Geology and Ore Deposits of the Itabira District Minas Gerais, Brazil
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Geology and Ore Deposits of the Itabira District Minas Gerais, Brazil GEOLOGICAL SURVEY PROFESSIONAL PAPER 341-C Prepared in cooperation with the Departamento Nacional da Producao Mineral of Br axil under the auspices of the International Cooperation Administration of the United States Geology and Ore Deposits of the Itabira District Minas Gerais, Brazil By JOHN VAN N. DORR 20 and ALUIZIO LICINIO de MIRANDA BARBOSA GEOLOGY AND MINERAL RESOURCES OF PARTS OF MINAS GERAIS, BRAZIL GEOLOGICAL SURVEY PROFESSIONAL PAPER 341-C Prepared in cooperation with the Departamento Nacional da Producao Mineral of Brazil under the auspices of the International Cooperation Administration of the United States UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. CONTENTS Page Page Abstract.__________________________________________ Cl Geology of the Itabira district Continued Introduction.______________________________________ 2 Structure Continued Acknowledgments....- _ _________________________ 3 Folds in other rocks_____-_--______-_________ C51 History_ _ ____________________________________ 4 Faults____________-___---_____---__-____- 51 Fieldwork and maps.___________________________ 6 Metamorphism. ________________________________ 52 General geology of the Quadrilatero Ferrifero___________ 6 Regional metamorphism_____________---_-__- 52 Topography and physiography ___________________ 7 Thermal metamorphism_____________________ 53 Stratigraphy_ __________________________________ 8 Iron ores-_________-___-_--_-------------_-_-----_- 54 Structure._____________________________________ 9 High-grade hematite ores. _____-___-_---_-__----_ 54 Metamorphism and the granitic-gneiss problem_____ 9 Chemical characteristics-____________________ 57 Other igneous rocks_____________________________ 10 Mineralogical characteristics of the hematite ore_ 58 Geology of the Itabira district__-___-_________________ 10 Gangue minerals___--_-__--__-----_---_--- 58 Stratigraphy. __________________________________ 10 Structures in the hematite ore________________ 59 Rio das Velhas series._______________________ 10 Hard ores_________--__-_-___-----------_- 61 Lithology__ _ ___________________________ 11 Massive ore_ ------------------------------- 61 Schists____________________________ 11 Schistose ore_ _________________---___------_ 61 Quartzite and meta-arkose(?)_________ 13 Thin-bedded ore_________--___-____----_---- 63 Dolomitic schist ____________________ 14 Soft hematite ore_ _______--___-_-__---_----- 64 Other rocks________________________ 14 Intermediate ore_____________________--_-_-- 64 Thickness and contacts..________________ 16 Size and shape of the high-grade hematite ore Minas series ________________________________ 16 bodies. _ ________--_-__----_----_--------- 65 Cara§a group __________________________ 16 Localization of the ore bodies-_______________ 66 Lithology_ _________________________ 16 Origin of the high-grade hematite ores___------ 66 Distribution, thickness, and contacts __ 17 Review of previous ideas_____----_------- 66 Itabira group Caue itabirite_ ___________ 18 Present hypothesis of origin.____-____---_ 68 Lithology_ _________________________ 18 Metasomatic origin of the high-grade Distribution, thickness, and contacts-- 22 hematite ores.______________--_-- 68 Origin of itabirite___________________ 23 Time of ore formation._______-____-- 69 Original composition________________ 24 Pressure conditions during ore forma Sedimentary environment__________ 24 tion. _______-____-___----_------- 70 Relations to volcanism______________ 25 Temperature conditions during ore Disaggregated itabirite______________ 26 f ormation__ _ _______________----_- 71 Piracicaba group______________________ 31 Source and nature of the fluids- ______ 72 Lithology__ _ _______________________ 31 Solubility of quartz and hematite_____ 72 Distribution, thickness, and contacts__ 32 Genesis of high-grade hematite deposits- 73 Talc veinlets_______________________ 32 Soft high-grade hematite ores.----------- 74 Tertiary and Quaternary deposits,.___________ 32 Present hypothesis-____________--_-- 74 Igneous and gneissic rocks _______________________ 34 Other hypotheses-____________-----_ 76 Altered ultramafic and mafic rocks____________ 34 Rubble ore___ -------------------------------- 76 Soapstone and serpentinite_______________ 34 Canga ore_ ------------------------------------ 77 Amphibolit e_ ___________________________ 35 Origin of canga_ ---------------------------- 77 Other amphibolitic rocks_______________ 36 Nature of canga____-__-_---_--__----------- 78 Granitic gneiss.