| Popis: |
For the past quarter century the Navan mine has exploited Europe’s premier Zn-Pb deposit. Its genesis is controversial, in part because it has not proved amenable to radiometric dating. The ore is epigenetic and is hosted in Lower Carboniferous platform carbonates of the early Courceyan Navan Group (~351 ± 4 Ma) and of the late Chadian-early Arundian boulder conglomerate (~338 ± 1 Ma). Paleomagnetic analyses were done on 282 specimens from 26 sites in the mine using alternating-field and thermal step demagnetization plus isothermal remanence analyses. Paleomagnetic fold tests on Navan Group ore and Arundian carbonate strata and conglomerate tests on mineralized and unmineralized clasts were negative, meaning that the clasts and adjacent rocks were magnetized at some time after clast deposition and that the magnetization postdates folding. Further, a contact test using a Tertiary dike that cut ore was positive, meaning that the remanence predates intrusion of the dike. Both the ore and host rocks retain a stable characteristic remanence direction in single- to pseudosingle-domain magnetite at D = 196.0°, I = 6.0°, (N = 21, α 95 = 2.6°, k = 147) that defines a pole position of 25.6° W, 31.8° S ( δ p = 1.3°, δ m = 2.6°). Comparison of the pole to the rotated Laurentian and unrotated eastern Avalonian apparent polar wander paths yields a late Arundian to early Asbian age of 333 ± 4 Ma for the chemical remanent magnetization that is primary in the ore minerals and a secondary remagnetization in the host rocks. The result implies that the ore mineralization event followed deposition and tilting of the overlying Arundian-Holkerian Lucan Formation at Navan. |
