Geology and Ore Deposits of the Nova Lima and Rio Acima Quadrangles Minas Gerais, Brazil GEOLOGICAL SURVEY PROFESSIONAL PAPER 341-A Prepared in cooperation with the Departamento N acional da Producao Mineral of Brazil under the auspices of the International Cooperation Administration of the United States Department of State Geology and Ore Deposits of the Nova Lima and Rio Acima Quadrangles Minas Gerais, Brazil By JACOB E. GAIR GEOLOGY AND MINERAL RESOURCES OF PARTS OF MINAS GERAIS, BRAZIL GEOLOGICAL SURVEY PROFESSIONAL PAPER 341-A Prepared in cooperation with the Departamento Nacional da Producao Mineral of Brazil under the auspices of the International Cooperation Administration of the United States Department of State UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1962 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 6 A-1 Intrusive rocks—Continued Introduction 6 2 Metamorphosed mafic and ultramafic rocks—Con. Location 6 3 Talc schist 6 A-45 Climate, drainage, and topography6 3 Metadiabase 646 Culture and accessibility6 4 Gneissic, granitic, and pegmatitic rocks 647 Mines and mineral exploration 6 4 Quartz veins 648 Field and laboratory work 6 4 Structure 6 48 Coordinate system for locations6 5 Bedding 648 Acknowledgments6 5 Schistosity 648 Geologic setting 6 5 Lineations_ 6 48 Previous investigations 6 6 Folds 6 49 Stratigraphy6 8 Rio das Velhas uplift 649 Structure6 8 Vargem do Lima syncline 649 Igneous rocks 6 8 Folds in the Santa Rita—Nova Lima—Raposos Regional deformation and metamorphism6 9 area 6 50 Rio das Velhas series6 9 Inferred fold in southwestern part of Rio Acima Nova Lima group6 9 quadrangle 650 51 Schists and phyllites 6 9 Faults 6 Iron-formation and associated quartzitic rocks_ _ 16 Metamorphism 6 52 Graywacke6 22 Geologic history 652 Quartz-dolomite and quartz-ankerite rock (lapa Ore deposits 653 seca) 6 23 Gold 6 53 Sericitic quartzite and schistose conglomerate6 26 Morro Velho mine 653 Age relations of rocks of the Nova Lima group_ 28 History 653 Maquine group_ 6 29 Geologic setting 655 Palmital formation6 30 Ore 6 56 Casa Forte formation6 31 Ore reserves 6 57 Age relations__ __ 6 33 Extensions of known ore bodies 658 Raposos mines 6'58 Minas series6 33 Bicalho mine 660 Caraga group6 33 Gaia, Gabirobas, and Faria mines 660 Conglomerate6 33 Other mines and explorations 661 Quartzite6 34 Morro da Gloria exploration 661 Schist and phyllite6 34 Urubu mine 661 Age relations6 34 Bella Fama mine 661 Itabira group6 34 Ribeirao da Prata explorations 661 Caue itabirite6 35 Age of gold mineralization 662 Piracicaba group6 38 Iron 6 62 Cercadinho formation6 38 Potential ore from rocks of Rio das Velhas age_ _662 Sabar a formation 6 40 Potential ore from rocks of Minas age 662 Quaternary(?) and Tertiary(?) rocks6 41 Itabirite 662 Clay beds6 41 Hard blue hematite 662 Mudstone6 42 Canga 663 Canga6 43 Clay 663 Recent deposits6 43 Yellow ochre 664 64 Intrusive rocks_ 6 44 Dolomite 6 6 64 Metamorphosed mafic and ultramafic rocks6 44 Manganese, cobalt, nickel 6 64 Serpentinite6 44 Literature cited 6 Metagabbro6 45 Index 67 6 IV6 CONTENTS ILLUSTRATIONS [Plates are in pocket] PLATE 1. Geologic map of the Nova Lima quadrangle, Minas Gerais, Brazil. 2. Geologic map of the Rio Acima quadrangle, Minas Gerais, Brazil. 3. Regional geologic map showing principal rock units of the Quadrilater° Ferrifero and location of Nova Lima and Rio Acima quadrangles. 4. Geologic map of Gaia workings, Honorio Bicalho. 5. Diagram of the Minas series along a portion of the Rio das Velhas northwest of Sabarg, Nova Lima quadrangle. 6. Geologic maps of 8, 12, and 14 levels, Morro Velho mine, Nova Lima. 7. Geologic maps of 2, 4-6 levels, Raposos mines. 8. Map of the Bicalho mine. Page FIGURE 1. Index map showing location of Nova Lima and Rio Acima quadrangles, Minas Gerais, Brazil A-3 2. Photomicrograph of metasedimentary quartz-carbonate-sericite schist, Nova Lima group 611 3. Photomicrograph of weathered iron-formation, Nova Lima group 618 4. Photomicrograph of unweathered carbonate-facies iron-formation, Nova Lima group 619 5. Photomicrograph of unweathered carbonate-facies iron-formation, Nova Lima group 620 6. Photomicrograph of unweathered carbonate-magnetite iron-formation, Nova Lima group 621 7. Photomicrograph of unweathered carbonate-facies iron-formation, with sulfide mineralization, Nova Lima group_ 622 8. Photomicrograph of quartz-carbonate rock (gray thin-layered lapa seca), Nova Lima group 625 9. Photomicrograph of quartz-carbonate rock (lapa seca), Nova Lima group 626 10. Photomicrograph of quartz-carbonate rock (lapa seca), Nova Lima group 627 11. Enlarged part of thin section shown in figure 10 628 12. Photomicrograph of weathered Calle itabirite 636 13. Photomicrograph of ferruginous quartzite, Cercadinho formation, Piracicaba group 642 14. Photomicrograph of staurolite-garnet schist, Sabara formation 6 43 TABLES Page TABLE 1. Comparative stratigraphic chart for Precambrian rocks of the QuadrilAtero