Uncommon metacarbonate rocks in the Nova Lima Group, Iron Quadrangle, Minas Gerais (original) (raw)

AngloGoldAshanti Córrego do Sítio Mineração Ltda. started in 2009 an exploration program near the abandoned São Bento gold mine (about 3 Moz @ 10-12 gAu/t), close to the Barra Feliz village, Santa Bárbara municipality, Iron Quadrangle, Minas Gerais State. Many older works, both open pit and underground, were made by informal prospectors (‘garimpeiros’) and companies. Northward to São Bento mine, the 2.85-2.61 Nova Lima Group, basal unit of the Archean Rio das Velhas greenstone belt, is composed of three packages, two of which are metapelites. A third package is present between these metapelites, and is essentially comprised of carbonaceous phyllites, metacherts and banded iron formations-BIF, whose thickness vary laterally. These packages are deeply meteorized, resulting in highly friable rocks down to depths of 400 meters or more, resulting in excessive core losses during drilling. These characteristics do not allow a satisfactory understanding of the stratigraphy and structural geology of the targets, despite the initial detailed mapping undertaken at the Anomaly 1. The drilled targets are aligned in a northeast direction, and are called Sangue de Boi, Shaft, Santana, Barra Feliz, Anomalia 1 and 2 and Jambeiro, totaling about 5.7 km in length x 300 km in width. Gold mineralization is structurally rather than lithologically controlled, despite a certain affinity to metacherts, iron formation-BIF lenses and quartz veins. Although rarely preserved, pyrite and arsenopyrite occur in fine grains, banded and disseminated, associated or not with hydrothermal zones, such as quartz, carbonate, sericite and sulfide alteration, both in the mineralized and barren rocks. An interesting feature in the area is the presence of metacarbonate rocks at the top of the chemical package of target Anomalia 1, which is a very uncommon rock in Archean greenstone belts. In 14 drillholes, this carbonate rock varies from 1 to 40 meters in thickness (folded bed) and can occur as laminated, banded, foliated or brecciated, locally with disseminated pyrrhotite and pyrite. Where unweatheared, these marbles have been interpreted to represent meta-limestones, associated with carbonaceous phyllites, metagreywackes and metacherts. Petrographic studies are needed to confirm this hypothesis. One interesting geological question that requires further academic research is how those carbonates were formed in a tectonically active Neoarchean basin associated with chemical rocks like metacherts and BIFs, considering that limestones are, in general terms, typically generated in shallow, calm and clean marine waters. Are we dealing with microbial carbonates precipitated in 100-200-meters-deep waters, not necessarily in abyssal depth, as the microbes would not have stand the ultra-violet exposure for a longer period?