Conodont color and surface textural alteration in the Muschelkalk (Triassic) of the Silesian-Cracow Zn-Pb district, Poland

Abstract

Limestone and dolostone samples were collected from sites within and adjacent to ore zones in the Trzebionka mine, Silesia-Cracow zinc-lead mining district, southern Poland, to assess the level of thermal alteration of the enclosed conodonts, via the color alteration index (CAI) technique, and to study any surface alteration effects on these microfossils. Additional conodont sampling from stratigraphic levels correlative with the horizons being mined in the Trzebionka mine was accomplished at four surface and two borehole localities in the district, to compare the CAI and surface alteration effects at these, commercially non-mineralized, localities with those effects in the mine. Data show that: 1. The overall background thermal level of the Triassic strata studied, presumably due to only shallow burial, is very low: CAI = 1; in the range of 50 to 80??C. 2. CAI values in the ore zones in the Trzebionka mine are slightly higher than this regional background: 1+ to 1.5 (in the range ???50 to 90??C minimum heating over geologic time of about 0.1 to more than 1 m. y.). This implies that heating "events" of higher temperatures affecting the conodonts, including the passage of the local ore-bearing solutions, were of rather short duration(s), on the order of about 1,000 to 50,000 years. CAI data from the Trzebionka Mine is consistent with temperature data from fluid inclusions, indicating ore-bearing fluid temperatures in the 100 to 138??C range, and the scenario that these fluids were resident in these strata for a geologically short period. 3. Conodonts from both surface and subsurface samples rarely show surface corrosion effects, but tend to show apatite overgrowths. These overgrowths vary in degree of development, but are consistent for each morphological type of conodont element, and qualitatively are consistent in each sample observed. 4. Ichthyoliths (fish teeth, spines, and scales), found in most of the samples, do not exhibit either mineral overgrowths or apparent corrosion effects to the extent seen in the conodont elements. 5. Ichthyoliths show color alteration effects that are consistent within-sample but which are very different from the CAI values of conodonts in the same sample. The color alteration of the fish teeth might be of value as a thermal maturation index in the future, if and when calibrated through controlled laboratory experimental testing, but at present cannot and should not be used as equivalent to conodont CAI.