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 retrograded, high-- grade assemblage, Wildcat body ••••••...... ••••..•.. 39 3.4. Photomicrograph of magnetite grains surrounded by quartz, Wildcat body •••••••••••••••••••••••••.•.•• 41 3.5. Photomicrograph of orthopyroxene and clino- pyroxene intergrowths, Wildcat body •••••••••••••••••• 45 3.6. Photomicrograph of orthopyroxene and clino- pyroxene as inclusions in each other, Wildcat body ••••••.•.•••••.•.•••••••••.•••••..•••••.• 46 3.7. Photomicrograph of intergrowth of olivine, ortho- pyroxene and clinopyroxene, Wildcat body ••••••••.•••. 46 3.8. Photomicrograph of olivine grains with alteration rims of grunerite, Wildcat body •••••.•.•••••••••••••• 51 3.9. Photomicrograph of progressive alteration of orthopyroxene to fibrous anphibole, Wildcat body ••••• 53 3.10. Photomicrograph of orthoarrphibole and grunerite, Wildcat body .••.•••.•••.•••.•.••••...•...••.••••.•••• 54 3.11. Photomicrograph of hastingsite and grunerite, Wildcat body ••••••••••••••••••••••••••.•••••••••••••• 54 3.12. Photomicrograph of multiphase pseudorro:r:ph after iron-silicate, Wildcat body ••••••••....•.•••...•••... 57 3.13. Photomicrograph of quartz and magnetite relations, Wildcat body .•••••••.••••••..•.••••••••••• 58
vii ILLUSTRATIONS CONT.
Figure Page 4 .1. Abandoned uranium mine within the Spruce Creek l::x:x1y •••••• 61 4.2. Plan of Spruce Creek l::x:xly •••••••••••••••••••••••••••••••• 63 4.3. Photomicrograph of orthopyroxene-rich assemblage, Spruce Creek l::x:x1y • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 6 8 4. 4. Photomicrograph of fine-grained pseudonorphic intergrc:wth of quartz and rragnetite, Spruce Creek oocy . . • ...... • . . . • . . . . • ...... • . . . • ...... 71 4.5. Photomicrograph of recrystallized pseudonorphic intergrc:Mth of quartz and rragnetite, Spruce CJ:eel< body ...... 7 2 4.6. Photomicrograph of pseudorrorphic intergravth suggesting replacerrent of both pyroxene and arnphil::ole, Spruce Creek l::x:xly •••••••••••••••••••••••••• 73 4.7. Photomicrograph of pseudorrorphic intergravth with fresh biotite and garnet, Spruce Creek l::x:x1y •••••• 7 6 4.8. Photomicrograph of rock type 3, Spruce Creek body •••••••• 77 5.1. Iandsat photograph of the Wann Springs field area .•••••.. 81 5.2. Iocation of lineament through the Wann Springs field area ...... •...... 8 2 5.3. Results of the rragnetorreter survey .....•..•.•...••.•..... 84 6 .1. Corrpositions of grains probed from the Wildcat body •.••.. 90 6.2. Corrpositional changes across probed grains •••.•..••.•.... 91 6.3. Results of geothenrorretry:- Area B .•....•••..•.•....•• 95/96 Area A •••••••••••••••••••• 97 /98 Area c ...... 99/100 6.4. Orthopyroxene-olivine geothel:JtOITEter •••.•••••••••.•••••. 105 6.5. Pigeonite geotbentD!lEter •••••..••••.•••••.•.•..••••••••. 105 6.6. Temperature, pressure and corrposition relations of olivine, orthopyroxene and quartz ••..•••.••••..••• 107 6. 7 • . Pressure-temperature projection shCMing stability of olivine, orthopyroxene and quartz ••.•••••••••••..• 108 6.8. Orthopyroxene geobarorreter ••••.•••••••.••••••••.•••••••• 112 6.9. Corrpositions of coexisting olivine and orthopyroxene •••. 112 6.lO. Corrpositions of coexisting grunerite and orthopyraxene ...... 113 6.ll. Corrpositions of coexisting grunerite and olivine •••••••• 113 6.12. Oxygen fugacity with respect to temperature for rretarrorphic iron-fonnations •••••••.••••••.••.••••••.• 116 TABLES
Table Page 2.1. M:xies of sarcples from Wann Springs Pluton ••••.•.••..••..• 18 3.1. Mineral assemblages in the Wildcat l:ody .•••••..•••.••.••• 40 3.2. Microprobe analyses of ga:i:net •.•••.•..••.••..••.••.•••••• 48 4.1. Mineral assemblages in the Spruce Creek l:ody •..••.••.•.•. 64 6.1. Previuos studies on high-grade rretarrorphosed iron-fonra.tion •.••.••••.••••.•••.••.•.••.•••..•••.•.•. 87
PLATES Plate Page 1 Geology of the Warm Springs field area ••.•••.••••••.• pocket 2 Sample map of the Wildcat l:::ody ••••••••••••••••••••••• in pocket 3 Sanple ma.p of the Spruce Creek l::xJdy .•.....••••.••••.• in pocket
ix INI'RQDUCI'ION
Problem
This thesis is ooncemed with the petrology and structure of an
Archean gneiss terrain near Dubois, Wyoming. The area is predominantly composed of migmatitic gneiss with arnphibolite, bounded unconformably to the east and northeast by Paleozoic sandstones and limestones. Within the area two unusual iron-rich bodies crop out, one associated with a small uranium prospect. The objectives of the thesis are to: 1) map and describe the regional geology; 2) determine the structural and metamorphic history of the area; 3) undertake a detailed petrographic study of the two major iron-rich bodies and determine the nature and origin of these rocks; 4) investigate the possibility of other economically viable bodies being found in the area.
Location .2ruJ Access
The Warm . Springs field area is located nine miles west of Dubois, in Fremont County, Wyoming in the northern part of the Wind River Mountains (Fig. 1.1). The area covers six square miles within the southern part of T.41N,R.107W and the northern part of T.40N,R.107W. Access to the area is good, via the Wildcat Loop forest road off the Union Pass road north of Dubois (Fig. 1.2). The northern iron-rich body, the Wildcat body, crops out on a sharp corner in Wildcat Loop, a mile south of Wildcat Creek. The
1 MONTANA -·-·-·-·-·-·-·-·-·-·-·-·WYOMING ---·-·-·-·-·-·-·-
us 16
... ----- ... " ' '. 0.,,,... ' . -; . \ "' ' ......
\ '·-\"¢.. '--i' ... -\ \
I 'I I .1 I, \ WARM SPRINGS FlELD\AREA ·.:'.
• .. '. 0(. • .... , .... , 9,.-:" I "" ·,·'-.. ) - Map area l ' ' Miias 0 30 WYOMING
Figure 1.1. Location of study area.
2 ' '
·., '•
,''-'.. _,'• .' ,'' I ' ... , ,• q,.. : ....0"' ,I' :;,r::: ,' I i ,-'' ·-;"\· ,._,, "-. .._ . /'.. .,,...... ·- .\ -. '· \ ) .I ( ") r i N
-.. .., Mlddle Min CO)\TINENT AL_,Dr'/iO'r:, ·--·-·-· .I
/ • Wiidcat Body I Spruce Creek Body \ \ ·, Downs Mtn
0 5
Miies
Figure 1.2. Location of iron-rich oodies.
3 southern body, the Spruce Creek body, lies five hundred yards southwest of where Wildcat Loop crosses Little Warm Springs Creek.
Methods S2f The thesis is based on seven weeks of field investigations during July and August of 1982. Regional mapping was done on a scale of 1:20,000 and the iron-rich bodies were mapped on a horizontal scale of 1 inch to 20 feet and a vertical scale of 1 inch to 5 feet. The bodies were sampled at approximately 2 foot intervals. Magnetometer and scintillometer surveys were performed over a small part of the field
area which incortx>rated the Spruce Creek body. Approximately 350 hand specimens were collected and brought back for laboratory study. Of these samples 145 were chosen for thin section study. Estimates of modal mineralogy for the plutonic rocks are given in Table 2.1 and these rocks are classified by the IUGS classification. Microprobe analyses were performed on olivine, orthopyroxene, clinopyroxene and garnet and the results are listed in Appendix A and Table 3.2.
Previous 1Qtk There has been little previous work in the field area. Keefer (1957) studied the Paleozoic sediments to the the Archean rocks were briefly mentioned. Other work on the Archean rocks has concentrated on other areas in the Wind River Mountains and will be discussed under regional geology. An unpublished manuscript by Ullrner and Honea (1981) included reconnaissance petrographic studies of the two major iron-rich bodies, suggested tx>Ssible structural controls and
4 \. _
emphasized the need for further study. That is the purpose of this thesis.
Regional Geology
The Warm Springs field area is situated within the Wind River Mountains, a major anticlinal structure with the axial plane dipping
to the northeast. The range is 150 miles long and 40 miles wide, with
a maximum elevation of 13 800 ft. The mountains have been eroded to
expose the Precambrian core, unconformably flanked Paleozoic sediments to the east and northeast. To the west of the range is the Wind River thrust. This has brought the Archean rocks westward over the Mesozoic rocks. Previous investigations of the Precambrian are scant, especially
in the north. The southern end of the range is composed of metasedirnents and rnetavolcanics metamorphosed to greenschist facies, and is considered to be a greenstone belt. The northern end is mainly gneissic and granitic rocks. Between these two areas lie the later- stage intrusions of the Lewis Lake, Bear's Ears and Middle Mountain Batholiths. These are composed of granitic to granodioritic rocks.
Bassett and Giletti (1963) and Naylor et al.(1970) dated these
intrusions at 2700 b.y. and suggested this dates a regional metamorphism that affected the core of the range at this time. Oftedahl (1953) studied the northern area to the west of the continental divide and recognized three major rock units. The major unit is a granitic gneiss complex with migmatites. These rocks are
5 very heterogeneous and are granitic to quartz-dioritic in composition. He noted both K- and Na-rich porphyroblasts. The second unit consists of dikes and other bodies of metagabbro which cross-cut the gneiss. The third comprises Late Precambrian diabase dikes which cut the first two rock types. The first two units have been metamorphosed to amphibolite to granulite facies; the diabase dikes are unaltered. Study of drill cores by Ebens and Smithson (1966), from the western edge of the Precambrian directly west of Lander in the southeast end of the range, showed rocks ranging from quartz-dioritic gneiss to quartz-monzonitic gneiss with no systematic relationship between depth and composition. They interpreted these rocks as a sequence of gently-dipping, layered, heterogeneous metamorphic rocks. The metamorphic grade did not appear depth controlled and they could not unravel the complex metamorphic history. Worl (1968) studied the area near Downs Mountain (Fig. 1.2). Here the rocks are granite, quartz-plagioclase-biotite gneiss, amphibolite and metadiabase. These are all part of a migmatitic complex in which mixing on various scales has taken place. At least two phases of folding were inferred in the gneiss. He noted late- stage, unmetamorphosed dikes of granite, pegrnatite and aplite. These rocks have been affected by at least three periods of deformation (Mitra and Frost, 1981), and two periods of metamorphism. Deformation zones (Mitra, 1978) formed during the Early Precambrian are characterized by recrystallized mylonites. These may have been f orrned during several different events, but they are now inseparable. However, at least two periods of folding are seen in the gneiss and,
6 according to Mitra and Frost (1981), each event was probably associated with the development of deformation zones. The second generation of deformation zones were formed during the Late Precambrian, 900-600 M.y (Condie, 1969). These cut the crystalline rocks whereas the Phanerozoic succession is unaffected. The third group of deformation zones was formed during the Laramide orogeny (Late Cretaceous to Early Eocene). The fractures trend in three main directions, N45°E, and N25°w (Worl, 1968). The earliest metamorphism was to granulite facies; the second varied over the area, from granulite to arnphibolite facies (personal communication, B.R. Frost, 1983). The time relations between these two periods of metamorphism and the Early Precambrian fractures is unclear. However, as the mylonite zones are recrystallized it appears that a least one of the metamorphic events post-dated this def orrration. There was widespread erosion during the Late Precambrian which resulted in a major unconformity. During Cambrian to Jurassic times a thick sedimentary sequence was deposited: this shows evidence of several marine transgressions and regressions. The Laramide orogeny in Late Cretaceous to Early Eocene times involved folding, overthrusting and faulting of the range. The general structure of the Wind Rivers is now a huge anticline, overturned and thrust to the southwest with the major thrust zone being to the west of the mountains. To the east side of the range, a series of smaller anticlines occur in the Paleozoic strata with their fold axes parallel
7 to that of the major fold. Post-Wasatch (i.e. post-Early Eocene) sediments lie horizontally against and over these features. Erosion during Oligocene times has exposed the Mesozoic beds to the east but not to the west of the Wind River Range.
8 Introduction The Warm Springs area is situated in the northern end of the Wind River Mountains in an Archean gneiss terrain. The major rock types are quartzo-feldspathic gneiss, amphibolite and felsic igneous rocks that form the Warm Springs Pluton (Plate 1). Within this terrain two unusual iron-rich bodies crop out; these will be described in the following chapters. The majority of the area is covered by glacial drift and outcrop is generally poor, however, near the canyon in the northeast there is almost 100% exposure.
Q:iartzo-felaspathic Q)eis..§ JJnit Introduction In the Warm Springs area the main map unit is dominated by quartzo-feldspathic gneiss. It also includes minor amphibolite, iron- rich granofels and late-stage felsic dikes. However these rock types only occur in relatively small bodies and it is not possible to map them on the scale used in this study.
(bartzo-feldspathic gneiss The gneiss varies in color from almost white to deep pink, yellowish green or black and white. The gneiss is weakly to well foliated. Typically the more biotite-rich samples show the better
9 . Figure 2.1. Mignatitic gneiss near Geyser Creek.
10 ...... f-'
Cabove) Conplexly folded gneiss the Spruce Creek body.
(left) Gneiss with planar foliation near the Wildcat body. foliation. Most of the rocks are relatively equigranular but some show alkali feldspar megacrysts. The composition varies from granitic to granodioritic. In small areas it is migmatitic, with alternating dark and light areas swirled in a complex pattern (Fig. 2.1). The gneiss is strongly folded in some outcrops (Fig. 2.2) but shows no folding in others (Fig. 2.3).
The gneiss is composed of quartz (20-50%), alkali feldspar (0-
60%), plagioclase (10-55%), biotite (5-30%), amphibole (0-8%), retrograde chlorite (0-20%), magnetite (l-8%), and trace amounts of apatite, epidote and sphen.e. The modal feldspars range from 100% plagioclase to 80% alkali feldspar and 20% plagioclase.
Quartz forms rounded, embayed grains up to 6 mm in size. They may be slightly to highly strained or partly recrystallized. The alkali feldspar is slightly perthitic microcline. It occurs both as large anhedral crystals reaching 20 mm in size and as smaller grains in the more equigranular gneiss. In thin section, several features suggest that the megacrysts are metamorphic and not phenocrysts. The grains have highly embayed outlines whereas the plagioclase typically forms almost euhedral laths. Much of the plagioclase is altered but the microcline is relatively unaltered.
Within the large grains, inclusions of quartz, plagioclase and chloritic material can be seen; these may show an alignment suggesting that an ,early internal foliation was overgrown by microcline.
Together, these features indicate that the alkali feldspar grains are metamorphic in origin.
In the equigranular gneiss, the grains of alkali feldspar are
12 also highly irregular. Generally they are approximately 3 mm in size.
The alkali feldspar shows evidence of strain and the porphyroblasts are locally fractured. The biotite defines the foliation in the gneisses. It is typically strongly pleochroic from pale yellow to dark red-brown. The biotite may be fresh or totally altered to chlorite. Rarely the biotite is kinked around the feldspar grains.
The amphibole is pleochroic from aark olive green to pale green and is probably hornblende. It forms small subhedral grains up to 2 mm in length. The hornblende also tends to define the foliation.
Magnetite forms small irregular grains. In certain areas it is fairly abundant, accounting for 8% of the assemblage. In the more altered gneisses, chlorite occurs as an alteration product of biotite. Trace amounts of sphene, apatite and secondary epioote occur. Anphibolite
The amphibolite occurs as small pods within the gneiss. Where contacts are seen, the two units are generally conf orrnable (Fig. 2.4).
The amphibolite units range in thickness from a few centimeters to a meter and up to 5 meters in length. Folding may or may not be seen depending on the cutcrop. There appear to be two different types of amphibolite. One is fine-grained and almost black (Fig. 2.4) whereas the second is normally coarser-grained, has a green color and is associated with pods and layers of felsic material (Fig. 2.5). Both show excellent foliation.
13 I-' ""'
(above) Dark, fine-grained amphibolite with gneiss on Wildcat IDop road south of the. Wild- cat body. 'Ihe two units are generally confonmble.
Figure 2.5. (left) Green, coarse-grained amphibolite with quartzo-feldspathic pods and layers. On Wildcat IDop road just south of Little Warm Springs Creek. The first unit is composed of amphibole (50-70%), plagioclase (30-40%), biotite (0-5%), quartz (5%), magnetite (3%), and trace amounts of apatite.
