Metallogenic Model of the Trepča Pb-Zn-Ag Skarn Deposit, Kosovo: Evidence from Fluid Inclusions, Rare Earth Elements, and Stable Isotope Data

Abstract

The Trepca Pb-Zn-Ag skarn deposit (29 Mt of ore at 3.45% Pb, 2.30% Zn, and 80 g/t Ag) is located in the Kopaonik block of the western Vardar zone, Kosovo. The mineralization, hosted by recrystallized limestone of Upper Triassic age, was structurally and lithologically controlled. Ore deposition is spatially and temporally related with the postcollisional magmatism of Oligocene age (2326 Ma). The deposit was formed during two distinct mineralization stages: an early prograde closed-system and a later retrograde open-system stage. The prograde mineralization consisting mainly of pyroxenes (Hd54100Jo045Di045) resulted from the interac- tion of magmatic fluids associated with Oligocene (2326 Ma) postcollisional magmatism. Whereas there is no direct contact between magmatic rocks and the mineralization, the deposit is classified as a distal Pb-Zn-Ag skarn. Abundant pyroxene reflects low oxygen fugacity (<1031 bar) and anhydrous environment. Fluid inclu- sion data and mineral assemblage limit the prograde stage within a temperature range between 390° and 475°C. Formation pressure is estimated below 900 bars. Isotopic composition of aqueous fluid inclusions hosted by hedenbergite (δD = 108 to 130‰; δ18O = 7.58.0‰), Mn-enriched mineralogy and high REE content of the host carbonates at the contact with the skarn mineralization suggest that a magmatic fluid was modified during its infiltration through the country rocks. The retrograde mineral assemblage comprises ilvaite, magnetite, arsenopyrite, pyrrhotite, marcasite, pyrite, quartz, and various carbonates. Increases in oxygen and sulfur fugacities, as well as a hydrous character of min- eralization, require an open-system model. The opening of the system is related to phreatomagmatic explosion and formation of the breccia. Arsenopyrite geothermometer limits the retrograde stage within the temperature range between 350° and 380°C and sulfur fugacity between 108.8 and 107.2 bars. The principal ore minerals, galena, sphalerite, pyrite, and minor chalcopyrite, were deposited from a mod- erately saline Ca-Na chloride fluid at around 350°C. According to the isotopic composition of fluid inclusions hosted by sphalerite (δD = 55 to 74‰; δ18O = 9.6 to 13.6‰), the fluid responsible for ore deposition was dominantly meteoric in origin. The δ34S values of the sulfides spanning between 5.5 and +10‰ point to a magmatic origin of sulfur. Ore deposition appears to have been largely contemporaneous with the retrograde stage of the skarn development. Postore stage accompanied the precipitation of significant amount of carbonates including the travertine deposits at the deposit surface. Mineralogical composition of travertine varies from calcite to siderite and all carbonates contain significant amounts of Mn. Decreased formation temperature and depletion in the REE content point to an influence of pH-neutralized cold ground water and dying magmatic system.