Mota-E-Silva J.,Votorantim Metais Ltda |
Mota-E-Silva J.,University of Brasilia |
Filho C.F.F.,University of Brasilia |
Giustina M.E.S.D.,University of Brasilia
Economic Geology | Year: 2013
The Limoeiro deposit, located in a high-grade mobile belt of the Brasiliano orogenic cycle (650-500 Ma), represents one of the few magmatic Ni-Cu(-PGE) sulfide discoveries in recent years. The deposit is hosted by an ultramafic chonolith consisting in a partly deformed and metamorphosed tube-like body affected by brittle faults within high-grade gneiss and schist. The absolute age of the ultramafic intrusion that hosts the Limoeiro deposit is not yet known, but regional geologic constraints suggest that both intrusion and host rocks belong to a terrane older than the orogenic cycle. Peak metamorphic parageneses of the ultramafic intrusion and country rocks indicate granulite facies of metamorphism, interpreted as the result of the Neoproterozoic Brasiliano orogenic cycle. The Limoeiro deposit is the first significant discovery of Ni-Cu sulfide in the Borborema province, opening a new window for exploration in this large orogenic belt. Despite the high-grade metamorphism and the tectonic deformation, the primary magmatic structure of the intrusion hosting the Limoeiro deposit is remarkably well preserved. This intrusion forms a tube-like conduit (chonolith) with cross sections a few hundreds of meters in diameter and with a considerable length of more than 4 km. The conduit consists of two distinct sequences of ultramafic rocks, denominated Upper and Lower sequences, each one consisting of a core of harzburgite enveloped by orthopyroxenite, with an irregular and discontinuous outer shell of amphibolite. These ultramafic sequences, characterized by similar composition and structure, have distinctively different S contents resulting from the presence of disseminated Ni-Cu sulfide mineralization only in the Upper sequence. The magmatic system of the Limoeiro deposit is interpreted to result from the input of two major pulses of magma with similar composition, but sulfide saturation and segregation is restricted to the second magma pulse (Upper sequence). Nickel-Cu(-PGE) sulfide mineralization is broadly concordant with the Upper sequence and different orebodies resulted from tectonic slicing of the originally continuous tube-like structure. The bulk of the mineralization in all orebodies consists of thick (up to 150 m) and elongated (up to 1 km) masses of rock containing disseminated sulfides broadly concordant with the chonolithic structure. Thin layers of massive ore occur in the lower portions of some of the orebodies, and represent about 3 vol % of the Limoeiro deposit. The intrusion hosting the Limoeiro deposit is probably the best preserved chonolith that has been overprinted by granulite facies metamorphism within an orogenic terrane. This study indicates that reconstruction of small magma conduits, an important tool for exploration for Ni-Cu sulfide deposits, may be successfully achieved within high-grade orogenic terranes. © 2013 Society of Economic Geologists, Inc.
Ferreira F.J.F.,Federal University of Parana |
Fornazzari Neto L.,Rua Deocleciano de Paula Xavier |
Szameitat L.S.A.,Petrobras |
Guimaraes G.B.,UEPG |
And 3 more authors.
Revista Brasileira de Geofisica | Year: 2014
The Aerogeophysical Project Serra do Mar Sul compiled gamma-ray data in the 70’s, for a large basement area (50,000 km2) in Paraná state, southern Brazil. The cps data were corrected in 1997 by BARMP, Brazil Airborne Radiometric Mapping Project, and are available now as TC (eU), K (%), eTh and eU (in ppm). The data for the two large intrusions in the region, the Cunhaporanga Granitic Complex (CGC) and Três Córregos Granitic Complex (TCGC) were used to generate maps for K, eTh, eU, total counts, and several derived parameters: eTh/K, eU/K, eU/eTh, F. Contrasts between the two complexes are explained as a result of outcrop pattern and weathering (CGC more weathered than TCGC) and differences in overall Th content (eastern TCGC domains richer in the element), but a more uniform U distribution. The differences are enhanced in the eTh/K map (TCGC with lower ratios). The late alaskitic Serra do Carambeí intrusion in the CGC shows the highest Th and U values. Gamma-ray patterns in granites subjected to strong subtropical alterations, with heavy seasonal rains, are more difficult to interpret, considering the possibility of strong leaching of K by weathering, and also supergene mobility of U, factors that are absent, or less important, in areas with drier climates. The gamma-ray patterns of the three elements are influenced by their original distribution in the granites, but can be drastically changed by late hydrothermal alteration and weathering. © 2014 Sociedade Brasileira de Geofísica.
Lopes J.A.,Votorantim Metais Ltda |
Rosas C.F.,Votorantim Metais Ltda |
Fernandes J.B.,Votorantim Metais Ltda |
Vanzela G.A.,Votorantim Metais Ltda
Journal of Mining Science | Year: 2011
The risk quantification in grade-tonnage curves is critical for capital investment in mining projects. Geostatistical simulations for orebodies can be used to obtain grade-tonnage curves, and determine uncertainty and risk assessments. Applying these in multi-element deposits can be a difficult practice, as the related attributes using the traditional co-simulation approaches require intensive computational work and may be impractical for use in the mineral industry. This paper presents the risk assessment for integrating grade-tonnage curves and resources categorization for lateritic nickel deposits in the central region of Brazil, by joint simulation of multiple correlated variables of interest: Ni, MgO and SiO 2. The joint simulation of these variables is based on Minimum/Maximum Autocorrelation Factors (MAF). Based on this approach, the resources are categorized honoring Ni joint simulated results by applying the 15 % rule, where it is considered there will be a statistical error < 15 %, with 90 % of a confidence interval per production period. © 2011 Pleiades Publishing, Ltd.