Paleomagnetism and the Origin of Mississippi Valley-Type Ore Deposits

Abstract

Paleomagnetism can provide useful information about the stratigraphic relationships between the host rocks and the ore of some ore deposits.Four North American mines with stratabound ore deposits of Mississippi Valley type were sampled and the direction and intensity of the natural remanent magnetization (NRM) were measured. Two of the sites sampled (Newfoundland Zinc Co. property near Daniel's Harbour in western Newfoundland and the St. Joe Minerals Co., #8 Mine in southeast Missouri) had a weak, but measurable NRM in both host and ore rocks. This magnetization proved to be highly stable upon alternating field (AF) demagnetization. The other two mines (Magmont Mine, southeast Missouri, and Pine Point Mine, Northwest Territories, Canada) had intensities of magnetization too low to be measured after demagnetization.The pole positions computed for the ores and their corresponding hosts are identical within the statistical uncertainty, strongly suggesting that the ore and the host are, geologically speaking, of roughly the same age. This study gives two reliable pole positions, one for late lower Ordovician dolostone and sphalerite ore from Newfoundland of 26 °N, 126 °E, and the other for the upper Cambrian, based on the Bonneterre dolostone and galena ore from southeast Missouri of 35 °S, 170 °W.Within the present limitations of the method the results agree with published opinions concerning the age of the ore, i.e. that host rock and ore formation were relatively close in time. Therefore, when significant time differences occur between epigenetic ores and their host rocks, the method may be expected to define this. The method will become progressively more valuable as the apparent polar wandering curves for various continental areas become better defined.