Arnphibole grains are subhedral and are up to 2 mm in length. The arnphibole is strongly pleochroic (a ,pale yellow, 8 ,olive green,y ,deep green) with a negative sign and a 2V of 40-50°. The mineral is probably hornblende. Plagioclase occurs as anhedral grains up to 1 mm in diameter. Quartz occurs as small equant grains typically under 1 mm across. Biotite is locally present and is strongly pleochroic from straw yellow to dark brown. The crystals are generally elongate parallel to cleavage, reaching 10 mm in length. The grains occur in random orientation, commonly at a high angle to the foliation and thus they appear to be later than the foliation. The second type of amphibolite is composed dominantly of amphibolitic bands with minor felsic bands or t=eds. Contacts with the surrounding gneiss are not seen. Within this arnphibolite the minerals are amphibole (40-70%), plagioclase (25-55%), quartz (0-5%) and biotite (0-5%) with minor magnetite and apatite. The felsic bands can range from 100% plagioclase to 75% plagioclase, 20% quartz and 5% rnicrocline. This amphibole is much paler in color, being pleochroic from medium-green to almost oolorless. Grains are subhedral and are up to
10 mm in size. Again the mineral is probably hornblende b.It more Mg- ri ch than that in the first amphibolite. Plagioclase occurs as anhedral grains up to 3 mm in size. It is typically :i;:artly altered to
15 sericite. Quartz occurs as small roundish grains less than 1 mm across and commonly occurs as inclusions in the arnphibole. The arnphibolite assemblage is altered near the felsic material, with the plagioclase reacting to sericite and the p:ile hornblende to a blue-green arnphibole, possibly ferroactinolite, and chlorite. Iron-rich Material Rarely, conformable iron-rich pods under 1 meter in size occur within the gneiss. The material is highly altered but it is distinctive due to its strong magnetism. These pods consist of quartz (30%), chlorite (50%), and magnetite (20%) with uncommon grains of unaltered biotite. Pieces of iron-rich float similar to these and to the rocks described in the following chapters occur sporadically throughout the field area.
NatID Springs Pluton Introauction The area to the east and northeast of Warm Springs Canyon is composed of igneous plutonic rocks, ranging in composition from granite to quartz monzodiorite (Fig. 2.6). Cross-cutting relationships are common, the more felsic rocks being the youngest. The actual contact of the intrusion with the surrounding gneiss was not seen. This is due to difficulties in mapping within the steep- walled canyon and the similarities of the two rock types. The gneiss and the felsic plutonic rocks are commonly very similar in outcrop,
16 Q Q
5
IUGS CLASIFICA TION
I-' -...J He JI. •P
•L c• •A
A p
Figure 2.6. M:>dal classification of felsic intrusive rocks from the Warm Springs
Pluton. See also Table 2.1.
Q = Quartz, A = Alkali feldspar, P Plagioclase. 11™ _OE SJ\MPTffl Em1 lIDEM SPRitNS PLUIPN
A c L M p R QUARI'Z 8.1 9.0 10.5 34.7 24.3 33.5 K-FELOSPAR 77.2 14.8 9.0 30.3 17.2 23.2 MYRMEKITE tr 1.3 tr PUJ:;IOCLASE · 11.l 50.2 52.3 32.3 32.7 36.5 BIOI'ITE 0.8 12.0 CHWRITE 0.7 18.3 17.2 1.0 6.0 5.5 MUS CCVI TE 0.1 tr AMPHIBOLE 0.3 1.5 MAGNETITE 0.8 4.0 3.2 0.3 0.3 0.7
EPIOOI'E 1.0 2.5 4.2 tr 4.2 0.3 SPHENE tr 0.5 1.2 1.2 0.1 APATITE 0.3 0.7 2.2 0.3
ZIRCOO tr
CALCTTE tr
QUARI'Z 8.5 12.2 14.6 35.6 32.8 36.0 K-FELI:SPAR 80.0 20.0 12.5 31.2 23.1 24.9 PLll.G IOCLASE 11.5 67.8 72.8 33.2 44.1 39.l
A Quartz syenite M Granite c Quartz nonzodiorite p Granodiorite L Quartz nonzodiorite R Granite 600 points counted for each sample. All samples from the Warm Springs Pluton to the east of the Warm Springs Canyon.
18 being distinguished mainly by the presence or absence of a foliation. The alkali-feldsp;ir phenocrysts within the Warm Springs intrusion show straight, regular grain boundaries and commonly have Carlsbad twins, whereas the porphyroblasts in the gneiss generally show irregular outlines. Warm Creek appears to follow the contact approximately (Plate 1). Granite The granitic rocks are p;ile pink, coarse-grained and relatively equigranular. They are composed of quartz, alkali feldspar, plagioclase and less than 6% maf ic minerals. Quartz is typically strained with most crystals 2 mm in size but locally reaching 10 mm. It is interstitial to the feldspars. The alkali feldspar is microcline showing excellent cross-hatch twinning. The crystals are anhedral and grain size is again typically 2 mm. The microcline is fairly fresh. The plagioclase is oligoclase and is generally highly altered, mainly to sericite with a little epidote. The plagioclase occurs as subhedral to euhedral laths up to 3 mm in length. The major mafic mineral was probably biotite. This is now altered to chlorite and a little magnetite. Trace amounts of primary magnetite, sphene and ap'3.tite are present. Granodiorite The granodiorite is a very coarse-grained, porphyritic rock. It is distinctive in having a yellow-green groundmass with large pink phenocrysts. The rock consists of quartz, alkali feldspar, plagioclase, hornblende and biotite with several minor alteration
19 '.
minerals.
Quartz is interstitial and strained with grains up to 4 mm. The alkali feldspar occurs only as large phenocrysts of microperthitic microcline. The phenocrysts have a euhedral form, show Carlsbad twinning and are up to 20 mm in length. Inclusions of plagioclase and quartz occur within the phenocrysts. The mineral is only slightly altered.
The plagioclase occurs as subhedral laths up to 8 mm in length. It is oligoclase to andesine in composition. The plagioclase is moderately altered to sericite, epiCbte and a trace of calcite• . Mafic minerals account for approximately 20% of the rock. The amphibole occurs as subhedral grains up to 2 mm in size. It is pleochroic from medium-green to pale brown with a negative sign and 2V of 50-60°. It is probably hornblende. Biotite is the major maf ic mineral with grains up to 10 mm. It is pleochroic from pale yellow to dark brown and is partially altered to chlorite. Epidote is the main alteration mineral, occurring as subhedral grains up to 3 mm in size. It shows a yellow-green to pale yellow pleochroisrn. Epidote occurs as small grains replacing the plagioclase; however, it appears to replace the biotite preferentially. Abundant elongate grains of sphene, up to 1 mm in length, occur aligned parallel to the cleavage of the chlorite which has partially replaced the biotite.
20 In thin section, the rock is compJsed of 77% slightly :perthitic microcline with grains up to 15 mm in length. These feldspars have inclusion of quartz, plagioclase and apatite. The plagioclase occurs as subhedral to anhedral grains. It is altered to sericite and a little fine epioote. CUartz is interstitial and strained, with grains up to 6 mm. The major mafic mineral is biotite, now partly altered to epidote, chlorite and a little magnetite. Again the epidote replaces the biotite preferentially. Q]artz monzoaiorjte The quartz monzodiorite is a medium- to coarse-grained, greenish- gray rock with pink phenocrysts of alkali feldspar • . The quartz occurs as strained anhedral crystals up to 3 mm in size. Alkali feldspar is only present as phenocrysts up to 15 mm in length. This feldspar is perthitic orthoclase and again it has inclusions of quartz and plagioclase. The plagioclase forms subhedral laths of andesine up to 5 mm in length. It is highly altered to sericite and a little epidote. Biotite is the main maf ic mineral, now partially altered to chlorite and epidote. The crystals are less than 1 mm in diameter and appear interstitial to the feldsi;:ars. A little hornblende is present, now highly altered. 21 Magnetite accounts for 4% of the rock and forms small euhedral grains 1 mm in size. Sphene, apatite and zircon are also present. As stated earlier, cross-cutting relationships of differing rock ty};es are common. In one outcrop a irreg.ilar block of tmder 1 meter in size composed of quartz monzodiorite is enclosed in a very coarse- grained granitic rock. In thin section abundant myrmekite is seen along the margin between these two rocks. One unusual rock is coarse-grained, greenish-yellow and strongly magnetic. It is composed 90% of plagioclase surrounded by interstitial magnetite (Fig. 2.7). A trace of microcline also is present. The plagioclase forms euhedral to subhedral crystals up to 20 mm in size. It is altered to sericite and fine-grained epidote. Epidote is also present within the magnetite, filling small fractures. It also forms small rnonomineralic patches in the rock. In one area a grain showing a rnicaceous cleavage appears to be completely pseudomorphed by epidote. It is plausible that this rock type is due to assimilation of some of the iron-rich material. The pegmatites that occur in the area are also plagioclase-rich. It is possible that the two rock types have a similar origin. Xenoliths of amphibolite can be seen within the intrusion. These were more altered than most of the amphibolite found within the gneiss. Near the contact with the igneous rock the amphibole is altered to chloritic material, however the actual contact is sheared and no other reactions can be seen. 22 Figure 2.7. large euhedral plagioclase laths surrounded by rragnetite within unusual pegrratite cutting the Wann Springs Pluton. 'Ihe plag- ioclase is partly altered to sericite and epidote. Crossed nicols. Field of view is 2.4 x 3.5 rnn. WSCE FL B. 23 Younger Dikes ,gnQ O.lartz Throughout the area dikes cross-cut the various Archean rock types. These range in composition from granite to granodiorite and texturally may be aplites and pegrnatites. The dikes can be seen cross- cutting both iron-rich bodies. The granitic dikes are very similar to those rocks found in the Warm Springs pluton. They are composed of strained anhedral quartz (20-40%), perthitic microcline (30-60%), altered plagioclase (20-40%) with magnetite, chlorite and apatite. The dikes that cross-cut in the Wildcat body vary between granodiorite and tonalite in composition. They are coarse-grained, grayish-white and composed of quartz, plagioclase, alkali feldspar and biotite. Quartz forms 40% of the rock and occurs as strained grains up to 1 cm across. Plagioclase forms up to 40% of the rock and varies from oligoclase to andesine in composition. Alkali feldspar accounts for 1- 15% of the rock; it is usually perthitic orthoclase. Both feldspars are altered, the plagioclase much more highly. Biotite is partly altered to chlorite and is highly kinked. Chlorite also occurs as a late-stage miner.al independent of biotite. Trace amounts of epioote, pyrite and zircon are present. In the Spruce Creek body a little tonalite is present. The rock is composed of quartz (30%), plagioclase (60%), alkali feldspar (5%), and chlorite (2%) with magnetite and epioote. The quartz is anhedral and highly strained. Plagioclase is completely altered to sericite and a little epidote. The alkali feldspar is microcline and occurs as 24 small anhedral crystals. Biotite is typically altered to chlorite. The epicbte occurs preferentially with the biotite. The tona.lite is very similar to that in the Wildcat body except that it is more highly altered. The tonalite cross-cutting the Spruce Creek body is commonly brecciated into small angular fragments. The plagioclase grains generally show intense fracturing and kinking of the twins. These fragments are surrounded by very fine rock powder which has abundant fine-grained magnetite and chlorite associated with it. These minerals are believed to have been deposited while the breccia was still very porous. The rock has since recrystallized and optically continuous overgrowths of the fractured quartz grains can be seen. These overgrowths are full of inclusions of the finely comminuted material. The whole assemblage is also cut by quartz veins. The pegrnatites are very coarse-grained igneous rocks, comp::>sed of quartz, plagioclase, alkali felaspar and biotite. Some of the biotite books attain a diameter of 8 cm. A crude alignment of these biotites can be seen in the pegmatite dike that cross-cuts the Wildcat body (Fig 2.8). This pegmatitic body is 18 feet high and ranges from 0-15 feet wide. Within the pegmatite monomineralic pods of biotite occur. Cross-cutting quartz veins are common. The veins typically show well-formed comb and free-growth structures (Fig. 2.9) but are also commonly cataclastic and show repeated fracturing and quartz veining through the same zone. Small fragments of the gneiss and other country rock types including rnylonite can be seen enclosed in these veins, the 25 Figure 2.8. Pegnatite vein cross-cutting the Wildcat body with large, aligned biotite crystals and biotite-rich tx>ds· Figure 2.9. Padiating, free-grc:Mth textures in quartz vein cutting the Wildcat body. Crossed nicols. Field of view is 2.4 x 3.5 nm. WSN 3L6. 26 quartz crystals radiating outwards from the rock fragment. The quartz veins in the Spruce Creek body show well-developed growth mosaic textures with the core of the grain being strain-free whereas the rest of the grain shows a complexly strained pattern (Fig. 2.10). These crystals also show overgrowths with the outer edge being full of inclusions. The mosaic pattern in the crystal continues into this overgrowth (Fig. 2.11). -- Often pieces of quartz showing growth mosaic texture are included as fragments in other quartz-rich veins suggesting repeated fracturing and infilling. Paleozoic Sediments To the northeast and east of the field area, Paleozoic sediments unconformably overlie the gneiss and the intrusion (Plate 1). The lowest formation of the succession is the Middle Cambrian Flathead Sandstone (Blackwelder,1918). This forms a prominent ridge in the area, trending approximately northwest-southeast. The sandstone dips at 30-40° to the east (Fig. 2.12). The lowest unit of the Flathead Sandstone is a gravelly arko_sic sandstone close to the contact with the Archean rocks, but the actual contact was not seen. The sandstone shows irregular beds up to 5 cm thick. Cross-stratification is poorly developed and some beds show weak normal grading. It is poorly to moderateiy sorted with grain size from fine sand to small pebble, with the typical size being coarse sand. The rock is composed of large angular to subangular grains of quartz, microcline and polycrystalline quartz, with finer- 27 Figure 2.10. Large quartz crystal with grcM'th nosaic rim, in brecciated tonalite, Spruce Creek l::ody. Crossed nicols. Field of view is 6 x 9 nm. WSS 19. Figure 2.11. Grcwthnosaic texture in quartz continuing into inclusion-full overgrc:Mth, brecciated tonalite, Spruce Creek body. Crossed nicols. Field of view is 0.6 x 0.9 nm. WSS 19. 28 Figure 2.12. Ridge of Flathead sandstone lying unconfonn- ably over Wann Springs Pluton to the east of Wann Springs canyon. 'Ihe sandstone dips away fran the canera viewp:>int. 29 grained quartz, microcline, muscovite and a little hematite which comprises the matrix. The upper units of the Flathead Sandstone are medium- to finely- bedded quartz sandstone. The rock is well sorted, with grain size varying from medium to fine sand. It is composed predominantly of rounded unit quartz grains with trace amounts of chert, polycrystalline quartz, microcline and hematite. The quartz is recrystallized at grain boundaries and quartz overgrowths are seen indicative of silica cementation. All the sandstones seen vary in color from a pale brown to a deep red-brown. This coloration is believed to be a secondary feature as it is not consistent within a given bed. Pleistocene Sediments Most of the area is covered by glacial drift of unknown thickness. The deposits are of till. The pebbles and boulders are granitic rocks or quartzite, some being over 1 meter in size. Structure The gneiss varies from being complexly folded to having just a planar foliation. Where possible measurements of the foliation and fold axes were recorded. Several measurements of mineral lineations in the amphibolite were also recorded. The results are shown on the stereonet in · Figure 2.13. As can be seen, due to the complex nature of the area with at least three phases of deformation ( Mitra and Frost, 1981), very 30 ',,, '•. ' .. .. __ ' ' ' ' ' ' \ ,,,/ \ , ' \ ,' ' ' \ ' ' \ ' \ I ' ' I ' \ : \ I 'I I I ' \ '' . I . I I ' ' ' 0 ' 0 ' 0 I 0 I ' * I' :I I I I ' 'I ... I ' ' \ I \ I ' I \ .. I I I \ I \ I ' \ I \. \ ' \ \ / ' \ ' \ ' ' ' , ' ', / ' ', \ \ \ \ ' ,';, ' ...... ' ', ',, ...... ,, ___ ------4------·- * Fold Axea $ O llneallon Figure 2 . .13 . :Plot of structural data from the Wann Springs area. 31 little can be obtained from these results. From the map (Plate 1) there appear to be two general trends to the foliation, one at approximately N60°w and a second at approximately N80°E. As stated earlier, in several places the gneiss shows complex folding (Fig. 2.2). At least two phases of folding can be inferred in the gneiss at the southern end of the field area shown by refolded folds. Mylonites are also seen in the area, especially near the Spruce Creek body. These show porphyroclasts of plagioclase and alkali feldspar and many of these are fractured. The rock has been recrystallized and the quartz grains now show little evidence of strain (Fig. 2.14). The quartz-rich layers, once probably ribbon quartz, still show abrupt changes in grain size suggesting the rock has not yet reached an equilibrium texture. A Landsat photograph of the area shows a lineament passing through both of the iron-rich bodies (Fig. 5.1). This trends approximately N25°w, the major Lararniae trend in the area. The direct cause of the lineam:nt on the ground has not been determined. Conparison .t.Q Regional Geology The geology of the Warm Springs area is comi;:atible with previous studies in the Wind River Mountains. The main lll'lit is a heterogeneous quartzo-feldsi;:athic gneiss ranging from granitic to granodioritic in composition and is locally migrnatitic. This agrees with other authors such as Oftedahl (1953), Ebens and Smithson (1966) Worl (1968) and Frost ( pers. comm., 1983). Oftedahl and Frost note bodies of 32 Figure 2.14. Pec:rystallized Iey"lonite within the Spruce Creek body. 