The Mississippi Valley-type (MVT) Pb-Zn ore district at Mezica is hosted
by Middle to Upper Triassic platform carbonate rocks in the Northern
Karavanke/Drau Range geotectonic units of the Eastern Alps, northeastern
Slovenia. The mineralization at Mezica covers an area of 64 km(2) with
more than 350 orebodies and numerous galena and sphalerite occurrences,
which formed epigenetically, both conformable and discordant to bedding.
While knowledge on the style of mineralization has grown considerably,
the origin of discordant mineralization is still debated. Sulfur stable
isotope analyses of 149 sulfide samples from the different types of
orebodies provide new insights on the genesis of these mineralizations
and their relationship. Over the whole mining district, sphalerite and
galena have delta(34)S values in the range of -24.7 to -1.5% VCDT
(-13.5 +/- 5.0%) and -24.7 to -1.4% (-10.7 +/- 5.9%), respectively.
These values are in the range of the main MVT deposits of the Drau
Range. All sulfide delta(34)S values are negative within a broad range,
with delta(34)S(pyrite) < delta(34)S(sphalerite) < delta(34)S(galena)
for both conformable and discordant orebodies, indicating isotopically
heterogeneous H(2)S in the ore-forming fluids and precipitation of the
sulfides at thermodynamic disequilibrium. This clearly supports that the
main sulfide sulfur originates from bacterially mediated reduction (BSR)
of Middle to Upper Triassic seawater sulfate or evaporite sulfate.
Thermochemical sulfate reduction (TSR) by organic compounds contributed
a minor amount of (34)S-enriched H(2)S to the ore fluid. The variations
of delta(34)S values of galena and coarse-grained sphalerite at orefield
scale are generally larger than the differences observed in single hand
specimens. The progressively more negative delta(34)S values with time
along the different sphalerite generations are consistent with mixing of
different H(2)S sources, with a decreasing contribution of H(2)S from
regional TSR, and an increase from a local H(2)S reservoir produced by
BSR (i.e., sedimentary biogenic pyrite, organo-sulfur compounds). Galena
in discordant ore (-11.9 to -1.7%; -7.0 +/- 2.7%, n=12) tends to be
depleted in (34)S compared with conformable ore (-24.7 to -2.8%, -11.7
+/- 6.2%, n=39). A similar trend is observed from fine-crystalline
sphalerite I to coarse open-space filling sphalerite II. Some variation
of the sulfide delta(34)S values is attributed to the inherent
variability of bacterial sulfate reduction, including metabolic
recycling in a locally partially closed system and contribution of H(2)S
from hydrolysis of biogenic pyrite and thermal cracking of organo-sulfur
compounds. The results suggest that the conformable orebodies originated
by mixing of hydrothermal saline metal-rich fluid with H(2)S-rich pore
waters during late burial diagenesis, while the discordant orebodies
formed by mobilization of the earlier conformable mineralization.