Paleomagnetic and mineral magnetic studies of zinc-lead ore deposits in the Metaline (Washington state, USA) and Midlands (Ireland) ore fields

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Keywords: Ore deposits, Ireland, Ore fields, Zinc-lead ore, Metaline ore field, Midlands ore field . Carbonate-hosted Zn-Pb deposits of the Metaline (USA) and Irish Midlands (Ireland) ore fields exhibit features of both MVT and SEDER deposits, and therefore, play an important role in the debate over genetic models for MVT - SEDER deposits, including the structural controls on ore mineralization, syngenetic versus epigenetic models, and the origin and migration pathways for hydrothermal fluids. The genetic controversy arises largely because of the lack of direct dates on mineralization. Paleomagnetic analyses on samples of host rock and ore mineralization from 38 sites (400 specimens) in the Metaline Zn-Pb district, Washington (USA), using the known thermal history and the paleoarc method of paleomagnetic dating, indicate coeval postfolding magnetizations acquired during the Middle Jurassic (166+-6 Ma), in the waning stages of the Nevadan Orogeny. The thermal (Th) and alternating field (AF) step demagnetization, saturation isothermal anaylses (SIRM), and artificial specimens' tests show that the characteristic remanent magnetizations (ChRM) is carried mostly by pseudosingle (PSD) to single domain (SD) pyrrhotite that records a primary chemical remanent magnetization (CRM) in ore and a secondary ChRM in host specimens. Furthermore, the paleomagnetic Middle Jurassic age suggests an epigenetic origin for ore formation of the Zn-Pb mineralization at the Pend Oreille Mine that coincides with the timing of cooling from the regional Nevadan orogenic heating episode. Paleomagnetic analyses of the least thermally affected (conodont alteration indices (CAI) of <3) Lower Carboniferous rocks at 18 sites (231 specimens) from Northern Ireland indicate posttilting ChRMs in magnetite and pyrrhotite that record a secondary CRM that was acquired ~3 to 4 Ma after limestone deposition. Also, paleomagnetic analyses of host rock and ore specimens in 46 sites (705 specimens) from the Galmoy and Lisheen Zn-Pb deposits from the Irish Midlands give stable postfolding ChRM, that reside in magnetite, were acquired during the Early Permian at 288+-8 Ma at Galmoy and 276+-6 Ma at Lisheen. These Early Permian paleomagnetic ages postdate the Asturian deformational phase of the Variscan orogeny and suggest an entirely epigenetic Variscan model of ore genesis in which the mineralization event occurred during the cooling from the regional Variscan thermal episode.