'Ihe feldspar porphyroblast is fractured but the quartz grains shew little evidence of strain. Crossed nicols. Field of view is 2.4 x 3.5 ran. WSS FL E. 33 ' . metagabbro, metadiabase and other mafic bodies. Worl indicates that there are toth orthoamtiiibolites and para-amtiiibolites based on relict diabase textures in some samples; however, the distinction was commonly imi;:ossible to make. In all the studies cited above large alkali-feldspar porphyroblasts occur in the gneiss. They report late-stage cross- cutting dikes, which include granites, p:gmatites and aplites. Oftedahl and Ebens and Smithson note that the metamorphic grade varies from arnphibolite to granulite facies. Frost (p:rs. comm., 1983) believes that the rocks in the northern Wind River Mountains are now of amphibolite facies with small relics of a prior granulite facies metamorphism locally preserved. This is precisely what has been found in the present study in the Warm Springs area. 34 _Tm; WIIJXM' IRON-RICH .W12X The Wildcat body crops out in a road cut on the Wildcat Loop forest road, one mile south of Wildcat Creek (Fig. 1.2). The outcrop is highly weathered to a rusty brown color (Fig. 3.1) and is very friable. Contacts with the surrounding gneiss are not exposed. A simple grid system was used for sampling the outcrop. This was set up by tape and compass and is shown in Figure 3 .2. Due to the intense weathering of the body, pieces of float.were also collected to complement the sampling. As can be seen in Figure 3.2, the exposure consists of iron-rich 11 pods 11 cut by granodiorite dikes and a pegmatite. There is a small outcrop of biotite gneiss which is very similar to the surrounding gneiss; however, the contact is covered. The granodiorite and pegmatite appear to have locally affected the iron-rich rock but the dominant mineral assemblages are due to the high-grade regional metamorphism. Although the outcrop is only 230 feet long by 20 feet high, it shows an unusual variety of metamorphic mineral assemblages and textures. Petrography Introauction The rock is a rusty-brown weathering, dark green to black, medium to coarse-grained granofels. Quartz, magnetite, olivine, garnet, pyroxene and locally amphibole can be seen in the hand specimen. All samples are highly magnetic. 35 Figure 3 .1. View of the Wildcat body looking south. The rusty- brown weathering rock is iron-rich granofels, the lighter colored rock is biotite gneiss. See also figure 3.2. 36 WILDCAT BODY (NORTHERN IRON - RICH BODY) PROJECTION ONlO VERTICAL PlAUE --- -·-- -1 ______j______---·--- _j _____ "->-----+------' 5 3 w 2 °'1 I E r G J K M SCALE KEY WEATHFRED ROCt< O SAMPLE TAKEN 70 FT 1 .. Mineral AsSerrblages All the mineral assemblages found in the 49 samples of the Wildcat body examined -p:!trographically are listed in Table 3.1. The highest-grade metamorphic assemblage seen consists of quartz, magnetite, orthopyroxene, clinopyroxene, olivine and garnet in a medium-grained, equigranular, granoblastic texture. The assemblage appears to have approached occasionally there is no trace of retrogressive alteration, however, the olivine is normally p:irtly altered to grunerite (Fig 3.3). This assemblage is relatively rare in the outcrop. The dominant assemblage is quartz, magnetite, orthopyroxene, clinopyroxene and garnet with some retrograde minerals, mainly arnphiboles. Mineralogy OJartz Quartz is ubiquitous throughout the body, commonly forming 40-50% of the rock arid rarely up to 75%. Nearly all the quartz shows evidence of strain, and locally mortar texture has begun to form. Grain size is typically 1 mm, however grains up to 8 mm across do occur. The quartz is typically evenly distr il:uted t.11roughout the rock, but local quartz-. rich bands can be seen. It is not certain whether these bands represent original sedimentary layering. Magnetite This mineral is second in abundance to quartz and it is present in all assemblages. Magnetite composes up to 40% of the rock but is 38 Figure 3.3. Slightly retrograded high-grade asserrblage in granofels, Wildcat body. 'lhe assemblage consists of olivine (high relief), orthopyroxene (pale bro.vn), clinopyroxene (;rredium green), quartz and rna.gnetite. A little retro- grade grunerite is also present as reaction rims on the olivine. Upp=r photograph is in plane light and the laver photograph is between crossed nicols. Field of view is 2.4 x 3.5 rrm. WSN 3F3. 39 MINERAL ASSEMBIAGF.s .IN .TEE WII.DCAT ,00,QX QI'Z + fw'Il' + OL + OPX + CPX ± Gr + [ GR.JN ] ± [ OAMPH ] ± [ CHL ] QTZ + MT + OL + CPX + GT + [ GR.JN ] QTZ + Ml' + OPX + CPX ± Gr ± [ GRJN ] ± [ OAMPH ] ± [ HAST ] ±[CHL]±[HEM] QTZ + MT + OPX + GT + [ OAMPH ] ± [ GR.JN ] QTZ + Ml' + CPX + Gr + [ GRJN ] QTZ + MT + GT ± [ GRJN ] ± [ HAST ] ± [ OAMPH ] ± [ HEM ] QTZ + Ml' + BIOT + CHL ± [ HEM ] ± ZR [ ] retrograde minerals QI'Z Quartz Ml' Magnetite OL Olivine OPX Orthopyroxene CPX Clinopyroxene GT Garnet GRJN Grunerite OAMPH Orthoamphibole CHL Otl.orite HAST Hastingsite HEM Herratite BIO'l' Biotite ZR Zircon K-SPAR Alkali f eldspir PI.Jl.G Plagioclase EPID Epioote PYR Pyrite 40 Figure 3.4. Subhedral rragnetite grains surrounded by quartz, granofels, Wildcat body. ]?ale-green nrineral with larrellae is clinopy.roxene, brc:M'l-green nrineral is orthopyroxene. Plane light. Field of view is 2.4 x 3.5 rem. WSN 1C6. 41 typically below about 15%. magnetite in equilibrium with the highest grade assemblage is subhedral to euhedral in habit. Magnetite is less abundant in the assemblage containing olivine. The grains are commonly seen completely surrounded by quartz and are rarely in contact with any other mineral (Fig. 3 .4). Locally these subhedral magnetite grains show a verrnicular intergrowth with quartz. Magnetite is also present as tiny rodlets, emphasizing the exsolution larnellae in pyroxene, or as veinlets cross-cutting all mineral grains. Orthopyroxene Orthopyroxene is present in most assemblages. It is typically associated with clinopyroxene, the two minerals being in approximately equal abundance. The orthopyroxene normally accounts for 20-30% of the rock, although this varies considerably throughout the body. Grain size is commonly 1 mm, but crystals up to 8 mm in length occur. Grain boundaries are commonly parallel to {100}. The orthopyroxene grains locally show low-angle tilt boundaries of a few degrees within the crystal, possibly due to deformation ( Spry, 1979, p. 20). The same boundary can be followed through adjacent grains. Some larger grains show a ragged, poikiloblastic texture. The mineral is moderately pleochroic, showing the distinctive mid-green to pink pleochroic scheme (a., pink; s , pale green-brown; y , green). Most of the orthopyroxene grains show only one set of fine exsolution lamellae parallel to {100} • No lamellae parallel to {001} have been found, indicatJ.ng that the orthopyroxene is not inverted 42 pigeonite ( Deer, Howie and Zussman 1966 p. 131). Most of the orthopyroxene has a 2V of 80-90°, a negative sign, and is the iron-rich orthopyroxene, eulite (Fs 70-90). However, locally the 2V varies from so 0 to 90° between adjacent grains, though there is no other .apparent difference between the grains. This is thought to be due to slight variations in composition as the 2V value is highly dependent on the chemistry in this range. Clincmyroxene The clinopyroxene typically occurs as subhedral grains of 1 mm, but grains of up to Smm are present in some samples. Rarely the clinopyroxene occurs without orthopyroxene, but normally, the two pyroxenes are intimately associated. This mineral is moderately pleochroic from medium-green to pale- green ( a, pale greenish; 13 , yellow green;Y , medium green). Exsolution larnellae occur in two directions. The lamellae parallel to {001} are very common, occurring in almost every grain. More rarely a finer second set of lamellae can be seen parallel to {100}. From the crystallographic orientation it is probable that the {001} larnellae are pigeonite the {100} orthopyroxene. Fine needles of opaques commonly emphasize the lamellae. The clinopyroxene has a positive sign and a 2V of 50-60°. It is probably ferroaugite. OrtbQP.Yroxeoe .aruJ ClinQP.Yroxeoe Relations The relationship of these two minerals is unusual. The typical texture seen is of a large orthopyroxene grain with "ears 11 of clinopyroxene on each side (Fig. 3.5). The clinopyroxene crystals 43 generally occur along rational {100} grain boundaries of the orthopyroxene. The { 001} lamellae of the clinopyroxene are clearly visible. This relationship can be further complicated by patches of clinopyroxene inside the large orthopyroxene crystal. These patches of 11 11 clinopyroxene are in optical oontinuity with the clinopyroxene ears • Also seen, although very rare, are alternating bands of the pyroxenes, the individual parts of the each mineral being in optical oontinuity (Fig. 3.5). The relative proportions of the two pyroxenes in this intergrowth is remarkably oonstant throughout the whole body, being approximatly 40% clinopyroxene, 60% orthopyroxene. Inclusions of one phase in the other are common. In adjacent grains in figure 3.6, both orthopyroxene in clinopyroxene and vice versa can be seen. The origin of this intergrowth is believed to be coarse exsolution of an original sub-calcic pyroxene but no other evidence for this higher-temperature pyroxene is present. Olivine Olivine occurs in the iron-rich granofels in some samples forming up to 50% of the rock. It is in textural equilibrium and, therefore, assumed to be chemically stable with quartz, pyroxenes, magnetite, and garnet. The grains are subhedral, typically 1 mm in size and grain boundaries are smooth and regular. The grains are slightly pleochroic with a pale brown color. The olivine has a negative optic sign and a 2V of 50-60° indicating at least 90% fayalite. Some alteration is common but not ubiquitous. Rarely a 44 a) b}_ Figure 3.5. Orthopyroxene and clinopyroxene intergrawths in iron- rich. granofels, Wildcat body. 3. 5. a (upper J_ shows a large orthopyraxene grain with "ears" of clinopyroxene • .3.5.b Clowerl shc:Ms alternating bands of the two pyroxenes. ·Both. are between crossed nicols and have a field of view of 0.6 x 0.9 nm. WSN 3K6, WSN 3Gl B. 45 Figure 3.6. Orthopyroxene (pale and clinopyroxene (nedium green). as inclusions in each other, granofels, Wildcat l:ody. Plane light. Field of view is 2.4 x 3.5 nm. WSN 3Ll Figure 3. 7. Intergrc:Mth of olivine (high reliefl, orthopyroxene (pale and clinopyroxene (iredium green) with quartz and a trace of nagnetite, granofels, Wildcat b:x:ly. Plane light. Field of view is 2,4 x 3.5 nm. WSN 6454 46 ragged poikiloblastic texture similar to that of the orthopyroxene can be seen. The relationship of the olivine and the two pyroxenes can be complex with the three minerals intergrown (Fig. 3.7). Garnet Gamet occurs in most of the iron-rich rocks and typically occurs concentrated in either pa.tches or bands. 'Ihe garnets form subhedral grains up to 8 mm across. They are pale pink in color. One sample from the Wildcat body was probed for garnet comi;:ositions. The results are listed in Table 3.2. The sample was found to be relatively homogeneous throughout. The composition is approximately 75% almandine and 23% grossular with trace amounts of pyrope and spessartine. Garnets commonly have inclusions of the two pyroxenes, quartz and rarely magnetite. Since garnet is seen also as inclusions in the pyroxenes, it is believed to be part of the high grade assemblage. Large garnets also occur in quartz veins that cross-cut the rock; these are probably of a later origin. Grunerite Grlll'lerite occurs in several differing habits within the body. It is typically a secondary mineral and will be discussed more fully in the section on alteration. The grlll'lerite is typically colorless but may show a weak, pale yellow pleochroism. It is typically fibrous, rarely platy and commonly shows the distinctive i;:olysynthetic twinning. It has a negative sign and a estimated 2V of 85-90° which indicates it is 85% Fe end member (Deer, Howie and Zussman 1966 p. 161). 47 MICRQPROBE ANALYSES .QF GARNEI' ANALYSIS 1 2 PHASE GARNET ------GARNET --- SiG-i 37.90 37.68 20.67 20.98 . 33.02 32.78 MgO 0.11 0.18 MnO 0.46 0.49 eao 7.84 7.89 ------Si 3.054 3.035 Al 1.968 1.991 Fe 2.226 2.208 Mg 0.013 0.022 Mn 0.031 0.033 Ca 0.677 0.681 ------'IDI'AL 7.969 7.970 OXYGENS 12 12 ------APPROXIMATE COMroSITION ALMJ1NDINE 75.5 75.0 PYROPE 0.4 0.7 SPESSARI'INE 1.1 1.2 GRCSSUI...AR------23.0 23.1 48 Ortboanphibole The orthoamphibole occurs only as an alteration product. The mineral is weakly pleochroic from a tan brown to pale brown, rarely colorless. It is fibrous and shows parallel extinction. Birefringence is 0.028 as determined by maximum interference colors in a standard thin section. No interference figure could be obtained due to the fibrous nature of the mineral. This high birefringence and fairly strong color suggests this is a very iron-rich anthophyllite. Hastingsite Hastingsite is a very distinctive amphibole because of the intense pleochroism from dark blue-green to pale straw-yellow (a,yellow; B ,dark olive-green; y ,smoky blue-green). The grains are subhedral to anhedral. Simple twinning is locally seen. It has a negative sign with a 2V estimated at 20°. Hastingsite is normally present in trace amounts in these rocks, however, it can account for up to 10% of some samples. It only occurs as an alteration product. Accesso.r.y Minerals Apatite is the most common accessory mineral encountered. The grains are typicaly rounded and are generally less than 0.1 mm in length although they can be up to l mm. Biotite is relatively rare and occurs only in the southern part of the road cut. It is strongly pleochroic from pale yellow to dark brown. Most of the biotite is altered to chlorite. It is present in the highly silicified rocks (discussed later), where it occurs with chlorite, magnetite and quartz. The biotite is highly kinked and is 49 commonly split along the cleavage forming a web-like :p;tttern. Chlorite is an alteration mineral formed after the biotite and rarely after other iron-rich minerals. Only one zircon was seen but it is of interest due to the optically continuous rim of zircon as an overgrowth on the primary grain and is presumably the result of high grade metamorphism. Hematite is rarely seen, forming as an oxiClation product of the rragnetite. A trace of calcite was seen in an assemblage with pyroxene, magnetite, quartz, garnet and hastingsite. Alteration A1teration .Qf Olivine The olivine is rarely fresh and is commonly i;:artly altered to a colorless, fibrous mineral with a high birefringence. This is probably grunerite, although some minnesotaite may be present. Magnetite is also a product of this reaction. Typically the olivine has a rim of the alteration mineral around it (Fig. 3.8), but rarely the olivine is completely replaced. The nature of this reaction can be illustrated by the simplified equations below. 7Fe2Si04 + 2H20 + = (OH) 2 Fayalite Vapor Quartz Grunerite 3Fe2sio4 + = 2Fe304 + Fayalite Vapor Magnetite Quartz The actual alteration is a combined reaction of these two 50 Figure 3.8. Olivine grains surrounded by alteration rims of gruner- ite and a little magnetite in iron-rich granofels, Wildcat l:x:xiy. Crossed nicols • Field of view is 0 • 6 x 0.9 rrm. WSN 2Fl. 51 hydration and oxidation equations, but as the relative proportions of the products is tmknown a more precise equation can not be given. A1teration S2f etroxenes Both grtmerite and orthoamphibole occur as a direct alteration of orthopyroxene. The orthoamphibole forms as a rim on the orthopyroxene with the fibers aligned parallel to the cleavage of the pyroxene. This feature appears to persists even where the pyroxene is completely replaced. The progressive alteration can be seen in Figure 3.9. Note that as the orthopyroxene alters, the clinopyroxene remains fresh. Grunerite also forms rims on orthopyroxene. The fibrous grunerite can be distinguished from orthoamphibole by the oblique extinction and lack of color in the former. Rarely both grunerite and orthoamphibole can be seen rimming different parts of the same orthopyroxene grain. The simplest end- member reactions likely to be involved are illustrated below. 