Stillwater Complex, Montana Structure, Mineralogy, and Petrology of the Basal Zone with emphasis on the Occurrence of Sulfides By NORMAN JPAGE GEOLOGICAL SURVEY PROFESSIONAL PAPER 1038 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1979 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress Cataloging in Publication Data Page, Norman J Stillwater Complex, Montana Structure, Mineralogy, and Petrology of the Basal Zone with emphasis on the Occurrence of Sulfides (Geological Survey Professional Paper 1038) Bibliography: p. 61 Supt. of Docs. No.: I 19.16:1038 1. Rocks, Igneous. 2. Intrusions (Geology) Montana. 3. Sulphides. 4. Ore-deposits Montana. I. Title. II. Series: United States Geological Survey Professional Paper 1038 QE461.P289 552'.l'09786 77-608351 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 024-001-03179-5 CONTENTS Page Page Abstract ............................................................................................ 1 Petrology and mineralogy of the Basal zone Continued Introduction ...................................................................................... 1 Rocks derived by partial melting ...................................................... 34 Geologic history of the Stillwater Complex and adjacent rocks 2 Occurrence ................................................................................ 34 Previous work and acknowledgments ............................................... 3 Petrography and mineralogy ..................................................... 35 Definition of the Basal zone .................................................................... 4 Characteristics of hornfelses adjacent to quartz, norite .............. 36 Distribution and thickness of the Basal zone ................................... 4 Origin of the quartz norites ....................................................... 36 Shape of the Basal zone ................................................................... 5 Other silicate rocks ........................................................................... 38 Description of contacts ..................................................................... 6 Ultramafic xenoliths .................................................................. 38 Contact of the Basal zone with metasedimentary rocks ............ 6 Autohornfelsed Basal zone rocks .............................................. 39 Contact of the Basal zone with the Ultramafic zone ................. 7 Sulfide-bearing rocks ........................................................................ 39 Contact of the Basal zone with younger intrusive quartz mon- Occurrence, general terminology, and distribution .................... 39 zonitic rocks .......................................................................... 7 Development of terminology for sulfide-bearing rocks ............. 41 Contact of the Basal zone with mafic dikes .............................. 7 Textures of sulfide-bearing rocks .............................................. 41 Contact of the Basal norite with the Basal bronzite cumulate .. 7 Disseminated and matrix sulfide textures .......................... 42 Age restrictions on the Basal zone ................................................... 7 Textures of massive sulfide rocks ....................................... 44 Petrology and mineralogy of the Basal zone ........................................... 7 Textural relations between silicate and Rock types in the Basal zone ........................................................... 7 sulfide minerals ............................................................... 44 Cumulate rocks of the Basal zone .................................................... 9 Mineralogy of the sulfide-bearing rocks .................................... 44 Cumulate sequences .......................................................................... 16 Pyrrhotite ........................................................................... 46 Size variation of orthopyroxene cumulates ............................... 17 Pentlandite ......................................................................... 47 Evidence for current structures ................................................. 18 Chalcopyrite family ............................................................ 48 Modal mineral variation in cumulates ...................................... 19 Magnetite, ilmenite, and hematite ...................................... 48 Lateral and vertical variation of orthopyroxene composition ... 21 Other opaque minerals ....................................................... 49 Compositional variation of plagioclase ..................................... 21 Proportions of sulfide minerals ................................................. 49 Discussion of cumulate rocks in the Basal zone ........................ 23 Interrelation between sulfide mineralization and Igneous-textured (noncumulate) rocks of the Basal zone ................. 23 silicate rocks .......................................................................... 50 Modal variation of noncumulate rocks ..................................... 23 Interpretation and model of sulfide crystallization .................... 52 Chemical composition of the noncumulate igneous-textured rocks 24 Mechanisms for separation of immiscible Viscosity, density, and settling velocities ................................... 28 sulfide-oxide liquids from basaltic magma ..................... 53 Contaminated igneous rocks ............................................................ 31 Collection and concentration models ................................. 54 Types, textures, and mineralogy of the contaminated igneous Crystallization of the collected sulfide-oxide liquid ........... 59 rocks ...................................................................................... 31 Limitations and speculations on origin of Basal zone ............................. 61 Relation of mixed rocks to inclusions of metasedimentary rocks 32 References cited ....................................................................................... 61 Processes for forming mixed or contaminated rocks ................ 33 Index ....................................................................................................... 67 ILLUSTRATIONS Page PLATE I. Interpretative map, cross sections, and long sections of the Basal zone ...................................................... In pocket 2. Vertical variation in orthopyroxene compositions and grain size from 15 cross sections of the Basal zone In pocket FIGURH 1. Index map of the Stillwater Complex ................................................................................................................................ 2 2. Geologic map of the Stillwater block ................................................................................................................................. 3 3. Frequency diagram of estimated stratigraphic thickness of Basal zone at 48 localities ...................................................... 5 4. Interpretative sketches of basin structures in Basal zone and Peridotite member .............................................................. 6 5. Sections showing rock types and mineralogy in Basal zone ............................................................................................... 9 6. Histograms of orthopyroxene and plagioclase compositions ............................................................................................. 16 7. Graph of mean length, width, and average grain size of orthopyroxene versus stratigraphic position for part of Basal bron/ite cumulate, upper Nye Basin ......................................................................................................................................... '8 IV CONTENTS Page FIGURE 8. Length-frequency distributions of orthopyroxene crystals for a segment of Basal bronzite cumulate, upper Nye Creek Basin .. 18 9. Photographs of scour and local unconformities in Basal norite ......................................................................................... 19 10. Plot of modes of cumulates from Basal zone ..................................................................................................................... 20 1 1. Orthopyroxene in Basal zone plotted against stratigraphic position. West Benhow, Golf Course section ......................... 20 12. Diagram of plugioclase composition plotted against stratigraphic position, upper Nye Basin section ............................... 22 13. Plot of coexisting plagioclase and orthopyroxene compositions in Basal /one .................................................................. 22 14. Diagram showing proportions, by volume, of clinopyroxene. plagioclase, and orthopyroxene in noncumulate rocks ...... 24 15. ARM diagram of noncumulate rock compositions compared with cumulate rock compositions ...................................... 27 16. Projections of analyses of noncumulate igneous-textured rocks in the system olivine-clinopyroxene - plagioclase - silica 27 17. Calculated dry density-temperature curves of possible Stillwater magmas ......................................................................... 28 18. Diagram of calculated viscosities of possible parent magmas for the Stillwater complex plotted against reciprocal temperature ................................................................................................................................................................