Smoky Butte

The Smoky Butte outcrops extend north-south for some 3.4 km and are mainly small plugs and thin dykes cutting Palaeocene sediments. Three varieties of lamproite are described by Velde (1975), all of which come from a quarry in a plug on the south side of Smoky Butte. The rock of the central part of the plug consists of olivine, usually replaced by pale green montmorillonite, zoned Ti-phlogopite, diopside, armalcolite, euhedral analcime, and minor quenched sanidine in a pale yellow glass. The rock surrounding this is similar but contains abundant groundmass sanidine and euhedral analcime pseudomorphs. The outermost rock of the plug is fine grained and contains numerous sedimentary xenoliths which are unaltered; it consists of phlogopite, K-richterite and riebeckite, armalcolite, priderite and sanidine, which is the most abundant phase. Analyses of rocks and minerals are given by Velde (1975). Sr, Nd and Pb isotopes, together with a range of trace element data, including REE, for six lamproites are presented and discussed by Fraser et al. (1985).