Paper No. 6
Presentation Time: 9:55 AM
GEOPHYSICAL IMAGING OF THE STILLWATER COMPLEX AND RELATION TO PLATINUM GROUP ELEMENT EXPLORATION
High resolution aeromagnetic and electromagnetic surveys along with gravity data aid in platinum-group element (PGE) mineral exploration and mine planning for the Stillwater Complex, MT, a layered mafic and ultramafic intrusion characterized by igneous layering and containing a reef-type deposit with the highest mineable grades of platinum and palladium worldwide. Linear aeromagnetic anomalies correspond to layering features in the Stillwater Complex. The aeromagnetic anomalies can be related to (1) the boundaries between major stratigraphic units, (2) some but not all, of the intervals, with olivine-bearing rock layers, and (3) a particular rock unit comprised of gabbroic rocks with minor olivine. Previous work on the Stillwater Complex showed that the distribution of olivine influenced the patterns in the aeromagnetic data over the complex because olivine commonly alters to a mixture of serpentine and magnetite. The interval of olivine-bearing rocks that host the J-M Reef, the PGE-enriched sulfide mineralization that is mined in the Stillwater Complex, shows up as a distinct linear anomaly. However, many of the anomalies correspond to contacts separating rock units with different proportions of plagioclase, bronzite, and augite.
The electromagnetic (EM) data may highlight reef-type and contact-type mineralization which contain low resistivity sulfide minerals. A low resistivity zone along the southern margin of the Stillwater Complex may correspond to mineralization in banded iron formation or contact-type sulfide mineralization in the Basal zone.
A 30-50 mGal, 30 km x 70 km ovate gravity high characterizes the exposed and interpreted buried extent of the Stillwater complex. Xenoliths of Stillwater Complex rocks from Cretaceous intrusions occur at least 15 km north of the last exposure of the Stillwater Complex, confirming the gravity interpretation of buried dense Stillwater Complex rocks. Interpretation of new gravity data will help identify the subsurface extent of Cretaceous intrusions that disrupt the Stillwater Complex, provide higher resolution information on the structure of the complex near outcrop and refine the lateral and depth extents of the Complex.