7FeSiDJ + Si 6FeSiDJ + Oz = + 6Si02 Opx Vapor Magnetite Quartz 52 a) b) Figure 3.9. Ortbopyroxene grains showing progressive alteration to fibrous arrphibole. In 3. 9. a (upper1 the orthopyroxene grain is slightly altered whereas in 3.9.b (lower) the ortbopyraxene is carrpletely altered. 'l11e clinopyroxene remains unaltered. Quartz and rnagnetite are also present. Both photographs are ·between crossed nicols with a field of view of 0.6 x 0.9 nm. WSN 3Kl. 53 Figure 3.:J_O. Orthoarrphil:x:>le fibers sho.ving parallel extinction overgrcMI'l by radiating grunerite fibers, iron-rich granofels, Wildcat body. Crossed nicols. Field of view is 0.6 x 0.9 rnn. WSN 3Kl. Figure 3 .ll. IntergrcMI'l green hastingsite and colorless grunerite . with quartz and magnetite, granofels, Wildcat body. Plane light. Field of view- is 0.6 x 0.9 nm. WSN 3K5. 54 Grunerite and orthoarnphibole intergrowths are common. In figure 3J.O the orthoami;:bibole fibers show an alignment , rarallel extinction and a pale brown color in contrast to the somewhat radiating fibers of colorless grunerite. Additionally, an intergrowth of these two arnphiboles was also seen showing the brown color, high birefringence and non-parallel extinction. Grunerite is also seen as an alteration product of the clinopyroxene. This is a more complex reaction than that of the opaques and other very fine-grained unidentified alteration products are formed together with the grunerite. Locally the {001} exsolution lamellae in the clinopyroxene can still be seen even when the grain is completely altered. Many thin sections show very fine asbestif orm needles that project into the quartz from the altered iron silicates. This is thought to be grunerite. Hastingsite is found in two different settings in the altered rock. It occurs with magnetite, commonly rimming the grains and also in a complex intergrowth with grunerite (Fig. 3J.l). Hastingsite is only found in assemblages bearing possibly the breakdown of the clinopyroxene would provide the calcium needed for the formation of this amphibole. Silicification The southern end of the Wildcat body appears to be affected by silicification. The original iron silicate, possibly grunerite, has been altered to a multiphase, microcrystalline pseudomorph composed of 55 quartz, magnetite and chlorite. The cleavage direction of the original iron-silicate is commonly preserved (Fig. 3.12). This reaction can be illustrated by the equations below. + 44H20.= + Grunerite Vapor Chlorite Quartz + = 14Fe304 + + 6H20 Grunerite Vapor Magnetite Quartz Vapor Using molar volumes for the phases invol_ved (Helgeson et al., 1978) the relative proportions of the products of these reactions can be calculated. Grunerite reacts with vapor to form chlorite and quartz in the ratio of approximately 4:1, whereas grunerite yields magnetite and quartz in the approximate ratio of 1:2. Figure 3.12 shows a pseudomorr:h composed of approximately 50% quartz, 35% chlorite and 15% magnetite. Quartz is present in excess compared to the other two r:tiases. Therefore, silicification of the rock, either due to addition of silica or leaching out of other elements, is believed to have taken place. Additionally, the surrounding quartz grains to grow into the pseudomorph replacing the alteration assemblage. The outline of the pseudomorph can be seen within the quartz grain. Locally just a faint trace of the original crystal can be seen within the quartz. Also seen is a rock that is composed of just magnetite and quartz. The original iron silicate is almost totally replaced, with only a "ghost 11 of the crystal remaining, now shown by a dusting of 56 Figure 3 •.12. Pseudarorph of quartz, m:i.gnetite and chlorite showing the previous cleavage. The clear grains appear to be growing into the pseudarorph. The upy;::er photo- graph is under plane light, the lover is between crossed nicols. Field of view is 2.4 x 3.5 rrm. WSN lCS. 57 a) b) Figure 3.J.3. Figure 3.l3.a shews quartz and .fine-grained magnetite aggregates with definate boundaries. Figure 3 .13 .b Sbc»..rs that the quartz.gram boundaries cut through the rnagnetite-rich areas. Figure 3 .13. a is with plane light and 3.l3.b is l:etween crossed nicols. Both photographs have a field of view of 2. 4 x 3. 5 rrm. WSN lC6. 58 fine magnetite grains. The grain boundaries of the quartz crystals cut through these magnetite-rich patches, indicating addition of silica into the system or removal of other elements (Fig. 3.13). 59 SPROCE IRON-RICH .WOY The Spruce Creek body is located 500 yards west of where Wildcat Loop crosses Little Warm Springs Creek (Fig. 1.2). The body is the host rock for uranium minerals which were mined in the late 1950's (Fig. 4.1). The outcrop is highly weathered, rusty brown, heterogeneous, iron-rich granofels in contact with granodioritic rocks with abundant quartz veins on the eastern edge of the body. No contact with the surrounding gneiss is visible. The area was sampled in detail (Fig. 4.2). Pieces of· float were also collected, some of which are believed to be from deeper within the mine as no similar rocks were seen in outcrop. PetrographY Introduction There is a wide range of iron-rich rocks in the Spruce Creek body showing various textures and mineral comi;ositions with no systematic relationship discerned. Where contacts between the differing rock types are seen, they appear to be conformable. The differences are thought to be due to either pre-metamorphic comi;ositional differences or metamorphic differentiation. There are three major rock types in the Spruce Creek body (Table. 4.1). The first type includes the highest grade assemblages, which contain pyroxene and amphiboles. These rocks are very similar to those in the Wildcat body. The other two rock types are very similar 60 Figure 4 .l. Abandoned uranium mine within the Spruce Creek iron-rich body. 61 mineralogically having abundant garnet and bioti te (Table. 4.1) , but Type Two shows a very distinctive quartz-magnetite intergrowth which is completely absent in Type Three. These two rock types can also be distinguished by textural api;:earance in hand si;:ecimen. In general, the rocks appear to be at a lower metamorphic grade than in the Wildcat body as no olivine is seen. They are more highly altered and are probably more aluminous, as shown by the abundant garnet and biotlte. They are very iron-rich. Layering within the rocks is uncommon. All t.."'le mineral assemblages found in the 58 samples of . the Spruce Creek b9dy examined petrographically are listed in Table 4.1. Pyroxene-rich rocks: l Introduction There are two tyi::es of pyroxene-rich granofels. Both are rnediurn- to coarse-grained, dark green to black and are highly magnetic. These two tyi::es represent the highest grade rocks seen in the Spruce Creek body and are distinguished by the pyroxene they contain; type l.A contains clinopyroxene whereas type l.B contains orthopyroxene. The two pyroxenes were never seen together but in some highly altered parts of the rock the alteration products and textures suggest that both pyroxenes may originally have been present. Mineralogy; LA.. This rock is layered on a scale of 5 cm, with fresh clinopyroxene-rich bands and highly altered layers consisting entirely 62 SPRUCE CREEK BODY (SOUTHERN IRON-RICH BODY) f'AOJCCHOH OHfO YCf'UCAL f'LAHE SECTION 1 SECTION 2 SECTION fl SECTION 4 SECTION 5 .. 0 0 ,··· :' O'\ .. w N 50"E H •o•w N rso•e N 5o•w N so"w PLAN VIEW SCALE 0 SAMPLE lAKfH HOfllZ VERT OF MINE IO•f • SAMl"lf TtttH SECHOHCO N 9ECTIOH .1 / "' IECHOH I Figure 4. 2. Spruce Creek body. Plan of mine as projected ...... "> ... onto a vertical plane. MINERAL ASSEMBIAGF.S .IN .Tl:lE SPROCE ™ TYPE l.a. QI'Z + Ml' + CPX + [ GRJN ] + [ OAMPH ] + [ HAST ] + [ '!'REM ] l.b. QTZ + MT + OPX + GT + [ GR.JN ] + [ CllOO'H ] + [ HAS!' ] + [CHL] + [HEM] 2. QTZ + MT + ± GT ± BIOT + [ CHL ] ± PLAG ± [ MUSC ] ± [ HEM ] ± [ EPID ] 3. QTZ + MT + ± GT± BIOT + [ CHL ] ± [ MUSC ] ± [HEI-1 ] ± [ EPID ] [ ] Retrograde minerals QTZ Quartz .rvn' Magnetite OPX Orthopyroxene CPX Clinopyroxene GT Garnet GR.JN Grunerite OAMPH Orthoarn'[ilibole CHL Chlorite HAST Hastingsite HEM Henatite BIOT Biotite K-SPAR Alkali feldsp;i.r PIJl1; Plagioclase EPID Epidote 64 of retrograde arnphiboles. At the amtact between the two layers, the clinopyroxene shows various stages of alteration. The unaltered areas are composed of 98% clinopyroxene and 2% quartz. The altered areas consist of grunerite (40-50%), hastingsite (15-25%) and orthoami;ilibole (15-25%) with a little tremolite, magnetite, chlorite, quartz and garnet. The clinopyroxene forms irregular, anhedral grains up to 4 mm in size. It is weakly pleochroic from pale green to almost colorless. Exsolution lamellae occur in two directions and are commonly emphasized by opaques. The mineral has a positive sign and a 2V estimated to be 500 and is likely to be augite. The quartz occurs as small rounded grains under 1 mm in size, showing slightly undulatory extinction. Where the clinopyroxene is only slightly altered it shows a rim of colorless amphibole. This amphibole has low second-order interference colors, a negative sign and a 2V of about 60-70°, indicating tremolite. With further alteration small irregular grains of hastingsite are formed with the tremolite and fine opaques are also produced. As the clinopyroxene becomes more altered, the opaques along the lamellae increase. The amphibole-rich layers are probably retrograde products of some earlier minerals. The three main amphiboles, grunerite, hastingsite and orthoamphibole are slightly altered and are intergrown in these layers. The orthoami;ilibole is pale brown and has a fibrous habit. It is 65 distinguished from the other amphiboles by its parallel extinction as well as its color. The mineral occurs as pseuoomorphs of some earlier grains, probably orthopyroxene. 'lhe texture is the same as that seen in the Wildcat body. The grunerite is colorless and shows the distinctive polysynthetic twinning. It is typically associated with the orthoami;:hibole, as radiating fibers overgrowing the aligned fibers of the orthoamphibole. Hastingsite occurs as irreg.ilar grains up to 3 mm in size. It is strongly pleochroic from pale green to dark smokey-blue, has a negative sign and an estimated 2V of 30-40°. It is concentrated in 0.5 cm layers in the amphibole-rich part of the rock. Chlorite occurs as a fine-grained aggregates and is an alteration mineral of all the iron silicates. It is seen only where the rock is fractured and cut by quartz veins. Garnet occurs as small subhedral grains throughout the rock with grains reaching l mm in diameter. 'Ibey are pale pink and are probably almandine-rich. Magnetite is fairly abundant in the retrograded part of the rock. It is very fine-grained and is probably a product of many of the retrograde reactions that took place. Mineralogy: J...lh. This rock· consists of orthopyroxene (25-35%), garnet (20-30%), orthoamphibole (10%), quartz (15%), magnetite (5%), and grunerite (10%) with minor bioite, chlorite, hastingsite and The rock 66 appears to show a slight foliation (Fig. 4.3). The orthopyroxene is pleochroic from pale pink to pale green. Grains are subhedral, commonly elongate parallel and are up to 5 mm in size. The grains have one set of exsolution lamellae parallel to {100} anq show low-angle tilt boundaries (Spry 1979, p.20). The mineral has an estimated 2V of 70-80° and a negative sign and is probably the iron-rich orthopyroxene eulite. The garnet forms large irregular embayed crystals of up to 10 mm in diameter. The grains have abundant inclusions of quartz, magnetite, biotite and chlorite. They are slightly elongate, almost oval in shape and appear to be aligned. Both grunerite and orthoamphibole are formed by the alteration of the orthopyroxene. The two minerals are seen both as rims to the pyroxene grains and in small patches where the two minerals are intergrCMn. A trace of hastingsite is also present. Quartz forms irregular, strained grains up to 5 mm in size. The magnetite is typically very fine-grained, formed by alteration, but rare euhedral grains also occur. The biotite is present as small crystals under 1 mm in size. It is pleochroic from orange to dark brown and is partly altered to chlorite. Garnet-biotite lXitll Intergrowt:hs: 2.& Introauction This rock is very fine-grained and varies from deep purple to deep green. In the field, only the distinctive large garnets could be 67 Figure 4.3. Orthopyroxene-rich assemblage with garnet :poikiloblast Within the Spruce Creek l:xxly. 'Ihe orthopyroxene is partly altered to fibrous arnphil:XJle. Crossed nicols. Field of view is 2.4 x 3.5 rrm. WSS 64. 68 recognized. The mineralogy is quartz, magnetite, garnet, chlorite, biotite and muscovite with trace amounts of epioote and plagioclase. In thin section this rock shows a distinctive quartz-magnetite intergrowth. Mineralogy: 2. C,;uartz accounts for 15-75% of the rock and is present in severcil forms: as irregular, slightly strained, clear crystals up to 5 mm in - size, as cross-cutting veinlets, and also intimately associated with the magnetite. This texture will be discussed later. Magnetite, which makes up 20-60% of the rock, also occurs in several forms. It is seen as euhedral crystals, commonly with the rim altered to hematite. It is an alteration product together with chlorite after garnet and occasionally biotite. It also forms an intergrowth with the quartz. Gamet is typically present in these rocks. wnere it Cbes occur, it accounts for 1-60% of the assemblage. The garnets are euhedral to subhedral and up to 8 mm in diameter. They are pale pink and are probably almandine-rich. Biotite is typically present with garnet and commonly a rim of biotite crystals surround the garnet. It accounts for up to 15% of the rock. The mineral is strongly pleochroic from pale yellow to dark brown and crystals are typically under 3 mm in length. Chlorite forms 1-15% of the assemblage. The mineral occurs as fine-grained aggregates and shows both anomalous brown and purple interference colors. It only occurs as an alteration product. Muscovite is present only as an alteration product after garnet 69 and accounts for up to 5% of these rocks. The muscovite occurs as small subhedral flakes under 0.1 mm in length. Epidote is very rare. It occurs as small dusty crystals less than 0 .4 mm in size. It typically occurs in patches of several grains. Plagioclase was found in one sample and the grains occur as inclusions in large altered garnets. The grains are up to 2.5 mm in size with an irregular outline and are full of inclusions. Quartz-rragnetite Intergrowths: 2 Intergrowtlis of quartz and magnetite define this mit. The tiny magnetite grains are aligned in thin layers under 0.1 mm in width and the quartz occurs between these layers (Fig. 4.4). This is believed to be a replacement texture. The original mineral is totally replaced and no relics were seen. Initially, the magnetite appears to have grown preferentially along the cleavage of the original crystal. The quartz replaces the rest of the crystal and occurs as tiny irregular grains (Fig. 4.4). In this photograph larger grains of "normal" quartz are also present. With recrystallization, the fine quartz grains anneal and grow. The alignment of the magnetite restricts their growth and they finally form fairly coarse-grained elongate crystals parallel to the alignment of the magnetite (Fig. 4.5). Also seen in this photograph is the more random pattern to the intergrowth where the cleavage is not present. The form of these magnetite-quartz pseudomorphs suggests that certain minerals are more likely to be the predecessor than others. 70 Figure 4.4. I?seudcxrorphic intergrowth. of fine-grained quartz and rragnetite with large' clear qufiltz crystals. The . rragnetite emphasizes a previous cleavage of the original mineral. lbck type 2, Spruce Creek body. Upper photograph is with plane light, lower between · crossed nicols. Field of vif'M is 2.4 x 3.S nm. WSS SSA. 71 Figure 4. 5. Intergrowth of quartz and rragnetite in the fonn of pseudc:m:n:phs.. 'Ihe quartz has rec:rystallized and occurs as elongated, parallel c:cystals. Ibck type 2, Spruce Creek body. Upper photograph is with plane light, lc:Mer is between crossed nicols. Field of view is 0.6 x 0.9 ran. WSS 49. 72 . Figure 4.6. Pseudamorphic intergrcwth of quartz and magnetite. 