Summary:
All gold mineralization at Cuiú Cuiú occurs in intrusive host rocks, and all basement rocks encountered at Cuiú Cuiú are intrusive rocks. All gold discovered to date in the project area, occurs in deformed zones, or smaller-scale structures, that transect, deform, and clearly post-date the emplacement of the igneous intrusions. The remainder of the gold mineralization that has been identified in the Project area occurs within the weathered sequences created by erosion and weathering of the primary basement mineralization.
The overall temporal and spatial relationships of gold mineralization to deformed zones and related deep-seated alteration and metamorphism suggest that classification as a mesozonal class (6 to 12 km) or hypozonal class (>12 km) orogenic gold deposit best describes the mineralization observed at Cuiú Cuiú, despite its occurrence within intrusive rocks.
Gold mineralization at Cuiú Cuiú is most likely associated with the Trans-Amazonian Orogen, a Paleoproterozoic regional compressional event caused by the convergence and ultimate collision of the Guiana Shield with the WestAfrican Craton from 2.26 to 1.95 Ga (Kroonenberg et al., 2018).
The entire surface of the Cuiú Cuiú property is heavily weathered to soil and saprolite, with much of the property covered by a layer of unconformable Miocene and recent sediments. As a result, there is no fresh outcrop, and surface geological mapping provides limited insight about the subsurface basement geology; even trench exposures are heavily weathered. This forces reliance on geophysical data, satellite imaging, radar and aerial photography, along with local confirmation from drilling and trenching activities.
The following sections discuss: 1) the general geology of the Proterozoic basement geological units, 2) the major Proterozoic structural trends and controls, 3) Proterozoic metamorphism and alteration, and 4) the impact of weathering and erosion on the older basement rocks and the formation of the extensive and well-developed younger Miocene unconformable sequence.
Mineralization
Several main types of primary gold mineralization are evident in the fresh basement rocks within the project area: pervasive replacement-style mineralization, sheeted and stockwork vein mineralization, and quartz vein or quartz flooding mineralization. All styles of gold mineralization are present in the larger gold deposits on the property, with higher grades typically associated with quartz-vein or quartz flooding mineralization. All of these styles occur within extensive hydrothermally altered envelopes with lower gold tenor. The sulphide content is generally a good indicator of gold grade.
Three deposit types also occur within the weathered rocks:
1. The up-dip projections of the fresh basement mineralized zones are weathered to saprolite at depths of up to 100 m from the surface. This basement saprolite exhibits a significantly lower density and completely different mineralogy compared to the unweathered primary mineralization, but it largely maintains the general orientation (strike, dip and widths) of the underlying primary mineralization.
2. Soil and colluvium have developed at the top of the saprolite, forming extensive, broad, sub-horizontal transported mineralized zones known as “blankets.” These blankets are angularly unconformable and overlie Type 1 deposits.
3. Over millions of years, the Blanket mineralization has been carried downslope into drainages by surface erosion, with both gold and colluvial materials eventually reaching larger streams and rivers to form placer deposits.
Cabral Gold is not currently assessing Type 3 placer mineralization, but Types 1 and 2 could represent significant future economic resources for the company. The Cuiú Cuiú property hosts approximately 50 gold target areas at various stages of exploration. Of those, six are relatively advanced, including the: 1) Central deposit, 2) MG deposit, 3) CN deposit, 4) JB deposit, 5) Machichie target area, and 6) PDM target area. The two largest primary fresh rock gold deposits outlined to date are Central and MG. These constitute the bulk of the current Cuiú Cuiú basement resources. The CN and JB deposits are considerably smaller. Although PDM and Machichie have much larger footprints than CN and JB, they have not been drilled sufficiently to determine resources. Section 7.5 offers a summary of the general geology for each of these targets, along with a discussion of their key attributes. Maps and sections are presented in subsequent Sections.
Primary Fresh Rock Proterozoic
Basement Gold Mineralization All fresh basement gold zones discovered at Cuiú Cuiú to date are associated with either extensive deformation zones or smaller-scale faults and fracture systems. The larger gold zones are mainly associated with extensive deformation and hydrothermal alteration.
Narrow mafic dykes are common within these gold zones. These dykes exhibit varying degrees of deformation, ranging from massive and undeformed to highly deformed chlorite schist. This variation may reflect a prolonged period of intrusion occurring before, during and after the deformation and gold mineralization events. These dykes are typically subvertical and steeply dipping, though some sub-horizontal mafic dykes have also been observed. Additionally, various felsic, pegmatite, and aplite dykes have been intersected in drill holes, with the exception of crowded diorite porphyries, which tend to be narrow and less abundant.