Geology of the Lead-Zinc Deposits in the Municipio De Januaria State of Minas Gerais Brazil
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-Sfc&T Geology of the Lead-Zinc Deposits in the Municipio de Januaria State of Minas Gerais Brazil GEOLOGICAL SURVEY BULLETIN 1110-B Prepared in cooperation with the Divisao a de Fomento da Producao Mineral, Departamento Nacional da Producao Mineral, Minister to da Agricultura, O5 H i i under the sponsorship of the Agency for OQ O PH International Development, U.S. De W O O partment of State S5 a EH Geology of the Lead-Zinc in « / State of Minas Gerais By JACQUES F. ROBERTSpN CONTRIBUTIONS TO BRAZILIAN GEQLQGY NQNFERROUS METALS G p, Q L 0 Q | C 4 L § I] R V g U T J N 1110- P"fep@red in cooperation with the Qivis&Q d.e Foment® $a P<fQ(i,uc%Q Mineral, MinMirk d® A under ike spmsor^hip of the Agtney f In$ern&ti@n@l Development,, partffient °J State UNITED'STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963 UNITEP STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL 3URVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.G. 20402 CONTENTS Page Abstract_______________________________________________________ 35 Introduction. _____________________________________________________ 37 Location of mineral deposits.___________________________________ 37 Topographic features__.___-_-__.-_--_-._____._____.___._____ 40 Geography and transportation routes.----_-_-___._____._________ 40 Previous geologic work.________________________________________ 43 Fieldwork and acknowledgments._-___---_-_-_---___________-___ 43 Geology._________________________________________________________ 45 Stratigraphy._-___-.____._____..____-_-.__._______._____._____ 46 Bambuf series_---_---___..______----___-_--_-_-______-_--__ 47 Historical conceptions of the Bambuf series_______-_-_-_ 47 Section near Itacarambf ________________________________ 49 Section on Morro Itapiracaba_-_--_--___-_-_______-_-___ 53 Section at Alto de Palmeiras-_.._____-_--___-____________ 56 Age of Bambuf series._________________________________ 57 Baurti formation. _________________________________________ 58 Structure.. ___________________________________________________ 59 Folds-___--.___-_._-___-_______-_-_-_.--_-_-_-____--____. 60 Faults and fractures..._-_-_.__.-..______.____._.___-_____._ 61 Physiographic development-____________________________________ 64 Mineral deposits_________________________________________________ 68 Lithologic and structural control.._____________________________ 69 Mineralogy________-__--___-___-___-_---_-_-_-_--___-_--___ 70 Altered wallrock_______________________________________________ 74 Classification and genesis._-__-_-___---------_--_________--____- 76 Summary and conclusions____________________________________ 79 Description of deposits.____________________________________________ 84 Mina Janelao______________________________________________ 84 Mina Grande__..-_______-_-________--_--___-__-_-_____-_-__. 90 Mina Sao Joao________________________________________________ 95 Pimenteira, Jacarezinho, and Taquarf prospects__________--_--__ 97 Riacho Se~co prospect--_________.___---_____-__________---_-__ 99 Mina Fabiao_-__---_______-______-____-___--____-_____---__- 101 Mina Capao do Porco__--__-_.___.-_____-__._______-__--_---_ 103 Alto de Palmeiras prospect-_____-..____--__--________-_____-_-_-_ 104 References cited.___________________________________________________ 107 Index________________________________________________________ 109 IV CONTENTS ILLUSTRATIONS [Plates are in pocket] PLATE 3. Geologic map of area west of Itacarambf, Minas Gerais. 4. Geologic map and section of Mina Grande. 5. Geologic map of Mina Janelao. Page FIGURE 4. Index map, showing location of the region of Januaria____ 38 5. Region of Janu&ria and Itacarambi, showing locations of principal deposits.. _________________________-_------_ 39 6. Stratigraphic sections of parts of the Bambuf series near Itacarambi___. ---_________________-_______--_----_ 50 7. Lapies surface of erosion developed on primary limestone and dolomite breccia__--________-___-_________------__ 51 8. View of Morro Itapiragaba, showing contact between the Bambuf series and overlying Bauru formation__________ 54 9. Geologic section from southeast side of Morro Itapirac.aba_-_ 55 10. Escarpment along Riacho Palmeirinha exposing the flat-lying clastic rocks and limestone of the Bambuf series..------.- 56 11. Slumped and rotated blocks of limestone at base of scarp, Chico Pacheco road___-.-_._--__-_.-___._____-__-___ 60 12. View of one of the highly silicified and leached limestone masses at Mina Sao Joao____-___--____________________ 75 13. Cliff along Riacho Embauba of limestone topped by dolomite, showing effects of solution weathering__-____-_____-.._ 85 14. View of Estaca Quatro zone at Mina Janelao, looking south. _ 86 15. Sketch of Estaca Onze, Mina Janelao, showing the results of geochemical prospecting._________...