Geologia do depósito LODE Au-As-Sb Laranjeiras, em metaturbitos do Grupo Nova Lima, Quadrilátero Ferrífero, Minas Gerais, Brasil

• Ano de defesa: 2011
• Autor(a) principal: Luiz Claudio Lima
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Minas Gerais; UFMG
• Palavras-chave em Português: Geologia econômica; Quadrilátero Ferrífero; Ouro; Ouro Quadrilátero Ferrífero (MG); Geologia econômica Quadrilátero Ferrífero (MG)
• Link de acesso: http://hdl.handle.net/1843/MPBB-8YXGJH

The Laranjeiras Gold Deposit (~1 Moz @6.32 g/t Au) is located at the eastern border of the Iron Quadrangle-QF, and is part of the Córrego do Sítio Lineament. It is hosted in intercalated Neo Archean, strongly foliated metasedimentary rocks, encompassing clastic and volcanoclastic units, which belong to the upper portion of the Nova Lima Group, Rio das Velhas Supergroup. Diamond drill core logs (>150,000 m) and geologic mapping in the open pit mines led to the definition of the deposits stratigraphy. The Nova Lima Group is represented in the area by the Córrego do Sítio Unit, informally subdivided into Upper, Intermediate and Lower. The rocks of the Upper and Lower portions are characterized by their essentially psammitic composition, with greywacke predominating and subordinated lenses of sericitic and/or carbonaceous metapelites, besides horizons of heterogeneous iron formations containing magnetite, carbonates and quartz. The Intermediate portion corresponds to the conduit of gold mineralization, where carbonaceous schists predominate. They contain heterogeneous shear zones, with thicknesses ranging from less than one meter to tens of meters. These obliquely host mineralized bands that form quartz-carbonate veins in regional-scale, tension gash zones and that marked the end of the gold mineralization. Hydrothermal alteration of rocks hosting quartz veins include intense leaching of the carbonaceous material, with carbonate, sericite and sulfides formation in the surrounding rocks. Laranjeiras shows lenticular oblate orebodies, arranged in narrow, en echelon quartz-carbonate veins, with 1-4-m thickness, 60-400 height and length of 60-1200 meters, respectively. The dip direction is ~S76°E to S49°E, dipping at 45°-75° to the SE and plunging at 30°-46° towards N39°E and N51°E, in opposition to other gold mineralized zones of the Nova Lima Group, QF, which in general display plunges to the east and southeast. The geometry of the orebodies is a result of polyphase structural evolution, with four phases of deformation. A mylonitic foliation developed in shear zones that are subparallel to the regional foliation (Sn) of the rocks hosting the mineralization. It displays a maximum of N29°E/68°SE.A cleavage of extensional, wide spaced crenulations, Sn+1, is developed during the event Dn+1, with a maximum at N16Eº/30ºNW. It is marked by carbonaceous matter accumulated on the cleavage planes, which allow for their easy identification. During the Dn+2 phase, folds developed and inverted the attitude and dip of both Sn and Sn+1 surfaces without changing the angle between the two. This phase did not imprint any cleavage or mineral lineation and, therefore, Sn+2 surfaces could not be measured. The Sn+3 surface is an axial plane of a wide-spaced crenulation cleavage at an attitude of N35ºW/79ºNE, accompanied by fracture cleavage and kink folds on a decimetric scale. Swarms of metabasic dikes, with differing stages of metasomatic alteration, obliquely cut across and are locally parallel to all Archean units. Petrographic and scanning electron microscopic (SEM) studies identified the following ore and gangue minerals associated with veins and their host rocks in decreasing order of abundance: pyrite (FeS2) , arsenopyrite (FeAsS), pyrrhotite (FeS), berthierite (FeSb2S4), chalcopyrite (CuFeS2), sphalerite (ZnFeS), ullmanite (NiSbS), gersdorffite (NiAsS), cubanite (CuFe2S3), tetrohedrite ((Cu,Fe)12Sb4S13), pentlandite ((Fe,Ni)9S8), hematite (Fe2O3), rutile (TiO2), minerals of the boulangerite group (Pb5Sb4S11), galena (PbS), stibnite (Sb2S3), cobaltite (CoAsS), cinnabar (HgS), electrum (Ag-Au) and gold (Au). These overall represent less than 1%. Multi-element geochemical analyses resulted in the establishment of a positive correlation between Au in veins and surrounding clastic metasedimentary and metavolcanic rocks, with the following elements, in descending order: Sb > As > Ag > Cd > Se >Hg > P> Te > W > F > Bi > In > Sr > Sc > Cl > Rb > Sn > Tl > Cu > Pb > Zn > B > Ge > Mn > Mo > Re. In metabasic dike rocks, Au has a positive correlation in with the following elements, in descending order: As > Sb > Ag > Cr > Cs > Tl > W > Zn > Ba > Hg > Li > Ni >Rb > Cd > F. The relatively high grades of Cr and Ni and the high La/Y ratios indicate that the original sediments at the Córrego do Sítio Lineament originated also from the erosion of mafic and ultramafic rocks. On the basis on the overall geological characteristics, such as highly deformed host rocks, abundant quartz-carbonate-sulfide veinlets, carbonate, sericite and sulfide hydrothermal alteration, plus the spatial association with large-scale compressional structures, the Laranjeiras Gold Deposit is classified as orogenic.