The garimpeiros can't extract the gold from the gabbro due to...

The garimpeiros can't extract the gold from the gabbro due to the complicated metallurgy and the ionic nature but they can mine the gold in the soils which tells you how rich the soil is!

It looks like the gold in soil that the garimpeiros have mined at Tres and Ema is produced by the weathering of the fresh rock and subsequent deposition of the gold in the surrounding iron rich magnetite-bearing soil. Magnetite oxidises very quickly when exposed to the weather which proves that the soil is very close to the source rock.

From the website:

Field reconnaissance conducted revealed that the extensive old gold workings (300m E-W length) mined in 2010 is on the western side of a major E-W analytical signal anomaly (fig. 3) defined in the geophysical airborne survey conducted by CPRM with flight lines N-S, 500m apart.

This anomaly is about 1 km wide and 4 km long and coincident with a dilation zone produced by the major NE-SW fault system; this zone is also quite distinct in its geomorphology with a different type of vegetation, darker green in the satellite image.

The soil where gold was mined is strongly red, likely due to iron rich mineral, quartz veins were not observed in this old gold working.

The gold mineralization associated with iron rich minerals (magnetite in soils) and this expressive magnetic anomaly on a E-W dilation zone in a region where abundant circular structures are also present indicates a good potential for the discovery of an IOCG (Au-Cu) deposit in this lease.