'Ihe textures suggest that there is replacerrent of both stumpy pJrOXe.Ile grains and fibrous amphil:x:lle grains. Rxk type 2, Spruce Creek lxx:1y. Upper photograph is with plane light, lower is crossed nicols. Field of view is 2.4 .x 3.5 nm. WSS 59B. 73 The dominant feature is the magnetite alignment in many of the pseudo morphs suggesting the original mineral had at least one good cleavage. '!he original grains varied in size from under 1 mm to over 8 mm. Certain pseuCbmorphs are short and stumpy in outline suggesting they were once pyroxene. Other grains show a splayed cleavage suggesting they were once grunerite. One sample shows both of these textures together, indicating an initial rock with large pyroxene grains surrounded by rore fibrous amphibole (Fig. 4.6). The nature of these reactions can be illustrated by the simplified equations below. Grunerite Vapor Magnetite Quartz Vapor Grunerite Vapor Chlorite Quartz 6Fesi0:3 + = 2Fe3o4 + 6Si2 Opx Vapor Magnetite Quartz 6FeSi0:3 + 4Hz0 = Fe6Si4CJ:i.o(OH)a + Opx Vapor Chlorite Quartz Using molar volumes (Helgeson et al., 1978) the relative proportions of the products of these reactions have been calculated. For both orthopyroxene and grunerite the ratio for magnetite and quartz is approximately 2:1 whereas chlorite and quartz are formed in the ratio of 4:1. In figures 4.4, 4.5 and 4.6, quartz is the dominant mineral and may form up to 70% of the intergrowth. This texture suggests, therefore, that silicification has occurred in these rocks. 74 Alteration: .2.... Garnet is typically partly to completely altered in this unit. Initially, the grains show fracturing and show slight alteration to chlorite along the fractures and around the edges. These fractures are commonly infilled with magnetite. The final alteration products are either an intergrowth of muscovite (40-50%), chlorite (35-45%), magnetite (10-20%) and quartz (5%) or to 100% chlorite. Commonly chlorite in the center of the pseudomorph shows anomalous purple interference colors whereas the edges and along the cross-cutting fractures the chlorite shows anomalous browns. This suggests the chlorite in the center of the grain is more iron-rich (Albee, 1962). The biotite also alters to chlorite and a trace of magnetite. The chlorite first forms along the cleavage of the biotite and around the edges. Rarely, the biotite is ex>mpletely replaced. The alteration of the garnet and the biotite is a later event than the development of the quartz-magnetite texture which is seen even where the garnet and biotite are completely fresh (Fig.4. 7). The late-stage quartz veins that cross-cut the rock cause further alteration of the rock. To each side of the veins abundant chlorite is present. In some cases the quartz crystals within the vein grow in optical ex>ntinuity with the quartz in the adjacent quartz-magnetite intergrowth, however the quartz-magnetite intergrowth is present where no quartz veins occur. 75 Figure 4. 7. Pseudcxro:r:phic intergrowth of quartz and magnetite with fresh garnet and biotite. Rock type 2, Spruce Creek I:ody. Upper photograph is with plane light, la.ver is between crossed nicols; Field of view is 2.4 x 3.5 rrm. WSS N4D. 76 Figure 4.8. Fresh gamet and biotite with "nonral" quartz and magnetite. RJck type 3, Spruce Creek body. Plane light. Field of view is 2.4 x 3.5 mn. wss 63. 77 Garnet-biotite la. This rock is medium-grained, greenish, weakly- to_ non-magnetic and is c:omEX>sed Cbminantly of quartz, garnet, biotite and chlorite. It is coarser-grained and paler colored than 'J:Ype 2 and never shows the quartz-magnetite texture (Fig. 4.8). Mineralogy: .3_s_ Quartz makes up 20-65% of the rock. It occurs as irregular, strained, clear grains up to 4 mm in size. It also occurs in cross- cutting veins. Magnetite occurs as small euhedral grains which are commonly partly altered to hematite. It appears to be dominantly an alteration product and forms up to 7% of the rock. Pyrite also occurs in trace amounts in these rocks. Where garnet occurs it makes up 20-50% of the rock. It forms rounded grains typically between 1-8 mm in diameter but locally up to 15 mm. The garnet is pale pink and is probably almandine-rich. The garnets are typically altered, some only along the edges and fractures whereas others are completely altered. The alteration products are either a muscovite-chlorite-rnagnetite intergrowth, or just chlorite. Biotite is pleochroic from p:ile yellow to brown. It makes up to 8% of the rock. It is normally found surrounding garnet. Commonly it is completely altered to chlorite. A trace of epioote is seen as small, slightly altered crystals up to 1 mm in size. 78 Uranium Mineralization Little work was done on the uranium mineralization in this study. The mine was worked by local in the mid to late 1950's and about 6 tons of ore with 2% u3o8 are thought to have been extracted (Morton,1955). Mine plans were not available and, therefore, the underground distribution of the mineralization is unknown. It appears to have occurred along a faulted zone to the eastern side of the main iron-rich body. Ullmer (per. comm., 1983), studied the uranium mineralization using a scanning electron microscope. He found the mineralization to be fine-grained uranyl-silicate aggregates as oxidation aggregates after equant grains of uraninite. There is virtually no thorium mineralization in the Spruce Creek body. He believes the mineralization was hydrothermally introduced, with the granitic rocks as a r:ossible source. 79 GEOPHYSICAL SURVEYS The purpose of these surveys was to establish whether any other iron-rich bodies occurred in the area that is 80 Figure 5.J.. Landsat photograph.of tbe Wann Springs area. North is toNards the top of the photograph. 'll'le linearrent trends approxinately north- south and passes throughooth the iron- rich bodies (circled}. '1he Wildcat body is the uppe:trrost circle. 81 N Creek " ", Lltlle Warm Springs Creek " I Trace of Lineament 0 5000 FT 0 1500 M Fj.gure 5. 2. location of linearrent through. the Warm Springs field area as sha-m on the Iandsat photograph. 82 began and ended on this base line where both a reading and time were noted. This system was used to check the diurnal range in the magnetic field in the area. As this variation was very small compared to the anomalies found, no correction for time variation was needed. The results of this sqrvey can be seen in Figure 5.3 and Appendix B. Expected total intensity background was between 55,000 and 60,000 gamma for this area of the United States (B.r;einer, 1973). The background of 56,500-57,000 gamma indicated by this survey is in accordance with his data. The anomalies are very high, especially over the Spruce Creek body, which was expected. An accurate reading could not be obtained in the area of the mine as the magnetometer did not register repeatable readings. This indicates the magnetic gradient in the area is over several hundred gamma per foot. The survey shows several en echelon linear trends in the expected direction. This indicates that the lineament could possibly control the relative positions of the two iron-rich bodies. The implications of this will be discussed later. A quantitative analysis of this survey is impossible due to lack of bedrock control in the area covered and an unknown depth of the glacial cover. However, the results strongly suggest that other iron- rich 11pods11 are present in the area. Scintillorret.er Suryey The scintillometer survey showed relatively little. The only real high was recorded standing at the entrance to the mine. Several smaller peaks were recorded but all these were on or near the forest 83 MAGNETOMETER SURVEY / I ).__ __,, ______/ 'I / ./ / r---- Go yser Creek / / o x Base line ·Lltlle Warm Springs Creek 0 Magnetic low 0 800 1600 FEET SCALE Figure 5. 3. Results of rnagneto:rreter sur1ey over linearrent to the north of Spruce Creek body. Contour interval is 500 t. 84 road and are believed to result from pieces of ore. dropped during transportation. 85 INI'ERPBETATION QF IRON-RICH EQDIFS Protolith Originally, Ullmer and Honea (unpublished manuscript, 1981), suggested that the iron-rich bodies were pods of ultramafic rocks. However, the abundance of quartz, commonly forming 40-50% of the rocks, makes this proposal unreasonable. The protolith for the iron- rich rocks described in chapters three and four is here proposed to be banded iron-formation because of the abundance of magnetite and iron- rich silicates within the rocks. The rocks are found to show several similarities to other high-grade metamorphic iron-formations which will be discussed later. Iron-formation is found elsewhere in the Archean of the Wind River Mountains. There is a major iron deposit within the greenstone belt near Atlantic City at the southeastern end of the range (Bayley, 1963). Worl (1968) reported taconite in the Archean gneiss near Downs Mountain (Fig. 1.2). He described assemblages including quartz, magnetite, grunerite, anthophyllite, blue-green hornblende and garnet but with only rare hypersthene. The taconite contains well-defined continuous layers which are possibly relict sedimentary layers and which grade laterally and across strike into ami;:tiibolite. The contacts of the taconite with the quartzo-feldspathic gneiss are sharp and typically conformable. This body seems similar to the Wildcat and Spruce Creek bodies, except that it is lower in metamorphic grade. A summary of previous work on high-grade metamorphosed iron- . formations is given in Table 6.1. In general, the rocks described by these authors are similar to those in this study. The highest-grade 86 Previous Studies Qn High-grade MetaIIQrpbose Author Area Highest Grade TeffiE:erature Asserrblage & Pressure COITT'ACT M£TNl08PHIC Berg, 1977. Nain Complex 01-+Cpx+Qtz 645-9150c Labrador Opx-+Cpx±Ol±Qtz:!:Mt±Grun 3.7-6.6 Kb Bonnichsen, Biwabik IF 650-7so0 c 1969;1977 Minnesota Opx+Qtz+Mt.±Cpx 1.5-3.0 Kb Floran and Gunflint IF Ol+Qtz+Mt±OPX;iCpx±Grun Papike,1978 Minnesota ±Hasti<)arrph French,1968 Biwabik IF < 6oo 0 c Minnesota Ol+Qtz+Mt 2-4 Kb Gunderson and Biwabik IF 01-+Cpx+Qtz+Mt.±Cpx±Grun Schwartz,1962 Minnesota Ol+Qtz+Mt Morey et al. Biwabik IF 01-+Cpx+Qtz+Mt-K;run±Gt 1972 Minnesota Sill1TODS et al. Gunflint IF >8000c 1974 Minnesota ±Grun >2 Kb Vanirre.n et al. Stillwater Opx+Qt:a+Mt.±Cpx±Grun >80o 0 c 1980 Canplex Ol+Mt±Opx±Cpx >2 Kb Montana Ol+Mt+Qtz (rare) REGIO!'AL METN10RPHIC Butler, 1969 Wal::osh IF Cpx+Qtz±Opit_+Mt.._-K;run Quebec Oiakraborty Wal::osh IF Opx+Grun.±Qtz±Mt 1966 Quebec Dahl, 1979 sq Montana Opx+Qtz+Mt.±CpJ<±Gt 745-67s0 c Gole and Yilgarn Block 670±500c Klein, 1981 W. Australia ±Grun 3-5 Kb Haase, 1982 Negaunee I.F Ol+Grun±Cpl(±Qtz±Mt.±Hbl 500-6250c Michigan 2-3 Kb Irmega and sq Montana Opx+Qtz+Mt.±Gt±Grun 650-1so0c Klein, 1976 ±Hen 4-6 Kb ¥.lein, 1978 Labrador Trough Opx+Qtz+Mt.±Cpx 700-7500c Canada 10-11 Kb Kranck, 1961 Mount Read Opx-+Cpx-tQtz+Mt+Grun QJebec Worl, 1968 Wind River Mts. Opx±Gt±Qtz±Mt.±Gruni 87 assemblages contain Fe-rich pyroxenes and olivines in various combinations along with quartz. AJnI::biboles are present in every area listed with grunerite being dominant and occurring both as a prograde and a retrograde i;:ilase. Other arni;:iliboles mentioned include hornblende, hastingsite, actinolite, cummingtoriite, gedrite and anthophyllite. Intergrowths of two amphiboles are commonly reported but only one occurrence of an intergrowth of two pyroxenes is recorded (Butler, 1969). Several areas have more aluminous rock with assemblages bearing biotite and garnet as well as clinopyroxene, hornblende, grunerite, actinolite and feldspar. Apatite is the most abundant accessory mineral. Assemblages containing the five phase assemblage olivine- orthopyroxene-clinopyroxene-rnagnetite-quartz are uncommon. Fayalite is present in several areas where iron-formation has been affected by contact metamorphism. However, in these assemblages there is evidence that most of the orthopyroxene is inverted pigeonite and not part of the highest-grade assemblage. Fayalite has only been reported twice in regional metamorphic areas (Haase, 1982; Gale and Klein, 1981). The assemblage reported by Haase does not contain any orthopyroxene whereas the assemblage described by Gole and Klein has the five-phase assemblage noted above. Klein (1973} recorded the changes in various types of banded iron-formations with progressive metamorphism. All types showed an increase in grain size with increasing temi;:erature. He states that an assemblage such as that seen in the Warm Springs area can be produced 88 I \ from two distinct types of iron-formation. Sedimentary carbonate facies iron-formation is comi:;osed mitially of carbonates, magnetite, chert or quartz and rarely local iron-silicates. If the chemical i:;otential of co2 remains high, the assemblage will recrystallize and grain size will increase with little change in the mineralogy. However, if the chemical t:etential of co2 is reduced, iron-silicates can be produced. The second type is the silicate facies composed originally of silicates such as greenalite with chert or quartz, iron oxides and calcite. With dehydration and decarbonation metamorphic iron-silicates can be formed. Geotherrrornetr:y sma Geobaronetr:y The highest-grade assemblage in the Wildcat body, olivine- orthopyroxene-clinopyroxene-quartz-magnetite, is excellent for obtaining both metamorphic temi;:eratures and pressures. Various metho& have been used to calculate these values. Electron microprobe analyses of one sample were kindly provided by Professor D. H. Lindsley, s.u.N.Y., Stony Brook. The sample, WSN 3F4A from the Wildcat body, was very fresh with no secondary minerals present. Three areas were probed and the data are shown m Figure 6.1, 6.2 and A{:pendix A. A relatively large electron beam width of 15-20 mµ was used to integrate small compositional variations in the mineral. The results are highly consistent between the various areas. The olivine is extremely iron-rich and varies from 95.7-96.5% fayalite. No significant comi:;ositional zoning is seen in these grains but the data 89 oC \ Ca \ AREA A AREA B AREA C \ \0 -4. , •• Cpx .. Cpx \ • '\. Cpx \ 'I l.O 0 Opx •S.o 01 Opx 01 Opxh 01 Fe 1·0 f"o 100 10 90 - 100 10 fO 100 _.Mg Figure 6.1. Iron-rich portion of Mg-Ca-Fe ternary diagram indicating the composition of the grains probed. (See also figures 6 .2 and 6. 3). CLINOPYROXENE OLIVINE AREA B AREA 8 AREA C AREA A 1.:i'o 1.0}0 Si 1.010 1.010 1.000 f------'·"'° Fe 1.uo l.1•01------ Mn 0.00<. o.00) 1------ Q,C'l'l Mg 0. JJU i I ! .,).00) Ca I . . _ I 21 31 22 28 30 27 15 16 20 37 38 60 61 73 74 88 89 Figure 6. 2. Compositional changes across grains (see also figure 6. 3) . For pyroxenes, the values plotted are cations per six oxygens, for olivines, cations per four oxygens. 't-bte changes in scale. .. are sparse. Variation in the comr::osition of the olivines across grains can be seen in figure 6.2. Fe and Mg show inverse correlation but this does not reflect zoning. Ca is essentially constant, varying from 0.002-0.003 per four oxygens except for the center of one grain that has a count of 0.005. The orthopyroxene data are also consistent. The comr::osition is En 12.6-14.5%, Wo 1.3-2.6% and Fs 83.5-85.5%. There appears to be very slight zoning in the grain with the center being higher in Ca and Fe and lower in Mg (Fig. 6.2). Antipathetic variation of Al and Si can be seen but this dJes not form a consistent pattern throughout the grain. The Mn shows no noticable trend. Figure 6.1 shows there is an exchange between the Ca and Fe components in the clinopyroxene with Fe/Mg being constant. The composition is En 10.4-11.4%, Wo 38.2-46.9% and Fs 42.9-50.8%. The clinopyroxene is zoned with ca generally being lower in the center of the grain although the ca distribution is somewhat erratic (Fig. 6.2). The Fe and Mg mirror the variation in Ca. Si and Al are fairly constant with a slight antipathetic exchange between them. The Mn is slightly variable with no consistent i;:attern. In addition, several grains of magnetite were probed and found to contain small amounts of Si, Al and Ti. The data are given in Appendix A. The metamorphic temi;:erature and pressure can be estimated using two-pyroxene geothermornetry, olivine-orthopyroxene geothermornetry and barometry and the presence or absence of pigeonite. Most of the published work has concentrated on minerals more magnesium-rich than 92 those in this study: results in the iron-rich range are less well controlled. Consequently, the results obtained can only give a general indication of the metamorphic conditions. The major method used in this study is the two-pyroxene thermometer of Lindsley (1983). This method involves the partitioning of Ca between two coexisting pyroxenes. 