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

Paleomagnetic mapping of the regional fluid flow event that mineralized the Upper Mississippi Valley Zn–Pb ore district, Wisconsin, U.S.A.

“…The PEMF component resides primarily in goethite, identified by rapid thermal demagnetization by ∼130°C (Fig. 4c), and in hematite, both of which AF demagnetization fails to Heyl and West (1982) and Pannalal et al (2004). The hatching extending northwest from the UMV district along the Mississippi River valley depicts the region of Jackson and Van der Voo's (1985) paleomagnetic study of the Oneota Formation.…”

“…Next, one typical specimen per site was alternating field (AF) demagnetized in 13 steps up to 120 mT using a Sapphire Instruments SI-4 demagnetizer and a second specimen was thermally demagnetized in 14 steps to 500°C using a Magnetic Measurements MMTD-80 demagnetizer. Based on the results of these test specimens, most remaining specimens were thermally demagnetized in 8 steps from 225 to 380°C to isolate the characteristic remanent magnetization (ChRM) carried by pyrrhotite that occurs preferentially with the sulphide mineralization in the UVM district (Pannalal et al, 2004), and that is used as the "tracer" for the mineralizing fluids in this study. Additional thermal steps were added as warranted.…”

“…The mineralization has given Early Permian ages of 270 ± 4 Ma by the 87 Rb-86 Sr sphalerite method (Brannon et al, 1992) and of 282 ± 10 Ma by paleomagnetism (Pannalal et al, 2004). Since the carbonate host rocks give conodont Colour Alteration Indices (CAI) of 1.0 (Sangster et al, 1994), they have never been heated above ∼ 60°C (Symons and Cioppa, 2002, and references therein), which is consistent with known burial depths of ≤2500 m. However, the vein sphalerites have a median range of fluid inclusion homogenization temperatures of 115 ± 30°C (Rowan and Goldhaber, 1996), implying that hydrothermal fluids with elevated temperatures must have ascended from depth through fractures and the Prairie du Chien Group to deposit the Zn-Pb mineralization.…”

“…Heyl and West (1982) speculated that minor subeconomic occurrences of Zn-Pb mineralization in the surrounding ∼ 100,000 km² were part of the UMV mineralizing event also. Pannalal et al (2004) reported paleomagnetic results from the core area of the UMV ore district, based on results from 33 sites of which 10 were in mineralization. They found that the ChRM of the 23 Middle Ordovician host rock sites resided mainly in pyrrhotite with magnetite being a carrier in only a few specimens.…”

“…1), including hundreds of large and small mining operations, with trace Zn-Pb mineralization extending reputedly well beyond the boundary (Heyl and West, 1982). The UMV ore occurs mostly in Middle Ordovician carbonates that were remagnetized on ore emplacement in the Early Permian (Pannalal et al, 2004). Suggested sources for the mineralizing fluids have been: 1) upward convection from underlying Precambrian heatproducing granites (e.g.…”