Mafic-ultramafic layered intrusion at Iron Mountain, Fremont County, Colorado

Abstract

Mafic and ultramafic rocks form a small layered funnel-shaped intrusive complex, including several smaller associated discordant intrusions, within Precambrian metamorphic rocks in the vicinity of Iron Mountain, northern Wet Mountains, Colo. The mafic rocks consist of several gradational varieties of gabbro, pyroxenite, dunite, anorthosite, and magnetite iron ore and are made up principally of bytownite-labradorite, monoclinic pyroxene, magnesiumrich olivine, and titaniferous magnetite. Brown amphibole, probably kaersutite, is a late-stage primary mineral.Part of the mafic complex is well layered, having originated through crystal settling in mafic magma. Conspicuous textures and structures that are characteristic of sedimentary rocks are graded bedding, crossbedding, and scour-andfill. Discordant bodies of pyroxenite and anorthosite intrude the layered rocks and offer evidence that later differentiation had proceeded below or beyond the chamber in whch the layered rocks accumulated. The mineralogic and chemical nature of the rocks and their close association with alkalic rocks of the McClure Mountain Complex indicate that these mafic and ultramafic rocks and the alkalic rocks are comagmatic.

Ediacaran‐Ordovician Magmatism and REE Mineralization in the Wet Mountains, Colorado, USA: Implications for Failed Continental Rifting

“…The surface lithology, however, consists of high-density mafic-ultramafic cumulates at Iron Mountain, low-density syenites within the majority of the felsic alkaline complex, and high-density ijolite of limited surface extent (Figure 2). One possible cause of the observed gravity anomaly is that fractional crystallization of the early melt produced a basal layer of mafic-ultramafic cumulates as opposed to a previously proposed subvertical funnel shape (Alexander, 1981;Shawe & Parker, 1967). Another possibility is that the ijolite body is wider in the subsurface than suggested by the surface extent.…”

“…Protracted alkaline magmatic rocks within the Wet Mountains are bounded on the east by the Ilse fault and on the west by the Texas Creek and Westcliffe faults (Figure 1; Armbrustmacher, 1988;Pivarunas & Meert, 2019). The Ediacaran-Cambrian McClure Mountain, Gem Park, and Democrat Creek alkaline complexes represent stocks and composite intrusions composed mostly of mafic-ultramafic cumulate rocks, nepheline syenite, syenite, and quartz syenite (Figure 2; Armbrustmacher, 1988;Christman et al, 1959;Olson et al, 1977;Shawe & Parker, 1967). The Gem Park complex is a stratified mafic-ultramafic instrusion composed of pyroxenite and gabbro (Armbrustmacher, 1984;Parker & Sharp, 1970).…”

“…The McClure Mountain complex can be divided into the mafic-ultramafic cumulates at Iron Mountain to the east and the younger felsic alkaline complex to the west (Figure 2). The mafic-ultramafic cumulates at Iron Mountain, which represent the first magmatic stage, are similar to the Gem Park complex and are composed of olivine gabbro, anorthosite, pyroxenite, and dunite (Alexander, 1981;Armbrustmacher, 1984;Shawe & Parker, 1967). The second magmatic stage at McClure Mountain formed the felsic alkaline composite complex, which varies in composition with mafic nepheline-clinopyroxene rocks (ijolite) in the southwest enclosed and truncated by younger nepheline syenite and hornblende-biotite syenite (Alexander, 1981;Armbrustmacher, 1984).…”