Geology and ore deposits of the Nova Lima and Rio Acima quadrangles, Minas Gerais, Brazil

Gair, Jacob Eugene

doi:10.3133/pp341a

Cited by 43 publications

(65 citation statements)

References 7 publications

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“…Similar relationships were deduced by Worthington and Kiff (1970), Mangan et al (1984), and Duke and Hodder (1987) for gold deposits in iron-rich exhalites in the central Appalachians. Strong supporting evidence for this concept of exhalative gold came from studies of the Homestake gold deposits by , and subsequently Sawkins and Rye (1974) applied the term Homestake-type deposits to gold ores of this type and suggested that their occurrence was worldwide, and included certain major deposits such as Morro Velho, Brazil (Gair 1962) and the Kolar goldfield, India. Strong supporting evidence for this concept of exhalative gold came from studies of the Homestake gold deposits by , and subsequently Sawkins and Rye (1974) applied the term Homestake-type deposits to gold ores of this type and suggested that their occurrence was worldwide, and included certain major deposits such as Morro Velho, Brazil (Gair 1962) and the Kolar goldfield, India.…”

Section: Gold Ores Of Probable Exhalative Origin: Homestake-type Depomentioning

confidence: 99%

Metal Deposits in Relation to Plate Tectonics

Sawkins¹

1990

Minerals and Rocks

170

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Section: Gold Ores Of Probable Exhalative Origin: Homestake-type Depomentioning

confidence: 99%

Metal Deposits in Relation to Plate Tectonics

Sawkins¹

1990

Minerals and Rocks

170

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“…1). In many cases they have been extensively described (Kitson, 1930;Emmons, 1937;Narayanaswami et al, 1960;Boyle, 1961;Harris, 1961;Gair, 1962;Dorr and Barbosa, 1963;Ferguson, 1968;Slaughter, 1968;Ridler, 1970;Woodall, 1975;Anhaeusser, 1976; Ntiamoah-Agyakwa, 1979; Pye and Roberts, 1981). It is neither the purpose nor within the scope of this paper to review thoroughly their geologic characteristics nor the genetic processes believed to have formed them, which remain controversial.…”

Section: Greenstone-hosted Lode Depositsmentioning

confidence: 95%

Metallogeny of Precambrian gold deposits; space and time relationships

Hutchinson

1987

Economic Geology

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A study of metallogenic relationships among Precambrian gold deposits provides some broad new insights regarding genetic problems. Three types of deposits are important: greenstone-hosted lodes, palcoplacers, and graywacke-or turbiditc-hosted ores. Epithermal deposits are present but are rare and of minor importance. Important gold lodes occur in greenstone belts of all Archcan cratons and are a worldwide element ofArchean metallogeny. Their temporal distribution, however, exhibits some enigmatic irregularities. They are apparently more important in late Archcan than in early Archcan belts, and much less so in otherwise similar early Proterozoic greenstone belts. Early Proterozoic oxygenation of the atmosphere-hydrosphere may explain the latter situation. The deposits are lacking or are very rare in middle-Proterozoic rocks, but they reappear occasionally in late Proterozoic and Phanerozoic orogenic belts. Their origin remains controversial, but broad metallogenic considerations favor original sea-floor hydrothermal exhalative deposition followed by extensive metamorphogenic redistribution rather than a single-stage epigenetic process.Palcoplacer deposits also occur on most Precambrian cratons at the base of Proterozoic successions which rest unconformably on older, deformed, and partially denuded Archcan rocks containing the greenstone-hosted lodes. Unlike the latter, however, these deposits are important only in the great Witwatersrand district of South Africa. If weathering of Archcan lodes in a hinterland terrane has provided the gold in the Witwatersrand deposits, then the spatial restriction of this palcoplacer type is unique. Moreover, only in the Witwatersrand district are the palcoplacer ores of an absolute age (2.8-2.6 b.y.) and associated with thick tholeiitic b•/salts similar to Archcan lodes in other cratons. These metallogenic relationships suggest a temporal and perhaps also a genetic link between the otherwise apparently very different palcoplacer and greenstone-hosted ores. The graywacke-or turbiditc-hosted lodes occur in late Proterozoic and Paleozoic sequences that are sediment dominated and lack the thick volcanic component of the greenstone-hosted type. Otherwise, they resemble the latter in their geologic, mineralogical, and geochemical characteristics and may represent a later evolutionary variant of it. However, they also exhibit many geologic similarities to deposits of the sediment-hosted or Carlin type, suggesting that they may belong to this last group, rather than to the epithermal deposits with which they are coregional and therefore commonly classified. Epithermal deposits, although rare in Precambrian terranes, are represented by the high-grade, silver-rich veins of the Cobalt and other similar Canadian districts. Their rarity in Precambrian rocks is due partly to erosional removal of these shallow, subaerially formed deposits but partly also to their later, evolutionary development within stabilized continental cratons. Gold-rich strata of the older types within these cratons may ...

