USING STREAM-SEDIMENT INDICATOR MINERALS TO EXPLORE FOR RARE EARTH ELEMENT DEPOSITS IN THE SHEEP CREEK AREA OF SOUTHWEST MONTANA

This study investigates using indicator minerals in stream-sediment samples as a potentially efficient and cost-effective tool for rare-earth element (REE) exploration. The main field site used in this study was the headwaters of the West Fork of the Bitterroot River, Ravalli County, MT, where several dozen small carbonatite dikes rich in REE and niobium (Nb) are known to occur (i.e., the “Sheep Creek” carbonatites). Stream-sediment samples were wet-sieved to < 0.5 cm in the field, dried, sieved, gravity separated using a Wilfley table, and passed through a hand magnet. The "heavy, non-magnetic" fraction of the sediment was mounted in epoxy and polished for automated SEM-EDS mineralogical analysis. Results confirm the hypothesis that higher concentrations of indicator minerals are found in stream sediment immediately below known REE-Nb-rich carbonatite deposits, with the most useful indicator minerals in the Sheep Creek area being monazite and columbite. Several detrital monazite grains in one of the stream-sediment samples were analyzed by the USGS LTrace laboratory in Denver, using LA-ICP-MS and the Th-Pb dating method. The results show that the age range of detrital monazites is very similar to the age range of monazites collected from the carbonatite outcrops. In addition, the monazites in the detrital vs. carbonatite samples are similarly enriched in light REE with no europium anomaly. This confirms that the monazites found in the heavy fraction of the stream-sediment samples are likely derived from carbonatites (the target rock type), and not from Precambrian granite or amphibolite country rock. Bulk chemical analysis of sieved but otherwise unprocessed stream-sediment samples from the same locations failed to show a strong REE or Nb signal in the Sheep Creek area. Some of the samples have low- to moderate-level concentrations of REE with a significant Eu anomaly, suggesting input of REE-bearing minerals from background (non-carbonatite) sources. Overall, the results of this study indicate that the indicator mineral approach has certain advantages over conventional stream-sediment sampling, especially in an area where the target rock type comprises a low percentage of the bedrock in the watershed.