Mississippi Valley-type Zinc Research Articles

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

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 C kerogen = −29.4 to −25.0‰, δ 15 N kerogen = −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 N 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 2 S in the pore water of the sediments. This biogenic H 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.

Palaeomagnetic dating of dolomitization and Mississippi Valley-type zinc mineralization in the Mascot-Jefferson City district of eastern Tennessee: a preliminary analysis

Palaeomagnetic analysis of 576 specimens (∼400 in detail) from 37 sites in four mines of the Mascot-Jefferson City Mississippi Valley-type zinc district indicates that the sphalerite mineralization in the Lower Ordovician carbonate host rocks was emplaced during the early Pennsylvanian at 314±6Ma during the Alleghenian Orogeny, and that peak dolomitization or peak recrystallization of the host rocks occurred about 20My earlier.

Enzyme leach anomalies associated with deep Mississippi Valley-type zinc ore bodies at the Elmwood Mine, Tennessee

Deeply buried Mississippi Valley-type deposits that have been or are currently being mined in North America were initially discovered by drilling. Conventional geochemical methods are ineffective for detecting these `blind' deposits when they occur deep within sequences of stable-platform carbonates and shales. The `enzyme' leach is a selective analytical technique for determining trace elements associated with amorphous Mn oxide coatings in soils. In many areas of the world, the enzyme leach method is useful for detecting low-level geochemical anomalies in soils, which are associated with blind mineral deposits. Enzyme leach analysis of soils, collected at the Elmwood Mine, Tennessee, revealed high-contrast anomalies over ore bodies 370 m below the surface. In areas where the soils are in chemical equilibrium, `combination' anomalies occur over Zn ore bodies. These are characterized by asymmetrical halogen halos which occur around a halogen `central low'. Commodity metals (Zn and Pb) and trace elements associated with the ore (Cd, Ba, and Mn) form apical anomalies, which occur over the ore bodies and within the halogen halo. Under most circumstances, agricultural practices do not affect enzyme leach results. However, agricultural activity in central Tennessee appears to have altered the proportion of amorphous Mn oxides in the soils in some locations. Where the MnO 2-form equilibrium of the soil has been disturbed, enzyme leach data are erratic. In the one instance where this was encountered, ratioing the data to Mn reveals anomalies which bracket the blind ore bodies.

Sphalerite-bearing detrital ‘sand’ bodies in Mississippi Valley-type zinc deposits Mascot-Jefferson City district, Tennessee

The Mississippi Valley-type sphalerite mineralization in the Mascot-Jefferson City zinc district of East Tennessee occurs as open-space fillings in breccia bodies within the upper part of the Knox Group (Lower Ordovician) which is truncated by a regional unconformity. A lower age limit of mineralization is constrained by the formation of solution-collapse breccia bodies, which are believed to be related to the post-Knox unconformity. The breccias contain irregularly distributed “sand” bodies that represent cavities filled with well-laminated and size-graded, sphalerite-bearing, detrital, internal sediments. The texture, composition, and fluid inclusion characteristics of the sphalerite, are consistent with its local derivation from the wallrocks as detrital grains. The conformability between the laminations in the sediments and the bedding planes of the host carbonate rocks suggests that the sand bodies formed prior to the regional deformation event (Alleghenian orogeny). The stylolitization of carbonate and sphalerite clasts in the internal sediments as well as the deformation of the sphalerite are also consistent with a pre-Alleghenian age for the emplacement of the main-stage sphalerite mineralization in the Mascot-Jefferson City district and, by analogy, in other Lower Ordovician-hosted Mississippi Valley-type districts of the southern Appalachians.