Precambrian Geology of the Elk Mountain - Spring Mountain Area, San Miguel County, New Mexico by Ingrid Klich, Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science in Geology New Mexico Institute of Miningand Technology Socorro, New Mexico February, 1983 i ABSTRACT Precambrian rocks of the Elk Mountain - Spring Mountain area, San Miguel County, New Mexico consist primarily of a bimodal (mafic-felsic) metavolcanic assemblage, plus locally abundant metasedimentary rocks (predominantly of volcanic provenance, plus ironstone, shales, and limestone) , and a complex of intrusives including meta- diabase and gabbro, tonal.f.te , granite , syenite and pegmatites. The metavolcanic suite is largely comprised of subaqueous basalts (with well-preserved pillow-structures), plus lesser amounts of felsic volcanic and volcaniclastic rocks ranging from rhyolites to dacites. The supracrustal succession has been subjected to at least two periodsof tight to isoclinal.folding, accompanied byintense tectonic transposition such that present orientations of 1ithol.ogic units no longer represent their original stratigraphy. Thus, delineation of a stratigraphic section for the supracrustal rocks within the study area is not yet possible. Rocks of the supracrustal succession have been variably metamorphosed to lower amphibolite facies with slight retrograde metamorphism to greenschist facies. Chemical analyses verify the compositional bimodality of the supracrustal assemblage. The mafic rocks are characterizedmainly as subaJ.kaline, low-K tholeiites, ii whereas the felsic rocks appear to be both tholeiitic and calc-al.kaline.Tectonic discrimination diagrams suggest formation of: these rocks at an island arc - destructive plate boundary. The submarine environment, bimodal volcanic suite, and relationship to associated sedimentary rocks are all compatible with deposition ina marginal. back-arc basin adjacentto an immature arc-system. The potentialfor volcanogenic base-metal sulfide deposits within the study area appears to be verylow. In addition, occurrences of ironstone and hydrothermal quartz veins are only localized, and cannot be considered economical.Mineralization associated with pegmatite intrusives (similar to that which was mined at the Elk Mountain deposit in the early 1940’s) occurs throughout the southern and western portions of the study area, but under present economic conditions is not considered an important resource. iii ACKNOWLEDGMENTS Financial support for this study was provided in part by a U.S. Department of the Interior Research Fellowship (Award No. G-5194015) and in part by the New Mexico Bureau of Mines and Mineral. Resources. I would like to acknowledge and greatfully thank Dr. James M. Robertson, not only for serving as my thesis adviser, but for suggesting this. project and sharing his knowledge of Precambrian geology, for providing continued support and enthusiasm, and finally forcritical.1~ reviewing. this manuscript. A special thanks also to Drs. J.A. Grambling, H.C. Condieand J.F. Callender for their invaluable contributions, suggestionsand enthusiasm in the fiel.d, and in addition to Dr. D.I. Norman for their helpful editorial comments. In addition, I thank Jeanette and Peter Abundis for their assistance in the field; Babette Faris, Phillip Allen, Gary Anderson, and Bob North for assistance with the geochemical anal.yses; and my colleagues to numerous to list, whose endless sense of humor make life enjoyable. Finally, I would like to thank my parents, who’s unfailing love and encouragement, not only for myself but for each other, mademy education possible. TABLE OF CONTENTS ABSTRACT ................................................. i ACKNOWLEDGMENTS ........................................ iii INTRODUCTION............................................. 1 Location .............................................. 1 Purpose ............................................... 3 Previous Work......................................... 4 Method of Investigation............................... 6 GEOLOGIC SETTING.......................................... Regional Geology...................................... 8 Phanerozoic Geologyof the Elk Mountain-Spring Mountain ....................Area 12 Quaternary Deposits............................... 12 Tertiary (?) Dikes (?) ............................ 12 Paleozoic Rocks................................... 16 PRECAMBRIAN GEO.OGY ..................................... 17 Geology of the Pecos Greenstone Belt................. 17 General Stratigraphyof the Elk Mountain-Spring Mountain ....................Area 21 Amphibolites...................................... 24 Aphanitic- to Fine-Grained Amphibolites........ 25 Medium- to Coarse-Grained Amphibolites......... 