Paleomagnetism of the Mississippi Valley-type ores and host rocks in the northern Arkansas and Tri-State districts

Abstract

A paleomagnetic study has been done on 452 specimens from the Early Ordovician Cotter and Middle Ordovician Everton formations in the northern Arkansas Mississippi Valley-type (MVT) lead–zinc ore district and from the Mississippian Warsaw and Keokuk formations in the Tri-State MVT district. Progressive alternating-field and thermal demagnetization first removed a steep normal modern viscous remanence component, leaving an underlying stable remanence of very low intensity (0.3 × 10−5 – 3 × 10−5 A/m). In the Tri-State area, further demagnetization of unaltered host rocks at seven sites in the Warsaw Formation yields a B component with a mean direction of declination (D) 146°, inclination (I) 14 °(A95 = 13°), corresponding to a paleopole of 130°E, 31°N (Dp = 7°, Dm = 14°), which is a primary Mississippian remanence. In northern Arkansas further demagnetization reveals an A component at 9 sites with a mean direction of D 156°, I 30° (A95 = 7°) and a C component at 14 sites of D 160°, I 1 °(A95 = 9°), corresponding to pole positions of 116°E, 33°N (Dp = 4°, Dm = 8°) and 119°E, 49°N (Dp = 4°, Dm = 9°), respectively. Almost all of the A-component sites are from unmineralized dolomite, whereas all of the C-component sites are from mineralized locations. This correlation suggests that the A magnetization is related to Late Devonian secondary dolomitization and the C magnetization to Permian mineralization produced by brines migrating from the Arkom basin upon deformation and uplift during the Ouachita orogeny.