HIGH-GRADE METAMORPHISM OF IRON-RICH BODIES IN ARCHEAN GNEISS, WIND RIVER MOUNTAINS, WYOMING. A Thesis Submitted to the Faculty of the Graduate School of the University of Minnesota BY JULIA ANN SYKES In Partial Fulfillment of the Requirements for the Degree of Master of Science April, 1985 To all those who waited so long ••. ABSTRACT The Warm Springs field area is situated in the northern Wind River mountains and is composed predominantly of Archean gneiss. Within this terrain, two unusual iron-rich bodies crop out. These are believed to be high-grade regionally metamorphosed iron-formation. The quartzo-feldspathic gneiss shows evidence of two phases of folding. It is occasionally migmatitic. Small conformable pods of amphiboloi te occur within the gneiss, indicating the regional metamorphic grade is now amphibolite facies. To the northeast of the area is the Warm Springs Pluton, composed of rocks of granitic to quartz-monzodioritic composition. Granitic dikes, plagioclase-rich pegmatites and quartz veins cut all these rock types. Paleozoic sediments unconformably overlie the gneiss and the pluton. The majority of the area is now covered by glacial drift. The northern iron-rich body, the Wildcat body, contains the highest grade assemblage consisting of olivine, orthopyroxene, clinopyroxene, quartz and magnetite. The equigranular, granoblastic texture indicates it is an equilibrium assemblage. The olivine and pyroxenes show retrograde reactions to amphiboles. This mineralogy indicates that the Wildcat body was metamorphosed to granulite facies. The southern body, the Spruce Creek body, contains more garnet and biotite than the Wildcat body indicating that it is more aluminous. Both pyroxenes occur but olivine is not seen. This body shows an unusual intergrowth of quartz and magnetite that is believed to be due to intense silicification of the iron-silicates. The assemblage olivine-orthopyroxene-clinopyroxene-quartz- magnetite is used for obtaining both fossil temperatures and pressures. Two pyroxene geothermometry gives a temperature range of 500-650°c and a zoning within the grains of 100°c. This is substantiated by other methods. The olivine, orthopyroxene and quartz geobarometer yields a pressure of 3.6-5.9 kbar for this temperature range. This temperature and pressure are within the amphibolite facies. Pigeonite, suggested by the intergrowth of orthopyroxene and clinopyroxene in the Wildcat body yield temperatures of goo 0 c and pressures of 8-9 kbar which indicate granulite facies metamorphism. The area has undergone two periods of metamorphism and several periods of deformation. The two iron-rich bodies crop out along a lineament and a magnetometer survey indicated other iron-rich bodies also conformable with this trend. It is suggested that the two bodies were originally part of one body, disrupted during the Late Precambrian. ACKNrn.JLEDGEMENTS I would like to take this opportunity to thank Dr J. A. Grant for being a wonderful advisor and for serving as chairman of my thesis committee. Ors J.C. Green, R.W. Ojakangas and C. Anderson are acknowledged for reviewing the thesis, serving on my committee and for their helpful criticism. I am indebted to Rocky Mountain Energy Company for funding the field season for this thesis. I am especially grateful to Dr D. Lindsley for providing the microprobe data used in this thesis. Special thanks go to my brother Alan Sykes who was my field assistant, also to Lawrence Rosen for his tremendous help during the final stages of thesis preparation. Lastly, I would like to thank all the faculty, staff and students at the University of Minnesota-Duluth for making my first experience of America so enjoyable. TABLE OF CONTENTS TITLE PAGE •• .i DEDICATION •. • ••••••••••• l...l. .ABSTRACT'. .. • • • • • • • • • • • • • • ••••••••••••••••••••••••••••••••••••••• • 111 ACKNCWLEI:GEMEN'IS •••••••••••• ..••..... iv TABLE OF CONTENTS • . • • • • . • . • • • . • • • • . • • • . •••.•.... v ILLUSTRA'J:'IONS •••••••.••••.••.••••••.•••••.• ·..••.•••••.•.••.•••.. vii TABI.&ES •••••••••••••••••••••••••• .........ix PIA.'I'ES • ..•••••.••••••••.•••.•••• . ........ix INTRODUCTION Problem ••.••..••••••••.•..•••.•••••• ........................ 1 Location and access ••.•.•.•.••.•.••• . .... 1 of study. .... 4 Previous work. • • • • • • • • • • . • • • . • . • • • • • • . • • • • . ...•.••••.••••.. 4 Regional geology •••••••••••.••••••••••..•.•. •••.•••••.•••.•• 5 GEOLOGY OF THE WAR1 SPRINGS .Z\REA Ir1. troduction . 9 Quartzo-feldspathic gneiss unit I:r1troduction . 9 Quartzo-feldspathic gneiss ...•.•..•••.•••..••. • •••• 9 .Arrpl1.il::x::>li 'te . ... 13 Iron-rich .material ••••••••.••••.........•••••.• ... 16 Wann Springs Pluton Introduction .••••....•.•••••.••••••••••...