Abstract

The Pine Point Zn–Pb deposits are Mississippi Valley-type (MVT) deposits in Middle Devonian dolostone of the Pine Point Formation. The median natural remanent magnetization intensity from 509 specimens from 40 sites is 3.6 × 10−5 A/m. This intensity is an order of magnitude lower than most MVT deposits and is close to the sensitivity of the cryogenic magnetometer, so only the stronger specimens could be analysed paleomagnetically. Isothermal remanent magnetization testing of ores, host rocks, and monomineralic specimens shows that single domain to pseudosingle domain magnetite in sphalerite is the main remanence carrier; magnetite in galena is the next most significant carrier. In contrast, barren dolostone, limestone, and calcite crystals have a much weaker remanence. Using less stringent criteria than is conventional, 17 acceptable site mean remanence directions were determined. Remagnetization circle data indicate that many of the poorly defined sites retain a reversed but nonisolated characteristic remanence. Of the 17 means, 8 form a coherent cluster, the A component, with a mean direction of D = 339°, I = 78° (α95 = 7°, k = 62), giving a pole position of 79°N, 197°E (α95 = 13°). The A pole indicates with > 95%confidence that the MVT ores at Pine Point are Late Cretaceous to Eocene in age, with a mid-Late Cretaceous to Paleocene age (84–58 Ma) being most likely. The dolostone host rocks were remagnetized at the same time. It is concluded that the A component, carried mainly by sphalerite and galena, was formed during the Laramide orogeny (97–58 Ma) when tectonic uplift forced Zn–Pb-rich basinal brines out of the Western Canada sedimentary basin into the Presqu'ile barrier reef on the adjacent foreland shelf.

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