Ouro em sericita-quartzo hidrotermalitos controlados por cisalhamentos conjugados brasilianos, na região de Cavalcante (GO), Brasil central

Abstract

In the Cavalcante area, north-northeastern part of the Goiás State, Central Brazil(coordinates 47° 30 - 47° 20 W and 13° 40 -13° 50 S), about eighty lens-shaped bodies of sericite-quartz S-C mylonites have been mapped andmost of them are along dextral, ENE- and sinistrai WNW-trending, sub-vertical, ductile-brittle shear zones that intercept each other and cross-cutboth the Archean-Paleoproterozoic basement and the Mesoproterozoic Araí Group. The shear zones stretch for 1-5 km whereas the Au-bearing bodies are much shorter and have an average width about 20-30 m. Seventy of these bodies have been sampled and contain gold in variableamounts, whereas the remaining ten bodies have not been prospected yet.The Cavalcante shear zone (dextral) is one of such structures and encloses the Cavalcante Mine, which has been producing gold (and platinum)since 1750, from underground operations in a 60 m-long segment of the Cavalcante sericite-quartz mylonite body, which is about 500 m-long,20-25 m-wide and displays a sub-vertical, N60°-70°E trending C foliation, and a N10°-30°E trending S foliation dipping around 75° -80° to ESE. Abundant geology and chemistry data from drill holes and underground galleries have allowed to state that the Cavalcante orebody is anamalgamation of several lenses displaying a map-view sigmoidal geometry with 10-20 m-long axis parallel to the S foliation and a 3-5 m-longshort axis in the perpendicular direction. They plunge about 80° to ENE and are continuous at depth for several tens of meters, as demonstratedby the mining operations. Gold occurs as generally sub-mm flattened grains controlled along the foliation planes and the higher grades areassociated with ore displaying a hydrothermal alteration characterized at least by pyrite, tourmaline, magnetite and carbonate minerals. Detailedstructural relationships clearly demonstrate that the mineralisation is syntectonic and the deformation was progressive, as Aubearing syn tectonicsiliceous veins emplaced subparallel to the S-C planes or along synthetic R planes (possibly R' planes as well) were also sheared and washedby Bobearing fluids. An absolute age determination of the mineralisation is still missing, but the conjugate pair of shear zones fits well with a nearly E-W trendingmaximum horizontal stress that is entirely compatible with the Brasiliano event already documented in the surroundings of Cavalcante. The tensof Aubearing bodies of hydrothermalite along the shear zones places the Cavalcante area in the rank of those with a very high potential for goldexploration.