Ferrifero6 In pocket 2. Petrographic data for schists and phyllites of the Nova Lima group6 A-12 3. Carbonates from rocks in the Nova Lima and Rio Acima quadrangles 614 4. Complete and partial chemical analyses of iron-formation from the Nova Lima group 619 5. Chemical analyses of weathered Caue itabirite and ocherous itabirite, Nova Lima quadrangle 637 GEOLOGY AND MINERAL RESOURCES OF PARTS OF MINAS GERAIS, BRAZIL GEOLOGY AND ORE DEPOSITS OF THE NOVA LIMA AND RIO ACIMA QUADRANGLES, MINAS GERAIS, BRAZIL BY JACOB E. GAIR ABSTRACT developed as a result of north- to northwest-directed movements The Nova Lima and Rio Acima quadrangles lie in the north- that accompanied formation of the Rio das Velhas uplift. These central part of the Quadrilitter° Ferrifero—the so-called iron movements also caused the northwestward overturning of quadrangle of central Minas Gerais, Brazil. The mapping of the Minas series in the northwestern part of the Nova Lima these quadrangles has been part of a joint project of topographic quadrangle. and geologic mapping of the iron region of central Minas Gerais, Rocks of the Rio das Velhas series have been divided into two done jointly by the U.S. Geological Survey and the Brazilian groups, named in earlier reports by members of the U.S. Geolog­ Departamento Nacional da Producao Mineral. The area is part ical ISurvey. The older of the two groups, and the oldest known of the Precambrian shield of Brazil and, except for superficial sequence of layered rocks in the Quadrilatero Ferrifero, is the deposits of Tertiary or Quaternary age, is underlain entirely Nova Lima group, and the younger is the Maquine group. by rocks of Precambrian age. The Nova Lima group consists of metasedimentary and meta­ In the Nova Lima and Rio Acima quadrangles, the rocks of volcanic schists and phyllites, with an estimated minimum thick­ Precambrian age occur in two series, the older Rio das Velhas ness of 4,200 meters. series and the younger Minas series,. separated by a major un­ Iron-formation and associated cherty and graphitic quartz­ conformity. Most of the area of the two quadrangles is under­ ite, graywacke, quartz-dolomite-ankerite rock (lapa seca), lain by rocks of the Rio das Velhas series ; the Minas series sericitic quartzite, and a locally developed schistose conglom­ occurs only in the northwestern part of the Nova Lima quad­ erate are interlayered with the schist and phyllite of the Nova rangle where it forms a northeastward-trending belt. Lima group. These rocks are mapped separately in the present The major structural feature in the two quadrangles is the study, but are not given formal formation names. Rio das Velhas uplift, which occupies much of the area. The At the surface, the iron-formation is weathered and consists Minas series in the northwestern part of the Nova Lima quad­ of alternating laminae of quartz and magnetite-hematite­ rangle lies on one flank of the uplift, dips generally southeast­ goethite. The fresh, unweathered, iron-formation, known only ward, and is overturned to the northwest. Another important from underground workings and drill cores, consists mainly of structural feature in the area is a north-northwestward-trending alternating laminae of quartz and sideritic carbonate or magne­ syncline located mainly in the eastern part of the Rio Acima tite-sideritic carbonate. The rock is similar to carbonate-facies quadrangle and downfolded in the central part of the Rio das iron-formation of the Lake Superior district. This rock was the Velhas uplift. This structure is here called the Vargem do Lima host rock for gold-sulfide mineralization at the Raposos mines syncline. It appears to converge on, and to be truncated by, in the Nova Lima quadrangle. rocks of the Minas series, some 20 to 25 kilometers southeast of The quartz-dolomite-ankerite rock (lapa seca ) is a recrystal­ the southeast corner of the Rio Acima quadrangle. The Vargem lized quartz-carbonate sediment. It is best known in the Morro do Lima syncline therefore antedates the deposition of the Velho gold mine at Nova Lima, and in several abandoned Minas series. mines, 3 to 8 kilometers south of Nova Lima, where it was the Rocks of the Rio das Velhas series are complexly folded in host rock for gold-sulfide mineralization. the northwestern part of the Rio Acima quadrangle and in the The Maquine group overlies the Nova Lima group, probably adjoining southwestern part of the Nova Lima quadrangle, as unconformably, and occupies the core of the Vargem do Lima shown principally by thin, mappable beds of iron-formation. syncline. The Maquine has been divided into two formations. The regional trend of this iron-formation is about northward The lower formation is called the Palmital, ranges in thickness from the Rio Acima quadrangle into the southwestern part of from 600 to 1,400 meters, and consists largely of schist and the Nova Lima quadrangle, then northwestward from the Nova phyllite with lenses of quartzite.