Geology of the Marysville Mining District, Montana : two phases of mineralization

Abstract

The Lewis & Clark line extending from Idaho to Eastern Montana is known for its numerous mineral deposits. Recent advances in our understanding of regional tectonics, the geological history of the Lewis and Clark line, and how mineral deposits form, provide an opportunity for a deeper investigation into this previously studied region. The Marysville Mining District (MMD) is located in the Lewis & Clark line and was chosen for this study. Renewed interest in the MMD allowed for access to the Drumlummon gold mine and the Bald Butte molybdenite deposit. This study applied recent advances in regional tectonics, knowledge of mineral deposit formation, age dating, and geochemical techniques, providing new insights into the MMD with an emphasis on the Drumlummon gold mine. The methods used to conduct this study included field mapping, geochronology, FEMEDS, and computer-based paleostress reconstruction. Results from this investigation were combined with previous work done to form ideas on the origins of mineral deposits in the MMD and the roles of regional tectonics on the emplacement of mineralized veins. The new age dates in the MMD reveal that mineralization occurred in two events: at ~80 Ma and ~40 Ma. Mapping results showed a deformation history in the MMD that included the transpression and transtension events experienced on the Lewis & Clark line. The mineralized veins at the Drumlummon gold mine and Bald Butte molybdenite prospect tend to follow the transtensional stress field. The ore mineralogy of the Drumlummon gold mine is most consistent with an intermediate sulfide epithermal system. Using the new data, a hypothesis was proposed that the origin of gold in the Drumlummon mine was the Silver City Stock.