•••••••••••..16 Gra.rtl te . .............................................. 19 GraI10diorite ........................................... 19 Quartz syenite .......................................... 21 Quartz ·rronz9diorite. ........ 21 Younger dikes and quartz veins •••.••.••.•...••. ••••••••• 24 Paleozoic sed.irn:=nts •••••••••••••.•••••••..••..•.••••••••.••.. 2 7 Ple·istocene sedi.rrErlts • .•....•.•....••.•.••••••.•..••.•.•.... 30 S"tru.ctl.J.re ••••••••••.••••••••• ·•••••• ·•••••••••••••••••••••••••• 3 0 Corrparison to regional geology •••••••.••..••••••••.••.•.•••• 32 THE WILCCAT IIDN-Rilli BODY Introduction •••••••.•••••••.••••••...•.••.•.••••••••••••.••. 35 Petrography Intl:oauct.ion . .......................................... 35 Mineral assemblages •••••••••••.••••••••••.••••••••.•••. 38 Mineralogy Qua.rtz • •.••.•.•.•..•.•• •.... ·• ·.• •••••••• 38 Magnetite •.••• • ••••••• 38 Orthopyroxene • . ...................... 42 Clinopyroxene. 43 v Mineralogy cont. Orthopyroxene and clinopyroxene relations •..••••.••••• 43 Olivirie, • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •••••••••••••••••• 44 Gamet . ................ ·· ......•.......................4 7 GJ::1.JI1eri te . • • . • . • • . • . • • • . • . • . • . • . • • • . • . • • • • • . • 4 7 Orthoarrphi.J:x:>le •.•••••••••••.••••••••••••..•.•.•••••••. 49 Ila.stingsi-te ......................................... " . 4 9 Accessory minerals .•.•.•.•..••••••.•..•••..••...•.•••. 49 Alteration Alteration of olivine .•..•.•••...•.•.••••....•...•.•.•. 50 Alteration of pyroxenes ••••.•..•••........••.••...••.• 52 Silicification •••••.••.•.••••..•••••••.••.••••••.••••.••..• 55 THE SPRUCE CREEK IRON-RICH BODY In-trOO.uction . .............................................. 60 Petrography Introduction ...••.••••.•••••••. ........ 60 Pyroxene-rich rocks: 'fype 1 In-trOO.uction ••...••.... ••.••..•.••..•.•••.•••.. 61 Mineralogy: 'fype lA.... ........................ 61 Mineralogy : 'fype .1B •••••••••••••••••••••••••••• • • • • • • • 66 Gamet-biotite rock with intergrcwths: 'fype 2 In-trOO.uction •••..••.•.••••.•..••..••.•..•••..•......•• 67 Mineralogy : 'fype 2 • • • • • • • • • • • • • • • • • • • • • • • • •••••• 6 9 Quartz-rragnetite intergrCMths: 'fype 2 .....•........... 70 Alteration: 'fype 2 •••••••••••••••••••••••••••••••••••• 7 5 Gamet-biotite rock: 'fype 3 In-trOO.uction ••..••••••..••..•••••••.•.••••..••••.•••.• 78 Mineralogy : 'fype 3 •••••••••••••••••••••••••••••••••••• 7 8 Uranium mineralization •...••.•.•...••••.••..•.•.•.•••••.•.. 79 GEOPHYSICAL SURVEYS Introduction . .............................................. 8 0 .Magnetorreter survey ••..•••...•••••..•••••••••• ·•••••.••.•••. 8 O Scintillorreter survey ••••••••••••.••••••••••••••••••••••••• 8 3 IN'I'ERPREI'ATION OF IRON-RICH BODIES Protolith.••••..•.••••.••••••••••. ••••••••• 86 Geothenrorretry and geobararretry .•...........•••.•. • •••••••• 89 Phase relations and alteration in the Wildcat l:ody •••.•••• 110 Phase relations and alteration in the Spruce Creek l:ody ••• llS Pelative position of the iron-rich bodies ••••.•••••••••.•• 119 . ...................................................... 121 REFERENCES CITED • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • .125 APPENDIX A ••••••••••••••••••••••••••••••••••••••••••••••••••••••Al APPENDIX B •••••••••••••••••••••••••••••••••••••••••••••••••••••• Bl vi ILLUSTRATIONS Figure Page 1.1. location of study area ••••.•.•••••..••••••••••••••..•...• 2 1.2. location of iron-rich bodies •••••••.••••••••••••••••••••. 3 2 .1. Migmati tic gneiss ••••••.••.••••••••••••••.••••••••.••••. 10 2. 2. Corrplexly folded gneiss •.••••.•••.•..••••.•.••..•••••••• 11 2.3. Gneiss with planar foliation ••.••••••••••.•••.•.•••..••. 11 2.4. :cark, fine-grained anphibolite •.•••••..•••••••••..•••••• 14 2.5. Coarse-grained anphibolite with felsic layers .•..•.••••. 14 2.6. IYbdal classification of felsic rocks from Wann Springs Pluton •.••..••...•••••.•..••••••••••.... 17 2. 7. Photomicrograph of plagioclase lat.1is sur- rounded by magnetite •.•••••.•••••••.••.•..•.•••.•.••• 23 2.8. Pegmatite ve.in •••••••••••••••.•••••••••••••.••••••••.••• 26 2.9. Photomicrograph of free-gmvth textures in quartz ••••.•. 26 2.10. Photomicrograph of grc:Mt.h-rrosaic textures in quartz ••••• 28 2.11. Photomicrograph of overgrowth on quartz ••.••••.•.••.. ·... 28 2.12. Ridge of Flathead sandstone •••..•••••••••••••••.•.••...• 29 2 .13. Plot of structural data for the Wann Springs field area ••••••••.••••.••.•••.•••.•••.•.••. • .••.••.• 31 2.14. Photomicrograph of rec:rystallized IT\Ylonite ••••••••.••••• 33 3.1. ViE!N of the Wildcat body •.•••••••.••••.••••••••.••...••• 36 3.2. Plan viE!N of the Wildcat body .••.•••••••••••••.••.•••••• 37 3.3. Photomicrograph of slightly