Hydrocarbon Biomarkers in the Topla-Mezica Zinc-Lead Deposits, Northern Karavanke/Drau Range, Slovenia: Paleoenvironment at the Site of Ore Formation

Abstract

The Mississippi Valley-type zinc and lead deposits at Topla (250,150 metric tons (t) of ore grading 10 wt % Zn and 3.3 wt % Pb) and Mežica (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 (δ 13 Ckerogen = -29.4 to -25.0‰, δ 15 Nkerogen = -13.6 to 6.8‰) 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 13 C compared to the associated kerogen, which is consistent with an indigenous source of the bitumens. The mineralization correlates with δ 15 Nkerogen values around 0 per mil, 13 C depleted kerogen, 13 C enriched n-heptadecane, and relatively high concentrations of bacterial 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.

Sulfur isotope variations from orebody to hand-specimen scale at the Mežica lead–zinc deposit, Slovenia: a predominantly biogenic pattern

“…In this communication, we compile the published sulfur isotope composition for the Mežica MVT deposit and present the results of a new comprehensive sulfur isotope study of sulfides from orebody down to hand-specimen scale, aimed to contribute to a better understanding of the sulfur sources and the relationship between the orebodies of different morphology and stratigraphic position. Our results complement previously reported organic and inorganic geochemical patterns of the Topla-Mežica MVT deposits (Spangenberg and Herlec 2006) and support the conclusion that biogenic reduction of sulfate was the main sulfide sulfur source to the ore fluid.…”

“…The combined data provided supporting evidence that Topla and Mežica were formed from a typical lowtemperature MVT brine in two different ore-forming processes. The early diagenetic Topla deposit formed in the Anisian stage in an anoxic supratidal saline to hypersaline environment, where ore deposition was caused by the mixing of the incoming hydrothermal metal-rich fluid with bacterial H 2 S-rich pore waters (Spangenberg and Herlec 2006). The knowledge of the clearly epigenetic mineralization at Mežica-mainly based on studies of orebodies conformable to bedding-has grown considerably recently (Kuhlemann et al 2001;Spangenberg and Herlec 2006).…”

Control of Reactivated Proterozoic Basement Structures on Sandstone-Hosted Pb-Zn Deposits along the Caledonian Front, Sweden: Evidence from Airborne Magnetic Data, Structural Analysis, and Ore-Grade Modeling

“…In both deposits, the highest Pb and Zn grades occur at the top of sandstone paleoaquifers in a similar manner as the variation in grades and paleoaquifer settings recognized in several carbonate-hosted MVT Zn-Pb deposits (e.g., San Vicente deposit in Peru; Fontboté and Gorzawski, 1990; Topla-Mežica deposits in Slovenia; Spangenberg and Herlec, 2006). This geometry is suggestive of a sour gas trap that accumulated at the top of paleoaquifers due to density.…”

Berriasian and early Valanginian environmental change along a transect from the Jura Platform to the Vocontian Basin

“…Carbon and oxygen‐isotope analyses were performed on whole‐rock samples following the procedures described by Spangenberg & Macko (), Spangenberg et al . () and Spangenberg & Herlec (). Visible calcite veins were removed during sample preparation in order to limit the diagenetic signature.…”