Ouro em formações ferríferas

Ouro em formações ferríferas

IRON FORMATION-HOSTED Au by R.H. McMillan Consulting Geologist, Victoria, British Columbia

Ref: formação ferrífera, BIF, Au, Ag, Cu, sulfeto maciço, veio, vênula, disseminação, ouro, greenstone

McMillan, R.H. (1996): Iron formation-hosted Au, in Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic Deposits, Lefebure, D.V. and Hõy, T, Editors, British Columbia Ministry of Employment and Investment, Open File 1996-13, pages 63-66. IDENTIFICATION SYNONYM: Mesothermal veins. COMMODITIES (BYPRODUCTS): Au (Ag, Cu). EXAMPLES (British Columbia - Canada/International): No B.C. examples; Lupin and Cullaton Lake B-Zone (Northwest Territories, Canada), Detour Lake, Madsen Red Lake, Pickle Crow, Musselwhite, Dona Lake, (Ontario, Canada), Homestake (South Dakota, USA), Mt. Morgans (Western Australia); Morro Vehlo and Raposos, Mineas Gerais (Brazil); Vubachikwe and Bar 20 (Zimbabwe); Mallappakoda, Kolar District (India). GEOLOGICAL CHARACTERISTICS CAPSULE DESCRIPTION: Gold in crosscutting quartz veins and veinlets or as fine disseminations associated with pyrite, pyrrhotite and arsenopyrite hosted in iron-formations and adjacent rocks within volcanic or sedimentary sequences. The iron-formations may vary between carbonate-oxide iron-formation and arsenical sulphide-silicate iron-formation. TECTONIC SETTING: In “greenstone belts” believed to be ancient volcanic arcs; and in adjacent submarine troughs. DEPOSITIONAL ENVIRONMENT/GEOLOGICAL SETTING: Sedimentary and submarine volcanic sequences in a range of mutually overlapping settings ranging from turbiditic clastic sedimentary environments to distal mafic (and komatiitic) environments with associated felsic tuffaceous and intrusive porphyries. AGE OF MINERALIZATION: Archean to Proterozoic. HOST/ ASSOCIATED ROCK TYPES: Contained mainly within various facies of Algoma-type iron-formation and cherts, although veins may extend into other units. Associated with variolitic, tholeiitic and komatiitic volcanic and clastic (commonly turbiditic) rocks, rarely felsic volcanic and intrusive rocks. Metamorphic rank ranges from lowest greenschist to upper amphibolite facies. Silicate-facies iron-formations are associated in some cases but are generally not gold-bearing. DEPOSIT FORM: In and near crosscutting structures, such as quartz veins, or stratiform zones within chemical sedimentary rocks. Host strata have generally been folded and deformed to varying degree, consequently the deposits may have developed in axial plane cleavage area or be thickened and remobilized in fold hinges. TEXTURE/STRUCTURE: Highly variable: gold mineralization may be finely disseminated in sulphide minerals in the stratiform examples or occur as the native mineral or in sulphides in crosscutting quartz veins. Sulphidization features such as pyrite overgrowths on magnetite are present in some deposits. ORE MINERALOGY (Principal and subordinate): Native Au, pyrite, arsenopyrite, magnetite, pyrrhotite, chalcopyrite, sphalerite, galena, stibnite, rarely gold tellurides. GANGUE MINERALOGY (Principal and subordinate): Vein quartz, chert, carbonates (calcite, dolomite or ankerite), graphite, grunerite, stilpnomelane, tourmaline, feldspar (albite). ALTERATION: In deposits at low metamorphic rank, carbonatization (generally ankeritic or ferroan dolomite) is generally prominent. Sulphidization (pyritization, arsenopyritization and pyrrhotitization) is common in wallrocks adjacent to crosscutting quartz veins. WEATHERING: Highly variable: sulphide-rich, carbonate-poor deposits will produce significant gossans. ORE CONTROLS: Mineralization is within, or near, favourable iron-formations. Most deposits occur adjacent to prominent regional structural and stratigraphic “breaks” and mineralization is often related to local structures. Contacts between ultramafic (commonly komatiitic) rocks and tholeiitic basalts or sedimentary rocks are important. All known deposits occur in Precambrian sequences, however, there are some potentially favourable chemical sediment horizons in Paleozoic rocks. Pinch outs and facies changes within geologically favourable units are important loci for ore deposition. GENETIC MODELS: One model proposed for iron formation-hosted Au is that the mineralization may form due to deformation focusing metamorphogenic or magmatic hydrothermal fluids, from depth, into a chemically and structurally (brittle- ductile transition zone) favourable depositional environment, late in the orogenic cycle. This theory is consistent with both the crosscutting relationships and radiometric dates for the gold mineralization. Another model emphasizes a syngenetic origin for the widespread anomalous gold values, similarity of the geological environments to currently active submarine exhalative systems, and the association with chemical sedimentary strata. Replacement features could be