Summary:
Deposit Types
The known gold occurrences in the Almas area are classified as orogenic, shear-hosted mesothermal gold deposits. Minor occurrences of lateritic or even placer gold are also found in the area but are typically small and not the target of current exploration.
The orogenic gold deposits targeted in current exploration in the Almas Project are hosted in Paleoproterozoic rocks, typically metabasalts and metasediments (commonly called greenstones). Exploration has also identified gold mineralization in granitic intrusives or granitoids, as in the case of the Vira Saia deposit. In all cases, the rocks have been metamorphosed to greenschist or lower amphibolite facies. Mineralization invariably forms along faults or shear zones; typically, the larger mineralized areas correlate with the larger shear zones. As well, flexures and intersection zones, where faults or shears cross, generally correspond to prime sites for these deposits.
The shear zones hosting gold mineralization typically show strong brecciation and mylonitization of the host rocks. Alteration of the host rocks is generally localized along the structural zones and is mainly silicification along with widespread carbonatization, potassic alteration, sericite alteration, and pyritization. Gold occurs in association with sulphides in quartz veins and veinlets. Sulphides are primarily pyrite with trace amounts of arsenopyrite, galena, and chalcopyrite. Gold is primarily free gold with an estimated 10% to 40% attached to sulphides, depending on location. Gold is primarily micron sized, though visible gold is locally present.
Exploration methods in the district typically start with magnetic surveys to identify major structures and magnetic alteration, followed by field mapping and soil sampling. IP surveys are often employed to further identify structures or resistive bodies. Trenching and drilling are used in the final phases.
Mineralization
Gold in the Almas Greenstone Belt occurs in three different associations:
• Gold associated with hydrothermally altered shear zones in basic to intermediate volcanic rocks.
• Gold associated with hydrothermally altered banded iron formation.
• Gold associated with smoky quartz veins in sheared granite gneiss.
Gold mineralization is closely associated with mylonitic banding in shear zones that cut mafic-tointermediate volcanic rocks, schists, and granite-gneiss, the latter being noted at the Vira Saia deposit. Gold occurs as free gold and as gold inclusions within sulphide minerals. Stronger gold mineralization is associated with faults and shear zones (Paiol and Cata Funda.
At Paiol and Cata Funda, individual mineralized shoots are shaped as lenses and/or anastomosing bodies within the shear zone. The mineralization shoots are typically steeply dipping and plunging lenses. Gold mineralization typically occurs in the centre of the alteration zone, associated with albite-quartz-ankerite (calcite) and the sulphide minerals, pyrite, chalcopyrite, and pyrrhotite. Some coarser grained gold has been observed at Cata Funda where it occurs primarily in quartz-carbonate veins within albite-sericite-pyrrhotite alteration envelopes developed in mafic to intermediate metavolcanic host rocks.
At Vira Saia, gold is closely associated with sulphide-bearing, quartz-sericite-rich ultramylonites formed in the core of shear zones developed in granodiorite. Chalcopyrite and galena are very rare. The intensity of the hydrothermal alteration is proportional to the progressive deformation in the shear zone. Quartz veins typically have saccharoidal (sugary) textures, believed to have formed by dynamic crystallization in shear zones, suggesting a syntectonic timing of vein formation. The Vira Saia deposit belongs to the lode gold, orogenic deposit type, with predominant quartz-sericite-carbonate alteration surrounding quartz veins with low iron sulphide content (<2%).
Paiol Mine
The Paiol mine is situated at the extreme north of the Brasília Fold Belt, at the base of the Riachão do Ouro Greenstone, which consists of oceanic volcanic rocks. These rocks were subjected to metamorphism, deformation, and hydrothermalism during the Paleoproterozoic collisional event, which provided migration of fluids mineralized in gold.
The main Paiol mineralized body extends approximately 650 m down dip, 1,250 m along strike, and has an average thickness of 30 m.
The orogenic mineralization is within a dextral transgressive fault marked by strong hydrothermal alteration. The alteration halos from the margin to the center of the fault are summarized as follows: (i) Epidote zone (quartz-amphibole-chlorite schist and quartzalbite-chlorite schist with pyrrhotite + chalcopyrite, waste rock); (ii) Chlorite zone (quartz-calcitechlorite schist with thick euhedral pyrite hanging and footwall); (iii) Ankerite zone (Chloriteankerite-quartz schist with disseminated fine pyrite + pyrrhotite, marginal halo, low grade); (iv) Sericite zone (sericite-ankerite-quartz schist with fine pyrite + pyrrhotite + arsenopyrite, central halo, medium grade); (v) Quartz zone (sericite-quartz schist with intense pyrite + pyrrhotite + arsenopyrite sulphidation, zone core, high grade).
Vira Saia
The Vira Saia deposit is hosted in the granitic gneiss complex. These complexes are composed of isotropic to weakly foliated granitoid plutons which have been variably classified as tonalites, trondhjemites, granodiorites, quartz monzonites, amphibole-quartz diorite, and monzogranites. A second granitoid suite, composed of the same lithologies but containing biotite as the primary mafic mineral, is recognized in the region.
At Vira Saia, a shear zone (N45°W) developed in granodiorite controls brecciation, alteration and gold mineralization.
The main Vira-Saia deposit body has overall dimensions of approximately 200 m in the down dip direction, 350 m along strike and averages 15 m in thickness. Exploration has also identified three smaller zones designated: East Body, NW Body, and NW Extension Body.
Hydrothermal alteration is well developed, and its intensity is proportional to the intensity of deformation in the granitic host rock. The outermost alteration zone is foliated and characterized by the appearance of muscovite, albite, and epidote. In an intermediate alteration zone, muscovite and albite still occur but are now associated with calcite and sulphide minerals, up to 1% by volume. Interfoliated quartz and recrystalized quartz veins with strong sericite on vein selvages occur in the core of the alteration zone. Sulphide mineralization is more intense in the central part of the alteration zone where very fine-grained pyrite occurs as inclusions in quartz and muscovite grains.
Cata Funda
The Cata Funda deposit is situated in the northern portion of the Almas Greenstone Belt, immediately southeast of the Almas town site. The deposit is hosted in metabasic and metasedimentary rocks that display hydrothermal alteration processes such as sericitization, carbonization, albitization and silicification. Host rocks are in contact with siliceous breccias and quartz-carbonate schists to the west and with tourmaline-bearing quartzites and metapelites of the Morro do Carneiro Formation to the northeast.
The gold mineralization occurs primarily in the central portion of the structure which displays zoned alteration assemblages like that previously described for the Paiol deposit.
The Cata Funda deposit has overall dimensions of approximately 240 m in the down dip direction, 230 m along strike and averages 10 m in thickness.
Bedrock is typically overlain by 2 m to 6 m of red, argillaceous soil, weakly magnetic, with low percentages of quartz fragments and pisolites. Beneath the soil horizon is 8 m to 30 m of red to yellow saprolite, locally sericitic and mottled containing Fe-Mn-oxides on relict foliations and fractures.