A TERTIARY AGE FROM PALEOMAGNETISM FOR MISSISSIPPI VALLEY-TYPE ZINC-LEAD MINERALIZATION IN UPPER SILESIA, POLAND

Ore and host rocks of the Middle Triassic Muschelkalk carbonates were sampled at 35 sites in three operating mines and several quarries in t he Cracow Silesia Mississippi Valley-type district of southern Poland. Paleomagnetic analysis was done using alternating field and thermal s tep demagnetization and saturation isothermal remanence tests. Specime ns from three limestone and two early dolomite sites retain a dual-pol arity A remanent magnetization component (D = 43 degrees I = 51 degree s, k = 40, alpha(95) = 12 degrees) which yields a Middle to Upper Tria ssic pole. A resides in single domain to pseudosingle domain magnetite in limestone and is probably primary, and. in single domain-pseudosin gle domain magnetite and hematite in early dolomite and is probably di agenetic. Two limestone sites carry a Tertiary remagnetization and mos t early dolomite sites carry a partial Tertiary remagnetization, or hy brid remanence, identified by paleomagnetic fold and breccia tests. La te dolomite and Mississippi Valley-type mineralization from 14 sites r etain a dual-polarity C remanence component (D = 3 degrees, I = 66 deg rees, k = 34, alpha(95) = 7 degrees), which defines a Tertiary pole po sition. Conglomerate and breccia tests confirm that the clasts in late dolomite were remagnetized. The Tertiary age for the Mississippi Vall ey-type ore deposits is within the post-mid-Jurassic to pre-Miocene wi ndow permitted by the geologic evidence for mineralization. The age al so supports gravity-driven fluid Bow models for ore genesis that are a ssociated with the Alpine orogeny. The dual polarities and elongate di stribution of site mean directions suggest a 1 to 20 m.y. duration for the late dolomitization and mineralization event.