Geology and Mineral Deposits of the Roseland District of Central Virginia
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Geology and Mineral Deposits of the Roseland District of Central Virginia U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1371 Geology and Mineral Deposits of the Roseland District of Central Virginia By NORMAN HERZ and ERIC R. FORCE U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1371 Relations among anorthosite, ferrodioritic rocks, and titanium-mineral deposits in Nelson and Amherst Counties in the Blue Ridge of Virginia UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1987 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Data Herz, Norman, 1923- Geology and mineral deposits of the Roseland district of central Virginia. (U.S. Geological Survey professional paper; 1371) Bibliography: p. Supt. of Docs, no.: I 19.16:1371 1. Geology-Virginia-Roseland Region. 2. Mines and mineral resources- Virginia-Roseland Region. I. Force, Eric R. II. Title. III. Series: Geological Survey professional paper ; 1371. QE174.R67H47 1987 557.55'49 85-600280 For sale by the Books and Open-File Reports Section, U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225 CONTENTS Page Abstract_____________________________ 1 Post-Grenville rocks-Continued Introduction_________________. 1 Surficial deposits ___ 33 General geologic and economic setting _. I Deposits of present valley systems _____________ 33 Previous geological work ____________ 3 Inactive boulder fans ______________________ 33 Mapping and stratigraphy _____. 3 Ridgetop gravel deposits ______________________ 33 Economic geology _________. 4 Radiometric age determinations ____________________ 33 Proposed lithologic units _______. 4 Previous determinations in the region ______________ 33 Field work _______________________ 5 New age data __________________ 34 Acknowledgments _________________ 5 Petrogenesis of the igneous rocks _____________________ 35 Pre-Grenville and Grenville rocks _________ 6 Origin of anorthosite and ferrodiorites ______________ 35 Banded granulites and associated rocks. 6 Liquid immiscibility in the Roses Mill and Turkey Banded granulite gneisses __ 7 Mountain plutons and the origin of nelsonite ________ 36 Quartz mangeritic and other mangeritic rocks _____ 10 Mobley Mountain Granite __________________ 37 Origin of the granulite unit ____________________ 10 Metamorphism ____________________________ 37 Stratigraphic equivalence _____________________ 11 Grenville-age metamorphism ___________________ 37 Leucocratic charnockites and associated rocks ________ II Granulite fades ________________________ 37 Roseland Anorthosite ____________________________ 11 Possible partial melting ____________________ 38 Petrography ________________________________ 12 Amphibolite-facies metamorphism 38 Field relations ______________________________ 12 Paleozoic greenschist-facies metamorphism __________ 38 Marginal impure fades _______________________ 12 Emplacement features ________________________ 12 Structure _______________________________ 39 Shaeffer Hollow Granite ________________________ 13 Pre-Grenvillian and Grenvillian structures _______ 39 Roses Mill pluton ________________________________ 15 Southeast-dipping foliation ___________ ___ 39 Characterization_____________________________ 15 Regional complex dome _______ ____ 39 Field relations ______________________________ 18 Mylonitic rocks and related deformation _____________ 40 Stratigraphic equivalence _____________________ 20 Possible diapirism ________________________ 42 Facies _____________________________________ 20 Mineral resources ____________ __ 43 Charnockitic ferrodiorite __________________ 20 Anorthosite-border rutile (+ilmenite) deposits ________ 43 Augen gneiss ____________________________ 21 Distribution and character _____________________ 43 Dikes __________________________________ 22 Origin _____________________________ 43 Nelsonite _______________________________ 22 Descriptions of resource data ___ _________ 43 Turkey Mountain plutons _________________________ 25 Ilmenite deposits related to nelsonite and Characterization_____________________________ 25 ferrodiorite ___________________________ 45 Stratigraphic equivalence ____________________ 25 Origin __________________________________ 46 Facies ____________________________________ 25 Disseminated ilmenite ________________________ 46 Massive charnockitic rocks _________________ 25 Impure concordant nelsonite ___________________ 46 Biotitic augen gneiss _____________________ 26 Nelsonite "dikes" ___________ ________ 47 Nelsonite ______________________________ 26 Other types of titanium-mineral deposits ____________ 47 Rocks of the Pedlar massif ________________________ 26 Feldspar resources ______________________________ 47 Post-Grenville rocks _______________________________ 27 Zircon resources _____________ _ 47 Mobley Mountain Granite _________________________ 27 Phosphate and rare-earth resources 47 Mafic dikes ____________________________________ 32 Graphite resources ______________ 48 Pyroxenite 32 Kaolin resources ________________ 48 Fine-grained mafic foliated dikes 32 References cited _____________. 