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University of Nevada Reno the Petrology of a Zoned Granitic Stock

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University of Nevada Reno the Petrology of a Zoned Granitic Stock University of Nevada Reno The Petrology of a Zoned Granitic Stock, Stillwater Range, Churchill County, Nevada A Thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology By Steven Wayne Nelson Hi May 1975 The thesis of Steven Wayne Nelson is approved: Thesis advisor /ft - ^ Department chairman Dean, Graduate School University of Nevada Reno May 1975 1 ACKNOWLEDGMENTS The writer is grateful for the assistance of many individuals during the course of this study. Many suggestions and much guidance were provided throughout the investigation by the thesis advisor, Dr. M. J. Hibbard. Many additional helpful suggestions were provided by Drs. L. C. Hsu, E. R. Larson and D. C. Noble of the University of Nevada, and by Dr. B. Page at Stanford University, California. The writer is indebted to Dr. John H. Stewart, Western Mineral Resources Branch, U.S. Geological Survey, Menlo Park, California, for providing the analytical support for this investigation under the Nevada State Map Project, No. 3-9320-0039. Drs. M. J. Hibbard and L. C. Hsu visited the field area in the fall of 1973. In the fall of 1974, graduate students Wayne R. Kemp and Andy B. Wallace accompanied the writer in the investigation of mineralized areas in the project area. Pat Walker provided recent aerial photographs which greatly assisted in the field work. And, to his wife, No&l C. Nelson, the writer is indebted for continuous moral and financial support through the graduate years. In addition, she has provided welcome assistance in portions of the field work. Special thanks is extended to Mrs. Sylvia Miller, the writer's mother-in-law, of the U.S. Geological Survey, for typing the final copy of this report. 1 1 ABSTRACT The epizonal Mud Springs stock is a zoned Tertiary intrusion consisting of six intrusive units. The initial intrusive unit was quartz monzodiorite (62.2% Si02) which was followed in order by a seriate granodiorite (65.0% SiC>2), a quartz monzonite porphyry (63.9% Si02), a quartz syenite porphyry (65.2% SiO ), and finally by a large composite dike consisting of alkali-feldspar granite (77.9% SiO ) and an alkali-feldspar granite porphyry (74.7% SiO ). 2 2 All contact relations between the units are well exposed and sharp. Minor aplite, granophyric dikes and hornblende andesite dikes are found. The granitic rocks of the stock have intruded Triassic slates and phyllites to the north and Tertiary volcanic rocks to the west and south. The stock is bounded on the east by the Dixie Valley fault and Holocene alluvial cover. The older I.X.L. Canyon quartz monzodiorite has been radiometrically dated at 28 m.y. All of the units are texturally homogeneous except for a zone of seriate porphyritic rocks bordering the more typical, hypidiomorphic- granular, I.X.L. Canyon quartz monzodiorite in the marginal areas of this unit. No mafic minerals are found in the alkali-feldspar granites, but biotite and hornblende make up approximately ten percent of the mafic mineral content of each of the other units. Minor rapakivi and antirapakivi textures are found in the Mud Springs quartz monzonite and quartz syenite porphyries respectively. Major oxide and trace element determinations were made on one representative sample from each of the units. Variation of the major oxides show that as SiC>2 increases A^O^, Ca0' M90 ' Ta°2 anc^ tota-*- iii iron (FeO+Fe^O^) decrease and Na20 remains nearly constant. K20 increases with increasing silica content. These trends correspond to an increase in potassium and a decrease in iron and calcium from the older to the younger units. Late potassium-feldspar replacement of plagioclase accounts for the high K 0 content (5.4%) of the Mud Springs quartz syenite. Trace element data suggest that simple progressive fractionation cannot be called upon to explain the variation in rock types found in the Mud Springs stock. The K/Rb ratios are higher than the average ratio for igneous rocks and indicate the late introduction of potassium-feldspar. Field mapping and mineralogical, textural and chemical data indicate that fractionation took place in an irregularly shaped magma chamber which was periodically tapped by fractures through the confining rocks. Economic mineralization is restricted to small local Cu, Pb, and Zn sulfide-bearing veins occurring within the seriate granodiorite and deposits of Pb, Zn, Cu, Ag, and Au found in the slates and phyllites of the contact zone in I.X.L. Canyon. An irregular zone of propylitic alteration of the granitic rocks occurs along the Dixie Valley fault. All of the units have well-developed joint sets, with northwest and northeast strikes predominating. The northwest joint set approximately parallels two faults in the area. Foliation and schlieren are rare. Two, previously unmapped normal faults have offset the alkali-feldspar granite dike and locally produced shearing and brecciation. IV TABLE OF CONTENTS Page ACKNOWLEDGMENTS....................................... • ABSTRACT........................................................ TABLE OF CONTENTS............................................... iv INTRODUCTION .................................................... x Name........................................................... i Location.................................................... i Accessibility ............................................. i Objectives of this study................................... X Methods of study............................................... 4 REGIONAL GEOLOGY ................................................... 3 Rock u n i t s ................................................... 8 Triassic rocks .......................................... 8 Jurassic rocks ....................................... 9 Tertiary.rocks ....................................... 10 Late Cenozoic lake sediments......................... 12 Major faulting of the Stillwater Range..................... 13 Thrust faults ....................................... 13 Cenozoic faulting..................................... 13 THE MUD SPRINGS STOCK........................................... 15 General statement ......................................... 15 Description of individual units ........................... 16 I.X.L. Canyon quartz monzodiorite.................... 16 Seriate granodiorite ................................. 25 Mud Springs quartz monzonite porphyry................. 29 Mud Springs quartz syenite porphyry.................. 38 Alkali-feldspar granites ............................. 45 Minor rocks of the stock............................. 52 PETROCHEMISTRY .................................................. 62 Introduction............................................... 62 Major oxide variation................................. 62 Trace element variation............................... 65 Normative mineral variation........................... 74 Mineralization ....................................... 82 STRUCTURES...................................................... 86 IGNEOUS ACTIVITY ................................................ 90 Other Tertiary (?) plutonic rocks in the Stillwater Range. 92 Emplacement history ....................................... 94 REFERENCES CITED 99 V ILLUSTRATIONS Figure Page 1. Location map of the Mud Springs stock...................... 2 2. Roads providing access to parts of the Mud Springs stock . 3 3. Modal plots of quartz and feldspars from the units of the Mud Springs stock......................................... 17 4. Photograph showing typical exposure of the I.X.L. Canyon quartz monzodiorite....................................... 18 5. Photograph showing contact zone in I.X.L. Canyon ........... 18 6. Photograph of zoned inclusion from the I.X.L. Canyon quartz monzodiorite....................................... 20 7. Photograph of a stained slab of I.X.L. Canyon quartz monzodiorite ............................................. 20 8. Photomicrograph of altered quartz monzodiorite ............ 23 9. Paragenetic sequence of minerals in the I.X.L. Canyon quartz monzodiorite....................................... 23 10. Photomicrograph of interstitial quartz in the I.X.L. Canyon quartz monzodiorite ............................... 24 11. Photograph of stained slab of seriate granodiorite ........ 27 12. Photograph of contact between the I.X.L. Canyon and seriate granodiorite units ............................... 27 13. Photomicrograph of adularia(?) in the mineralized seriate granodiorite ..................................... 28 14. Photomicrograph of quartz filling a fracture in the seriate granodiorite ..................................... 28 15. Paragenetic sequence of minerals in the seriate granodiorite ............................................. 30 16. Photomicrograph of rapakivi texture in the Mud Springs quartz monzonite porphyry................................. 32 17. Photograph of stained slab of the Mud Springs quartz monzonite porphyry ....................................... 33 18. Photomicrograph of granophyric groundmass in the Mud Springs quartz monzonite porphyry.................... 33 Vr ILLUSTRATIONS (Cont.) Page Figure 19. Photomicrograph of kink banding in biotite................ 34 20. Paragenetic sequence of minerals in the Mud Springs quartz monzonite porphyry ............................... 34 21. Modal abundances of phenocrysts from the various portions of the Mud Springs quartz monzonite porphyry............. 36 22. Photograph of stained slab of the Mud Springs quartz syenite porphyry........................................
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