Contact skarn formation at Elkhorn, Montana; I, P-T-component activity conditions of early skarn formation

Abstract

Black Butte Stock. The initial stage of skarn development (I) is characterized by a variety of three-phase and less often four-phase assemblages which usually include garnet and pyroxene and occasionally epidote, vesuvianite, or clintonite. The second stage of skarn development (II) was marked by local replacement of garnet and pyroxene upon cooling by one or more of epidote, amphibole, clintonite, chlorite, calcite, and quartz. Electron microprobe analyses indicate wide variations in pyroxene and garnet compositions in stage I. Application of experimental and calculated phase equilibria to observed skarn assemblages has allowed estimations of general physical-chemical conditions of formation for stage I skarns: P = 1.0 + or - 0.3 kb, T = 525 + or - 35 degrees C, 0.04 <X(CO 2 ) <0.30, -22.7 <log f(O 2 ) <-19.0, log f(S 2 ) <-3.3, -1.65 <log a(SiO 2 ) <0.0, -1.85 <log a(Al 2 O 3 ) <+0.1. The hydration and carbonation reactions that characterize stage II occurred at lower temperature and in a water-rich fluid. Phase equilibria applications indicate the entire zoning sequence of the massive skarns could have equilibrated within a continuous gradient in X(CO 2 ) from 0.30 + or - 0.05 at the skarn-marble interface to 0.03 to 0.07 at the intrusive contact.--Modified journal abstract.