Summary:
The geology of the Vazante Operation area consists of a sequence of pelitic carbonate rocks belonging to the Serra de Garrote and Serra do Poço Verde formations of the Vazante Group. The currently known mineralization has been traced along a strike length of approximately 10.5 km, extending from the southern end of the Vazante Mine to the northern limits of the Extremo Norte Mine.
Mineralization
The zinc-lead-silver mineralization at the Vazante Operation is hosted by the Vazante Shear Zone which has been traced by drilling and sampling along a strike length of approximately 12 km. The Vazante Shear Zone has a general strike of azimuth 50° and dips approximately 60° to the northwest at surface. The hanging wall lithologies of the Vazante Shear Zone are comprised of dolostone and sericitic phyllite, slates and marl units of the Serra do Poço Verde Formation while the footwall lithologies to the Vazante Shear Zone are dominated by dark grey dolostones of the Upper Morro do Pinheiro Member. Drilling information indicates that the dip of the zinc mineralized zone gradually decreases with depth in the southern portions of the structure.
The zinc mineralization at the Vazante and Extremo Norte mines is composed largely of hypogene zones that are composed mainly of willemite (Zn2SiO4) veins, veinlets, and stockworks that are hosted by sphalerite-rich carbonate. The mineralization typically contains willemite (50% to 70%), dolomite (10% to 30%), siderite (10% to 20%), quartz (10% to 15%), hematite (5% to 10%), zinc-rich chlorite (5% to 10%), barite (<5%), franklinite (<5%), and zincite (<5%), with subordinate concentrations of magnetite and apatite (Monteiro et. al., 2006). Lead and silver are also recovered from the hypogene mineralization is produced from the Vazante Operation. While no detailed studies regarding the specific lead and silver bearing minerals have been carried out on samples of the hypogene mineralization, several detailed mineralogical studies have been conducted using concentrate samples. RPA notes that the majority of the lead mineralization in the concentrates has been found to be related to galena (PbS), with lesser amounts of lead being contained in cerussite (Pb(CO3)). Mineralogical studies have indicated that the silver values are contained in the minerals acanthite (Ag2S) and jalpaite (Ag2CuS2).
Supergene zones of zinc-rich mineralization have been developed in the near-surface portions of the hypogene mineralized zones. These supergene zones are referred to as the calamine zones at the Vazante Operation. The calamine mineralization is composed principally of smithsonite (ZnCO3) that includes subordinate amounts of hemimorphite (Zn4(Si2O7)(OH)2.H2O) and quartz. The calamine mineralized zones were derived from weathering of the primary willemite mineralization.
Deposit Descriptions
Vazante
The zinc silicate mineralization of the Vazante deposit is hosted in a tectonic hydrothermal breccia zone that is found near the contact between the Lower Pamplona and the Upper Morro do Pinheiro Members of the Vazante Group. Metamorphosed dikes are tectonically imbricated with the carbonates and hydrothermal breccias in the main orebody.
The Vazante portion of the orebody, which is approximately seven kilometres in length, has a variable thickness, and has been traced by drilling to a depth of approximately 800 m below surface. Willemite can form as pods, veinlets, and metre wide veins within the hydrothermal breccia and is locally controlled by antithetic faults in the deposit. The willemite composition, according to electron micro-probe (EMP) studies conducted by Monteiro et. al. (2006), has been shown to contain minor amounts of FeO (up to 0.3 wt%) and Ca (up to 0.13 wt %).
Willemitic mineralization is tectonically imbricated with small sulphide orebodies, metabasites, and brecciated metadolomites. The sulphide orebodies are offset by normal and reverse faults and are cut by late hydrothermal veins. This can cause a complex relationship between orebodies and host rocks. The sulphide bodies consist of iron poor sphalerite with round inclusions of galena, and occasional inclusions of hematite, quartz, and dolomite. The relationship between sphalerite and willemite is not well understood.
Extremo Norte
The Extremo Norte mineralization is associated with the Vazante Shear Zone, and is primarily hosted in the Lower Pamplona Member, or along the contact between the Lower Pamplona and Upper Morro do Pinheiro Members. The mineralization consists of willemite (in variable quantities, but can reach up to 40% in abundance), specular hematite (1% to 50%), and minor franklinite (ZnFe2O4).
The mineralized zones occur as a primary gently undulating, northwest dipping tabular sheet that has been traced by drilling along a strike length of approximately four kilometres and to a depth of approximately 600 m from surface. The mineralization at the Vazante Mine is observed to generally contain more sulphide minerals associated with the willemite, and observations suggest that it has experienced a higher degree of ductile deformation than is observed at the Extremo Norte deposit. Metamorphosed and hydrothermally altered basic dikes are tectonically imbricated with the carbonates and the hydrothermal breccias in the Vazante Mine (Babinski et. al., 2005), but these have not been observed at the Extremo Norte deposit (McGladrey, 2014).
Calamine
The majority of the existing calamine mineralized zones are located along the Extremo Norte portion of the Lumiadeira-Sucuri-Extremo Norte mineralized trend. Several minor occurrences of calamine are located along the northeastern sections of the Sucuri trend of the Vazante Mine. The calamine mineralized zones consist of fully weathered supergene material that is derived from the primary willemite zinc mineralization and its enclosing host rock units.
Deposit Types
The zinc deposit found at the Vazante Operation is the largest known non-sulphide zinc deposits in the world and is the type example for this class of mineral deposit. The Vazante Operation mineralization is composed primarily of hypogene zinc silicate minerals dominate in abundance by willemite. The primary hypogene willemite mineralization has been subjected to supergene weathering processes to produce secondary zinc mineralization, referred to as the calamine mineralization, which is composed of smithsonite (ZnCO3) and hemimorphite (Zn4Si2O7(OH)2).
Non-sulphide hypogene zinc deposits are known to occur either as stratiform deposits, or structurally controlled deposits. The mineralization at the Vazante Operation is an example of a structurally controlled deposit.
The hypogene mineralization has two strong controls, lithostratigraphic and structural. The Vazante Operation deposit is clearly confined to a clay-dolomite interval near the contact between the Lower Pamplona Member and the Lower Morro do Pinheiro Member. Mineralization is restricted to the brecciated area in the fault zone that has had a long history of reactivation (Monteiro, 2002). This hypogene mineralization is interpreted to result from deposition of metals from hydrothermal fluids that have moved through the Vazante Shear Zone.
The morphology of the Vazante Operation deposit consists of veins and anastomosing podiform bodies that are fault bounded. The geochemical controls on the deposition of the mineralization are not well understood and the source of the metals is not known. The deposit genesis is thus in debate, as it may have originally represented a sulphide deposit that was subsequently silicified, or it may have consisted of primary zinc silicate mineralization that was later re-concentrated into the Vazante Shear Zone.