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Hydrocarbon biomarkers in the Topla-Mezica zinc-lead deposits, northern Karavanke/Drau Range, Slovenia; paleoenvironment at the site of ore formation

Jorge E. Spangenberg and Ursos Herlec
Hydrocarbon biomarkers in the Topla-Mezica zinc-lead deposits, northern Karavanke/Drau Range, Slovenia; paleoenvironment at the site of ore formation
Economic Geology and the Bulletin of the Society of Economic Geologists (August 2006) 101 (5): 997-1021

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 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 (super 13) C (sub kerogen) = -29.4 to -25.0 per mil, delta (super 15) N (sub kerogen) = -13.6 to 6.8 per mil) 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 (super 13) C compared to the associated kerogen, which is consistent with an indigenous source of the bitumens. The mineralization correlates with delta (super 15) N (sub kerogen) values around 0 per mil, 13C depleted kerogen, 13C 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 H (sub 2) S in the pore water of the sediments. This biogenic H (sub 2) S 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.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 101
Serial Issue: 5
Title: Hydrocarbon biomarkers in the Topla-Mezica zinc-lead deposits, northern Karavanke/Drau Range, Slovenia; paleoenvironment at the site of ore formation
Affiliation: Universite de Lausanne, Institut de Mineralogie et Geochimie, Lausanne, Switzerland
Pages: 997-1021
Published: 200608
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 114
Accession Number: 2007-010820
Categories: Economic geology, geology of ore deposits
Document Type: Serial
Bibliographic Level: Analytic
Annotation: Includes two appendices
Illustration Description: illus. incl. 4 tables, sect., strat. col., sketch map
N46°30'00" - N46°30'00", E14°46'00" - E14°46'00"
Secondary Affiliation: University of Ljubljana, Department of Geology, SVN, Slovenia
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200707
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