Summary:
The Ore occurs mainly at the top of the Serra da Saudade Formation and underlies the Areado Group sandstone. The Ore occurs in extensive outcrops, along both banks of the Indaiá River, over an area of approximately 120 km x 20 km. It covers the regions and municipalities of Santa Rosa da Serra and São Gotardo (SW), Matutina, Quartel de São João and Cedro do Abaeté (center), Paineiras and Biquinhas (NE) in the state of Minas Gerais.
The thickness of the Ore unit varies from 15 m to 80 m in the southernmost domain, to over 50 m in the northern half of the Serra da Saudade range. The lower contact with the siltstone of the Serra da Saudade Formation is transitional (2 m to 3 m in width) and contains intercalations of limestone lenses and calciferous siltstone.
The Serra da Saudade Formation was eroded during the Gondwana Cycle (KING, 1956), probably during the Jurassic Period, and it was over this extensive peneplain that the Cretaceous sandstone beds of the Areado Group were deposited.
The upper contact is transitional with rhythmic intercalations of Ore and siltstone of various colors (predominantly pink when weathered), defined informally as the transition zone. These intercalations vary from millimeters to meters in thickness.
Mineralization
After performing detailed mineralogical studies using a combination of optical microscopy, X-ray diffraction, electron microprobe analysis and scanning electron microscopy, it was determined that the Ore is a silty-clayed sedimentary rock. Despite the presence of folds in the outcrops, no minerals, metamorphic structures or evidence of deformation were identified in the thin sections. The natural fragmentation in the outcrops is due to the fractures and bedding surfaces. Therefore, the mineralization in the project area is now referred to as a glauconitic siltstone. Despite this change in nomenclature, the project mineral resources were not affected, as the Ore deposit is homogeneous in its glauconitic siltstone content.
The Ore is the target rock type across the VERDE group of permits, as it contains a high content of K2O. It is a fine-grained siltstone, usually laminated, alternating with more massive levels and a few intercalations of dark-green argillite sheets.
The Ore unit shows millimeter- to centimeter-thick bands that are rich in glauconite, dark green in color and interbedded with quartz-rich layers.
A petrographic study using optical microscopy, X-ray diffraction, an electron microprobe and a scanning electron microscope was carried out upon fresh samples of Ore containing more than 10% K2O, collected from diamond drilling half core samples. Thin section studies detected significant quantities of K-bearing minerals and quartz: glauconite (40%-80%), K-feldspar (10%-15%), quartz (10%-60%), muscovite-sericite (5%), biotite (2%), titanium oxide (<1%), manganese oxide (<1%), goethite (<1%), barium phosphate and rare-earth element phosphates (trace amounts).
Mineralized Zones
As stated previously, potassium mineralization in the ore occurs as mineral-forming elements of glauconite, K-feldspar and muscovite-sericite. The Ore located within VERDE’s permit area can be traced along the entire 120 km strike length and has a potential width of up to 500 m. Enriched levels of potassium with K2O grades from 8% to 12% are associated with the glauconitic levels, which are dark green in color.
Surrounding Rock Types
To the east, there are green to yellowish sandstone beds with hummocky cross-stratification showing low-angle truncation surfaces and wavelengths of between 1 to 2 m and forming beds that are between 5 cm and 2 m in thickness with a fine medium-grained sandy base and a silt-mud top. The base of the Serra da Saudade Formation is composed of a typical gray siltstone. The gray siltstone grades upward into mud-silt-sand rhythmite, characterized by laminated rhythmite with varying proportions of mud, silt and fine sand grains. It may be intercalated with beds of fine-grained sandstone. The laminations are formed by interbedded green fine-grained siltstone and greenish gray to white, soft mudstone. These lithotypes form the transition zone.
Deposit Types
Glauconite is considered a diagnostic element indicative of continental shelf marine environments with slow rates of accumulation. It develops as a consequence of diagenetic alteration of sedimentary deposits, bio-chemical reduction and subsequent mineralogical changes affecting iron-bearing micas such as biotite and is also influenced by the decay process in organic matter degraded by bacteria in marine animal shells. Glauconite forms under reducing conditions in sediments (AMOROSI, 1995).