explained as normal diagenetic features and contact areas between sulphide-rich ore and carbonate wallrock as facies boundaries. ASSOCIATED DEPOSIT TYPES: Au-quartz veins , turbidite-hosted Au-quartz veins , Algoma-type iron-formations . COMMENTS: This type of deposit has not been documented in British Columbia. The closest analogy is the 900 zone on the Debbie property (092F331) which contains gold in magnetite-jasper-sulphide-bearing bedded chert, in quartz veins and in stockworks cutting ankeritic aphyric pillow basalt. Some workers consider auriferous stratiform pyrite bodies, such as Bousquet, Doyon, and Agnico Eagle in the Canadian Shield, to be closely related to iron formation-hosted Au. EXPLORATION GUIDES GEOCHEMICAL SIGNATURE: Si, Fe, S, As, B, Mg, Ca, Au and Ag generally show strong enrichment in the deposits, while Cu, Zn, Cd, Pb and Mn generally show moderate enrichment. GEOPHYSICAL SIGNATURE: Airborne and ground electromagnetic and magnetic surveys and induced polarization surveys can be very useful to detect and map the high sulphide and magnetite content of many of the deposits. OTHER EXPLORATION GUIDES: Standard prospecting techniques to trace mineralization directly or in float trains in glacial till, talus or other debris derived from the gold mineralization remains the most effective prospecting tool. Areas with gold placers are potential targets. Exploration programs should focus on the primary depositional environment for stratiform deposits. ECONOMIC FACTORS TYPICAL GRADE AND TONNAGE: The more significant deposits fall in the ranges from 6 to 17 g/t Au and 1 to 5 Mt (Thorpe and Franklin, 1984). At the adjacent properties of Morro Velho and Raposos in Brazil, approximately 10 million ounces of gold have been produced at a grade of between 15 and 16 g/t since 1834. In Ontario, the Detour Lake mine contains a resource of 48 t Au and the Madsen Red Lake deposit produced 75 t, the Pickle Crow Deposits 45 tonnes and the Central Patricia 19 tonnes. At the Lupin mine 6.66 Mt of ore grading 10.63 g/t Au were produced between 1982 and the end of 1993 with remaining reserves of 5.1 Mt averaging 9.11 g/t. ECONOMIC LIMITATIONS: The narrow veins in some deposits require selective mining techniques which are no longer highly profitable. On the other hand, deposits, such as Lupin, are sufficiently large to be mined very profitably utilizing modern mechanized equipment. IMPORTANCE: Although attention in recent years has been focused on the large epithermal volcanic-hosted gold deposits of the circum-Pacific Belt and on Carlin-type deposits, iron-formation hosted gold deposits, such as Lupin, rank as world class and remain attractive exploration targets. For example, the Homestake mine has produced approximately 300 t of gold since starting production in 1876. REFERENCES ACKNOWLEDGMENTS: Chris Ash, Dani Alldrick, Andre Panteleyev and Howard Poulsen reviewed the profile and provided constructive comments. Berger, B. R. (1986): Descriptive Model of Homestake Au; in Mineral Deposit Models, Cox, D.P. and Singer, D.A., Editors,U.S. Geological Survey, Bulletin 1693, pages 245- 247. Boyle, R.W. (1979): The Geochemistry of Gold and its Deposits; Geological Survey of Canada, Bulletin 280, 584 pages. Fyon, J.A., Breaks, F.W., Heather, K.B., Jackson, S.L., Muir, T.L., Stott, G.M. and Thurston, P.C. (1992): Metallogeny of Metallic Mineral Deposits in the Superior Province of Ontario; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 2, pages 1091-1174. Fripp, R.E.P. (1976): Stratabound Gold Deposits in Archean Banded Iron-Formation, Rhodesia; Economic Geology, Volume 71, pages 58-75. Kerswill, J.A. (1993): Models for Iron-formation-hosted Gold Deposits; in Mineral Deposit Modeling, Kirkham, R.V., Sinclair, W.D., Thorpe, R.I. and Duke, J.M., Editors, Geological Association of Canada, Special Paper 40, pages 171-200. Padgham, W.A. and Brophy, J.A. (1986): Gold Deposits of the Northwest Territories; in Gold in the Western Shield, Canadian Institute of Mining and Metallurgy, Special Volume 38, pages 2-25. Rye, D. M. and Rye, R. O. (1974): Homestake Gold Mine, South Dakota: I. Stable Isotope Studies; Economic Geology, Volume 69, pages 293-317. S iddaiah, N. S.; Hanson, G.N. and Rajamani, V. (1994): Rare Earth Element Evidence for Syngenetic Origin of an Archean Stratiform Gold Sulfide Deposit, Kolar Schist Belt, South India; Economic Geology, Volume 89, pages 1552-1566. Thorpe, R.I and Franklin, J.M. (1984): Chemical-sediment-hosted Gold; in Canadian Mineral Deposit Types: A Geological Synopsis, Eckstrand, O.R., Editor, Economic Geology Report 36, Geological Survey of Canada, page 29. Vielreicher, R.M., Groves, D.I., Ridley, J.R. and McNaughton, N.J. (1994): A Replacement Origin for the BIF-hosted Gold Deposit at Mt. Morgans, Yilgarn Block, W.A; Ore Geology Reviews, Volume 9, pages 325-347.

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