Summary:
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.
The Pau-a-Pique and Ernesto (Lower Trap) deposits are similar in that they both occur at the contact between the Aguapeí Group and the basement meta-tonalite. At both locations, the contact is associated with shear zones and hydrothermal alteration assemblages with pyrite, sericite and hematite. However, there are some differences between each deposit including the Ernesto deposit having a low-angle dip (±15°-25°), and being hosted in the contact between the Aguapei Group and metatonalite (Lower Trap) and areas of weakness within Aguapei Group. While Pau-a-Pique mineralization has a high-angle (±80°) and is hosted only in the contact of the Aguapei Group and metatonalite.
The Middle Trap in Ernesto and São Francisco deposits are similar in that they both occur within Aguapeí Group psammitic rocks affected by hydrothermal alteration resulting in assemblages rich in silica, sericite and hematite. The Ausenco (2010) report draws parallels between the shallow dipping Ernesto Deposit and detachment-style gold deposits in the south eastern California and to bedding plane parallel shears in Tarkwa sediments in Ghana. Reid et al. (2012) consider the São Francisco Mine (currently in care and maintenance and owned by a third party), located north of Ernesto, to be a shear hosted lode gold deposit. São Francisco is located approximately 60 km northwest of Ernesto and displays similar host Aguapeí Group lithologies and structural controls at the deformed basement/Aguapeí Group contact. São Francisco is considered by Reid et al. (2012) as epigenetic, structurally controlled, and composed of narrow, 1 cm to 5 cm wide, and quartz veins containing free gold. The veins, and vein systems and stockworks both parallel and crosscut the bedding planes and appear to represent separate but closely related mineralizing events. The São Francisco, Ernesto-Nosde_Lavrinha and Pau-a-Pique Deposits are broadly similar in host lithologies, structural style, alteration, and mineralization and all share characteristics of shear hosted lode gold deposits. At Lavrinha and Nosde, mineralization occurs within a schist sub-member of the Aguapei Group. Mineralization is often associated with narrow quartz vein and veinlets in phyllonitic matrix with strong sericitization and chloritization. The thickness and size of quartz veins are smaller than Ernesto and rarely exceed 1 m in true thickness. Pseudomorphs of pyrite and strong sericitization with the presence of quartz are good indicators for mineralization intervals. Strong foliation and kink-band structures disrupted mineralized shoots both along strike and down dip of the deposit. The style of mineralization and deposit type is very similar to Ernesto and detachment style faults are marked with pervasive alteration along the contacts and within a sericite schist package. Mineralization and alteration both developed in contact meta-arenite with sericite schist and also within the sericite schist.
Mineralization and Alteration
Ernesto Deposits
The Lower Trap consists of an intensely altered mylonitic zone developed along detachment structures between the Lavrinha tonalite and the feldspathic metasandstone of the Fortuna Formation. Alteration associated with gold mineralization within the mylonitic zone includes abundant quartz veins and veinlets with coarse-grained euhedral pyrite and fine-grained bipyramidal crystalline magnetite, along with visible gold. Additionally, there is fine-grained sericite, chlorite, specularite, and fissural hematite and limonite.
The presence of extensional faulting at the time of mineralization caused the alteration of the footwall tonalitic unit. The tonalite is extensively altered and historically logged as saprolite. However, the mineralogical composition of this altered footwall unit is completely different from the saprolite on the surface. The footwall saprolite is mainly composed of clay minerals produced by alteration of feldspar and mica from tonalite groundmass and gradually changes to weakly altered tonalite.
The Middle Trap is restricted to a permeable conglomeratic horizon, where it intersects dilation structures developed by folding and faulting. It comprises milky quartz veins with fresh and weathered pyrites, with sericite and chlorite alteration of the matrix and fissural hematite. This trap crops out in the Ernesto pit, and is intersected in drill holes at the Nosde and Lavrinha deposits.
The Upper Trap is widely developed in the Lavrinha and Nosde deposits, occurs in metapelitic rocks (hematite sericite schist) in dilation zones of the intensely deformed synclinal troughs. The Upper and Intermediate traps share similar alteration and mineralization suites. The Upper Trap seems to be eroded in the Ernesto deposit area.
The Bonus Trap consists of centimetre-thick cross-cutting quartz veins hosted by the upper metasandstone in the Nosde and Japonês deposits. These milky quartz veins include fresh and weathered pyrite and box works, along with visible gold. Hematite and limonite occur as fissure-filling and halos around the mineralized quartz veins.
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.