'As the Clecreases, the Ca content in the clinopyroxene increases while that in the orthopyroxene Clecreases. For the components Cao, MgO, FeO and Sio2 and the phases clinopyroxene, orthopyroxene, olivine and quartz the phase rule states there are two degrees of freecbm, a divariant field. This means that if two intensive variables are specified, all the variables are fixed. Therefore, for a given composition and pressure, the temperature of the assemblage is fixed. In this method, the analyses are plotted on the pyroxene quadrilateral at constant pressure. The composition and temperature may vary but they are dependent on each other. '!be effect of pressure on the temperature obtained by this method is per Kbar, which is negligible for this study. The results are plotted on the 5 Kbar diagram of Lindsley (1983) (Fig. 6.3). This was chosen since it is close to the pressures obtained by Gole and Klein (1981) for their similar assemblage. The orthopyroxene and clinopyroxene in this sample from the Wildcat body are very pure and have few components outside the pyroxene quadrilateral. Wo + En + Fs for the clinopyroxene varies from 98.5-98.0% and from 99.0-98.5% for the orthopyroxene. As the 93 percentage of "others" is rarely above 2%, the diagrams from Lindsley (1983) can be used directly with no special projection needed. As there is compositional zoning in both the orthopyroxene and the clinopyroxene, the center of the grain is likely to indicate an earlier, higher temperature than the outside of the grain. The diagram had to be extended in the orthopyroxene range as the orthopyroxene analyses plotted slightly beyond Lindsley' s contours for 5Kbar. Three different areas from one sample were probed and the results can be seen in figure 6.3. The figure shows a sketch of the grain with the points analyzed, the results on the 5Kbar graphical thermometer (Lindsley, 1983) and the temperatures indicated. Both intergrown orthopyroxene-clinopyroxene grains and discrete grains were analyzed. The temperature range indicated by the clinopyroxene varies from 450- 7600c and that of the orthopyroxene from 505-645°c. The clinopyroxene probed shows exsolution lamellae whereas the orthopyroxene shows no visible exsolution. One intergrown orthopyroxene-clinopyroxene grain was probed in area B which indicated temperatures ranging from 450-650°c. The central area of the clinopyroxene (points 21, 31 and 32) gives temperatures of 615-645°c. The adjacent central area of the orthopyroxene (points 23, 24 and 35) yields temperatures of 620-63o0 c. These six points give the highest temperatures within the grain and correlate well. The points close to the edge, 27 and 29 in the clinopyroxene and 18 in the orthopyroxene, give temperatures of 505- 5300c. This indicates a possible l00°c zonation from core to edge. Several points appear anomalous. Point 19 in the clinopyroxene gives 94 . •30 •28 23 . •22 Opx 24. :is. 25• Cpx (]Cpx .32 6 •37 . .12 13 01 Mt 20 0.5mm Clinopyroxene Orthopyroxene # TC # TC 21 645 28 630 31 630 35 630 32 625 24 620 28 615 34 615 33 600 25 605 30 560 26 595 22 545 18 520 29 530 27 505 .19 405 Figure 6.3.a Area B. Pyroxene geothe.r:rraretJ:y of sample WSN 3F4 A. 'Ibp diagram shavs the points probed in each grain. 'Ihe compositions are plotted on the contoured-pyroxene quadrilateral on page 96. 'Ihe temperatures indicated on this diagram are shown above. 95 =- .19 -\ - 29:27 .22 , . / JQ• , 3J I 29•._..31 --- . I'• I --· -32 '21 I ·--··------..._I - - \ '-- \, \ \ "· \ \\ "'\ I \ \ \\ I \.D \X1 (j\ I \ ',,, / / \ \ \\ , \ I I I I i I --- '"- - " 3 0 3 Fs -- -- c,- h 2?'"'-25£4 600 . ; k18 Figure 6. 3. a ,A,rea B. Graphical geothenrorreter (from Lindsley, .1983) with corq;iositions shCMn of po.ints probed .in area B. See also page 95. Opx 83 . or 73 •71 Mt •72 Opx 0 .5mm 0.5mm Clinopyroxene Clinopyroxene # TC j TC 76 760 84 670 77 640 87 600 78 585 86 600 75 550 85 490 Orthopyroxene j TC 83 603 82 565 ·Figure 6. 3 .b Area A. Pyroxene geothenmrreb:y of sarrple WSN 3F4 A. 'Ibp diagram shc:Ms the points probed in each grain. 'Ihe compositions are plotted on the contoured pyroxene quadrilateral on page 98. The temperatures indicated on this diagram are shc:Mn above. 97 --, lid ?_ISOQ!OL_ _ _:.______I -- soo""' ,,. ... , - 9QQ - - -- ·- .19'_ ,' \ -- --- 7\ -- \ i\\". "'\ \ I \ ' \\ I \\ \0 OJ \X1 ' / \ \ ',, \ \ \ \ I ' \) ! " \I y Figure 6.3.b Area A. Graphical geothenrorreter (from Lindsley, 1983) with compositions shONn of points probed in area A. See also page 97. JI 0 .5mm Opx •2. .49 01 •50 •47 •3· 55 .51 .!J• 58• • 69 . ·•8 • 52 0 45. 56 • 70 46• • 53 • ,Cpx 5 Opx 01 60 01 0.5mm Clinopyroxene Orthopyroxene Clinopyroxene Orthopyroxene # TC # TC # TC # TC 57 725 44 630 65 630 70 650 49 670 48 595 67 600 69 630 53 600 47 590 64 580 68 600 54 535 43 560 62 530. 56 510 45 560 63 505 51 495 42 550 52 480 46 550 59 480 50 475 58 460 Figure 6.3.c Area C. Pyroxene geothenrorretry of sample WSN 3F4 A. 'Ibp diagram shows the points probed in each grain. 'Ihe compositions are plotted on the contoured pyroxene quadrilateral on page lOO. The terrperatures indicated on this diagram are shc:Mn above. 99 _"___ _ - -- ,.------"I\___ /'\ ___ ---" -- --/\------7\------. \ lld _ SQQ 5a• •_50 / , 55 52 '\ --uoa.___ 59• "•.;:;?1 I ' ·56 ' 700 54° 5 J ,_ - - .6 4 . ,_ - - \ / I \ - -- -\---=-----/------'\ ,' - -\- - ---7\-----:__------\ -. -.'\ I • -. -._,\ :' ---- ...__ \\'·- '-.., \\. :' ...__ '-.., I \\ · I " I \\ "\ ,' \\ \ I \\ ' . \\ \ . \\ '! \ \\ I I I I-' 8 \>x \ ' \,A x \ ' '// / \ \ \','x / / \ \ ', \\ " ',··, \ \ \ I I \ ": \I I \ I \ I' _J l 1000 qoo -.- - of p:>ints prcbed in area C. See also page 99. gives a temperature of 60o0 c. '!his extreme change in temperature over such a small area is unreasonable. Point 22 in the center of the clinopyroxene also gives a relatively low temperature of 545°c. The erratic variation in temperatures on a small scale may be due to exsolution lamellae in the crystal. The {100} lamellae in the clinopyroxene are orthopyroxene (Deer, Howie and Zussman 1966 p. 123). The presence of these orthopyroxene lamellae lower the ca CX)ntent of the clinopyroxene in this area and therefore, this area would yield low temperatures. One intergrown grain and one discrete clinopyroxene grain were probed in area A. The intergrown grain yields temperatures of 490- 6700c for the clinopyroxene and 565-603°c for the orthopyroxene. Relatively few points have been analyzed and they are probably too sparse to detect zoning. The i;:oints probed show no CX)rrelation between temperature and relative :i;x:>sition of the point. The single clinopyroxene grain is not adjacent to orthopyroxene. Therefore, it is not ideal for this method as it may not have reached equilibrium with the orthopyroxene in the assemblage. The grain appears to show zoning with 760°c in the core and 550-585°C at the edge. This grain gives the highest temperature for the assemblage, however, the implications of this are unknown. In area c, both an intergrown grain and adjacent coexisting grains were probed. Temperatures from the intergrown grain range from 450-725°c for the clinopyroxene and 550-63o0c for the orthopyroxene. The orthopyroxene shows zoning. '!he more central points, 44, 48 and 101 47 yield temperatures of 590-630°c. The next points, 43 and 45 give 560°c and the outer points, 42 and 46 yield temperatures of 55o0 c. 'Illus the orthopyroxene shows temperatures of 630-55o0 c f rem core to edge. The clinopyroxene temperatures are less a:msistent. The highest temperature recorded in this grain is ns0 c at point 57, however the adjacent point, 56, gives a temperature of 510°c. This probably reflects exsolution. Along the edge next to the orthopyroxene, p'.)int 49 gives a high temperature of 670°c, however the total weight i;:ercent for this analysis is only 96.6% and thus it may not be trustworthy. If this analysis is not used, the grain does show a crude temperature zoning along t..-Us edge but the changes are very irregular. The second group of crystals in Area C again show zoning in the orthopyroxene with temperatures of 600, 630 and 6so0 c at p'.)ints 68, 69 and 70 respectively, going towards the center of the grain. The edge of the grain is The clinopyroxene grain gives temi;:eratures of 580-63o0 c close to the edge. It appears that the orthopyroxene is much more consistent than the clinopyroxene and, therefore, probably more reliable. The lack of consistent data from the clinopyroxene is believed to be due to exsolution. The intergrown grain in area B gives a temperature in the core of 615-645°c and a temperature of 505-53o0 c at the edge. The intergrown grain in area C yields a core temperature of 590-63o0 c and an edge temperature of sso0 c. These two grains give similar results and api;,::ear trustworthy indicating a range between 550-6S0°c for the assanblage. 102 Several other methods of geothermometry were also applied. The well known pyroxene geothermometer of Wood and Banno (1973) revised by Wells (1977) was not used. Recent work has shown that some of the calculations are in error and, although the thermometer is still useful for high temp:ratures, it gives results over 100°<: too high for metamorphic pyroxenes (Lindsley, 1983). The Ross and Huebner (1979) graphical pyroxene thermometer is very similar to the one used and gives identical results (Lindsley, 1983Y. Kretz (1982) uses formulae to model another pyroxene thermometer. He used both Ca exchange and Fe-Mg exchan_ge, however this second method does not give good results (Lindsley, 1983). The Ca exchange method uses the formula below for temp:ratures less than 1080°c. T(°K) = 1000 / (0.054 + 0.608 xOPX - 0.304 ln (1 - 2 [Ca]cpx)) where xOPX = Fe / Fe + Mg, [Ca] = Ca I Ca + Mg + Fe xOPX for this and the following methods has been calculated from values obtained by projecting onto the Fe-Mg composition line from wollastonite. The values for xOPX range from 0.85 - 0.87. The values of [Ca] were obtained using the most extreme results and give values of 0.38-0.46. Temperatures ranging from 450-725°c are obtained. If values are taken for the clinopyroxene from probed points yielding reliable results from the previous method, [Ca] varies from 0.41-0.45. These four values give temperatures of 521, 528, 634 and 644 °c. The range in temp:ratures for both results is similar to that found using LindsleY!_ '.s two-pyroxene method, discussed above. 103 Coexisting orthopyroxenes and olivine are used for the thermometer of Sack (1980). The values used for olivine were XFe 0.95 - 0.97 and Xpe 0.85 - 0.87 for the orthopyroxene. These figures give yalues of K D (01 - Opx) between 2.84 and 4.93.; This indicates temperatures of 560-730°c at one atmosphere (Fig. 6.4). Again this corresp:mds to the results already obtained by the last two methods. The presence or absence of pigeonite is also useful as an indication of temperature. In the rocks from the Wildcat body, the orthopyroxene appears to be primary and shows no evidence that it has inverted from pigeonite. Much work has been oone on the magnesium-rich pigeonites and very little on the iron-rich. In general, pigeonite becomes stable at lower temperatures with increasing iron but, since iron-rich pigeonite is stable at high temperatures relative to the temperatures already obtained for this assemblage it indicates a maximum temperature for the assemblage. Ross and Huebner (1979) developed a pigeonite thermometer which is shown in figure 6.5. This gives a maximum temperature of 90o 0 c for the Wildcat assemblage. Lindsley and Grover (1980) state that at Fe contents greater than = 0.7, the temperature of the five phase assemblage of olivine- orthopyroxene-clinopyroxene-pigeonite-quartz remains constant at 825° ± 15°. This is over the pressure range of 1-llKbar. Therefore, the Wildcat assemblage must have reached equilibrium below 825°C. It was suggested that the intergrowth of clinopyroxene and orthopyroxene seen in the Wildcat body was possibly due to coarse exsolution of a original sub-calcic phase. This sub-calcic pyroxene would have been an iron-rich pigeonite and, therefore, would indicate 104 Figure 6.4. Orthopyroxene-olivine geothenroneter at 1 atm. after Sack, 1980. Shaded area sba.vs composition range for the Wildcat body indicating a xfx temperature range of 570-730 C. Ill x 0.. I 0 c 2..0 ::.: Figure 6 .5. (}:JelON) Pigeonite geotherrro- rreter from R:>ss and Huebner, 1979. Due to the lack of pigeonite in the Wildcat 0.2 o.• 0.0 o.• body this the:rrroneter indicates a X°'.. maximum temperature of 900 C. iI •JOO L I i \. Il ! ;200 0 •0 ! • • • ,. i l I I I ,. """ \ \ 1 I \ O•P \ I \,. I ' lOS a temperature of above 825°C. Using the ratio of 40% clinopyroxene and 60% orthopyroxene as seen in thih section and plotting this on the 5 Kbar graphical thermometer (Lindsley, 1983) a temperature of approximately 900° is obtained. The assemblage quartz-olivine-orthopyroxene is very useful for indicating pressures of metamorphism. The method is based on the divariant reaction, Olivine + Quartz = Orthopyroxene. Orthopyroxene is stable on the high pressure side of the reaction. Magnesium lowers the stability field of iron-rich orthopyroxene with resi;ect to temi;erature and pressure. The relationships of temperature, pressure and composition for the assemblage can be seen in figure 6.6. This is based on experimental studies on the Enstatite-Ferrosilite join. At high XFe values for olivine and orthopyroxene and low temperatures and pressures, only olivine and quartz are stable. As pressure increases with or without an increase in temperature, the three-phase assemblage of olivine, orthopyroxene and quartz becomes stable. With further increase in pressure, just orthopyroxene is stable. For a given composition two lines can be drawn in P-T space representing two divariant reactions. The first reaction is between the two-phase field , olivine-quartz, reacting to produce the three phase assemblage, olivine-quartz-orthopyroxene. The second reaction produces the single-phase field of orthopyroxene (on the enstatite- fer ros ilite join). For different compositions, a series of these lines can be plotted (Fig 6.7). The isocomi;x:isitional lines denoted by 106 Figure 6.6. Tertperature, pressure and corrq:;;osition relationships for iron-rich olivine and orthopyroxene with quartz rrodified after Lindsley, 1980). Ruled plane indicates ferrosilite (Fs) 97. 01 - olivine Opx - orthopyroxene Q - quartz l07 Low Quartz / High 0""" 15 100 11. Fs /,'' Q5 13 Fs QO 12 / Fa / /' 05 11 \) 10 7' CJ Ill.., 9 8 7 6 5 4 3 2 / / / , I 0 300 l.00 500 600 700 BOO 900 1000 1100 T oc Figure 6.7. Pressure - temperature projection sho.ving the stability of iron-rich orthopyroxene with respect to iron-rich oliVine and quartz. Shaded area covers the fields of ferrosilite (ps) 85-87 and fayalite (pa) 95-97. (M'.xlified after Bohlen, Essene and Eloettcher, 1980}. l08 the Fa com:i;:onent represent the temp:rature and pressure where olivine and quartz begin to be unstable and the three-phase assemblage is formed. The lines denoted by Fs show the temperature and pressure above which only orthopyroxene of that oomposition is stable. As the compositions of the olivine and orthopyroxene become richer in magnesium, the temp:ratures and pressures of these reactions decrease. This figure has been drawn using data obtained from Bohlen and Boettcher (1981), Bohlen et al. (1980) and Cohen and Klement (1967). Their data have been extrapolated to lower temp:ratures and pressures by thermodynamic calculations. The stability fields of the various assemblages are also temperature dependent and, therefore, the temperature has to be obtained first. Using the previous values for the orthopyroxene, XFe = 0.85-0.87, and olivine, XFe = 0.95-0.97, pressures of 3.6 to 5.9 Kbar are obtained for temp:ratures ranging from 550-645°c. The iron-rich pigeonite suggested by the intergrowth seen in the Wildcat body yields a pressure of 8-9 kbar for a temperature of 900°c and a value of XFa = 0.95. In addition, the orthopyroxene geobarometer of Jaffe et al. (1978) is used (Fig. 6.8). This gives pressures of 4.4 to 6.8 Kbar, rather similar to the pressure obtained by the previous method. The first method is probably more accurate as the data are more recent and account for a greater number of variables. In summary, the Wildcat assemblage, olivine-orthopyroxene- clinopyroxene-quartz-rragnetite, indicates a temp:rature range of 550- 6450C and a pressure range of 3.6-5.9 Kbar. This pressure and temperature are within the field of .amphibolite metamorphism. The 109 '· pigeonite yields a temperature of 90o 0 c and a pressure of 8-9 kbar which indicates granulite facies rnetarnorphisrn. Relations ,gruJ Alteration .in .the Wildcat The phase relations between magnesium-rich olivine and orthopyroxene have been studied in detail but little is known about the iron-rich members. Miyano and Klein (1983) collected the available data on the distribution of Fe and Mg between coexisting iron-rich olivines and pyroxenes (Fig 6.9). This figure indicates that the partitioning of iron and magnesium between the two phases is relatively insensitive to pressure. The data from the Wildcat body fall on the curve but are much more iron-rich than previously reported values. A similar problem of lack of data is encountered when studying the amphiboles. Much work has been done on coexisting amphiboles, for example, by Stout (1972) and Klein (1968). The minerals include anthophyllite, cummingtonite-grunerite, gedrite and calcic amphibole, generally in homotaxial intergrowths with contacts parallel to the prismatic zone. They studied the distribution of the various elements between pairs of amphiboles tut this would apply only to amfbiboles in prograde assemblages where equilibrium between the various amphiboles was attained. In the Wildcat area, the amfbiboles are all retrograde and their chemistry will be influenced by the primary mineral phase as well as the physical conditions. Equilibrium is more likely to be obtained between the amphibole and the pyroxene or olivine it is 110 '· replacing than between the different retrograde amphiboles. All the amphiboles found in the Wildcat area are stable in the amphibolite facies. Grunerite-cummingtonite is the most common amphibole in metamorphic iron-formation. Previous studies all show evidence of prograde grunerite in equilibrium with the highest-grade, amJ;ilibole bearing assemblage. The prograde grunerite described has a tabular habit and occurs as discrete grains with smooth grain boundaries. Grunerite, in this form, does not occur in the Wildcat body. Bonnichsen (1969) reported both prograde and retrograde grunerite. '!he retrograde grunerite replaces both orthopyroxene and olivine. He found that retrograde grmerite replacing orthopyroxene is richer in iron than the prograde. He suggests that the stability field of grunerite extends into the iron-rich field with decreasing temperature or an increase in the activity of water. In all assemblages, the grunerite is lower in Fe/Mg than the coexisting orthopyroxene. Miyano and Klein (1983) collected data for coexisting orthopyroxene-grunerite pairs from various metamori;ilic iron-formations (Fig. 6.10). Optical examination indicated the grunerite from the Wildcat area has a Xpe value of 0.8-0.9. Using the orthopyroxene data, Xpe = 0.85-0.87, this figure would suggest an XFe of 0.84 ± 0.03 for the retrograde grunerite which agrees with the optical data. Many studies show olivine altering to grunerite (Fig. 6.11). 