Metal redistribution during fluid exsolution and migration in the Middle Banded series of the Stillwater Complex, Montana

Olivine-bearing zones III and IV (OB-III and -IV) of the Stillwater Complex, MT preserve evidence of fluid migration particularly in the variation of apatite compositions. This feature, coupled with the lack of evidence of the addition of substantial amounts of primitive magma, allows these zones to be used to test the importance of volatile fluids in redistributing metals. The concentrations of Cu, Au, AE, and As all show marked increases at and/or below the levels inferred to represent the top of the fluid migration front in the crystal pile. However, no significant sulfide enrichment is associated with the maximum metal enrichments. The Au concentration peak is much sharper and offset stratigraphically above that of Cu and corresponds with the maximum Cl content of interstitial apatite. These concentration profiles are modeled numerically, assuming fluid transport and chromatographic concentration of the metals. Mass-balance calculations indicate that cumulates significantly below the level of the inferred fluid-saturation front lost Cu while those at or immediately below gained Cu. We conclude that Cl-rich fluids redistributed the background metal concentrations. Although no dramatic enrichments are observed (enrichment factors range from two to three), the results demonstrate the potential importance of fluids in redistributing and concentrating metals in layered intrusions. (C) 1999 Elsevier Science B.V. All rights reserved.