____________________________ 37 Itabirite. _ ------------------------------------- 78 Lithology._____________________________ 37 Grade of itabirite__ ------------------------- 79 Distribution_ _ _ _______________________ 41 Residual enrichment of itabirite-_______--__-_ 79 Contacts and age relations.___--________- 41 Intermixing of ore types________________--_-----_ 80 Borrachudos granite_______________________ 42 Descriptions of the high-grade ore bodies.______-_- 80 Lithology._____________________________ 42 Caue" area___ ----------------------------- 80 Age relations.__________________________ 44 Mining-___________----__-_------------ 80 Dikes_ ____________________________________ 45 Exploration. --------------------------- 82 Metamorphosed mafic dikes___-________-- 45 Structure- ____________--___--_---_----- 82 Pegmatites_ _ _________________________ 46 Distribution of ore______________________ 84 Postmetamorphic mafic dikes___________ 46 Reserves_ _ --------------------------- 86 Quartz veins_______________________________ 47 Ore classification._________________ 87 Structure._____________________________________ 49 Folds in the Minas series.___________________ 49 Method of calculation.-_________---_ 88 Linear structure and depth of synclines____--_- 50 Tonnage estimates, Caue area-___--_----- 88 Origin of crenulations in iron-formation______ 51 Chacrinha area_____________________--_----- m IV CONTENTS Iron ores Continued Manganese deposits Continued Descriptions of the high-grade ore bodies Con. Page Types of occurrence Continued Page Onga area________________________________ C91 Deposits in soil and canga_---------------__- C103 Piriquito area. _____________________________ 92 Summary. _________________________________ 103 Dois C6rregos area-__-----_-----____________ 94 Gold deposits-___--------__----___-_-------__--___- 103 Conceigao area_____________________________ 95 Alluvial gold-__________________________________ 104 District reserve estimate.._"______________________ 98 Gold in canga__________________________________ 104 Summary of reserve estimates-_______________ 98 Gold in quartz veins in the schistose rocks_________ 104 Geologically possible ore_____________________ 99 Gold in "jacutinga"___----__--------------_____- 104 Economic and technical considerations-_______ 100 Origin.________________________________________ 105 Manganese deposits.-__--__-_-_-_--___________._____ 101 Future of the gold ores-_______------_-___-____-_ 106 Types of occurrence..___________________________ 102 Water analyses.____________________________________ 106 Shear zone deposit-_________________________ 102 Selected bibliography______________________________ 106 Reserves. _,-___________________________ 102 Index_____________________________________________ 109 Disseminated deposits.______________________ 102 ILLUSTRATIONS [All plates are in plate volume] PLATE 1. Geologic map and sections of the Itabira district. 2. Mineral deposits map of the Itabira district. 3. Geologic map of mine 6, Cau6 area. 4. Geologic map of mine 7, Cau6 area. 5. Geologic map of new adit, Conceigao area. 6. Columnar sections of ore on Cau6 Peak. 7. Old underground workings in the Conceic.ao area. 8. Sections across ore bodies in the Cau6, Piriquito, Dois C6rregos, Onc.a, Chacrinha, and Conceic.ao areas. Page FIGURE 1. Index maps of Brazil and the Quadrilatero Ferrffero_____________---_-_-_-_-_____---_---_-_______-_ C3 2. Sketch map showing the distribution and outcrop pattern of the Itabira group____-_-_-_-_-_________-- 7 3-6. Photomicrographs. 3. Chlorite-muscovite schist-__________________________________________________________________ 12 4. Chlorite-feldspar schist___________________-_--_-________---_-__-_____-__---_-_______-___-_ 12 5. Mafic metatuff________________________________________________________________________ 13 6. Dolomitic chlorite schist -----_-_-_______---------------_--------------_----------_---_---- 14 7. Chemical analyses of soft hematite by screen fractions._____________________________________________ 20 8. Chemical analyses of itabirite by screen fractions, Cau6 Peak____---_--_____--___-_-_-___-_______-- 21 9. Geologic map of the Santa Barbara mine, Cau6 area-__-_________-___-___-_-_____---------_-_-_____ 23 10-13. Photomicrographs. 10. Cau6 itabirite showing typical distribution of quartz and hematite in itabirite-___________________ 29 11. Showing normal distribution of quartz and hematite in itabirite.-____________---_-_---_-___-_-- 29 12. Showing irregular degree of recrystallization of the quartz.____________________________________ 30 13. Showing fine hematite dust in quartz_______--__-_--____-__-_-__-_-_____-_-------------_---_ 30 14. Talc veinlets on bedding planes and crosscutting