'Ille XFe of the retrograde grunerite is generally lower than that of the prograde grunerite in contrast to the data for orthopyroxene (Bomichsen, 1969). Both are lower than the roexisting olivine. Figure 111 ·c 500 600 700 800 900 1000 1100 2 Figure 6 • 8 . Orthopyroxene geo- 4 barorreter (from Jaffe et al. ,19781 with isocoinfOsitional lines shaving 6 the composition of orthopyroxene 8 coexisting with. olivine and quartz. Shaded area is for ferrosili te (.fs) _ 12 85-87. A pressure range of 4.4- 14 6.8 Kbar is indicated for 16 tures of 550-650 C. 18 1.0 I A I I Figure 6 • 9 . CoinfOSi tions of co- 0.9 I I m existing olivine and orthopyroxene 0.8 ,,.,1 'li.2 I (;from Miyano and Klein, l983)_. 0.7 10 9' 0 / 0 xoFo .. / 0 Shaded area indicates composit- /o 0.6 / ions from the Wildcat body. 0.5 0.4 0.4 0.5 0 .6 0.7 0.8 0 .9 1.0 1. 0 0 .9 0.8 0.7 XG'"' "' 0.6 0.5 0.4 Figure 6.10. Compositions of coexisting grunerite (Gru) and orthopyroxene (Opx) from Miyano and Klein, 1983. Sfladed area indicates orthopyroxene comf,Qsitions from the Wildcat l:ody. 1.0 0.9 0.8 ;° I • 0.7 • I I • I I d,._ I I Proqrocle I I pairs I I 0.6 I I I I I I I 0.5 /'· I . I 0 .4 pa1t1 I I 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Figure 6 .ll. COroposi tions of coexisting grunerite and olivine l011 after .Miyano and Klein, 1983. Shaded area indicates olivine carq;ositions from the Wildcat l:ody. ll3 6.11 suggests a probable value of Xpe of 0.83 ± 0.05 for the retrograde grtmerite of the Wildcat body using Xpe for the oliv:ine of This is very similar to that obtained from the orthopyroxene data and the optical data. As Miyano and Klein (1983) point out, reactions to form grunerite from olivine depend on the availability of water as the temi;;erature is lowered, and on the temperature and oxygen fugacity of the original assemblage if one assumes f02 is l::uf f ered by a metamorphic assemblage such as quartz-fayalite-magnetite. If the assemblage originally consisted of olivine, magnetite and quartz, grtmerite can be formed :in two ways. If the water fugacity was originally high, grunerite will directly replace olivine. This is the case in the Wildcat body. If however, the water fugacity is low, orthopyroxene will replace the olivine. This in turn can react to form grunerite at a lower temperature, assurn:ing appropriate water f ugacity. The stability of anthophyllite has only been studied for the magnesium-rich end member (Greenwood, 1963, 1971). Pure magnesian anthophyllite is stable between 600-800°c up to pressures of 3Kbar. More iron-rich anthophyllite will be stable at lower temperatures. Anthophyllite is rare in metamorphosed iron-formation. Worl (1972) reported anthophyllite grains with cores of hypersthene or grunerite similar to those seen in the Wildcat body. Vernon (1972) studied aluminous hypersthene altering to anthophyllite. Grant (1981) studied coexisting orthopyroxene and orthoamphibole and noted rims of aluminurrrpoor orthoamphibole on the 114 pyroxene. They suggest that this is late hydration of the orthopyroxene with little participation of other ftlases. Hastingsite is also relatively rare in iron-formation. It was noted by Floran and Papike (1978) and Gole and Klein (1981). Bonnichsen (1969) reported hornblende replacing clinopyroxene. He suggested that with a decrease in temp:rature less aluminum is needed to stabilize hornblende coexisting with pyroxenes and olivine. Kranck (1961) noted the association of cummingtonite and green calcic amphibole in low grade rocks. Much work has been done on the composition of fluids . in equilibrium with iron-f orrnation during metamorphism. Iron-formations that contain the assemblage quartz-magnetite-fayalite (QMF) are buffered to a certain oxygen fugacity for a certain temperature and pressure. Figure 6.12 shows various buffers that affect iron- formation. For a temperatures of 645°C, a pressure of 6 Kbar, and an assemblage of QMF, as in the rocks studied here, the oxygen fugacity will have a value lo-17 to lo-18 bars. This figure indicates grunerite would be stable in these rocks, but if no water was present gnmerite could not f orrn. None of the amphiboles in the Wildcat body appear as if they are prograde. '!his absence of prograde arnphiboles suggests that the p:ak metamorphic temp:ratures and pressures of the assemblages were higher than the stability fields of all the amphiboles in these iron-rich bulk comp:isitions. It also suggests that the rocks were anhydrous as water greatly extends the stability fields of the amphiboles at 115 -12 P5 = 6 Kb (+magnetite) -13 -14 Magnetite ..§ c:- - 16 £ -19 -20'---_.__ __.__ ....._ _ __;.: __.L.._ _ _.__ ___J 600 700 750 Temperature, °C Figure 6.12. Oxygen fugacity with respect to temperature for rretarrorphic iron-fonnations (from Miyano and Klein, 1983) • Mineral assemblages and terrperatures inferred in this study imply -17 -18 an oxygen fugacity of 10 to 10 ll6 elevated temperatures. Haase (1982) indicated a temperature of approximately soo 0 c and 2-3Kbar for the maximum stability for his iron-rich, amphibole-bearing, 07roxene-free assemblages. No similar work has been done on amphibole-free, pyroxene-olivine assemblages with a high Xpe value but Haase's work provides a minimum temi;:erature. The alteration of pyroxene and olivine to form amphiboles is a dominant feature in the Wildcat body. The rocks in the area are in the amphibolite facies. Frost (pers.comm., 1983) stated that there were two regional. metamorphic events in the Wind River Mountains. The first was to granulite facies and the second varied over the area from amphibolite to granulite facies. This appears to be consistent with the present findings in the Warm Springs area described above. The metamorphosed iron-formation shows no textural evidence of being affected by the second metamorphic event. The absence of such evidence suggests that the iron-rich bodies may have been anhydrous and, therefore, unaffected by the amphibolite facies metamorphism. Since there are now retrograde amphiboles in the iron-rich bodies, it appears that either they were produced by partial equilibrium at amphibolite facies, limited by HzO supply during the waning of the second metamorphic event or they were due to a later event. If these fibrous, retrograde amphiboles were formed prior to the second metamorphism, they would have been recrystallized to some degree. Furthermore, the amphibolites studied in the surrounding gneiss do not show any mineralogy or textures that would be indicative of a prior granulite facies metamorphism. This suggests that the amphibolites 117 were more hydrous than the iron-formation and completely equilibrated with the secorxi amt:Oibolite metamorphism. Relations ,aruJ Alteration .Q.f .the Spruce The assemblages at the Spruce Creek body are different from those of the Wildcat body. No mineral analyses are available from this area. The assemblage is rich in garnet and biotite which indicates that these rocks are more aluminous than the Wildcat body. This type of assemblage has been noted by .Gole and Klein (1981) who suggested that the rocks they studied were originally iron-rich shale. The unusual silicif ication texture of iron-silicates suggests replacement of either pyroxene or amphibole or both. Where the pseudomorphed iron-silicate occurs with fresh garnet and biotite, the mineral textures indicate they were once in equilibrium. This suggests that the original metamorphic assemblage was biotite with either pyroxene or amphibole or both. This assemblage was then affected by silicif ication. It is not plausible that the biotite and garnet were formed after the alteration of iron-silicates. After the intense silicification of the pyroxene and/or amphibole, only fine-grained quartz and magnetite remained. A large influx of material would be needed to form the biotite and garnet from the assemblage now seen. Furthermore, if the post-silicification metamorphism was strong enough to produce the biotite and garnet, the fine-grained quartz and magnetite would be totally recrystallized. So, it is suggested that preferential 118 silicification affected only the pyroxene and arnphibole but left the biotite and garnet unscathed. The silicification must post-date the granulite facies metamorphism that produced the pyroxene and amphibole. Much of the quartz and magnetite is very fine-grained and suggests, therefore, that silicification occurred after the amphibolite metamorphism. Rarely, the quartz does appear to be recrystallized. This local recrystallization may be due to contact metamorphism during the intrusion of the later dikes. The Spruce Creek body is the host to a small uranium aeposit. It is possible that the uranium was hydrothermally introduced along fractures in the Spruce Creek body and the source could possibly be the Warm Springs Pluton, the closest granitic body. Relative Position Qf .the Iron-rich 6odies The magnetometer survey indicated the possibility of additional iron-rich pods beneath the glacial cover (Fig. 5.3). The pods located by the survey and the major iron-rich bodies appear to be conformable with the trend of a lineament seen on the Landsat photograph. Mitra and Frost (1981) studied the deformation zones in the Wind River Mountains. They found three generations of deformation: an Early Precambrian, a Late Precambrian and a Lararnide deformation. The Early Precambrian shear zones are characterized by recrystallized mylonites but due to the recrystallization, there is no preferential weathering and therefore they do not stand out on air photographs. The Late 119 Precambrian zones are characterized by retrograde assemblages of chlorite, epidote and actinolite. These zones weather relatively easily and may produce distinct lineaments. The zones vary in width from microscopic to tens of kilometers. The Laramide deformation zones are not easily seen on air photographs. They are characterized by brittle deformation and cut the Paleozoic sediments. It is possible that the lineament seen on the Landsat photograph could be due to a Late Precambrian shear zone. Recrystallized rnylonites in the Spruce Creek body indicate that the area was affected by deformation during the Early Precambrian, It is probable that the area was active again during the Late Precambrian deformation. The fractured rocks in the Spruce Creek body typically have chlorite filling in the areas between the fragments, characteristic of the Late Precambrian deformation. It is suggested that the two bodies studied were originally part of the same body and were moved apart during the Late Precambrian deformation. The additional pods indicated by the magnetometer survey show an en echelon pattern trending in the same direction as t..11e lineament. These pods could be pieces of the original body smeared out along the deformation zone. 120 SIM1ARY '!he Warm Springs area, underlain pre00minantly by ·Archean gneiss with amphibolite, is situated in the northern Wind River Mountains in Wyoming. The granitic Warm Springs pluton intrudes the gneiss in the northeast. Within the gneiss terrain ·two unusual iron-rich bodies crop out. The earliest event for which evidence was seen in these rocks is a regional metamorphism to granulite grade which is observed in the mineral assemblages preserved in the Wildcat body. There is no textural or mineralogical evidence of any assemblage prior to the high grade assemblage seen now. This suggests that the temperature and pressure of this early event were higher than the stability field of r anq:hiboles. The second metamorphism in the area was to amphibolite grade, which proeuced amphibolites in the country rock, however, it did not significantly affect the iron-rich bodies. It is believed that the iron-rich bodies were anhydrous and therefore not affected. The surrounding gneisses and amphibolites show no evidence of the early granulite-facies metamorphism. The quartzo-feldspathic gneiss may show a planar foliation or be complexly folded and is comioonly migma.titic. The gneiss shows at least two of folding. The mylonites are therefore, at least one of the regional metamorphic events post-dated the Early Precambrian deformation. Large, irreg.ilar crystals of alkali feldspar are common and these are believed to be of metamorphic origin. Within 121 the gneiss small conformable pods of amphibolite occur. To the east and northeast of the Warm Springs canyon is the intrusive Warm Springs pluton, composed of rocks ranging in composition from granite to quartz monzodiorite. This was intruded after the second regional metamorphism. The contact between the intrusion and the gneiss is not exposed but xenoliths of the cotmtry rock are seen within the pluton. 