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

Abstract

Four stages of skarn formation are recognized in outcrops near Elkhorn, exposing the contact aureole of the Black Butte quartz diorite stock intruding late Precambrian-Palaeozoic dolomitic country rock. The first and most important stage involved replacement of dolomitic marbles by massive skarn dominated by garnet and pyroxene. Detailed study of mineralogical zoning, mineral compositions and compositional variations of minerals and phase assemblages indicates that, during development of the zoned magnesian skarns, significant fluctuations in the activities of several chemical variables took place, rather than simple diffusion and/or infiltration involving gradients of only one compositional variable. Application of experimental and calculated phase equilibria permitted estimation of the physical and chemical conditions during skarn formation. The study includes tables of representative electron-microprobe analyses of skarn minerals, as well as a discussion of microprobe and data-reduction procedures.-M.S.