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Geology and SHRIMP U-Pb Geochronology of the Igarapé Bahia Deposit, Carajás Copper-Gold Belt, Brazil: An Archean (2.57 Ga) Example of Iron-Oxide Cu-Au-(U-REE) Mineralization

Fernando H. B. Tallarico

Instituto de Geociências, Universidade Estadual de Campinas, Caixa Postal 6152, 13.083-970, Campinas, SP, Brazil, and Centre for Global Metallogeny, School of Earth and Geographical Sciences, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia

Bernardino R. Figueiredo

Instituto de Geociências, Universidade Estadual de Campinas, Caixa Postal 6152, 13.083-970, Campinas, SP, Brazil

David I. Groves, Natalie Kositcin, Neal J. McNaughton and Ian R. Fletcher

Centre for Global Metallogeny, School of Earth and Geographical Sciences, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia

José L. Rego

Companhia Vale do Rio Doce, Caixa Postal 51, Serra dos Carajás, 68.516-000, Parauapebas, PA, Brazil

View larger version (72K): [in a new window]   FIG. 1. Geologic map of the Carajás Copper-Gold belt, showing the major iron, manganese, Fe oxide Cu-Au-(U-REE) deposits and the Breves Cu-Au-(W-Bi-Sn) deposit. Modified from Tallarico et al. (2004). The Itacaiúnus shear zone is a broad zone centered on the Carajás fault and bounded by the two major faults (bold lines) shown to the north and south.

View larger version (70K): [in a new window]   FIG. 2. A. Geologic map of the Igarapé Bahia Cu-Au deposit and geologic cross section A-A’ through the Acampamento Sul orebody (modified from Companhia Vale do Rio Doce/Docegeo, 1996, unpub.). Cpy = chalcopyrite; mgt = magnetite; sid = siderite. B. Structural map of the Igarapé Bahia copper-gold deposit. Coordinates and depth in hole for drill holes from which dated samples were taken are given in Table 2.

View larger version (165K): [in a new window]   FIG. 3. A. Photomicrograph of the primary sedimentary bedding that has been bent around a chalcopyrite (cpy) nodule and is continuous with discordant veins containing chalcopyrite (plane-polarized light, 384/141.50 m). B. Photomicrograph of a set of discordant and concordant chlorite + chalcopyrite veins crosscutting the sedimentary bedding (plane-polarized light, 384/141.20 m). C. Photomicrograph of a mineralized Fe chlorite breccia with a fragment of laminated sedimentary rock from the hanging wall in a chlorite ± magnetite ± chalcopyrite matrix (plane-polarized light, 353/168.70 m). D. Magnetite breccia with abundant chalcopyrite (cpy), crosscut by carbonate veins (BHMAG-003/407.25 m). E. Photomicrograph of the matrix of a magnetite breccia with abundant biotite (bi) showing pleochroic halos due to uraninite inclusions (plane-polarized light, BHMAG-003/407.25 m). F. Photomicrograph of the matrix of a magnetite breccia showing the textural relationship between chalcopyrite (cpy), bornite (bn), and magnetite (mgt; reflected light, BHMAG-001/476.00 m). All drill core is stored by Companhia Vale do Rio Doce at Igarapé Bahia.

View larger version (32K): [in a new window]   FIG. 4. Schematic distribution of the hydrothermal alteration zones of the Igarapé Bahia Cu-Au-(U-REE) deposit (not to scale).

View larger version (45K): [in a new window]   FIG. 5. Representative rare earth element (REE) distribution patterns for host rocks and mineralized breccias from the Igarapé Bahia Fe oxide Cu-Au-(U-REE) deposit, showing the extreme low REE enrichment of the breccia ores and surrounding volcanic rocks. Chondrite values are from Evensen et al. (1978).

