Mafic-Ultramafic Layered Intrusion at Iron Mountain Fremont County Colorado GEOLOGICAL SURVEY BULLETIN 1251-A Mafic-Ultramafic Layered Intrusion at Iron Mountain Fremont County Colorado By DANIEL R. SHAWE and RAYMOND L. PARKER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY BULLETIN 1251-A Mafic and ultramafic rocks occur in a small layered funnel-shaped intrusive complex associated with younger alkalic rocks of the McClure Mountain Complex UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967 UNITED STATES DEPARTMENT OF THE INTERIOR STEW ART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Abstract_________________ Al Introduction, ______________ 1 Structural features.________ 4 Petrography and mineralogy- 8 Chemistry. ________________ 22 Conclusions, _______________ 26 References cited.___________ 27 ILLUSTRATIONS Page PLATE 1. Geologic map of the mafic-ultramafic layered intrusion at Iron Mountain, Fremont County, Colo____- ________ In pocket FIGTJRE 1. Geologic map of the McClure Mountain Complex showing location and geologic setting of the mafic-ultramafic complex at Iron Mountain-__________________________ A2 2. Photograph showing rhythmic layering in gabbro of the layered intrusion at Iron Mountain.__________________ 5 3. Photograph showing graded bedding in gabbro of the lay ered intrusion at Iron Mountain...___________________ 6 4. Photographs showing scour-and-fill structure and cross- bedding in gabbro of the layered intrusion at Iron Mountain. ________________________________________ 7 5-8. Sketches of thin sections of the layered series: 5. Pyroxenite____________________________________ 9 6. Olivine-rich gabbro______________-_---__-------- 10 7. Titaniferous magnetite iron ore_-__-___---_------ 11 8. Fine-grained crossbedded gabbro_________________ 12 9. Sketch showing mutual boundary relations of closely packed plagioclase crystals (sodic bytownite) in gabbro of the layered series______________________________________ 13 10. Sketch of thin section showing hourglass structure resulting from schiller in monoclinic pyroxene of pyroxenite of the layered series._____________________________________ 17 11. Composition diagram showing field of clinopyroxenes from Iron Mountain and crystallization trends of alkalic and tholeiitic clinopyroxenes_______________-_-_-_----_--- 18 12. Sketch of thin section showing euhedral brown amphibole core in euhedral monoclinic pyroxene in fine-grained pyroxenite of the discordant pyroxenite intrusion_-_____ 19 in IV CONTENTS TABLES Page TABLE 1. Estimated modes of Iron Mountain mafic and ultramafic rocks__ ____________________________________________ A14 2. Optical data for clinopyroxenes from Iron Mountain rocks_ 16 3. Chemical analyses, norms, modes, and spectographic analyses of Iron Mountain mafic and ultramafic rocks___ 23 CONTRIBUTIONS TO GENERAL GEOLOGY MAFIC-ULTRAMAFIC LAYERED INTRUSION AT IRON MOUNTAIN, FREMONT COUNTY, COLORADO By DANIEL R. SHAWE and RAYMOND L. PARKER 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, magnesium- rich 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 charac teristic of sedimentary rocks are graded bedding, crossbedding, and scour-and- fill. 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. INTRODUCTION Mafic and ultramafic rocks in the form of a layered, funnel-shaped intrusive complex underlie an area of 3% square miles in the vicinity of Iron Mountain in the northern Wet Mountains, Fremont County, Colo., about 11 miles southwest of Canon City. Parker and Hildebrand (1963) noted that these rocks form the northeastern part of a larger alkalic complex at McClure Mountain, which includes also various types of syenite, nepheline syenite, and mafic nepheline-bearing rocks (fig. 1). The complex at McClure Mountain was originally described in detail by Parker and Hildebrand (1963). Rocks of the complex exposed in the type area shown in figure 1, Avhich encompasses about 20 square miles, are here formally designated the McClure Mountain Ai A2 CONTRIBUTIONS TO GENERAL GEOLOGY 105°30' 105°25' R. 73 W. R. 72 W. EXPLANATION MMcouCnltuarile OCAMBRRIAh Complex PPRREECCAAMMBBRRIIAANh Alluvium Disccrdant rocks of ' mafie complex Nepleline syenite Layered rocks of SSiggggwB mafie complex _> ) T PRECAEMBRIAN r Mai ic nepheline- CQOAMBURRIAN MMcoCunltuarine bearing rocks Complex Gran itic gneiss and Y reated rocks ^ Bioti;e-hornblende Contact syenite FIGURE 1. Location and geologic setting of the mafic-ultramafic complex at Iron Mountain. Modified from Parker and Hildebrand (1963). MAFIOULTRAMAFIC INTRUSION, IRON MOUNTAIN, COLORADO A3 Complex. The complex, originally shown as Precambrian in age (Parker and Hildebrand, 1963), is now considered to be Cambrian or Precambrian. Significantly, mineralogy and chemistry of the mafic and ultramafic rocks of the layered intrusion, as determined in the present (1966) study, suggest that the layered intrusion and the alkalic rocks of the McClure Mountain Complex are comagmatic. The mafic and ultramafic