On July 1, 2024, Nexa Resources S.A. completed the previously announced sale of the Morro Agudo Complex situated in the state of Minas Gerais in Brazil to Casa Verde Holding Ltda.
Summary:
The mineralization at Morro Agudo and the Ambrosia Trend are considered to be examples of Irish-type Zn-Pb deposits hosted by carbonate rocks which are a subclass of Mississippi Valley Type deposits.
MORRO AGUDO MINE
The Morro Agudo zinc and lead deposit comprises a number of concordant stratabound sulphide bodies, non-concordant remobilized sulphide bodies, and intraformational dolarenites and breccias of the Morro do Calcário Formation.
Mineralization
The combined length of the known mineralized bodies is approximately 1,700 m, the width is about 1,200 m, and the bodies have a variable thickness with a maximum of about 10 m. Mineralization is bounded to the northwest by the Main Fault. The western limit has not yet been defined, but drilling has shown continuity of mineralization at depth. Drilling deeper than 1,000 m has shown continuity of mineralization at depth, but with thinner intercepts (1 to 5 m) and lower grades.
The mineralized bodies are separated from each other by waste intervals of stratified dolarenites at upper and intermediate levels, and by waste breccias at lower levels. Sulphide levels occur as concordant stratabound lenses in dolarenite, and subordinately in dolomitic and dolarenitic breccias, in addition to occurring as late tectonic structure fill (faults/fractures). These sulphide lenses are typically less than 4 m thick, and separated by barren intervals that range from a few centimetres to several metres, depending on the lithology where they are deposited.
Mine geologists have identified eight mineralized strata, denominated from G to N, from the base to the top, respectively that are consistent across the deposit despite being divided by faults (Figure 7-8).
According to Rubo and Monteiro (2010), three episodes of mineralization are identified at the Morro Agudo deposit:
• Event 1: replacement of breccia matrix for chalcedony with very fine to finegrained sphalerite and other sulphides, mostly preserving the micritic carbonate particles and giving rise to mineralized dolarenites
• Event 2: filling of voids (veins and larger cavities) by fine to coarse-grained sphalerite, pyrite, and galena
• Event 3: Formation of late veins with sulphides cutting the veins that were deposited in the second event.
AMBROSIA TREND
The Ambrosia Trend deposits (Ambrosia Sul, Ambrosia Norte and Bonsucesso deposits) occur in the pelite–carbonate rocks of the Vazante Group in a similar stratigraphic position to Morro Agudo. Lower greenschist facies metamorphism has affected the Vazante Group in the area. When it is possible to recognize sedimentary textures, the prefix “meta” is not used for lithological descriptions.
Mineralization
Mineralization is hosted in dolomitic breccias along the Ambrosia Fault. At Ambrosia Norte and Bonsucesso, mineralization is predominantly vein-like, and is associated with brecciated dolomites of the Morro do Calcário Formation. In most cases, there is a single mineralized structure, but occasionally, two or more mineralized structures are present. At Ambrosia Sul, mineralization is controlled by hydrothermal breccias.
There are two types of mineralization associated with the Ambrosia Fault:
• Oxide mineralization: smithsonite and cerussite. The calamine mineralization (a mixture of the zinc secondary minerals hemimorphite (Zn4(Si2O7)(OH)2·H2O) and smithsonite (ZnCO3), was generated by weathering and supergene enrichment of the sulphide mineralization
• Sulphide mineralization: sphalerite and galena. Sulphide mineralization mainly fills microfractures in the rock and is associated with dissolution areas (cavity filling) together with sparry dolomite and pyrite.
The main mineralization stage is represented by the formation of sphalerite and significant concentrations of pyrite with subordinate galena concentrated in thin veinlets and vein networks that cut the recrystallized dolomite. Near the surface, there is a significant concentration of calamine (Monteiro, 2002).