Structure, petrology, and petrogenesis of the Treasurevault stock, Mosquito Range, Colorado

Abstract

The Treasurevault stock intrudes high-grade metamorphic rocks approximately 10 km east of Leadville, Colorado, in the northern Mosquito Range. The stock is roughly circular in exposed plan and about 3 km in diameter. It consists of quartz monzonite that is remarkably uniform in mineralogical and chemical composition. Three textural facies occur in roughly concentric zones: medium-grained rocks occur between fine-grained rocks at the contacts and coarse-grained rocks near the center of the stock. Primary flow structure near the contact suggests that the upper part of the stock is hemispherical. Primary radial and diagonal joints are well developed near the contact. Both flow structures and joints probably formed just before and after consolidation as a result of compression perpendicular to contacts. The K-Ar age of the stock is 1,430 ± 45 m.y. (Precambrian Y), indicating emplacement during the Silver Plume magmatic event. A model for the petrogenesis of the Treasurevault stock includes derivation of the parent magma by partial melting of lower crustal rocks at a depth of ∼40 km, emplacement of the magma due to gravitational instability to a depth of ∼10 km, and crystallization of most of the magma under conditions of water undersaturation. A water-saturated pegmatite magma developed after more than 97 percent of the stock had crystallized.