Summary:
Deposit Type
Gold deposits in the Tapajós Gold Province can be broadly classified into three main types:
- Mesozonal deposits;
- Epizonal intrusion centered or intrusion related deposits; and
- Alluvial, colluvial and supergene enriched saprolitic deposits.
The mineralogy and textures of the deposits at the Palito Mining Complex is consistent with a model for an intrusion related mesothermal gold-copper mineralization. This relatively new classification of gold deposits is associated with granitic rocks and are best developed above and surrounding small, granitic intrusions. Mineralization styles can manifest as stockworks, breccia, skarns and lode style veins, and have a clear metal association zonation.
Secondary gold deposits are also encountered immediately above the mineralised veins, within the regolith profile. These deposits generally manifest themselves within the residual saprolite and laterite portions of the profile and are developed through the weathering process and often upgrade the primary gold grades within the narrow-weathered vein through a supergene enrichment process, which also enlarges the footprint of the sulphide vein mineralisation.
The secondary deposits contain free primary gold within the oxidised sulphides in the vein material and free gold associated with the secondary iron oxides (goethite, hematite) along fractures and joint planes within the saprolite adjacent to the veins and in pisolites and lateritic cements at the paleosurface above the vein.
Mineralization at the Palito and São Chico deposits is hosted in granite and granodiorite of the Paráuari suite.
Existing bonanza gold grades are associated with massive chalcopyrite-pyrite blowouts within the quartz veins. The principal vein system at Palito has a strike length more than 900m along broadly N306°, however this varies from N300° to N325° locally. Individual veins average widths of approximately one metre, varying between 20 cm and up to 4 m.
Palito Mine
The mineralization control found on the Palito property is related to the schematic mega-system of strike-slip faulting and Riedel fracture systems of the Tapajós Province as described by the CPRM (2008). The mineralized veins appear related to the intersection of “Y” and “P” and/or “R” (sinistral strike-slip) faulting on 305° and 315° and/or 295° orientations respectively within “R” 070° trending structural corridors.
Mineralization at the Palito Mine is hosted within all three granitoids encountered in the immediate vicinity and is intimately associated with northwest-southeast vertical to sub-vertical mesothermal quartz-chalcopyrite-pyrite veins and pyrite disseminations filling the brittle-ductile fault sets. The principal vein system has a strike length over 900 m, with mineralized veins typically averaging approximately 1 m in width with grades between 15 and 30 g/t. Grades in excess of this are associated with semi-massive chalcopyrite-pyrite blowouts within the quartz veins, typically on the intersections of “Y” or “P” and “R” veins.
Gold mineralization associated with quartz and hydrothermal alteration assemblages is reported in all fracture orientations of the Riedel system and are dominated by fractures oblique to the principal strike-slip shear orientation.
Gold mineralization occurs within pyrite and chalcopyrite associated with sphalerite, argentite and tellurobismuthite/tetradymite. Gold occurs as fine grains of 10 to 15 microns in size, with the highest gold grade typically found in zones of hydraulic brecciation, where the alteration zones, sulfides and vein quartz show multiple stages of reactivation. In the granodiorite, the dominant sulfide is pyrrhotite with lesser pyrite and chalcopyrite compared to the granites, typically resulting in pyrrhotite-pyrite sulfides with lower gold grade. Sulfides occur as segregates in the quartz veins, as disseminations within the grey hydrothermal alteration selvedges, or as decimeter to meter wide massive sulfide veins.
The nature of sulfide mineralization varies along the strike and plunge extends of the deposit. Pyrite and chalcopyrite dominate in the granites, whereas pyrrhotite and pyrite with lesser chalcopyrite are found in the granodiorite. Within the granodiorites, the pyrrhotite-pyrite sulfides tend to be lower grade due to lower chalcopyrite content and is likely a result of greater mafic mineral content in the granodiorite affecting the paragenesis of sulfide deposition.
Secondary gold deposits are encountered directly above the mineralized veins within the residual saprolite and laterite portions of the regolith profile. Free gold is contained within the oxidized sulfides in the vein material, and with secondary iron oxides along fractures and joint planes within the saprolite adjacent to the veins, and in pisolites and lateritic cements at the paleo-surface above the vein. The secondary deposits are generally not high tonnage, as the residual saprolite profile is only approximately 5 to 8 m depth underlying up to 5 m of barren transport cover. They can however be nearly 10 times broader than the source mineralized vein due to weathering dispersion of the gold.
São Chico Mine
Mineralization is hosted in west-northwest striking, steeply south dipping quartz-sulfide veins at the São Chico Mine. Individual veins can range from under 0.2 to over 3 m in width, averaging roughly 0.8 m, and are observed over an area of approximately 1 km by 300 m in size. The shear zones hosting mineralization correspond to the “P” shear orientation in a pure east-west lateral Riedel system.
The Main Vein at São Chico strikes in a broadly west-northwest direction, dips steeply to the south, and ranges from approximately 1.0 to 3.9 m in apparent width. Mineralization extends approximately 140 m down dip and approximately 100 m along strike and is open down dip and plunging to the west; to the east, mineralization is open for a minimum of 80 m along strike. The fault zone is variably mineralized, with both sinuous and regular quartz veining, pyrite, sphalerite, galena, chalcopyrite, and electrum. Along the vein, high grade lodes are developed where a high degree of brecciation is observed and are manifested as moderately east plunging shoots within the plane of the Main Vein.
These lodes exhibit a higher chalcopyrite content than what is observed along the strike of the fault zone. The dip extent of mineralization remains open along the fault and within numerous higher-grade ore shoots.
The Western Zone is a sub-parallel fault zone developed in the footwall of the Main fault zone. The quartz veins display a similar alteration assemblage to the Main Vein, though a narrower width, less brecciation and less sulfide content is observed. The fault zone extends along strike for over 100 m to the northwest of the Main Vein.
The Highway Vein strikes roughly west-northwest, dips steeply to the south, and varies from 1.6 to 2.9 m in apparent width. The mineralized zone extends 75 m below the surface and remains open along strike and down dip. High gold grades are associated with brecciated granite supported by chlorite-sericite-quartz cement with blebby pyrite and galena.
The Parallel Vein is found 60 m south of the Main Vein fault, within the hanging wall. The vein averages 90 cm in width, and has been defined over a strike length of approximately 80 m to a depth of 75 m.
Mineralization comprises quartz-sulfide veins and breccias, with narrow chlorite-sericite-pyrite selvedges within a wider potassium-feldspar alteration halo. Although the mineralized interval may be 1 to 2 m in width, individual veins are often less than 30 cm wide with cross-cutting veins within the mineralized zone. Veins are composed of white to grey, fine-grained, massive to banded quartz with chlorite stylolites.