PETROLOGY OF A SECTION OF DRILL CORE AT THE CONTACT BETWEEN STILLWATER CUMULATES AND BANDED IRON FORMATION, MONTANA

The Archean Stillwater Complex is a large layered mafic intrusion located in south-central Montana and is exposed for nearly 50 km along the north flank of the Beartooth Mountain Range. A section of drill core (~ 1m long) thought to represent a transition zone between pyroxene cumulates of the Stillwater Complex and adjacent banded iron formation was selected for detailed study. The rocks within the core have been affected by alteration such that non-hydrous minerals have undergone reactions to form amphiboles and clays and calcic plagioclase has been largely altered to sodic plagioclase. The length of the core was characterized for geochemical variations using a hand-held Bruker XRF (X-ray fluorescence) spectrometer and six thin sections of somewhat variable lithology were selected and are representative of fine-to-medium grained Stillwater pyroxene cumulate (A), a very fine-grained igneous rock oriented 30° relative to the core axis (B), coarser-grained pyroxene cumulate (C and D), pyroxenite with magnetite stringers (E), and very fine-grained iron formation (F). The sequence in the core represents the transition from cumulates in the Basal series of the Stillwater Complex (A) to Archean iron formation (F) of the country rock. The intervening lithologies may represent either (1) a Stillwater chill zone (B) and different types of iron formation and related rocks or (2) a metasomatic reaction zone with B representative of modified Stillwater rock and C-D, modified iron formation. Samples A and E are characterized by albitized plagioclase, orthopyroxene with fine exsolution lamellae, relict clinopyroxene, quartz, abundant clay minerals and two varieties of amphibole (Fe-Mg-Mn and Ca amphibole subgroups). Accessory minerals include Fe, Cu-Fe and Ni-Fe sulfides, Fe-oxides and chromite. Fine-grained sample B was much less altered and characterized by very calcic plagioclase (An_>90), orthopyroxene, calcic amphibole, and clay minerals. Accessory minerals include chromite, ilmenite and magnetite with few sulfides. Samples C and F lack both plagioclase and pyroxene, and have amphiboles (primarily Fe-Mg-Mn with lesser Ca amphibole subgroups) and unidentified clay minerals. Accessory minerals include Fe oxides, and Fe-Ni sulfides.