48 Diabasic dikes _______________ 32 Appendix 1. Simplified log of 11 core holes. 52 Gabbro ____________________ 32 Appendix 2. Locality descriptions ____. 53 III IV CONTENTS ILLUSTRATIONS Page PLATE 1. Geologic map and cross section of the Roseland district _____________________________________ In pocket FIGURE 1. Index map of the study area _________________________________________________ _ 2 2-8. Photographs of: 2. Landslide scar on ridge north of Bryant Mountain ___________________________________ 6 3. Streambed exposures in Jennys Creek _________________________________________ 6 4. Outcrop showing banding in granulite _______________________________________ _ 7 5. Specimen of banded granulite and leucocharnockite ______________________________ _ 7 6. Specimen of platy-textured leucocharnockite __________________________________ _ 7 7. Specimen showing isoclinal fold in garnetiferous banded granulite __________________________ 7 8. Outcrop of foliated banded granulite __________________________________________ 9 9. Photomicrograph of banded granulite __________________________________________ _ 9 10-20. Photographs of: 10. Specimen of anorthosite dikelet in quartz mangerite ________________________________ _ 13 11. Coarse impure anorthosite in bodies intrusive into quartz mangerite _____________________ _ 14 12. Outcrop of coarse-grained impure marginal facies of anorthosite ______________________ __ 14 13. Outcrop of folded Shaeffer Hollow Granite and Roses Mill dikes ___________________________ 14 14. Outcrop showing texture of Shaeffer Hollow Granite _____________________________ _ 15 15. Road cut at Roses Mill _____________________________________________ __ 15 16. Roses Mill specimen showing ilmenite rimmed by sphene ____________________________ _ 18 17. Anorthosite xenolith in Roses Mill unit _________________________________________ 18 18. Anorthosite xenolith in augen gneiss __________________________________________ 19 19. Leucocharnockite xenoliths in augen gneiss ______________________________________ 19 20. Roses Mill dike in quartz mangerite _______________________________________ _ 19 21. Annotated photomicrograph of Roses Mill dikerock _____________________________________ 20 22-24. Photographs of: 22. Textural facies of Roses Mill unit ____________________________________________ 21 23. Contact of Roses Mill sill and banded granulite _____________________________________ 23 24. Impure concordant nelsonite at hand specimen and microscopic scales ________________________ 24 25. Photomicrographs of impure nelsonite of Piney River mine _______________________________ _ 25 26-28. Photographs of: 26. Outcrop of cumulate banding in Turkey Mountain unit ________________________________ 26 27. Dikes of Mobley Mountain Granite ___________________________________________ 29 28. Outcrop of migniatitic Mobley Mountain Granite ___________________________________ 29 29. Gravity model of Mobley Mountain Granite __________________________________________ 30 30. AFM diagram of Grenvillian igneous rocks from Roseland _________________________________ 36 31. Diagrams showing pyroxene compositions in ferrodioritic rocks _______________________________ 36 32. ACF diagram for Grenvillian granulite-facies assemblages _________________________________ 37 33. Annotated photograph of mylonite outcrop along Roses Mill-leucocharnockite contact ___________________ 40 34. Photograph of outcrop of cataclastic facies of leucocharnockite _______________________________ 41 35. Photograph of specimen showing shear fabrics in leucocharnockite _____________________________ 41 36. Photomicrograph of mylonite _________________________________________________ 41 37. Photograph of specimen of sheared Pedlar charnockite ___________________________________ 41 38. Stereonet plots of mylonitic foliations and lineations _____________________________________ 42 39. Photograph of outcrop showing granulated leucocharnockite xenolith in augen gneiss ___________________ 43 40. Map and cross sections of the Roseland rutile quarry _____________________________________ 44 CONTENTS TABLES TABLE 1. Lithologic units used by previous workers in the study area ___________________________________ 3 2. Terminology of this report and a modern study of an adjacent area for the Lovingston massif _________________ 5 3. Analyses of granulites, mangerites, and Shaeffer Hollow Granite _______________________________