If yon do not need this publication after it has served your purpose, please return it to the Geological Survey, using the official mailing label at the end UNITED STATES DEPARTMENT OF THE INTERIOR ALKALIC ROCKS OF IRON HILL GUNNISON COUNTY, COLORADO GEOLOGICAL SURVEY PROFESSIONAL PAPER 197-A UNITED STATES DEPARTMENT OF THE INTERIOR Harold L. Ickes, Secretary GEOLOGICAL SURVEY W. C. Mendenhall, Director Professional Paper 197-A ALKALIC ROCKS OF IRON HILL GUNNISON COUNTY, COLORADO BY ESPER S. LARSEN Shorter contributions to general geology, 1941 (Pages 1-64) UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON : 1942 For sale by the Superintendent of Documents, Washington, D. C. ......... Price 40 cents CONTENTS Page Preface, by G. F. Loughlin_________________________ v Other dike rocks_______________-____________________ 29 Abstract____"_--__-___--__________________________ 1 Augite syenite and shonkinite ____.__--___--__--__ 29 Introduction. ______________________________________ 1 Olivine gabbro. ________________________________ 30 Location and topography. _______________________ 1 Carbonate veins_________----__---------__--_---___- 31 Field work and acknowledgments.________________ 2 Character __ __________________________________ 31 Geology of the surrounding area__________________ 2 Origin _______ _ ____ _ __ _.. __ __. ___ .__. 31 Age of the Iron Hill stock_____________________ 2 Hydrothermal processes- ______--_-___-_---_--__-___- 31 General geology of the Iron Hill stock _____________ 3 The hydrothermal products. _____________________ 32 Hydrothermal activity___.= ______________________ 4 Carbonate type _ __._--_______-___-_-_____- 32 Special features of the stock____________________ 4 Apatite-magnetite-carbonate veins and iron oxide "Marble" of Iron Hill_____________________________ 4 veins. ___________________________________ 32 Distribution__ _ _______________________________ 4 Aegirite-amphibole. _ ________________________ 32 Character___ _________________________________ 5 Hastingsite and diopside_________________-_._ 33 "Marble"—--------------------_--------_ 5 Melanite __ _ _ __ --____--__-___________ 33 Martite-apatite veins___-_____-___-_-______ 5 Alterations of melilite_ ______________________ 33 Siliceous iron oxide veins____________________ 5 Zeolites, cancrinite, and anal cime _____________ 33 Origin of the iron ore______________________ 5 Other types _____ __ _ , ______ - _ __ _ ___ 33 Silicate rock______________________________ 5 Conclusions. __________-_---_-_---_----__-__-___ 33 Carbonate bodies in the granitic rocks_______ 6 Chemistry ________ _ _ _ _ _ __ ___ __ _ _ 33 Origin of the marble__________________________ 6 Comparison with other areas_ ____________________ 34 Available facts _____________________________ 6 General character of the stock_______________-__._____ 34 Modes of origin___________________________ 7 Succession of events______-____-__-__---___---___ 34 Uncompahgrite. _____-_-_-_______----__ _ ___________ 9 Relative size of formations. ______________________ 35 Occurrence...--________________________________ 9 Mineral character of the stock __ ________________ 35 Fresh Uncompahgrite________________________ 9 Chemistry of the stock_______-___________-__- 35 Altered uncompahgiite__________________________ 10 Petrogenesis ___ _ __________________________________ 36 Chemistry of the alteration products._____________ 11 Comparison with similar masses. _________________ 36 Chemical analyses, norms, and modes.____________ 12 Rocks.. ___________ _ ___ _ ______ 38 '13 Pyroxenite_______________________________________ Mineralogy. _______________________________ 39 Distribution and age_______-_____-_-___________- 13 Chemical character. ________________________ 39 Character____________________________________ 14 Succession of intrusions______________________ 39 General features.___________________________ 14 Origin of the rocks-________---_--__--_---__---_- 39 Common pyroxenite_______________________ 14 Uncompahgrite-- __________---_-_--_________ 40 Apatite-rich rock___________________________ 14 Pyroxenite- ________________________________ 40 Biotite rock._______________________________ 14 41 Magnetite-perofskite rock.—________________ 14 Syenite and nepheline syenite. _______ 41 Olivine rock______________________________ 15 Nepheline gabbro_-__---_____-_---__ 42 Garnet pyroxenite________-------___________ 15 Quartz gabbro __ ____________________ 42 Nepheline pyroxenite (melteigite)_____________ 16 Conclusions as to origin. ____________ 42 Feldspar-nepheline rock (shonkinite)__________ 16 Mineralogy, by E. S. Larsen and W. F. Jenks__ 43 Feldspar-bearing rock_______________________ 16 Sulfides. _ ___-________-_______--___-__ 43 Sphene pyroxenite-_______________________ 17 Fluorite... ___ ___ __ ______ _ _ . 