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 80-m-thick upper metasandstone exposed in the Nosde and Japonês open pit (Malheiros and Garcia, 2023; unpublished manuscript).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.
Ernesto Geological and Domain Modelling
The geological layout of the Ernesto deposit area is subdivided into seven lithological domains, two of which are potentially mineralized. These domains are metaconglomerate (MGL) and Mylonite (MYL). Furthermore, local mineralization is observed in the metarenites (MAR), higher than MGL and lower than MGL, but has not been considered as a potentially mineralized host.
All seven units exhibit dip (foliations) normally between 20 and 45º to the NE and deepen more significantly in the continuity of the N-NE trend.
The two potentially mineralized units (MGL and MYL), respectively Middle Trap and Lower Trap, have a permeable conglomeratic horizon, where it crosses dilatation structures developed by folding and faults. This horizon is composed of milky quartz veins with fresh and weathered pyrites, with alteration of the sericite and chlorite matrix and fissure hematite. Mylonite, on the other hand, consists of an intensely altered mylonite zone developed along detachment structures between the Lavrinha tonalite and the feldspathic metarenite of the Fortuna Formation. Alteration associated with gold mineralization within the mylonitic zone includes abundant veins (i.e. parallel to foliation) of quartz, and veins with coarse-grained euhedral pyrite and fine-grained bipyramidal crystalline magnetite, along with visible gold. In addition, there is sericite, chlorite, specularite and fissure hematite and fine-grained limonite present. The presence of extensional faults at the time of mineralization caused the alteration of the tonalitic unit of the footwall. Tonalite is extensively weathered and historically recorded as saprolite. Lithological models were used to confine the grade shell models in Leapfrog Geo® Software. A nominal 0.2 g/t Au cut-off was used to constrain mineralization within two main mineralized units (MYL and MGL). The local mineralization within metarenites (MAR) and Saprolite (SAP) was also modeled using the same nominal cut-off grade.
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.
Pau-a-Pique deposit
The Pau-a-Pique deposit is hosted by a mica-rich mylonitic zone in the sheared contact between the footwall Fortuna Formation and the hanging wall Mesoproterozoic igneous basement. The metasediments comprise metaconglomerates and arkosic metasandstones, while the basement includes diorites, tonalites, and granodiorites. These units were affected by the D2 shearing, and the igneous rocks were transformed into schistose layers of biotite, quartz, chlorite, muscovite, epidote, calcite, ilmenite, rutile, magnetite, apatite, and titanite. Toward the center of the shear zone, the biotite transitions into gray muscovite in zones with subsequent hydrothermal alteration and high-grade gold mineralization. The sheared metasediments in the hanging wall are described as centimetric to metric layers of muscovite with quartz, magnetite, rutile and ilmenite.
A silicified breccia occurs to the southwest of the Pau-a-Pique orebody, with no gold mineralization. It is uncertain whether it is related to an earlier rifting-related hydrothermal event, during the deposition of the Aguapeí sediments, a late D3 event, or a later Neoproterozoic event.
Mineralization Pau-a-Pique deposit
The Pau-a-Pique ore zone is a 3- to 15-m-wide schistose layer with biotite and muscovite, along with multiple generations of quartz veins. The gold mineralization includes disseminated coarse pyrite with associated swarms of quartz veins. Gold occurs as inclusions in the pyrite and native gold within the quartz veins or disseminated in the mylonite. Gangue minerals comprise biotite, muscovite, magnetite, albite, chlorite, calcite, apatite, ilmenite, and rutile.
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.