View larger version (18K): [in a new window]   FIG. 6. Geochemical plots showing the relationship between Au and Cu enrichment and enrichment in Ce and La in the host volcanic rocks and the mineralized breccias. A. Log-log plots: A. Ce (ppm) vs. Au (ppm). B. La (ppm) vs. Cu (wt %). Plots illustrate the strong correlation of REE with Au and Cu and hence the likelihood that monazite is related to Cu-Au mineralization.

View larger version (53K): [in a new window]   FIG. 7. Representative cathodoluminescence (CL)-SEM images of analyzed zircon grains from host metavolcanic rock samples B18C and B18D and diabase dike samples B36A and B21D. The locations of SHRIMP analyses for dated grains are shown as circles. All ages are 207Pb/206Pb ages. Discordant data are marked with a (d). A. B18C.1-1 and B18.C1-2. B. B18C.3-1. C. B18C.6-1. D. B18D.2-1 and B18D.2-2. E. B36A.6-1. F. B36A.3-1. G. B36A.5-1. H. B21D. 6-1. I. B21D.7-1. J. B21D.2-1 and B21D.2-2.

View larger version (20K): [in a new window]   FIG. 8. Concordia plot of SHRIMP data from zircons from the host metavolcanic rocks of the Igarapé Bahia Fe oxide Cu-Au-(U-REE) deposit. The weighted mean 207Pb/206Pb age from the concordant analyses (shown as unshaded) is 2748 ± 34 Ma (MSWD = 2.7). Samples UWA-B18C and UWA-B18D.

View larger version (115K): [in a new window]   FIG. 9. Representative backscattered electron images of sample UWA-B34, showing analyzed monazite grains (white) in apparent textural equilibrium with chalcopyrite (cpy) and magnetite (mgt) in the matrix of ore-bearing breccias of the Igarapé Bahia Fe oxide Cu-Au-(U-REE) deposit. Other associated minerals are siderite (sid), apatite (ap), and chlorite (chl). The locations of SHRIMP analyses for dated grains are shown as circles. All ages are 207Pb/206Pb ages. Discordant data are marked with a (d). A. B34.5.3-1. B. B34.5.2-1. C. B34.4.2-1. D. B34.7.3-1. E. B34.4.1-1. F. B34.6.2-1.

View larger version (18K): [in a new window]   FIG. 10. Concordia plot of SHRIMP data for monazite from the matrix of mineralized magnetite breccias (sample UWA-B34) of the Igarapé Bahia Fe oxide Cu-Au-(U-REE) deposit. The weighted mean 207Pb/206Pb age of the main concordant cluster (unshaded), shown in the inset, is 2575 ± 12 Ma (MSWD = 1.19).

View larger version (21K): [in a new window]   FIG. 11. 87Sr/86Sr vs. time diagram showing the isotopic evolution of seawater (Veizer and Compston, 1976), continental crust and mantle (Faure and Powell, 1972) relative to the carbonate minerals from the matrix of mineralized breccias and crosscutting veins of the Igarapé Bahia Fe oxide Cu-Au-(U-REE) deposit. The Igarapé Bahia samples clearly indicate a crustal derivation. ACPS = Acampamento Sul orebody.

View larger version (36K): [in a new window]   FIG. 12. Summary of the SHRIMP ages obtained in this study compared to conventional U-Pb zircon ages and Rb-Sr bulk-rock ages of the Estrela Granitic Complex, the Old Salobo Granite, the Itacaiúnas Granite, Re-Os ages of molybdenite, and Pb-Pb model ages of bornite-chalcopyrite-magnetite mineralization at Salobo, and the host metavolcanic rocks of the Igarapé Bahia deposit.

View larger version (14K): [in a new window]   FIG. 13. 18O vs. 13C diagram showing the field for carbonates from the Igarapé Bahia Fe oxide Cu-Au-(U-REE) deposit (Oliveira et al., 1998; Tazava, 1999) compared to carbonates from carbonatites, freshwater limestone, marine limestone, and the Olympic Dam deposit. The data are consistent with an association with alkaline rocks. Modified after Oreskes and Einaudi (1992).