A Brasiliano Age (500 ± 5 Ma) for the Mina III Gold Deposit, Crixás Greenstone Belt, Central Brazil

Abstract

K-Ar and 40 Ar/ 39 Ar geochronological studies were performed on selected minerals from the Mina III gold deposit, Crixás greenstone belt, state of Goiás, central Brazil. They include amphibole and biotite from amphibole schists; biotite from quartz-chlorite-carbonate-muscovite schists; chloritoid, muscovite, and paragonite from muscovite schists; and biotite from biotitemarbles. Analyses yielded ages between 750 and 500 Ma, indicating that the Brasiliano orogenic event affected rocks of the Archean greenstone belt. It is proposed that the gold mineralization, closely related to a post-metamorphic peak hydrothermal alteration and spatially associated with muscovite schists, is 505 ± 10 Ma, whereas older ages may indicate an excess of argon.

The Orogenic Crixás Gold Deposit, Goiás, Brazil: A Review and New Constraints on the Structural Control of Ore Bodies

“…All results presented by these authors point to a Neoproterozoic age for gold mineralization. Ages of 726-660 Ma and 518-576 Ma were obtained from amphibole porphyroblasts and from biotite and chloritoid porphyroblasts, respectively, but both were related to excess argon [48]. A 500 Ma age obtained from muscovite and biotite, related to a post-metamorphic/hydrothermal peak, were interpreted as the age of the ore [48].…”

“…2.3-2.1 Ga [44][45][46]. [48] Schist 505 ± 7 Sm-Nd (grt&WR) [24] The age of gold mineralization was first suggested to be Archean in view of the general characteristics of the greenstone belt and the presence of meta-komatiites in the basal section of the Crixás Group [7,49]. Later, a Neoproterozoic age for gold mineralization, related to the Brasiliano Orogeny, was discussed, and was largely based on the relative timing of some structural arguments and metamorphic events [1].…”

“…Ages of 726-660 Ma and 518-576 Ma were obtained from amphibole porphyroblasts and from biotite and chloritoid porphyroblasts, respectively, but both were related to excess argon [48]. A 500 Ma age obtained from muscovite and biotite, related to a post-metamorphic/hydrothermal peak, were interpreted as the age of the ore [48]. Later, a Sm-Nd composite isochron (garnet and whole-rock) yielded an age of 505 ± 7 Ma and was interpreted as the age of gold mineralization [24].…”

Linking Gold Systems to the Crust-Mantle Evolution of Archean Crust in Central Brazil

“…Subsequent studies proposed that massive sulfide orebodies in the Crixás greenstone belt (CGB) formed post-peak metamorphism associated with the Brasiliano orogeny [125]. Neoproterozoic ages for gold mineralization were reinforced by K-Ar and Ar-Ar ages from amphibole at 660-730 Ma, biotite and chloritoid at 520-580 Ma, and biotite and muscovite at~500 Ma [133], together with a whole-rock Rb-Sr isochron of Au-bearing chlorite-garnet schist at 505 ± 7 Ma [87]. However, ages obtained from these methods are easily disturbed by subsequent thermal events and thus considered unreliable to constrain ages within polymetamorphic terranes such as the GAB [21,22].…”