'Ihe gneiss and granite are cross-cut by late-stage granitic dikes, plagioclase-rich pegrnatites and quartz veins. During the Late Precambrian a major period of erosion occurred. Paleozoic sedimentary rocks (Cambrian Flathead Sandstone) tmconformably overlie the gneiss and the pluton. The lowest tmit seen is a gravelly, arkosic sandstone whereas upper units are medium- to finely-bedded quartz sandstone. The majority of the area is covered by glacial drift. The Wildcat body is composed of iron-rich granofels cut by granodioritic dikes and a pegrnatite. No contact with the surrounding gneiss is exposed. The highest-grade assemblage found is olivine- orthopyroxene-clinopyroxene-quartz-magnetite in a medium-grained, equigranular, granoblastic texture. Olivine is fairly rare and the dominant assemblage consists of orthopyroxene-clinopyroxene-quartz- rnagnetite with some garnet and retrograde amphiboles. The pyroxenes show a range of tmusual textures suggesting exsolution of an earlier, sub-calcic pyroxene. The pyroxenes and olivine show retrograde reactions to amphiboles. The rock shows evidence of local 122 siiicification with multicrystalline, multiphase pseudomorphs of chlorite, quartz and magnetite after iron-silicate. The Spruce Creek body is composed of heterogeneous iron-rich granofels. The iron-rich granofels shows various textures and mineralogy with no systematic relationship between the various rock tyi;:es. The highest-grade assemblages seen are similar to those in the Wildcat body but olivine is absent. Both orthopyroxene and clinopyroxene are present but the two minerals are not seen in the same assemblage. In general the rocks have more biotite and garnet suggesting they are more aluminous. The Spruce Creek body shows unusual pseudomorphic intergrowth of quartz and magnetite, believed to be formed by the silicification of iron-silicates, possibly pyroxene and arnphibole. The body shows evidence of deformation. The retrograde amphiboles seen in both iron-rich bodies were produced either during the waning of or after the second metamorphism. The silicification post-dates the granulite-facies metamorphism that produced the pyroxenes and amphiboles and it is probable that it is later than the amphibolite-facies metamorphism. The protolith of these iron-rich bodies is believed to have been banded iron-formation that could have originally been either carbonate or silicate facies. This is suggested by the abundance of quartz, magnetite and iron-silicates and the textures and mineralogy are similar to other high-grade metamorphic iron-formation described in the literature. The assemblage olivine-orthopyroxene-clinopyroxene- ' quartz-magnetite is relatively rare in metamorphosed iron-formation. It has been reported only once before in a regionally metamorphosed 123 area but several studies note this assemblage in contact metamorphic areas. The assemblage olivine - orthopyroxene - clinopyroxene - quartz magnetite is well suited for obtaining estimates of both metamorphic temperatures and pressures. Since much of the work has been done on more magnesium-rich minerals, the results in the iron-rich range are less well controlled. The two-pyroxene geothermometer of Lindsley (1983) was the main method used; it indicated a temperature of 550- 6500c and a zoning of loo0 c within some of the grains from core to edge. The geothermometers of Kretz (1982) and Sack (1979) both agree with this temperature range. The olivine-orthopyroxene-quartz geobarometer of Bohlen and Boettcher (1981) was also used and, for the temperatures obtained, this method gave pressures of 3.6-5.9 Kbar. This pressure and temperature are within the field of amphibolite metamorphism. The oxygen fugacity was probably lo-17 to lo-18 bars. The pigeonite, suggested by the orthopyroxene and clinopyroxene intergrowth that occurs in the Wildcat body, ,yiel&3 a temp:rature of 9oo 0 c and a pressure of 8-9 Kbar which indicates granulite facies. These two sets of results correlate well with both the field observations and with other work in the area (Frost, per. comm., 1983). The relative positions of the two bodies is believed to be influenced by a Late Precambrian deformation zone underlying the lineament. 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(197 8) Regional metamorphism of Proterozoic iron-formation, Labrador Trough, Canada. American Mineralogist, 63, 898-912. Kranck, s.H., (1961) A Study of Phase F.quilibria in a Metamorphic Iron Formation. Journal of Petrology, 2, 137-184. Kretz, R. (1982) Transfer and exchange equilibria in a portion of the pyroxene quadrilateral as dedlced from natural and experimental data. Geochimica and Cosmochimica Acta, 46-1, 411-421. 128 Labotka, T.C. Vaniman, D.T. and Papike, J.J. (1982) Contact metamorphic effects of the Stillwater Complex, Montana: the concordant iron-formation: a reply to the role of buffering in metamorphism of iron-formation. Americam Mineralogist, 67, 149- 152. Leake, B.F.. (1978) Nomenclature of amphiboles. American Mineralogist, 63, 1023-1052. Lindsley, D.H. (1981) The formation of pigeonite on the join hedenbergite-ferrosilite at 11.5 and 15 kbar: experiments and a solution rrodel. Arrerican Mineralogist, 66, 1175-1182. Lindsley, D.H. (1983) Pyroxene thermometry. American Mineralogist, 68, 477-493. Lindsley, D.H. and Grover, J.E. (1980) Fe-rich pigeonite: a geobarometer (abs). Geological society of America, Abstracts with Programs, 12, 472. Miller, B.M. (1936) Cambrain stratigraphy of northwestern Wyoming. Journal of Geology, 44, 113-144. Mitra, G. and Frost", B.R. (1981) Mechanisms of deformation within Laramide and Precambrian deformation zones in basement rocks of the Wind River Mountains. Contributions to Geology, University of Yijoming 19,161-173. Mitra, G. (1978) Ductile deformation zones and mylonites: The mechanical processes involved in the deformation of crystalline basement rocks. Arrerican Journal of Science, 278, 1057-1084. Miyano, T. and Klein, c. (1983) Phase relations of orthopyroxene, olivine, and grunerite in high- grade metamorphic iron-formation. American Mineralogist, 68, 699-716. Morey, G.B., Papike, J.J., Smith, R.W. and Weiblen, P.W. 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(197 9) Temperature-composition relationships between naturally occuring augite, pigeonite and orthopyroxene at one bar pressure. American Mineralogist, 64, 1133-1155. Sack, R.O. (1980) Some constraints on the thermodynamic mixing properties of Fe-Mg orthopyroxenes and olivines. Contributions to Mineralogy and Petrology, 71, 257-269. Saxena, S.K. (1976) Two pyroxene geothermometer: a model with an approxinate solution. Arrerican Mineralogist, 61, 643-652. Saxena, S.K. (1983) Problems of two pyroxene geothermometry. Earth and Planetary Science Letters, 65, 382-388. 130 Simmons, E.C., Lindsley, D.H. and Papike, J.J. (1974) Phase relations and crystallization sequence in a oontact metamorphic rock from the Gunflint iron formation, Minnesota. Journal of Petrology, 15, 539-565. Smith, D. (1971) Stability of the assemblage iron-rich orthopyroxene- olivine-quartz. American Journal of Science, 271, 370-382. Smithson, S.B., Brewer, J.A., Kaufman, s., Oliver, J.E. and Zawislak, R.L. (1980) Complex Archean lower crustal structure revealed by COCORP crustal reflection profiling in the Wind River Range, Wyoming. Earth and Planetary Science Letters, 46, 295-305. Spry, A. (1969) Metamorphic textures. Pergamon Press Ltd. Oxford, Great Britain. Stout, J. H. (1972) Phase Petrology and Mineral Chemistry of Coexisting Arnphiboles from Telemark, Norway. Journal of Petrology, 13, 99- 145. Vaniman, D.T., Papike, J.J. and Labotka, T. (1980) Contact-metamorphic effects of the Stillwater complex, Montana: the roncordant iron formation. American Mineralogist, 65, 1087-1102. Vernon, R.H. (1972) Reactions involving hydration of cordierite and hypersthene. Contributions to Mineralogy and Petrology, 35, 125- 137. Wells, P.R.A. (1977) Pyroxene thermometry in simple and complex systans. Contributions to Mineralogy and Petrology, 62, 129-139. Williams, W.C. (1980) Precambrian geology of the Dinwoody Lakes Region, Wind River Range, Fremont County, Wyoming. (M.S. Thesis) Laramie, Wyoming. The University of Wyoming. 85p. Wood, B.J. and Banno, s. (1973) Garnet - orthopyroxene and orthopyroxene - clinopyroxene relationships in simple and complex systems. Contributions to Mineralogy and Petrology, 42, 109-124. Worl, R.G. (1968) Taconite in the Wind River Mountains, Sublette County, Wyoming. 'Ihe Geological Survey of Wyoming, Preliminary Report 10. 131 ---16_ __ ANALYSIS 12 13 14 15 1'{ 18 19 20 21 PHASE Mt Ht Mt 01 OJ Mt Opx Cpx 01 Cpx 1 Si02 0.95 0.56 0.57. 30.93 30.98 (•. 58 n .05 48.58 30.29 118. 22 ----- 1. 19 1. 4 3 ------1 .09 0.73 1. 5 3 ---- - 1 .59 Tl 2 ----- 0.95 0 . 68 ------0.71 ------FeO 91.25 90.98 90.37 68.65 68.93 9 1 . 4 1 47.37 25 . 118 68.59 27.86 MnO ----- 0.06 0.03 0.20 0 . 24 0.08 0. 111 ----- 0.24 MgO ------1. 711 1.70 -- - - - 4.30 3 .5 5 1. 3 9 3. 15 Cao 0. 111 ------0. 10 0. 10 - -- - - 0.58 20.84 0. 10 18. 4 4 TOTAL 92. 3lj 93. 7 ll 93.08 101. 62 102 . GO 93.87 100. 17 99.98 100 . 61 99.26 Si 0. 049 0.028 0.028 1. 013 1. 012 0.029 1. 986 1. 959 1 .G07 1.961 Al ----- 0.070 0.084 ------0 .0611 0.036 0.073 --- -- G.076 Ti ----- 0.035 0.025 ------0.026 Fe 3.895 3.760 3.765 1. 881 1. 8811 3. 79 1 1. 67 2 0.859 1. 907 0.948 Mn ----- 0.003 0.002 0.005 0.007 0.003 0.005 ---- - 0.007 Mg ------0.085 0.083 -- --- 0.271 0.213 0.069 0 . 213 Ca 0.007 ------0.004 0.004 ----- 0.026 0.901 0.003 0.804 TOTAL 3.951 3.896 3.904 2.988 2.990 3. 913 3.996 4.005 2.993 4.002 OXYGENS 4.0 4.0 4.0 11. 0 4.0 6 . 0 6.0 6.0 4.0 6.0 Appendix '/\. Micropr:obe analyses of olivine (_01), orthopyroxene (Opx), clinopy.roxene (Cpx) and magnetite (Mt) from sanple WSN 3F4A. Analysis nurrbers correspond to the points in figure 6.3. ANALYSIS 22 23 211 25 20-- -2-1 28 29 30 31 PHASE Cpx Opx Opx Opx Opx Cpx Cpx Cp x Cpx Cpx Si02 IJ8. 36 lj6. 91J IJ6. 79 IJ6. 82 116. 81 48 .51 48. 30 48. 38 48. 38 48. 39 1.55 1. 13 1.06 l. 06 1 . 01 1. 62 1 .51 1 .60 1. 60 1.64 Tl 2 ------feO 26.47 ll6.93 IJ7. 67 117. 78 117. 56 26 .Oil 27.25 26.06 26.60 28.01 MnO 0.03 0. 12 0. 1 IJ G. 10 0. 14 0.03 0.03 0.03 0.05 0.02 MgO 3.39 4. 31 lj. 31 lj. 19 lj . 33 3.39 3.56 3. 116 3 .119 3, IJ!j Cao 19.88 1.05 1 .DO 0.93 0.93 20.33 19.22 20.23 19 .97 18. 86 TOTAL 99.68 100.ll8 100.97 100.88 100.78 99.92 99. 87 99.76 100.09 100.36 Si 1. 961 1 .976 1. 961l 1. 967 1.967 1 .960 1. 958 1 .958 1. 955 1. 956 Al 0 .0'/4 0.056 0.053 0.053 0.050 0.078 0.072 0.077 0.076 0.078 Ti Fe 0.898 1 .650 1 .671l 1 .679 1 .672 C.880 0.92ll 0.882 0.899 0.91l7 Hn 0.001 O.OOll 0.005 0. DOil 0.005 0.001 0.001 0.001 0.002 0.001 Mg 0.205 0.210 0.269 0.263 0.271 0.20ll 0.215 0.209 0.210 0.207 Ca 0. 861l O.Oll7 0.045 0.042 O.Oll2 0.880 0.835 0.877 0.865 0. 817 TOTAL- lj .003 11.003 lj .o 10 4.008 4.007 4.003 lj .005 lj .0011 4.007 lj .006 OXYGENS 6.0 6.0 6.0 6.C 6.0 6.0 6.0 6 . 0 6.0 6.0 Appendix A. (.continued) . ANALYSIS 32 33 34 35 36 37 38 111 112 43 PHASE Cpx Cpx Opx Opx Cpx 01 01 Opx Opx Opx Si02 48.29 48.45 q7. 16 116. 70 116. 3q 30. 77 30.67 46.79 q6 .81 ll6.10 1. 61 1.55 0.89 1 .01 1. 27 ------Q.95 1.27 0.91l Ti 2 ------FeO 27. 71 27.ll6 46.76 ll7.40 26. 19 69.02 69.29 47.66 47.ll6 ll7.83 Mr.0 0.01 0.01 0.13 0. 11 0.00 0.24 G.25 0. 13 0. 11 0. 11 MgO 3.ll6 3.ll6 q. llO 4.32 4.00 1.62 1 . 4 1 4.04 ll.29 4. 1q Cao 18.85 19 .53 0.94 1.03 19. 71 0. 15 0.09 0.73 0.78 0.81 TOTAL 99.99 100.46 100.28 . 100 .57 99.51 101. 80 101. 71 100.30 100.72 100.53 Si 1.957 1 .955 1. 983 1 .967 1. 961 1 .008 1 .008 1. 977 1.965 1.970 Al 0.017 0.074 0.044 0.050 0.061 ------0 .0117 0.063 0.047 Ti Fe 0.940 0.927 1 .6115 1. 670 0.889 1. 892 1 .905 1. 684 1. 667 1.688 Hn 0.002 0.000 0.0011 0.004 0.000 0.001 0.001 0.004 0.004 0.004 Mg 0.209 0.208 0.276 0.271 0.242 0.079 0.069 0. 2511 0.269 0.261 Ca 0.819 0.845 0.042 O.Oll7 0.857 0.005 0.003 0.033 0.035 0.036 TOTAL lj .004 4.009 3. 9911 ll.009 4 .010 2.991 2.992 3.999 4.003 4.006 OXYGENS 6.0 6.0 6.0 6.0 6.0 11.0 ll.O 6.0 6.0 6.0 Appendix A. (continued) . ----- ___lj_8 ___ -- ANALYSIS llll 115 lf6 -- 47 49 50 51 52 53 PllASE Opx Opx Opx Opx Opx Cpx Cpx Cpx Cpx Cpx Si02 46.11 116.95 lj 7. 3 3 116 . 74 !i6 .51 118.117 118.116 118. 45 4 8. 15 47 .95 0.98 0.85 0.86 1. 11 1.00 1.68 1.68 1.61 1. 56 1.65 Ti 2 ------FeO 117.98 47 .118 47.67 117. 97 47 .119 25.56 25.56 26.09 25. 73 25.80 HnO 0 . 11 0. 11 0. 1 ll 0. 15 0. 10 0 .01 0.01 0.02 0 . 01 0.03 HgO 11.23 11.116 4.63 3.98 II .33 3.50 3.50 3 .119 3.48 3. ll6 Cao 0 . 95 0.83 0.75 0.94 0.93 20.86 20 . 86 20.63 20.82 18.69 - TOTAL 100 . 96 100.38 101 . 38 100.89 100.36 100.08 100.07 100.29 99.75 97.58 Si 1 .961l 1. 973 1. 9711 1 .966 1. 9611 1 . 991 1. 953 1. 952 1. 951 1. 976 Al 0.0119 c .0112 0.0112 0 . 055 0 . 050 0.079 0.080 0.01'"( 0 . 075 0.080 Ti ------Fe 1.688 1 .669 1 . 663 1 .688 1. 678 0.923 0.862 0.879 0.872 0.889 Hn 0.004 0.004 0.005 C.006 0.004 0.003 0.000 0.001 0.000 0.001 Hg 0.265 0.279 0.288 0.250 0.212 0. 211 0.211 0.209 0.210 0.213 Ca 0.0113 0.037 0.034 0.042 0.042 0.762 0.901 0.891 0.904 0.825 TOTAL 4.013 11.0011 4.006 4.007 4 .010 3 .969 lj .007 4.009 4.012 3.9811 OXYGENS 6.0 6.0 6.0 6 . 0 6.0 6.0 6 . 0 6 . 0 6.0 6.0 Appendix A. (continued}. AllALYSIS 511 55 56 57 53 59 60 61 62 63 PHASE Cpx Cpx Cp x Cp x Cpx Cpx 01 01 Opx Opx Si02 118. 110 118 . 38 118.11 2 117. 98 118. 611 119. 11 30.59 30.83 116.65 116. 70 1.57 1.55 1. 55 1 . 53 1. 110 1.27 ------0.98 1.05 T1 2 ------feO 26.05 25.28 26.09 28 . 76 25 . 36 25.33 68. 72 69.03 117 . 60 117.82 MnO 0.01 0.00 0.01 0.03 0.03 0.00 0.20 0 . 23 0 . 15 0 .12 MgO ].119 3.113 3.118 3 . 50 3.68 3.79 1.69 1. 67 11.20 lj. 22 cao 20 . 20 20.90 20.53 16. 90 20 . 89 21. 011 0.09 0. 11 0 . 73 0.68 TOTAL 99. 72 99 . 511 100. b8 98 . 70 100.00 100.511 101 . 29 lOl .87 100 . 31 100 . 59 Si 1 . 959 1.959 1. 955 1 . 971 1. 961 1. 966 1 .007 1 .009 1 . 971 1 .968 Al 0.075 0.0711 0.0711 0 .0711 0.067 0.060 ------0 . 0119 0 . 052 Ti Fe 0.882 0.857 0.881 0.988 0 . 855 0. 8118 1. 89 3 1. 890 1 . 682 1. 686 Mn 0 . 000 0.000 0.000 0.001 0 . 001 0.000 0 . 005 0.006 0.005 0.0011 G'i Hg 0.210 0.207 0 . 209 0.2111 0.221 0.226 0.083 0.087 0.2611 0. 265 Ca 0.876 0 . 907 0 . 888 0. 711 lj 0.902 0.903 0.003 0.0011 0 . 033 0.031- TOTAL 11 . 002 11 . 0011 11.007 3.992 11 . 005 11 . 003 2.991 2.996 11.0011 lj .006 OXYGENS 6.0 6.0 6.0 6 . 0 6.0 6.0 " . 0 ".o 6.0 6.0 Appendix A. (continued) . 68 AllAL'iSIS 61l 65 65 6T ------69 70 71 72 73 PHASE Cpx Cpx Cpx Cpx Opx Opx Opx Mt Mt 01 Si02 ll8. llll ll8. ll6 lt8. Ill! 118. 68 ll6 .82 ll6. 63 ll6. 58 0.53 0. ll7 30.93 Alb03 1.58 1.58 1.51l 1. 70 0.99 1.08 1.07 1.08 2.59 Ti 2 ------2. 12 0.82 feO 26.36 27.27 27.22 26.82 117. 18 lt6.98 ll7. 88 89.75 39.88 66.2lt MnO 0.00 O.Oll 0.00 0.02 o. 10 0.11 0.08 0.07 0.04 0. 19 MgO 3. ll lj 3.58 3. 6lt 3.59 lj. 18 11. 30 lj. 25 0.00 0.00 1. lt8 cao 19.69 19.50 19 .61l 19. 7 !J 0.98 1, 10 1. 17 ------0. 11 TOTAL 100.01 100.IJ3 100.ll8 100.55 100.25 100.20 101 .03 93. 55 93.80 - -913. 95 Si 1. 959 1. 95 IJ 1. 951J 1 .956 1. 975 1.968 1. 958 0.026 0.023 1.033 Al 0.075 0.075 0.075 0.080 O.OIJ9 0 .0511 0.053 0.063 0. 1ll9 Ti ------0.078 0.030 Fe 0.909 0.920 0.919 0.901 1 .665 1 .659 1 .681l 3. 6911 3.669 1. 851 Mn 0.000 0.001 0.000 0.000 0.003 0 .OO!J 0.003 0.003 0.002 0.005 Mg 0.208 0.215 0.219 0.215 0.263 0.271 0.266 0.000 0.000 0.071J Ca 0.853 0. 8lt3 0.8lJ3 0.850 O.Olt5 0.050 0 . 052 ------O.OOll TOTAL lJ .OO!J IJ.008 lj .010 ll .OOll lj .ooo lj .006 lJ .016 - f.8T3 ___ OXYGENS 6.0 6.0 6.0 6.0 6.0 6.0 6.0 lj .o lj .o lj .o Appendix A. (continued). ANALYSIS 71l 75 76 77 78 80 81 82 83 81l PHASE 01 Cpx Cpx Cpx Cpx Opx Opx Opx Opx Cpx Si02 30.66 118. 57 1l8 . 09 1l8. 28 48.24 1l6 .65 1l6. 98 47.08 46.72 48. 17 ---- - 1. 52 1.59 1, 116 1. 61 0.93 1. 03 1.02 0.93 1.1l7 Ti 2 ------FeO 67.82 25.93 29.28 27. 61 27 .Oll 1l6. 97 1l7 .00 1l7. 39 IJ6 . 58 28 . 1l5 MnO 0.20 0.02 O.Oll 0.02 0.00 0. 10 0. 11 0 . 11 0. 12 0.03 MgO 1.1l5 3, 118 3 . 1j lj 3. 61 3. IJ 8 11. 32 1j. 49 1j. lll 1j. 29 3.52 CaO 0. 10 20.llO 17 . 117 18.93 19. 71 0.93 1.01 0.92 1.06 18.78 TOTAL 100.23 99 . 92 99.91 99 . 91 100.08 99 . 90 100.62 100 . 66 99.70 100 .1l2 Si 1. G18 1. 962 1. 958 1. 958 1. 952 1. cj75 1. 975 1. 977 1. 979 1. 951 Al ----- 0 . 012 0 .077 0 .067 0 . 077 0 . 01l6 0.051 0.050 0.01l6 0.070 Ti Fe 1 .833 0.876 0 . 997 0.937 0.916 1. 663 1. 650 1. 665 1.650 0. 961J Mr. 0.006 0 . 001 0.001 0 . 001 0.000 O.OOll 0 . 0011 O. OOll 0 .OOll 0.001 Mg 0.012 0 . 209 0.209 0.218 0.210 0. 27 3 0.281 0.259 G. 27 1 0.212 Ca 0.004 0.883 0.762 0.823 0.855 0 .01l2 0 .01l5 0. Qlj 1 0.048 0.815 TOTAL 2 . 933 lj .003 1l .OOll 1l .OOIJ 4. 010 lj .003 lj .006 3.996 3. 998 1j .013 OXYGENS 1j .0 6.0 6.0 6.0 6.0 6.0 6.'0 6.0 6.0 6.0 Appendix A. (continued). ANALYSIS- -- 8 7 PHASE Cpx Cpx Cpx Si02 118 .so 48.38 48.32 1 .6 1 1 .119 1 .119 Ti 2 ------FeO 25.95 26.91 26.93 MnO 0.02 0.03 0.02 HgO 3. 41 3.39 3 .114 cao 20.61 19.47 19.50 TOTA!.. 100. 10 99.67 99.70 Si 1. 957 1.9611 1.962 Al 0.076 C.071 0.071 Ti Fe 0.876 0.914 0.914 Mn 0.001 0.001 0.001 ii; Mg 0.205 0.205 0.208 Ca 0.891 0. 3117 0. 8118 TOTAL 4 .000-- 4 .002 -.r:oo11 OXYGENS 6 . 0 6.0 6.0 Appendix A. (continued) . ----DATA FROM MAGNETOMETER --SURVEY STATIOll LETrER LINE NUMBER y z {\ B c D E* f G H .J K L M N p Q R s 0 5665 5673 5656 5671 5663 I 56117 5684 5690 570'( 5702 5681 5677 4 5670 5681 5656 5669 56'{4 5670 5690 56811 5690 5707 5712 5720 5674 3 5662 5656 5681 5637 5649 5671 5690 5665 5685 5679 5686 5713 5730 5767 5731 12 5656 5662 5647 5665 5673 5664 5683 5679 • 5707 5698 5726 5675 . 15 5657 5662 5666 5669 565ll 5662 5663 5650 5666 5675 5686 5667 5685 20 5656 5662 5642 5651 5624 5568 5673 5652 56711 5692 5578 5690 5717 24 5586 5670 5675 5689 5530 55115 5664 566'7 5684 5687 5658 5661 5675 28 5653 56'(5 • 5649 5571 5675 5684 5691 5684 5709 5680 5705 5732 32 5671 5706 5545 56119 5659 5668 5689 5674 5671 5685 5723 5754 5630 "tJj 36 5633 5642 5663 5679 5681 5735 5660 5662 5679 5636 5672 5678 5702 .I-' 40 5642 5646 5685 5708 5687 5718 5652 5645 4681 44 5651J 5670 5653 5730 5673 561J7 5646 561J7 5638 5656 5623 561J7 5651 !JS 5677 5659 5642 5872 5633 5662 5636 56lt6 5633 5631J 561J3 5650 5651 52 5651 5675 5641 5810 5750 5636 56117 5649 56811 5652 5651 5645 56115 56 5651 5647 5651 5650 5652 5641J 5667 5699 5691 5719 5668 5651J 5670 60 5651 5660 5668 5659 5665 5663 5657 5667 5675 5720 5702 5678 57511 Appendix B. Data from magnetorreter survey. Readings are in 10 gamna measurements. Line numbers and station letters are plotted on the figure on page B2. The results of this survey are shown on page 84. MAGNETOMETER SURVEY / I / \ _,.,,, I' ------./ 800 1600 FEET Spruco Creek Body X corresponds to station letter E nurr.bers by this correspond to the line nurrber Appendix B. Diagram to show line numbers and station letters that corres:pond to the data on page Bl. See also page 84. B2