LARAMIDE OROGENIC FLUID-FLOW INTO THE WESTERN CANADA SEDIMENTARY BASIN - EVIDENCE FROM PALEOMAGNETIC DATING OF THE KICKING-HORSE MISSISSIPPI VALLEY-TYPE ORE DEPOSIT

The Kicking Horse and Monarch Mississippi Valley-type zinc-lead ore de posits 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 Middl e Cambrian dolostones of the Cathedral Formation on the southwestern m argin of the western Canada sedimentary basin. These strata were folde d, thrust faulted, and uplifted during the Laramide orogeny. Paleomagn etic analysis was done on 316 specimens from 32 mineralized and host-r ock sites, using alternating held and thermal step demagnetization ana isothermal remanence methods. The results show that the characteristi c remanent magnetization is carried by both pyrrhotite and magnetite. The increase in characteristic remanent magnetization intensity near M ississippi Valley-type mineralization ties the magnetization to the mi neralization event. Fold tests on a small scale (similar to 10 m) and a large scale (similar to 10 km) show that the characteristic remanent magnetization postdates the main Laramide deformation event. Comparis on 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 Hors e mineralization and adjacent host rocks form the bulk of the collecti on and give a Late Cretaceous pole position of 64.0 degrees N, 152.8 d egrees 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 c onforms to the finding of all normal characteristic remanent magnetiza tion 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 Mississ ippi Valley-type mineralizing and dolomitizing event in the western Ca nada sedimentary basin is reviewed, and it is concluded that the main fluid flow event in the basin occurred during the Late Cretaceous-Pale ocene Laramide orogeny.