Sulfide recrystallization and gold remobilization during the 2.0 Ga stage of the Minas Orogeny; implications for gold mineralization in the Quadrilatero Ferrifero area, Brazil

The historic Quadrilatero Ferrifero mining district in Brazil is a polydeformed Archean terrane recording several tectonic events. This history has hampered the accurate dating of its important gold deposits, given that most of the geochronological clocks have been affected by multiple thermal events. To determine an accurate time constraint for the mineralizing events in this region, we provide the first Re-Os dating for a gold deposit in the Quadrilatero Ferrifero, obtained from 13 mineral separates of disseminated sulfide phases (pyrrhotite, coarse-grained arsenopyrite, fine-grained arsenopyrite, and pyrite) at the recently discovered Sao Sebastiao deposit (northwest Quadrilatero Ferrifero). Three distinct successive sulfidation stages are interpreted at the deposit. The final stage, texturally associated with gold, is marked by fine-grained arsenopyrite and pyrite (assemblage III) and is associated with high-temperature ( approximately 600 degrees C) features. A Re-Os errorchron is obtained when considering the data of all samples collectively; however, a comprehensive Re-Os isochron age of 1987+ or -72 Ma (2sigma ; n=4; IsoplotR model 3; initial (super 187) Os/ (super 188) Os=2.1+ or -0.7) is obtained from assemblage III. A (super 187) Re- (super 187) Os (super r) isochron at 1988+ or -56 Ma (mean square of weighted deviates=0.1, initial (super 187) Os (super r) =-0.1+ or -2.6 ppt; n=4) is obtained if an initial (super 187) Os/ (super 188) Os composition of ca. 2.1, is used, this being consistent with the fact that the (super 187) Os in the sulfides in assemblage III is largely radiogenic ( (super 187) Os (super r) ). Earlier assemblages (I and II) individually show variable initial (super 187) Os/ (super 188) Os, indicative of disturbance to the Re-Os systematics at approximately 2.0 Ga. This age is considered the best estimate for the timing of gold mineralization at Sao Sebastiao, being coincident with the waning stages of the Minas orogeny, the thermal effects of which are restricted to the southern Quadrilatero Ferrifero. Hence, we consider that the hot fluids from which assemblage III crystallized were responsible for resetting the Re-Os systematics in assemblages I and II and were central to Paleoproterozoic gold deposition in the region. This approximately 2.0 Ga age challenges previous notions of a late Archean, approximately 2.7 Ga mineralizing event in the Quadrilatero Ferrifero and makes it possible that gold remobilization occurred elsewhere in the region, particularly in areas representing deeper crustal levels.