Laramide orogenic fluid flow into the Western Canada sedimentary basin; evidence from paleomagnetic dating of the Kicking Horse mississippi valley-type ore deposit

Abstract

The Kicking Horse and Monarch Mississippi Valley-type zinc-lead ore deposits are in the southern Rocky Mountains of the foreland belt in the Canadian Cordillera of British Columbia. These epigenetic sphalerite-galena deposits are hosted by a massive sparry dolomite sheet in Middle Cambrian dolostones of the Cathedral Formation on the southwestern margin of the western Canada sedimentary basin. These strata were folded, thrust faulted, and uplifted during the Laramide orogeny. Paleomagnetic analysis was done on 316 specimens from 32 mineralized and host-rock sites, using alternating field and thermal step demagnetization and isothermal remanence methods. The results show that the characteristic remanent magnetization is carried by both pyrrhotite and magnetite. The increase in characteristic remanent magnetization intensity near Mississippi Valley-type mineralization ties the magnetization to the mineralization event. Fold tests on a small scale ( approximately 10 m) and a large scale ( approximately 10 km) show that the characteristic remanent magnetization postdates the main Laramide deformation event. Comparison of the characteristic remanent magnetization directions shows that the host rocks were remagnetized when the mineralization was emplaced with a primary characteristic remanent magnetization. The Kicking Horse mineralization and adjacent host rocks form the bulk of the collection and give a Late Cretaceous pole position of 64.0 degrees N, 152.8 degrees W (semi-axes of oval of 95% confidence: delta p = 5.4 degrees , delta m = 5.8 degrees ; number of sites (specimens) = 19 (180)). The age of this pole coincides with the Cretaceous normal superchron and conforms to the finding of all normal characteristic remanent magnetization directions except for one sample with an antipodal direction. No site retains a characteristic remanent magnetization that predates the Laramide orogeny. The evidence for and against a pre-Laramide Mississippi Valley-type mineralizing and dolomitizing event in the western Canada sedimentary basin is reviewed, and it is concluded that the main fluid flow event in the basin occurred during the Late Cretaceous-Paleocene Laramide orogeny.