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Geology and Mineralization

The Porquinho-Chico Torres district hosts a mineralized shear zone system at least 10 km in length, that has produced in excess of 1 million ounces of gold, about 95% of it from alluvial/saprolitic sources. Numerous primary sources have been identified by the garimpieros, but always in the drainages due to lack of exposure on the slopes and ridges.

The bedrock exposures in some of the old garimpeiro pits indicates that gold mineralization occurs within quartz veins and quartz stockworks hosted in granites.

The host granites are typical of the Tapajós, usually red to grayish in color, coarse to medium grained, with 20% or greater coarse gray, subhedral quartz, 10% dark ferromagnesian minerals, mostly biotite, and accessory magnetite (distal to the hydrothermally altered zones). Alteration is generally weak to moderate sericitization of feldspars and chloritization of ferromagnesian minerals. Strongly altered and mineralized areas are intensely sericitized and weather to a white-gray plastic clay, with only the quartz remaining as relict mineral grains and veins. Most mineralized areas also contain narrow 0.2 to 1.5m wide andesite dykes with mineralisation often occurring on the contacts between the andesites and the enclosing granite. This is possibly due to the fact that the dyke margins are zones of rheological contrast and are therefore ideal sites for mineral deposition. .

The observed quartz veins are several millimeters to 25 cm wide, but average 2 to 10 cm width, and occur in sub-parallel arrays and coarse stockworks. The vein mineralogy consists of hard, gray translucent quartz, often weakly banded, with accessory calcite, ubiquitous sericite, and 2% to 10% sulfides. Sulfide mineralization is predominately pyrite, with rare base metal sulfides. Occasionally visible gold is observed. Alteration selvedges consist of sericite, clays, iron oxides and minor chlorite.

Magellan's has defined a north-south trending mineralized corridor comprising six garimpo workings and target areas which together extend over an area of 7km. From south to north, these are;
  • Facão
  • Mutuca
  • Galeria
  • Melexete
  • Cetrífugo
  • Macarrão
Subsidiary structures within the pits controlling veins and dykes are generally northeast or northwest striking. All of the pits have been sampled and the major ones have been mapped. Channel sample results include;
  • 4m @ 5.71g/t Au and 3.5m @22.7 g/t Au at Centrífuga
  • 3.8m a 6.03 g/t Au at Macarão
  • 3.0m @ 4.7 g/t Au at Chico Torres
  • 1.0m @ 78.1 g/t Au at Melexete.
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