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“…2400 Ma (Babinski, 1991) or 2100 Ma (Herz, 1970;Teixeira, 1982;Delhal and Demaiffe, 1985;Machado et al, 1989). The mapping of the area by different members of the Dorr team (Guild, 1957;Simmons and Maxwell, 1961;Gair, 1962;Johnson, 1962;Dorr and Barbosa, 1963;Pomerene, 1964;Wallace, 1965;Reeves, 1966;Simmons, 1968a, b;Alves, 1969;Barbosa, 1969;Moore, 1969;O'Rourke, 1969;Herz, 1970Herz, , 1978Maxwell, 1972; A.N. Cunha, B. Ashley, and A.…”

Section: Minas Supergroupmentioning

confidence: 99%

Genesis of banded iron-formations

Castro¹

1994

Economic Geology

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A definitive interpretation of the genesis of banded iron-formations has long been sought. The explanation presented here involves the interpretation of an active continental margin as the depositional environment of the Minas Supergroup in Brazil, with the results that the same environment could explain the origin of the Cau• Itabirite, a well known iron-formation. A model is proposed to cover the deposition of banded iron-formations in this setting, and also, in that of a passive con tinental margin, believed to be the two most important sites of precipitation of such ores.In active margins, during the powerful process of subduction, the ocean lithosphere is cut by faults and fractures through which seawater gains access and hydrothermally leaches the brittle yet hot lower layers. The process is analogous to that currently being studied near midocean ridges when iron and silica, among other compounds, are driven by thermal convection to the sea floor. It occurs in present subductions as it occurred in past ones, but in early Precambrian time, both hydrosphere and atmosphere were anoxic: ferrous iron was soluble in seawater and the earth's surrounding ozone layer had not yet been formed. A plume of FeO-SiO2-rich seawater was concentrated above and along the trench, covering the nearby cratonic border. During the sunny hours of the day, iron was oxidized into ferric hydroxide before precipitation. Most banded iron-formations laid down in early Precambrian time were subducted with their ocean plates and the only fraction preserved was that deposited above cratonic borders. The break in iron-formation deposition at the end of that era was due to seawater and atmosphere oxygenation which produced the ozone-filtering layer around the earth and also caused extinction of marine anaerobic microorganisms. Banded iron-formations deposited in active or passive continental margins may assist in the interpretation of the palcoenvironment of the regions where they occur. Attempts have been made to classify some well-known Precambrian iron-formations based on this assumption. Foreword As ?•mT of a joint project of the Brazilian and American governments, more than 100 man-years of geologists' time were spent, from 1946 to 1965, on different phases of the geologic study of the area of occurrence of the Minas Supergroup in the State of Minas Gerais, usually known as Quadril•ttero Ferrffero (Fig. 1). Under the leadership of John Van Nostrand Dorr II, 13 American (U.S. Geological Survey) and three Brazilian professionals (DNPM) were responsible for the conduct of the project, one of the most important contributions Brazil has received from the earth science sector. For many reasons, this work remains a landmark, coinciding with the establishment of the first schools of geology and with the rapid development of this science in Brazil. The event is remembered because one of its main products was the deciphering of the geology of the area where the Minas Supergroup occurs, and in this sense, was fundamental for hundreds of other works...

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Geology and ore deposits of the Nova Lima and Rio Acima quadrangles, Minas Gerais, Brazil

Abstract: A-1 Introduction 6 2 Location 6 3 Climate, drainage, and topography6 3 Culture and accessibility6 4 Mines and mineral exploration 6 4 Field and laboratory work 6 4 Coordinate system for locations6 5 Acknowledgments6 5

Cited by 43 publications

References 7 publications

Metal Deposits in Relation to Plate Tectonics

Metal Deposits in Relation to Plate Tectonics

Metallogeny of Precambrian gold deposits; space and time relationships

Genesis of banded iron-formations

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