MAPS, SECTIONS, AND STRUCTURE-CONTOUR DIAGRAMS SHOWING THE GEOLOGY AND GEOCHEMISTRY OF THE MOUAT NICKEL-COFFER PROSPECT, STILLWATER COMPLEX, STILLWATER COUNTY, MONTANA

ABSTRACT Deposits of magmatic nickel-copper (Ni-Cu) sulfides are concentrated near the base of the Stillwater Complex, a Late Archean mafic to ultramafic layered intrusion exposed on the north edge of the Beartooth Mountains, Montana. Extensive drilling and limited underground development work by the Anaconda Minerals Company has delineated the sub-surface extent of one of these deposits, the Mouat Ni-Cu prospect or deposit. Structural interpretation shows that rocks of the Mountain View area of the Stillwater Complex are folded into a broad syncline which is inclined and cut by several generations of faults. The oldest faults strike northeast and have near vertical dips. These faults are cut by northweststriking, near vertical faults showing left-lateral separation. Apparent separation on these faults are less than 500 ft. Two high-angle reverse faults, the Lake fault and the Bluebird thrust, truncate the Mouat deposit on the northeast and the southwest, respectively. Sulfide minerals that constitute this deposit are concentrated at the lower contact of the Stillwater Complex within the Basal series and within and adjacent to very discontinuous, irregularly shaped Stillwater-associated sills and dikes that intrude the metasedimentary rocks underlying the complex. The base of the Stillwater Complex shows considerable relief over short distances; as a result, the thickness of the Basal series ranges from 0 to 450 ft. However, there is no clear evidence that sulfides at the base of the complex pooled in depressions on the floor of the complex. Intensity of faulting and the discontinuous nature of the matrix-to-massive sulfide mineralization do not allow probabilistic tonnage and grade estimates to be made despite the density of drilling