Metal source and fluid-rock interaction in the Archean BIF-hosted Lamego gold mineralization: Microthermometric and LA-ICP-MS analyses of fluid inclusions in quartz veins, Rio das Velhas greenstone belt, Brazil

摘要

The Lamego orogenic gold deposit (440,742 oz gold measured reserves and 2.4 million t measured resources, with an average grade of 5.71 g/t Au and a cut-off grade of 2.15 g/t Au; AngloGold Ashanti Corrego do Sitio Mineracao S/A (AGA) personal communication, 2014) is located in the 5 km-long trend that includes the world-class Cuiaba deposit. It is hosted in the Neoarchean metavolcano-sedimentary rocks of the Rio das Velhas greenstone belt, Quadrilatero Ferrifero, Brazil. Mineralization is associated mainly with metachert-banded iron formation (BIF) and carbonaceous phyllites in the reclined Lamego fold, in which the Cabeca de Pedra orebody represents the hinge zone. Mineralization is concentrated in silicification zones and their quartz veins, as well as in sulfide minerals, product of BIF sulfidation. Hydrothermal alteration varies according to host rock, with abundant sulfide-carbonate in BIF, and sericite-chlorite in carbonaceous phyllite. Quartz vein classification according to structural relationships and host rocks identified three vein systems. The V1 system, mainly composed of smoky quartz (Qtz I) and pyrite, is extensional, crosscuts the bedding plane So of BIF, and is parallel to the fold axis. The V2 system, of the same composition, is represented by veins that are parallel to the S1-2 foliation and S-0. 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 (Qtz II) veins, which result from the reciystallization of smoky quartz, located mainly in shear zones and faults; these veins form structures en echelon and vein arrays. The most common ore minerals are pyrite, As-pyrite and arsenopyrite. Fluid inclusion-Ft trapped in all quartz veins present composition in the H2O-CO2 +/- CH4-NaCl system. Fluid evolution can be interpreted in two stages: i) aqueous-carbonic fluid trapped in Qtz I, of low salinity (similar to 2% equiv. wt.% NaCl), and ii) carbonic-aqueous fluid, of moderate salinity (average 9 eq. wt.% NaCl) hosted in Qtz II. Both stages are characterized by decrepitation temperatures in the range of 200 to >300 degrees C, and suggest a fluid of metamorphic origin. Applying an arsenopyrite geothermometer, the calculated formation temperature for the Cabeca de Pedra orebody is 300 to 375 degrees C. The vertical intersection of the isochors allows a minimum pressure calculation of 2.6 kbar. The composition of individual Fls of this orebody, obtained by LA-ICP-MS analyses, compared with results of Fls for the Carvoaria Velha deposit, Corrego do Sitio 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. Trace elements vary according to fluid-rock reactions, and are directly related to the host rock composition. The comparison of data sets of the two deposits shows that the Cabeca de Pedra Fls have 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 at the Cabeca de Pedra orebody, Lamego gold deposit are: i) hydrolysis of the carbonaceous matter of phyllite and BIF, affecting fO(2), destabilizing sulfur complexes and enhancing gold precipitation; ii) replacement of BIF iron carbonates by sulfides; and iii) continuous pressure changes that lead to silica precipitation and free gold.