AGE AND DURATION OF MISSISSIPPI VALLEY-TYPE ORE-MINERALIZING EVENTS

Statistics of a combined paleomagnetic data set from six studies of Mi ssissippi Valley-type deposits and their surrounding host rocks show t hat the characteristic remanent magnetization of the host rocks predat es the magnetization of the mineralization, providing a positive conta ct test confirming that the ore magnetization is primary. Either multi ple fluids or dramatic changes to a single fluid through time are requ ired to account for the separate magnetization ages observed in the ho st rocks and ores. The dispersion of the characteristic remanent magne tization directions provides an estimator for the duration of the mine ralizing process, suggesting a mineralizing event of about 4 m.y. dura tion.