rocks at Iron Mountain discordantly intrude granite gneiss, amphibolite, and other metamorphic rocks of Precambrian age. The Iron Mountain rocks cut across foliation of the metamorphic rocks and are clearly younger than the regional meta- morphism that has affected the gneisses and related rocks. At its west edge, the intrusive complex at Iron Mountain is intruded by biotite-hornblende syenite and locally by dikes and irregular bodies of nepheline syenite. These syenites are believed to be closely related in age and genesis to a syenite body which occurs about 6 miles to the southwest, and which has been dated by the Larsen zircon method as about 595 million years old (Christman and others, 1959). The Iron Mountain mafic and ultramafic rocks and other rocks of the McClure Mountain Complex are cut by conspicuous syenite porphyry dikes. The only known commercial mineral deposit in the mafic and ultra- mafic rocks of the Iron Mountain area consists of titaniferous mag netite ore at the Iron Mountain mine, about a mile south of Iron Mountain on the south side of Sandy Creek. A few prospects are widely scattered in the mafic and ultramafic rocks; some of these expose iron ore, others contain minor iron and copper sulfides, and at least one exposes a small vein containing fluorite and galena. None of the prospects indicate minable deposits. The Iron Mountain mine, which probably was operated on a very small scale as early as 1872, was described by Endlich (in Hayden, 1874), Putnam (1886), Chauvenet (1886, 1890), and Kemp (1899). The ore was described in detail and previous works were reviewed by Singewald (1913), and more recently the deposit was the subject of magnetic and gravity studies by the U.S. Bureau of Mines (Becker and others, 1961). Rocks similar to those of Iron Mountain constitute a smaller mafic- ultramafic complex at Gem Park about 8 miles to the southwest. This complex, which is the subject of a separate study and is like that at Iron Mountain, is funnel-shaped and layered and contains local deposits of titaniferous iron ore. It differs, however, in the absence of much associated syenite, in the development of vermiculite in the mafic and ultramafic rocks, and in the development of abundant carbonatite dikes and associated niobium and rare-earth mineralization. A4 CONTRIBUTIONS TO GENERAL GEOLOGY STRUCTURAL FEATURES The stratified complex at Iron Mountain consists of a sequence of igneous rock layers several thousand feet in total exposed thickness, which forms a spoon- or funnel-shaped body (now disrupted by faults) whose long axis trends northwestward (pi. 1). Gabbro, dunite, anorthosite, and pyroxenite are interlayered in all levels of the in trusive complex. The layers range in thickness from a fraction of an inch to several tens of feet; thicker layers appear to extend laterally thousands of feet and vary little in thickness. The rocks are generally medium grained (2-5 mm) and contain different proportions of the principal minerals plagioclase, pyroxene, olivine, and magnetite. Small coarse:grained pegmatitic bodies composed principally, of plagioclase and pyroxene occur locally in the layered series. The layered rocks appear to be nearly conformable to the contact because their strikes are everywhere nearly parallel to the surface trace of the contact with gneissic country rocks, except where both rock types are in fault contact. The layers commonly show a foliation, parallel to the layering, defined by the orientation of tabular mineral grains; no lineation is evident within layers. Rock layers along the west margin of the layered intrusion com monly diverge strongly from the nearly conformable inward-dipping attitudes present throughout most of the remainder of the body. The local irregularities in attitude probably resulted from disruption during intrusion of the large biotite-hornblende syenite body against this contact. Local strong brecciation of layered rocks and intimate penetration by syenite along the west edge of the mafic and ultramafic complex also attest to the disruption effected by the syenite intrusion. Probably part of the original layered intrusion here has been displaced by syenite. A large part of the layered rocks, in an area covering about one-half square mile in the southern part of the complex and adjacent to its west margin, is grossly out of orientation with the remainder of the complex. Here rock layers consistently strike nearly north and dip steeply to the west. The rock layers in this area are distinctive litho- logically in that they contain abundant titaniferous magnetite; the Iron Mountain magnetite mine is located near the north end of this area. Compositional layering, which is attributed to crystal settling and other phenomena closely analogous to processes forming sedimentary rocks (Wager and Deer, 1939), is an outstanding feature of the com plex. Rhythmic layering (fig. 2) is common, as is graded bedding in some layers as much as 6 inches thick in Avhich pyroxene and sparse