Abstract

Ore and host rocks of the Middle Triassic Muschelkalk carbonates were sampled at 35 sites in three operating mines and several quarries in the Cracow-Silesia Mississippi Valley-type district of southern Poland. Palcomagnetic analysis was done using alternating field and thermal step demagnetization and saturation isothermal remanence tests. Specimens from three limestone and two early dolomite sites retain a dual-polarity A remanent magnetization component (D = 43 degrees , I = 51 degrees , k = 40, alpha 95 = 12 degrees ), which yields a Middle to Upper Triassic pole. A resides in single domain to pseudosingle domain magnetite in limestone and is probably primary, and in single domain-pseudosingle domain magnetite and hematite in early dolomite and is probably diagenetic. Two limestone sites carry a Tertiary remagnetization and most early dolomite sites carry a partial Tertiary remagnetization, or hybrid remanence, identified by palcomagnetic fold and breccia tests. Late dolomite and Mississippi Valley-type mineralization from 14 sites retain a dual-polarity C remanence component (D = 3 degrees , I = 66 degrees , k = 34, alpha 95 = 7 degrees ), which defines a Tertiary pole position. Conglomerate and breccia tests confirm that the clasts in late dolomite were remagnetized. The Tertiary age for the Mississippi Valley-type ore deposits is within the post-mid-Jurassic to pre-Miocene window permitted by the geologic evidence for mineralization. The age also supports gravity-driven fluid flow models for ore genesis that are associated with the Alpine orogeny. The dual polarities and elongate distribution of site mean directions suggest a 1 to 20 m.y. duration for the late dolomitization and mineralization event.

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