Paper No. 8
Presentation Time: 10:45 AM


DUARTE, Beatriz Paschoal and PIRES Sr, Fernando R.M., Geology, State University of Rio de Janeiro, Rua São Francisco Xavier, 524, sala 4006 A, Rio de Janeiro, 20550900, Brazil,

The famous gold mineralization related to stratabound tourmalinite of Passagem de Mariana deposit contains a complex suite of metamorphic and hydrothermal minerals formed in the lower to intermediate sections of the Proterozoic Minas Supergroup. The mineralized sequence lies at the southern flank of the Mariana Anticline. The metasedimentary sequence is strongly folded and metamorphosed in medium grade characterized by the presence of staurolite, almandine and kyanite. Underneath the tourmalinite layer occur an impure quartzite, a fuchsite-rich zone and, in direct contact with the tourmalinite, a garnet-staurolite-chloritoid-kyanite graphitic micaschist. Overlying the tourmalinite, occur a 1-3m thick sequence including a quartz-carbonate-sulphide BIF succeeded by Mg-rich ankeritic dolomites, increasing the amount of sulphides upwardly, resulting in a dark and dense carbonatic rock. The carbonatic sequence is overlain by a biotite-muscovite schist and BIF with magnetite, cummingtonite and subordinate carbonate. Gold mineralization is characterized by interspersed arsenopyrite crystals, concordant with foliation of massive and weakly banded tourmalinite, displaying finely dispersed gold specks. Quartz and Mg-sideritic carbonate invade tourmalinite at the mineralization zone as concordant veins and boudins. Discordant, steep dip, narrow quartz veins transect all rock succession. Pyrrhotite, arsenopyrite, pyrite, bismuthinite, Bi-native, tetradymite, chalcopyrite and gold occur in the discordant quartz veins. The gold-tourmalinite mineralization is thought to have formed in the amphibolite-greenschist transition (300-450oC, 3-6 kbar?) by extremely reducing and acid solutions, with As, S, C and B. The source of gold is supposed to be the Archean Rio das Velhas greenstone which formerly spread detrital gold together with Ni-Fe-Cu in detrital pyrite later converted as first step, in arsenopyrite, pentlandite, gersdorfffite, chalcopyrite and pyrrhotite under anoxic conditions in the underlying Moeda Formation. An excess of metals, such as Fe, Bi, Pb, Te, Mo, accumulated with S, As, B and C-rich fluids in a coastal sabkha environment. Metamorphism and associated hydrothermal process were responsible for element remobilization and the formation of the present parageneses.