Geology of part of the Alder Creek mining district, Custer County, Idaho

Abstract

In the Alder Creek mining district, stocks of quartz monzonite, granite, leucogranite porphyry and related dikes, and volcanic rocks, all of early Tertiary age, seem to be differentiates of a single parent magma. The stocks have metamorphosed sedimentary rocks of the Copper Basin Formation and the White Knob Limestone, both from Early Mississippian to Early Permian in age; limestones have been in part isochemically metamorphosed to marble, and in part metasomatized to skarn. Valuable concentrations of copper, lead, and zinc minerals occur in the skarn, and mostly within skarn at the contact between the leucogranite porphyry and limestone. Glacial deposits of two ages and alluvial deposits of two ages occur in the report area. INTRODUCTION Metamorphic rocks at the contact between sedimentary and intrusive rocks are magnificiently exposed in the cliffs, ridges, and cirques of the Alder Creek mining district. The sedimentary rocks are limestone, siltstone, and quartzite, and some mixed argillaceous silty and limy rocks. The intrusive rocks are a variety of compositions all possibly differentiates of a single parent magma. Of the metamorphic rocks, skarn is of greatest interest because it contains most of the copper, lead, and zinc ores that are found in the district. Most of the concentrations of ore minerals are confined to skarn that is adjacent to the leucogranite porphyry, one of the varieties of intrusive rock. The present summary of data on the Alder Creek mining district is a product of the observations by the authors plus those of T. H. Kiilsgaard, and published data cited below. Nelson constructed the detailed a real map (pi. 1) of most of the productive part of the district, made observations and formulated ideas as to the origin of the ore deposits during the summer of 1958. The mines were inactive during the summer of 1958, and Nelson did not visit the underground workings; control