__._.__.____._.___ 89 16. Highly silicified rib of limestone with discernible bedding in center of mineralized zone, Mina Grande________________ 92 17. Looking east along fracture at south edge of Mina Grande mineralized zone___-___--__--__________.___--__-___ 92 18. Geologic sketch map of Sao Joao mine and Pimenteira prospect__ _ --------------__-_-___-_---__---_..__--_- 96 19. Geologic sketch map of the Riacho S6co occurrence...-_____ 100 20. Geologic sketch map of Mina Fabiao___________________ 102 21. Geologic sketch map of the Alto de Palmeiras.------------- 105 CONTRIBUTIONS TO BRAZILIAN GEOLOGY NONFERROUS METALS GEOLOGY OF THE LEAD-ZINC DEPOSITS IN THE MUNICIPIO DE JANUARIA, STATE OF MINAS GERAIS, BRAZIL By JACQUES F. ROBERTSON ABSTRACT Lead-zinc mines and prospects in the Municipio de Januaria, in. northern Minas Gerais, Brazil, are scattered along both sides of the Eio SSo Francisco for more than 100 kilometers. Seven deposits of lead and zinc and one fluorite prospect are grouped in a small area from 10 to 15 kilometers west of the village of Itacarambi. Others lie in a wide area surrounding the city of Januaria, of which only two small occurrences are described in this report, Production of lead from these deposits has been small and discontinuous, to taling only a few tons of lead and several hundred kilograms of silver. In 1957, the surface zone of residual blocks of oxide ore at Mina Janeiro, west of Ita carambi, was explored; several thousands tons of hand-sorted ore was stockpiled and a few hundred tons was shipped to Bio de Janeiro for pilot-plant extraction of zinc. In the same year, a fluoride deposit near Mina JanelSo was explored. Reserves of the deposits west of Itacarambi are judged to be small on the basis of surface showings; those of deposits near Janudria are considered to be of no commercial importance. Sedimentary rocks in the Januaria region include the Bambui series and the Bauru formation. The Bambui series consists of flat-lying carbonate and fine grained clastic rocks of possible Ordovi'cian or Silarian age. The base of the series is not exposed. A section of part of the Bambui series west of Itacarambi is 520 meters thick. The lower half includes six limestone and dolomite units of various types; the upper half is composed mainly of calcareous siltstone, and mudstone. West of the city of Januaria, the Bambui series consists of sub stantially the same units but the section is only 104 meters thick. The Bam bui series east of the Rio Sao Francisco in the section at Alto de Palmeiras is 210 meters thick and copsists mainly of calcareous mudstone and siltstone, with some interbedded. dark-gray limestone. The Bauru formation of Cretaceous age overlies the Bambui series. It is con tinental in origin and consists of massive, fine-grained red-bed sandstone and small amounts, of siltstone containing variable proportions of calcareous, si liceous, and limonitic cement. The Bauni formation characteristically caps buttes and mesas; near the city of Januaria it is 90 ineters thick. M.ost strata of the Janu&ria region are only slightly warped or faulted. West of Itacarambi, a broad anticline that is 6 tP 8 kilometers in length, trends N, 35 36 40° W. Drag folds, longitudinal and cross faults of small displacement, and monoclinal flexures occur along the limbs of this fold. Northeast and west of the structure the strata are virtually flat lying. A low dome is defined at the south east end of the anticline. Northwestward, the anticline dies out and the beds flatten. The Baurti formation is thought to have been involved in the deforma tion, but the evidence is inconclusive. The mineral deposits in the Municfpo de Januaria contain the common primary and secondary lead-zinc minerals and occur as replacements and as vein fillings in fractures and faults; at a few places there are mantolike masses of silicified primary- or pseudo-breccia within, flat to gently dipping carbon ate rock. Most deposits west of the Rio Sao Francisco occur in a dark-weather ing crystalline dolomitic limestone unit of the Bambul series. West of Itacar- ambf they are related to the northeast-striking cross faults or to the northwest- trending faults and fractures that flank the anticlinal block. The location of these deposits seems to have been controlled by a combination of structure and lithology; clastic beds have not been favorable to the deposition of lead and zinc minerals. All deposits are similar mineralogically. Primary sulfide minerals include argentiferous galena, sphalerite, and traces of chalcopyrite and bornite. The sulfide minerals were transported by solutions that silicified and dolomitized