A mineral system approach on the Paleoproterozoic Au-bearing quartz veins of the Jacobina Range, northeastern of the São Francisco Craton, Brazil.

Abstract

The Jacobina Range mountain chain is located in the northeastern part of Bahia state, Brazil. It is 250 km long, with N–S direction located at the eastern border of the Gavi˜ ao-Lençois ´ block, and corresponds to the northern portion of the Contendas-Mirante-Jacobina Lineament. There are several structurally controlled Au-bearing quartz veins hosted by metasedimentary rocks of the siliciclastic Jacobina Group and Vale do Coxo metaultramafic schists that are exploited by artisanal miners. Four of these deposits were mapped. They are hosted by two Paleoproterozoic fault systems, named from east to west, as Pindobaçu and Maravilha. The Pindobaçu fault system, D1 phase, second-order thrust, and reverse faults hosts both central and oblique mineralized veins at Morro da Palmeirinha and Maravilha deposits, respectively. The Maravilha fault system, D2 phase, second-order transpressive fault, hosts the mineralized central shear vein at the Jaqueira deposit. The D3 phase, second-order normal faulting hosts mineralized central shear vein at Mina Velha. The sericitic alteration is dominant in the D1 and D2 phase deposits, while sulfidation prevails in the D3 phase. Gold grades are lower at the deposits with schist as host rocks when compared to deposits were quartzite is the predominant host rock. Chlorite geothermometry showed that the hydrothermal system temperature conditions ranged between 303 ◦C and 346 ◦C at Jaqueira and the primary H2O + CO2+NaCl (type I) and H2O + NaCl (subtype II.a) fluid inclusion assemblage seems to be heterogeneously entrapped during the phase separation, which was triggered by a decrease in pressure, with retrometamorphic conditions, during the orogenic uplift at the Jacobina Range. Later, this assemblage was reequilibrated according to the hydrothermal system temperature conditions. The vapor-rich H2O + NaCl fluid inclusion assemblage subtype II.d suggests a possible magmatic contribution in fluid composition at the Mina Velha deposit. The H2O + NaCl, secondary subtypes II.b and II.c fluid inclusion assemblages from Jaqueira and Mina Velha deposits are of uncertain origin. The hydrothermal mineralization at Jacobina Range is interpreted as an orogenic mineral system, and the following elements were characterized: (1) participation of orogenic and possibly magmatic fluid composition; (2) the fault systems in a compression-driven fluid flow type acted as conduits of the mineralizing fluids, and the second-order structures were the throttle of the hydrothermal system; (3) the later phases of tectonic evolution were the driver; (4) the inductor of gold precipitation was fluid-rock interaction; (5) dispersion in the gold content occurred when schist host rocks are present.