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Lead-isotopic characteristics of the Cracow-Silesia Zn-Pb ores, southern Poland

1992; United States Department of the Interior; Linguagem: Inglês

10.3133/ofr92393

2332-4899

S.E. Church, R.B. Vaughn,

Geology and Environmental Impact Studies

New lead-isotope analyses of a suite of samples of ore collected from a paragenetic sequence in the Cracow-Silesia MVT district show no lead-isotopic variation in carbonate-hosted ores and are in agreement with lead-isotopic results from previous work by Zartman et al. (1979).Two well-studied MVT districts in the world show uniform lead-isotopic compositions, the Cracow-Silesia and the Pine Point districts.The lead-isotopic data require that the saline hydrothermal fluids which formed these deposits be prevented from circulation through crystalline basement rocks.This requirement is met in both the Cracow-Silesia and Pine Point districts because thick sequences of Devonian shale and carbonate rocks containing shale aquacludes separate the aquifers from the basement in both basins.We postulate two sources of lead which might account for such uniform lead-isotopic signatures.Hypothesis one would require that the ore lead is bound in iron-oxide phases in multicycle molasse sedimentary rocks.In this case, leaching of ore lead must have occurred shortly after deposition to prevent measurable contributions of radiogenic lead which would form because of radiogenic decay of uranium and thorium in the silicate components of the sediments, and which, upon leaching by the hydrothermal fluids, would result in a radiogenic lead-isotope array such as seen in most other MVT deposits.The time span between deposition and leaching would be limited by the mass balance between lead in the iron-oxide phases and the radiogenic lead in the silicate components.This hypothesis is supported by the lead-isotopic data from the Kozieglowy mine.Hypothesis two calls for leaching of lead from evaporite sequences in the Upper Silesia Basin.Concentrations of lead from the evaporite sequence in the Western Canada sedimentary basin average nearly 1 ppm in halite and 10 ppm in anhydrite (Thiede and Cameron, 1978).Dissolution of the evaporite sequences in the Upper Silesia Basin by meteoric recharge in the Pre-Carpathian Mountains would produce the necessary hydrodynamic fluid flow producing the highly saline brines currently present in the Upper Silesia Basin.Mixing of these saline brines with sulfur-rich waters would cause precipitation of the ores.Lead in the evaporites would be uniform in composition because it was homogenized in solution and precipitated along with the evaporites.Furthermore, lead would be decoupled from uranium and thorium so there would be no radiogenic growth of lead in the source rocks.Under these conditions, the 230 Ma model age of the lead in the Silesia-Cracow ores would reflect the age of the source rocks and provide no constraints on the time of formation of the ore deposits themselves, but does constrain the age of the source of lead.Further research is proposed to test these two hypotheses.