43 Variations in the pyroxenite____________________ 17 Qu_jrtz__ __ ____ ___ _ __ ___ __-- _ 43 Order of crystallization and intrusion of the vari­ Spinel.. __________________ --__ 43 eties.. ______________________________________ 19 Oxides of titanium_________-__________-_ 43 Origin of the varieties_________________________ 19 Iron oxides __ ________________________ 43 Chemical analyses, norms, and modes.____________ 20 Carbonates _____________-_____-_-___-_- 4:4 Ijolite (melteigite)_______________________ T ________ 22 Brugnatellite_ _______--__-_-_-_______-__ 44 Distribution._________________________________ 22 Feldspar _ _________ __________-____-__- 4.4 Petrography ___________________________________ 22 Lime-soda feldspar _ _______________ 44 Chemical analyses, norms, and modes.____________ 24 Albite, microcline, and orthoclase _____ 44 Crystallization of ijolite.________________________ 24 Nepheline ___ _________________________ 44 Soda syenite.______________________________________ 25 Melilite. ___________ ____ __ _______ 44 Nepheline syenite_________________________________ 26 Pyroxenes. ____________________________ 45 Nepheline gabbro and quartz gabbro._______________ 28 Amphiboles. _-_______---____--_____-__- 48 Nepheline gabbro—____________________________ 28 Soda-tremolite and glaucophane group 50 Quartz gabbro——____________._,_.________ 28 Hastingsite group _ _____________--- 52 Chemical analyses, norms, and modes. ____________ 29 Wollastonite..- __ ________ __ _. ___ _ 52 m IV CONTENTS Mineralogy, by E. S. Larsen and W. F. Jenks—Continued. Page Mineralogy, by E. S. Larsen and W. F. Jenks—Continued. Page Olivines_ __________________________ 52 Analcime _______________________--__---.---_--- 55 Micas _________-___-____-__-_-_---_ 53 Juanite and cebollite_---___----__--_------------ 56 Muscovite, ____________________ 53 Minor constituents, by E. S. Larsen and George Steiger__ 56 Biotite and phlogopite_ ________ 53 Titanium __„____________________-_-----_--__--- 57 Apatite ____________________________ 53 Phosph orus__________»_____._-__-___--_-_-__--_ 58 Manganese__- _ __-____-__--__--__-__------------ 58 Sphene_ _.-________-_-_-___-___-___ 53 Barium and strontium__________-___-___--__-__ 59 Perofskite__ __ _ _____________________ 54 Zirconium, nickel, chromium, and vanadium.______ 60 Idocrase.__________________________ 54 Chlorine and fluorine-.______---__---_--__-_---__ 60 Melani te_____ ______________________ 54 Bearing on petrogenesis__________-__--_-"--___.- 60 Chlorites, serpentine, and vermiculites. 54 Spectroscopic tests.___________.__--___-__---__-_ 61 Cancrinite and sulfatic cancrinite_ _.__ 54 Index _______________-__________--_--___-___--___-_ 63 Natrolite __________________________ 55 ILLUSTRATIONS Page PLATE 1. Geologic map of Iron Hill area, Gunnison County, Colo____---__,______-.___-___-_--___-_-_--_---___-__ In pocket 2. Panorama of the Iron Hill area_________________________________________________________________________ 2 3. A, B, Pyroxenite on south side of Stone Gulch; C, Banded syenite.______________-____--_-___-__-__----_____ 18 FIGURE 1. Marble-pyroxenite contact_____________________________________________________________________________ 5 2. Section south of Stone Gulch, showing variation in pyroxenite._____________________________________________ 17 3. Section on north side of Stone Gulch, showing variation in pyroxenite._______________________________________ 18 4. Section south of Deldorado Creek, showing variation in pyroxenite,_________________________________________ 18 5. Section northeast of gully B, showing complex relations of uncompahgrite and varieties of pyroxenite_--__-___-__ 19 6. Pseudo-binary diagram of equilibrium in mixtures of nepheline and diopside______.-___--______-___-______-___ 24 7. Optical properties of the pyroxenes carrying acmite plotted against content of Fe2O3 and acmite________________ 48 8. Soda tremolite and glaucophane amphiboles, atomic ratios plotted against Mg+Fe"-Al-Fe"'______ ____________ 51 9. Soda tremolite and glaucophane amphiboles, ft, YAc, and specific gravity plotted against Mg-j-Fe"-Al-Fe'"____ 51 PREFACE By G. F. Loughlin The author of this paper gives a thorough description growth of the science of petrology and help to guide of a complex of very unusual igneous rocks and associ­ future investigations. The description of this unusual ated hydrothermal deposits. Their unusual character complex and the author's interpretation of its origin, involves difficulties in interpretation of genesis and therefore, should be most interesting, especially to those modes of intrusion and deposition, and even in definition. studying the origin of igneous rocks. Obviously, the Perhaps the most puzzling deposit of all