Abstract

Paleomagnetic data are reported for Mississippi Valley-type zinc-lead mineralization and associated sparry to microdolomite from the Robb Lake showing in the Rocky Mountains. The data define a stable characteristic remanent magnetization (ChRM) direction of Dec = 351 degrees , Inc = 65 degrees , alpha 95 = 2.5 degrees , k = 290, and N = 12 (n = 125). It resides mostly in pyrrhotite with minor magnetite and postdates Laramide orogenic folding. Also the intensity of the remanence increases with increasing sphalerite content showing that the pyrrhotite-borne magnetization and mineralization are genetically related. Thus it is a chemical remanent magnetization that is primary in the mineralization and sparry dolomite and a secondary remagnetization in the host dolostones. Following a tilt correction of 9 degrees for late thrust fault movement, the ChRM direction gives a pole that indicates that the dolomitization and mineralization is Eocene in age, coinciding with the waning stages of the Laramide Orogeny.

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