Summary:
DISTRICT GEOLOGY
The Almas Gold Project is situated in an historical gold mining area with numerous small artisanal mines (garimpos) and the former Vale Paiol Gold operation. Gold occurs in sheared metavolcanic rocks within the greenstone sequence, as well as in sheared felsic intrusive rocks. The rocks are, in general, Paleo-Proterozoic in age (~2.2 billion years old) and have undergone regional metamorphism ranging in intensity from greenschist- to amphibolite-facies. The metamorphism resulted in deep-seated, shearhosted, mesothermal, gold deposits which have more recently been referred to as orogenic gold deposits. The gold-mineralized zone occurs in the core of hydrothermal alteration zones, generally associated with variable amounts of quartz, carbonate, albite, sericite and sulphide minerals.
DISTRICT MINERALIZATION
Gold mineralization is found in three groups of rocks, in metabasalts of the Córrego Paiol Formation, in metasedimentary rocks of the Morro do Carneiro Formation and in granite-gneiss complexes.
Several occurrences of gold are hosted in the metabasalts, mainly south of Almas, the most important being that of the Córrego Paiol mine. The occurrences hosted in metasedimentary rocks are dominant in the southern portion of the terrain, Conceição do Tocantins region, while the occurrences in granite-gneissic rocks are distributed throughout the granite-greenstone terrain.
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-to-intermediate 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 sulfide minerals. The stronger gold mineralization is associated with faults and shear zones.
DEPOSIT GEOLOGY
The Cata Funda - Paiol Trend is a structural domain within the Almas volcano-sedimentary sequence that contains the primary and secondary gold targets. The Paiol and Cata-Funda deposits have many similarities with respect to mineralization type, geometry and lithologies, although some distinctive characteristics are noted.
The Paiol and Cata Funda deposits are situated on the same trend that generally strikes N10°-20°E at Paiol but rotates to N10°- 20°W at Cata Funda. The dips range from 60° to 80° northwest and 40 to 60 southwest, respectively. Strong gold mineralization is associated with hydrothermal alteration centered on mylonitic bands and sulfide-bearing quartz veins in strongly sheared metavolcanic rocks.
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 is also found in the area but are typically small and not the target of current exploration.
Mesothermal gold deposits are a distinctive type of gold deposit which are typified by many consistent features in space and time. Perhaps the single most consistent characteristic of the deposits is their consistent association with deformed metamorphic terranes of all ages. Observations from throughout the World’s preserved Archaean greenstone belts and most recently active Phanerozoic metamorphic belts indicate a strong association of gold and greenschist facies rocks. However, some significant deposits occur in higher metamorphic grade Archaean terranes or in lower metamorphic grade domains within the metamorphic belts of a variety of geological ages. Pre-metamorphic protoliths for the auriferous Archaean greenstone belts are predominantly volcano-plutonic terranes of oceanic back-arc basalt and felsic to mafic arc rocks. Clastic marine sedimentary rock-dominant terranes that were metamorphosed to graywacke, argillite, schist and phyllite host younger mineralization and are important in some Archaean terranes.
These deposits are typified by quartz-dominant vein systems with =3 to 5% sulfide minerals mainly Fe-sulfides and =5 to 15% carbonate minerals. Albite, white mica or fuchsite, chlorite, scheelite and tourmaline are also common gangue phases in veins in greenschist-facies host rocks. Vein systems may be continuous along a vertical extent of 1 to 2 km with little change in mineralogy or gold grade. Mineral zoning does occur, however, in some deposits. Au/Ag ratios range from 10 (normal) to 1 (less common) with ore in places being in the veins and elsewhere in sulfurized wallrocks.
Deposits exhibit strong lateral zonation of alteration phases from proximal to distal assemblages on scales of meters. Mineralogical assemblages within the alteration zones and the width of these zones generally vary with wallrock type and crustal level. Most commonly, carbonates include ankerite, dolomite or calcite; sulfides include pyrite, pyrrhotite or arsenopyrite; alkali metasomatism involves sericitization or, less commonly, formation of fuchsite, biotite or K-feldspar and albitization and mafic minerals are highly chloritized. Amphibole or diopside occur at progressively deeper crustal levels and carbonate minerals are less abundant. Sulfidation is extreme in banded iron formation (BIF) and Fe-rich mafic host rocks.
The orogenic gold deposits targeted in current exploration in the Almas Gold 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.