Paleomagnetic dating of Upper Mississippi Valley zinc–lead mineralisation, WI, USA

• Published in: Journal of applied geophysics 2004-06, Vol.56 (2), p.135-153
• Main Authors: Pannalal, S.J, Symons, D.T.A, Sangster, D.F
• Format: Article
• Language: English
• Subjects: Age dating; Alleghenian–Ouachitan orogeny; Applied geophysics; Earth sciences; Earth, ocean, space; Exact sciences and technology; Internal geophysics; Paleomagnetism; Upper Mississippi Valley Zn–Pb district; Wisconsin
• ISSN: 0926-9851; 1879-1859
• DOI: 10.1016/j.jappgeo.2004.04.006

The Upper Mississippi Valley (UMV) zinc–lead district of Wisconsin is located on the northern fringe of the Illinois basin. It was the first major Mississippi Valley-type (MVT) Zn–Pb district to be mined in the United States. Its mineralisation is hosted by Ordovician limestones and dolostones, and has given a 87Rb– 86Sr sphalerite age of 270±4 Ma. One of the few MVT deposits worldwide to be directly dated by this method, it merits comparison with the paleomagnetic dating method for such deposits. Specimens (359) from oriented cores from host rock outcrops and mineralised blocks from mine waste piles at 33 sites have been analysed paleomagnetically using alternating field and thermal step demagnetisation, and some specimens were tested using saturation isothermal remanence methods. The specimens' characteristic remanent magnetisations (ChRM) are carried mostly by pyrrhotite with minor magnetite with a mean direction for the: (a) host limestones at Dec.=151.6°, Inc.=−7.3°( N=5 sites; α 95=6.3°, k=149.8) and (b) dolostones at Dec.=154.8°, Inc.=−7.6°( N=16 sites; α 95=3.5°, k=112.0). An increased ChRM intensity for the mineralised specimens ties their magnetisation to the mineralisation event. The limestones' paleopole plots off the Early Permian and towards the Ordovician portion of the apparent polar wander path for North America, indicating a secondary chemical remagnetisation, possibly with a minor residual primary Ordovician remanence. The dolostones' paleopole at 45.1°N, 126.7°E (δp=1.8°, δm=3.5°) records a secondary chemical remagnetisation from regional dolomitisation during the Early Permian (282±10 Ma). Comparison of the ChRM inclinations for the dolostones and mineralised blocks indicates that the host rocks were remagnetised during mineralisation; that is, the dolomitisation and mineralisation events were about coeval. Thus, the paleomagnetic age confirms the radiometric age for UMV mineralisation. Furthermore, the Early Permian paleomagnetic age agrees with ore genesis models that relate dolomitisation and mineralisation to topographically driven fluid flow during Alleghenian/Ouachitan orogenic uplift.