31 Supracrustal Rocks................................ 35 Felsic Volcanic and Volcaniclastic Rocks....... 35 Metasedimentary Rocks.......................... 41 Intrusive Rocks................................... 55 Tonalite ........................................ 57 Granites ....................................... 60 Syenite ........................................ 64 Pegmatites..................................... 68 Age Relations Among Intrusive ............Rocks 69 STRUCTURE AND METAMORPHISM.............................. 71 Structure............................................ 71 Folding and Tectonic Transposition................ 71 Foliations and Lineations......................... 75 Faults and Shear Zones............................ 77 Joints ............................................. 80 Discussion........................................ 80 Metamorphism ......................................... 83 GEOCHEMISTRY............................................ 90 Amphibolites.......................................... 93 Felsic Volcanicand Volcaniclastic Rocks................................ I03 Intrusive Rocks..................................... 108 Discussion.......................................... 115 MINERALIZATION ......................................... 117 DISCUSSION............................................. 119 Relation to Pecos Greenstone. Belt ................... 119 Comparison to Proterozoic Rocksof the Southwestern United States...................... 122 Tectonic Setting.................................... 128 Volcanogenic Massive-Sulfide Potential.............. 132 SUMMARY AND CONCLUSIONS................................ 135 Geologic History.................................... 135 Conclusions ......................................... 137 REFERENCES ............................................. 139 APPENDIXES APPENDIX A: Sample Location Map........................ A-1 APPENDIX B: Petrologic Descriptionsof Geochemical Sampl-es........................ 3-1 APPENDIX C: Sample Preparation......................... C-1 APPENDIX D: X-ray Fluorescence Analysis................ D-1 APPENDIX E: Instrumental Neutron Activation Analysis...E-1 PLATES I. Geologic Map................................. in pocket FIGURES 1 . Location Map......................................... 2 2 . Geologic Provincesof North-Central New Mexico....... 9 3 . Regional Geologic Mapof the Southern Sangre de Cristo Mountains. New Mexico........................ 10 4 . Si02 vs K20 Diagram for Cenozoic Volcanic Rocks..... 15 5 . General-ized Geologic Mapof the Pecos Greenstone Belt............................... 18 6 . Schematic Cross-Section Showing Relationships Among Major Lithologic Units........................ 23 7 . Isolated Pillow-Structurein Aphanitic- to Fine-Grained Amphibolite............................ 26 8 . Pillow Breccia in Aphanitic-to Fine-Grained Amphibolite............................. 26 9 . Photomicrograph of Relict Vesicles.................. 29 10 . Photomicrograph of Pillow-Selvage................... 30 11 . Relict Subophitic Texture in Metadiabase............ 34 12 . Cumulate Plagioclase in Porphyritic-Amphibolite..... 34 13 . Relict Plagioclase Phenocrysts in Metadacite........ 38 14 . Mafic Fragmentin Metadiabase....................... 39 15 . Compositional Layering in Felsic Gneiss............. 45 16 . Lenticular Fragments of Felsic Granofels in Pelitic-Schist......................... 45 17 . Transposition Layeringof Rhyodacite and Ferruginous Quartzite........................... 48 18 . Photomicrograph of Cross-Laminations in Ferruginous Quartzite............................ 50 19 . CaJc-Silicate Interlayered with Para-Amphibolite....53 20 . Isoclinal Fold-Nose in Calc-Silicate................ 54 21 . A-Q-P Modal Analysis Classification of Granitic Intrusive Rocks......................... 56 22 . Tonalite with Amphibolite Xenoliths................. 58 23 . Photomicrograph of Catacl-astic Texture in Tonalite................................. 58 24 . Foliated-Granite .................................... 63 25 . UnfoLiated-Granite .................................. 63 26 . Coarse-Grained Syenite.............................. 67 27 . Medium-Grained Altered Amphibolite.................. 67 28 . Small-ScaLe IsoclinalFo1.d in Felsic Gneiss......... 73 29 . Sma1.l-Scale Tight Foldin Metasediments............. 73 30 . Equal-Area Stereographic Projection Polesof to Foliation from the Southeastern Portionthe of Study Area.......................................... 76 31 . Equal-Area Stereographic Projectionof Poles to Foljation from Entire StudyArea. Excluding the Southeastern Portion................................ 76 32 . Enveloping Surfaces for Felsic Volcanic. Volcaniclastic. and Metasedimentary Units..........