Summary:
Ernesto-Lavrinha-Nosde deposits are situated in the Middle Proterozoic Aguapeí belt, along the southwestern margin of the Amazon Craton, in the Sunsás-Aguapeí province.
In the Ernesto-Lavrinha-Nosde area, the Middle Proterozoic Fortuna Formation crops out at the Cágado Anticline. The Formation consists, from base to the top, of a 40- to 60-m-thick basal feldspathic metasandstone and a 10- to 30-m-thick overlying metaconglomerate that serves as a stratigraphic guide. The above 60- to 120-m-thick lower metasandstone comprises interbedded metasandstone and muscovite schist, followed by a 20- to 60-m-thick intensely altered muscovite schist exposed in the Lavrinha and Japonês open pits. The overlying upper feldspathic metasandstone is ~30 m thick, followed by the < 80-m-thick upper metasandstone exposed in the Nosde and Japonês open pit (Malheiros and Garcia, 2023; unpublished manuscript). All of the rocks have been subjected to regional low-grade greenschist facies metamorphism but still display many well-preserved sedimentary structures such as graded bedding, crossbedding, and load deformation features.
The Lavrinha tonalite underlies the sediments. This unit is composed of fine- to medium-grained plagioclase and quartz into an aphanitic matrix, being weakly foliated and hydrothermally altered. The contact with the Fortuna Formation exhibits 0.2 m to 10 m mylonitic and ultramilonitic fabrics, and a metric layer of intensely altered, crushed, and decomposed metatonalite or saprolite. These features are observed only in drill cores under the Lavrinha and Nosde deposits and in the Ernesto open pit.
Mineralogy of Ore-Bearing Rock in Ernesto, Lavrinha, and Pau-a-Pique
The XRD analysis indicates that Ernesto and Lavrinha mineralized rocks are composed of 50 to 70% quartz, 25 to 40% muscovite, 5 to 7% hematite, and 1 to 4% kaolinite, with minor goethite and microcline. Ernesto and Lavrinha show typical mica-schist mineral composition that has been metamorphosed under lower greenschist facies. Most of the feldspar has been altered to sericite. Sulphide minerals such as pyrite are not identified by the XRD method. However, the presence of iron oxides in Ernesto and Lavrinha suggests that most of the pyrite is altered and converted to iron oxides. The lithological logging of mineralized intervals supports this since, in most cases, pyrite is replaced by iron oxide.
Lavrinha Geological and Domain Modelling
The geological layout of the Lavrinha deposit area is subdivided into seven lithological domains, three of which are potentially mineralized. These domains are inferior and superior metarenites (MAR) – Upper Trap Schists. Furthermore, local mineralization is observed in the metarenites (MAR), higher than metaconglomerate (MGL), but has not been considered as a potentially mineralized host.
Deposit Types
Ernesto, Lavrinha, Nosde and Japonês and Pau-a Pique deposits are described as a detachment-style gold deposit that typically has the following characteristics:
• Gold mineralization is associated with low angle to flat detachment faults, generally with a normal (extensional) sense of movement that consistently places younger units over older units.
• Mineralization is commonly characterised by quartz-rich vein and veinlet zones (in the ±25% range) with magnetite or hematite, coarse euhedral pyrite (in the ±1% range), sericite, some clay mineral, some late stage calcite and gold. The gold is commonly associated with only very small amounts of silver.
• Mineralization is typically located along a 3 m to 8 m thick of mylonitic rocks of a detachment (or thrust) fault that intersects high angle structures, either faults or folds. The detachment is commonly within a deformed zone 10 m to 30 m thick.
• The continuity of the mineralization within the detachment zone is normally quite good, extending over 100 m.
• Detachment–style gold mineralization is in altered rock parallel to anticline axes and faults.
• Multiple styles of mineralization are common with local stacked mineralized zones.
• Fluid inclusion studies indicate temperatures of formation about 200°C to 250°C.