic texture. Partly preserved sanidine, accompanied with the mixture of muscovite and quartz, reflects a near-neutral to weakly acidic environment. The clasts of country rocks and skarn mineralization underwent intense milling and mixing due to repeated magmatic penetrations. Sericitization of the breccia matrix, locally accompanied with minor kaolinitization, point to an increased water activity under near-neutral to weakly acidic conditions. Large fragments originally composed of anhydrous skarn minerals (pyroxenes) are usually completely altered to a mixture of fibroradial magnetite, quartz and various amount of carbonates suggesting an increase in oxygen fugacity. Their pyrite rims reflect that the increase in oxygen fugacity was followed by an increase in sulfur fugacity. The clast predominantly composed of Fe-sulfides and minor Bi-sulfides point that the increase in sulfur fugacity was locally sufficient to complete sulfidation of hedenbergite to pyrrhotite and/or pyrite.

Although the phreatomagmatic breccia at the Trepča Pb-Zn-Ag skarn deposit does not carry significant amounts of the ore mineralization, its formation was crucial for the ore deposition. Phreatomagmatic explosions and formation of the breccia turned the system from the lithostatic to hydrostatic regime and triggered the retrograde stage increasing the water activity and oxygen fugacity in the system. In addition, cooling and decompression of the system contributed to more effective degassing of magmatic sulfur increased the sulfur fugacity. 

Keywords

phreatomagmatic breccia; Pb-Zn-Ag skarn deposit; transition from lithostatic to hydrostatic regime; fluid inclusions; stable isotopes; K/Ar dating

Hrčak ID:

155682

URI

https://hrcak.srce.hr/155682

Publication date:

29.2.2016.

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