Petrology and Geochemistry of Selected Talc-bearing Ultramafic Rocks and Adjacent Country Rocks in North-Central Vermont GEOLOGICAL SURVEY PROFESSIONAL PAPER 345 Petrology and Geochemistry of Selected Talc-bearing Ultramafic Rocks and Adjacent Country Rocks in North-Central Vermont By ALFRED H. CHIDESTER GEOLOGICAL SURVEY PROFESSIONAL PAPER 345 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1962 UNITED STATES DEPARTMENT OF THE INTERIOR STEW ART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director The U.S. Geological Survey Library has cataloged this publication as follows : Chidester, Alfred Herman, 1914- Petrology and geochemistry of selected talc bearing- ultra- mafic rocks and adjacent country rocks in north-central Vermont. Washington, U.S. Govt. Print. Off., 1961. vii, 207 p. illus., maps (7 fold. col. in pocket) cliagrs.. tables. 29 cm. (U.S. Geological Survey. Professional paper 345) Bibliography: p. 205-207. 1. Petrology Vermont. 2. Talc Vermont. 3. Mines and mineral resources Vermont. 4. Geochemistry Vermont. 5. Rocks, Igne­ ous Vermont. I. Title. (Series) For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, B.C. CONTENTS Page Geology of the Barnes Hill, Waterbury mine, and Mad Abstract. __________________________________________ 1 River localities Continued Introduction. ______________________________________ 3 Petrography Continued Location and history. ___________________________ 3 Schist Continued Barnes Hill locality _________________________ 3 Mineralogy, etc. Continued Page Waterbury mine locality_____--_---__________ 4 . Ilmenite, rutile, and sphene_ _________ 52 Mad River locality_________---_----__-______ 4 Garnet.___________________________ 52 Previous investigations __________________________ 5 Apatite__. _________________________ 53 Fieldwork and acknowledgments. -___-_---________ 5 Epidote and allanite _______________ 53 Geologic setting. ___________________________________ 6 Other minerals _____________________ 53 Regional setting. _______________________________ 6 Petrogenesis-___________________________ 53 Waterbury- Waitsfield area_ ______________________ 7 Quartzite __________________________________ 55 Metamorphosed sedimentary and volcanic rocks_ 8 Mineralogy, textural features, and para- Intrusive igneous rocks_ _____________________ 9 genesis ______________________________ 55 Ultramafic rocks_ _______________________ 9 Petrogenesis-___________________________ 55 Mafic hypabyssal and granitic rocks_______ 9 Greenstone. ________________________________ 55 Metamorphism_ ____________________________ 10 General features._______________________ 55 Structure, _ ________________________________ 10 Mineralogy, textural features, and para- Geology of the Barnes Hill, Waterbury mine, and Mad genesis. _____________________________ 57 River localities. __________________________________ 10 Albite_ ____________________________ 57 General geology ________________________________ 10 Chlorite. __________________________ 57 Barnes Hill locality ________-----_---________ 11 Amphibole_ ________________________ 57 Waterbury mine locality.- _ __________________ 12 Epidote and allanite ________________ 58 Mad River locality_-________----___-________ 14 Biotite ____________________________ 58 Structure. _ ____________________________________ 16 Carbonate.________________________ 58 Major features- _--_______--__--____________ 16 Ilmenite, rutile, and sphene._________ 59 Structural details. __________________________ 16 Other minerals. ____________________ 59 Bedding _______________________________ 17 Petrogenesis-____-_-____-_-__-_-___-_-__ 59 Layering in ultramafic rocks_._ _ __________ 17 Carbonate rock_____________________________ 60 17 General features-_______________________ 60 Schistosity. ____________________________ 22 Barnes Hill.. ______________________ 60 Slip cleavage. __________________________ 23 Waterbury mine_______________-___- 61 Fracture cleavage. ______________________ 23 Mad River__---_-_-___-_-_____-___- 61 Other cleavage _________________________ 24 Mineralogy, textural features, and para- Lineation_ _________________^___________ 24 genesis-______-_---_________-__-----_ 61 Shear polyhedrons- _____________________ 24 Carbonate.________________________ 61 Joints. -______________---__--_-_-_-____ 24 Amphibole_ ________________________ 62 Faults. ________________________________ 24 Chlorite ___________________________ 62 Structural features of the Sterling Pond area. _ _ 24 Other minerals. ____________________ 62 Origin and relations of structural features______ 25 Petrogenesis____ _ _______________________ 63 Petrography ___________________________________ 27 Albite porphyroblast rock____________________ 64 Methods and procedures.