| dc.description.abstract | The Lamego gold deposit is hosted in Neoarchean metavolcano-sedimentary rocks of the Rio das Velhas greenstone belt. Mineralization is associated mainly with metachert-banded iron formation (BIF) and carbonaceous phyllites in a reclined fold, the Lamego fold, in which the Cabeça de Pedra orebody represents the hinge zone. Gold mineralization is concentrated both in quartz veins and in silicification zones, as well as sulfide minerals, product of replacement of BIF during hydrothermal activity, particularly sulfides. This alteration varies according to host rock, with abundant sulfide-carbonate in the BIF, and sericite-chlorite in carbonaceous phyllite. Quartz veins are classified into three main systems, according to their structural relationships and host rock. The V1 system, mainly composed of smoky quartz and pyrite, is extensional and crosscuts the banding of BIF, and is parallel to the fold axis. The V2 system, of same composition, is represented by veins that are parallel to the S1-2 foliation and S0 bedding of BIF, This system is also characterized by silicification zones in the BIF-carbonaceous phyllite contact that has its maximum expression in the hinge zone of folds. The V3 system has milky quartz veins, which result from recrystallization of smoky quartz, located mainly in shear failure zones and forming structures en echelon and vein arrays. The most common ore minerals are pyrite and arsenopyrite mainly in BIF. Fluid inclusion-FI microthermometric studies demonstrate that IFs trapped in all vein types present composition in the H2O-CO2 ± CH4-NaCl system. Fluid evolution can be interpreted in two stages: i) aqueous-carbonic fluid of low salinity (~ 2% equiv. wt% NaCl) trapped in smoky quartz (Qtz I), suggests a metamorphic origin, and a decrepitation temperature in the range of 200 to 300°C; and ii) carbonic-aqueous fluid, with moderate salinity (average 9 % equiv. wt% NaCl) hosted in milky quartz (Qtz II), with decrepitation temperatures in the range of 200 to 300°C. The deposit formation temperature is calculated using the arsenopyrite geothermometer at 300 to 375°C. The vertical intersection of the isochors allows the calculation of the pressure between 2 to 3.5 kbar. The composition of individual FIs of the Cabeça de Pedra orebody,obtained by LA-ICP-MS analyses, compared with results obtained in inclusions of the Carvoaria Velha deposit, Córrego do Sítio lineament, highlights a standard composition typical of metamorphic fluids with Na> K> Ca> Mg, which increase or decrease in concentration as a function of salinity in both deposits. Smaller elements vary according to the fluid-rock reactions, and are directly related to the composition of differences in host rock. To compare the data sets of the two deposits, it is clear that the Cabeça de Pedra FIs has a higher enrichment in Zn, while Cu, As and Sb are richer in Carvoaria Velha, suggesting influence of the host rock geochemistry. The suggested mechanisms for gold precipitation are: i) hydrolysis of the carbonaceous matter of phyllite and BIF, slightly changing the O2 in the fluid, ii) replacement of BIF iron carbonates by sulfides, and iii) the continuous pressure changes that lead to silica precipitation and free gold. | |
