Paleomagnetism of the Mississippi Valley‐type Newfoundland zinc deposit: Evidence for Devonian mineralization and host rock remagnetization in the northern Appalachians

Abstract

Newfoundland Zinc Mines (NZM) are exploiting sphalerite‐rich Mississippi Valley‐type (MVT) zones in platformal carbonate rocks of the Humber Zone in western Newfoundland. The zones are stratabound within the upper part of the Lower Ordovician St George group. Geologic studies in the mine area simply require ore genesis to postdate the Early to Middle Ordovician Taconic orogeny. Thermal and alternating field (AF) demagnetization reveal that both the ore and adjacent host rocks at the NZM have similar characteristic remanence magnetizations (ChRM) with normal polarity. The ChRM direction from both ore and host rocks is at 9°/30° (declination/inclination) (N = 16 sites, α95 = 6°, k = 46) which gives a pole position of 22°N, 113°E (dp = 3°, dm = 6°). This pole falls on the boundary between the Middle and Late Devonian on the North American apparent polar wander path and corresponds in time to the Acadian orogeny. Paleomagnetic breccia tests show that the host rocks were remagnetized after the Middle Ordovician and that this remagnetization coincides with or postdates MVT mineralization. The ChRMs paleoinclination indicates that both remagnetization and mineralization are not younger than Late Devonian. The similar magnetization characteristics of the adjacent and remote host rocks in the Humber Zone and the authigenic nature of the magnetic carriers revealed by scanning electron microscopy with energy dispersive analysis suggest that the NZM host rock has a secondary magnetization. From the pervasive nature of both the secondary remagnetization and the trace MVT mineralization in the northern Humber Zone, we argue that both events are coeval. We suggest that Acadian orogenic uplift forced hot basinal brines into the porous and fractured Ordovician shelf carbonates of the northern Humber Zone.