- _ __-____---________ 27 Mineralogy, textural features, and para- Determination of mineral compositio ns__ ______ 41 genesis-________-___-_______-_------_ 64 Chlorite_ ______________________________ 44 Albite. ____________________________ 64 Serpentine. ____________________________ 46 Chlorite- __________________________ 64 Talc_ _________________________________ 48 Accessory minerals._________________ 64 General features. ______________^___---_-_--_ 48 Petrogenesis_ _ __________________________ 64 Schist_ _ ____:______________________________ 49 Rocks of the blackwall zone__________________ 65 Mineralogy, textural features, and para- General features-_______________________ 65 genesis. _____________--_-_--_-----_-- 49 Blackwall chlorite rock ______________ 65 Tremolite rock _____________________ 65 Quartz ____________________________ 49 Talcose carbonate rock ______________ 66 Albite__ __________-------_-_----___ 50 Mineralogy, textural features, and para- Sericite__ _ _________________________ 50 genesis-_____________________________ 66 Chlorite_ ______________-_-__--_--__ 51 Chlorite. __________________________ 66 Biotite_ -_________---_-_-_----_---_ 51 Ilmenite, rutile, and sphene__________ 69 Graphite. __________--__-_--__--_-_- 52 Magnetite. ________________________ 69 in IV CONTENTS Geology of the Barnes Hill, Waterbury mine, and Mad Petrology and geochemistry Continued River localities Continued Steatitization Continued Page Petrography Continued Relation of steatitization to structure and Rocks of the blackwall zone Continued regional metamorphism____________________ 92 Mineralogy, textural features and para- Volume relations in steatitization _____________ 93 VI CONTENTS TABLES Page TABLE 1. Modes of rocks from the Barnes Hill, Waterbury mine, and Mad River localities---------------------------- 29 2. Optical data on minerals from the Barnes Hill, Waterbury mine, and Mad River localities__ _________________ 34 3. Chemical and spectrographic analyses of rocks and minerals. __________--__--_________-__--__--_-_--__ _ 42 4. Formula compositions and optical properties of blackwall chlorite _______________________________________ 68 5. Formula compositions of antigorite ___________________________________________________________________ 74 6. Formula representations of the average composition of intermixed magnetite and chromite in serpentinite______ 75 7. Formula compositions of carbonate in serpentinite __ ___________________________________________________ 76 8. Formula compositions of talc _______________________________________________________________________ 80 9. Formula compositions of carbonate in talc-carbonate rock and talc-carbonate veins. ______________________ 81 10. Modes of suites of specimens across the contacts of ultramafic bodies____ _________________________________ 96 11. Calculated modes, rock formulas, and gains and losses during steatitization, for specimens across the contact of the main ultramafic body at the Waterbury mine locality______________________________________________ 97 12. Gains and losses during steatitization, per zone, for selected elements and radicals, for specimens across the con­ tact of the main ultramafic body at the Waterbury mine locality. _____________________________________ 109 13. Calculated modes, rock formulas, and gains and losses during steatitization, for specimens across the contact of the main ultramafic body at the Waterbury mine locality_______________________-______-__--__-----_--_ 110 14. Gains and losses during steatitization, per zone, for selected elements and radicals, for specimens across the con­ tact of the main ultramafic body at the Waterbury mine locality_____ ______________________^___________ 112 15. Calculated modes, rock formulas, and gains and losses during steatitization, for specimens across the west con­ tact of the ultramafic body at the Barnes Hill locality.___:. ___________________________________________ 113 16. Gains and losses during steatitization, per zone, for selected elements and radicals, for specimens across the west contact of the ultramafic body at the Barnes Hill locality.- __________________________________ __________ 114 17. Calculated modes, rock formulas, and gains and losses during steatitization, for specimens across the east con- tack of the ultramafic body at the Barnes Hill locality _______________________________________________ 115 18. Gains and losses during steatitization, per zone, for selected elements and radicals, for specimens across the east contact of the ultramafic body at the Barnes Hill locality. ____________________________________________ 116 19. Calculated modes, rock formulas, and gains and losses during steatitization, for an idealized suite of specimens across the contact between