The Mississippi Valley-type zinc and lead deposits at Topla (250,150
metric tons (t) of ore grading 1.0 wt % Zn and 3.3 wt % Pb) and Mezica
(19 million metric tons (Mt) of ore grading 5.3 wt % Pb and 2.7 wt %
Zn) occur within the Middle to Upper Triassic platform carbonate rocks
of the northern Karavanke/Drau Range geotectonic units of the Eastern
Alps, Slovenia. The ore and host rocks of these deposits have been
investigated by a combination of inorganic and organic geochemical
methods to determine major, trace, and rare earth element (REE)
concentrations, hydrocarbon distribution, and stable isotope ratios of
carbonates, kerogen, extractable organic matter, and individual
hydrocarbons. These data combined with sedimentological evidence provide
insight into the paleoenvironmental conditions at the site of ore
formation. The carbonate isotope composition, the REE patterns, and the
distribution of hydrocarbon biomarkers (normal alkanes and steranes)
suggest a marine depositional environment. At Topla, a relatively high
concentration of redox sensitive trace elements (V, Mo, U) in the host
dolostones and REE patterns parallel to that of the North American shale
composite suggest that sediments were deposited in a reducing
environment. Anoxic conditions enhanced the preservation of organic
matter and resulted in relatively higher total organic carbon contents
(up to 0.4 wt %). The isotopic composition of the kerogen (delta
C-13(kerogon) = -29.4 to -25.0 parts per thousand, delta N-15(kerogen) =
-.13.6 to 6.8 parts per thousand) suggests that marine algae and/or
bacteria were the main source of organic carbon with a very minor
contribution from detrital continental plants and a varying degree of
alteration. Extractable organic matter from Topla ore is generally
depleted in C-13 compared to the associated kerogen, which is consistent
with an indigenous source of the bitumens. The mineralization correlates
with delta N-15(kerogen) values around 0 per mil, C-13 depleted kerogen,
C-13 enriched n-heptadecane, and relatively high concentrations of
bacteria] hydrocarbon biomarkers, indicating a high cyanobacterial
biomass at the site of ore formation. Abundant dissimilatory
sulfate-reducing bacteria, feeding on the cyanobacterial remains, led to
accumulation of biogenic H2S in the pore water of the sediments. This
biogenic H2S was mainly incorporated into sedimentary organic matter and
diagenetic pyrite. Higher bacterial activity at the ore site also is
indicated by specific concentration ratios of hydrocarbons, which are
roughly correlated with total Pb plus Zn contents. This correlation is
consistent with mixing of hydrothermal metal-rich, fluids and local
bacteriogenic sulfide sulfur. The new geochemical data provide
supporting evidence that Topla is a low-temperature Mississippi
Valley-type deposit formed in an anoxic supratidal saline to hypersaline
environment. A laminated cyanobacterial mat, with abundant
sulfate-reducing bacteria was the main site of sulfate reduction.