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 (23-26 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
(Hd(54-100)Jo(0-45)Di(0-45)) resulted from the interaction of magmatic
fluids associated with Oligocene (23-26 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 (<10(-31) bar) and
anhydrous environment. Fluid inclusion data and mineral assemblage limit
the prograde stage within a temperature range between 390 degrees and
475 degrees C. Formation pressure is estimated below 900 bars. Isotopic
composition of aqueous fluid, inclusions hosted by hedenbergite (delta D
= -108 to -130 parts per thousand; delta O-18 = 7.5-8.0 parts per
thousand), 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 mineralization, 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 degrees and
380 degrees C and sulfur fugacity between 10(-8.8) and 10(-7.2) bars.
The principal ore minerals, galena, sphalerite, pyrite, and minor
chalcopyrite, were deposited from a moderately saline Ca-Na chloride
fluid at around 350 degrees C. According to the isotopic composition of
fluid inclusions hosted by sphalerite (delta D = -55 to -74 parts per
thousand; delta O-18 = -9.6 to -13.6 parts per thousand), the fluid
responsible for ore deposition was dominantly meteoric in origin. The
delta S-31 values of the sulfides spanning between -5.5 and +10 parts
per thousand 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.