Shasket Creek

Three irregular plugs and a number of associated dykes extend over 6. 5 km in the Curlew Quadrangle of Ferry County. Two major rock units are distinguished: the first, comprising the major parts of the plugs, varies from monzonite, through nepheline syenite to shonkinite, while the second, which comprises parts of the principal plug and the dykes, is a syenite porphyry. The alkaline rocks intrude Permian and Triassic sedimentary rocks and greenstones, and are themselves cut by later quartz monzonites. At the contact Triassic limestones have been altered to marbles containing wollastonite, phlogopite, actinolite, melanite and idocrase, and greenstones have been horfelsed. The monzonites are trachytoid rocks of oligoclase-andesine, perthitic orthoclase and microcline, amphibole and many accessory minerals including quartz. The syenites are mesocratic rocks of alkali feldspar, subordinate albite, and abundant poikilitic amphibole, with, in the nepheline-bearing (altered) varieties, pale green pyroxene rimmed by amphibole and poikilitic melanite. The shonkinite comprises 50% pale green pyroxene, about 30% amphibole and perthite. Syenite porphyry consists of aligned, tabular orthoclase crystals up to 5 cm in length and a little oligoclase-andesine, biotite, amphibole and poikilitic melanite. Hexagonal masses of natrolite, calcite and sericite pseudomorph nepheline.