Formation of scheelite-bearing and scheelite-barren skarns at Lost Creek, Pioneer Mountains, Montana

Abstract

The three garnet and pyroxene zones which comprise the scheelite-bearing skarn in the study area show similar variations in mineral percentage and chemistry when compared to an adjacent garnet and pyroxene scheelite-barren skarn. Each of the zones in this dark brown skarn exhibits a smooth increase in percent total garnet toward the intrusive and then drops abruptly on the intrusive side of each zone. The three generations of garnet and pyroxene of the three zones show smooth sinusoidal variation in their major oxides across each zone and both minerals are relatively iron rich. In contrast, the beige scheelite-barren skarn shows random mineralogical and chemical trends across its width. The major minerals in this unzoned skarn are aluminum and magnesium rich relative to the zoned skarn minerals. The three garnet-rich zones are thought to have formed in a clean marble from solutions in which the availability (activity) of each major oxide varied in a smooth sinusoidal manner across each zone during the growth of the three generations of garnet and pyroxene. In the unzoned skarn, the presence of aluminum and magnesium as contaminants in this portion of the marble alters the metasomatic pattern of the diffusing oxides and their interference patterns which form the repeated mineral and chemical zoning found in the zoned skarn.